This file documents the internals of the GNU compilers.
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Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being “Funding Free Software”, the Front-Cover Texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled “GNU Free Documentation License”.
(a) The FSF’s Front-Cover Text is:
A GNU Manual
(b) The FSF’s Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.
collect2gcc.target/i386gcc.target/spu/eagcc.test-frameworkdg-add-optionsdg-require-supportdg-final
gcovGIMPLE_ASMGIMPLE_ASSIGNGIMPLE_BINDGIMPLE_CALLGIMPLE_CATCHGIMPLE_CONDGIMPLE_DEBUGGIMPLE_EH_FILTERGIMPLE_LABELGIMPLE_NOPGIMPLE_OMP_ATOMIC_LOADGIMPLE_OMP_ATOMIC_STOREGIMPLE_OMP_CONTINUEGIMPLE_OMP_CRITICALGIMPLE_OMP_FORGIMPLE_OMP_MASTERGIMPLE_OMP_ORDEREDGIMPLE_OMP_PARALLELGIMPLE_OMP_RETURNGIMPLE_OMP_SECTIONGIMPLE_OMP_SECTIONSGIMPLE_OMP_SINGLEGIMPLE_PHIGIMPLE_RESXGIMPLE_RETURNGIMPLE_SWITCHGIMPLE_TRYGIMPLE_WITH_CLEANUP_EXPRdefine_insnenabled attributetargetm Variable__attribute__collect2Next: Contributing, Up: (DIR) [Contents][Index]
This manual documents the internals of the GNU compilers, including how to port them to new targets and some information about how to write front ends for new languages. It corresponds to the compilers (Ubuntu/Linaro 4.6.4-6ubuntu2) version 4.6.4. The use of the GNU compilers is documented in a separate manual. See ‘Introduction’ in Using the GNU Compiler Collection (GCC).
This manual is mainly a reference manual rather than a tutorial. It discusses how to contribute to GCC (see Contributing), the characteristics of the machines supported by GCC as hosts and targets (see Portability), how GCC relates to the ABIs on such systems (see Interface), and the characteristics of the languages for which GCC front ends are written (see Languages). It then describes the GCC source tree structure and build system, some of the interfaces to GCC front ends, and how support for a target system is implemented in GCC.
Additional tutorial information is linked to from http://gcc.gnu.org/readings.html.
| • Contributing: | How to contribute to testing and developing GCC. | |
| • Portability: | Goals of GCC’s portability features. | |
| • Interface: | Function-call interface of GCC output. | |
| • Libgcc: | Low-level runtime library used by GCC. | |
| • Languages: | Languages for which GCC front ends are written. | |
| • Source Tree: | GCC source tree structure and build system. | |
| • Testsuites: | GCC testsuites. | |
| • Options: | Option specification files. | |
| • Passes: | Order of passes, what they do, and what each file is for. | |
| • GENERIC: | Language-independent representation generated by Front Ends | |
| • GIMPLE: | Tuple representation used by Tree SSA optimizers | |
| • Tree SSA: | Analysis and optimization of GIMPLE | |
| • RTL: | Machine-dependent low-level intermediate representation. | |
| • Control Flow: | Maintaining and manipulating the control flow graph. | |
| • Loop Analysis and Representation: | Analysis and representation of loops | |
| • Machine Desc: | How to write machine description instruction patterns. | |
| • Target Macros: | How to write the machine description C macros and functions. | |
| • Host Config: | Writing the xm-machine.h file. | |
| • Fragments: | Writing the t-target and x-host files. | |
| • Collect2: | How collect2 works; how it finds ld.
| |
| • Header Dirs: | Understanding the standard header file directories. | |
| • Type Information: | GCC’s memory management; generating type information. | |
| • Plugins: | Extending the compiler with plugins. | |
| • LTO: | Using Link-Time Optimization. | |
| • Funding: | How to help assure funding for free software. | |
| • GNU Project: | The GNU Project and GNU/Linux. | |
| • Copying: | GNU General Public License says how you can copy and share GCC. | |
| • GNU Free Documentation License: | How you can copy and share this manual. | |
| • Contributors: | People who have contributed to GCC. | |
| • Option Index: | Index to command line options. | |
| • Concept Index: | Index of concepts and symbol names. | |
Next: Portability, Up: Top [Contents][Index]
If you would like to help pretest GCC releases to assure they work well, current development sources are available by SVN (see http://gcc.gnu.org/svn.html). Source and binary snapshots are also available for FTP; see http://gcc.gnu.org/snapshots.html.
If you would like to work on improvements to GCC, please read the advice at these URLs:
for information on how to make useful contributions and avoid duplication of effort. Suggested projects are listed at http://gcc.gnu.org/projects/.
Next: Interface, Previous: Contributing, Up: Top [Contents][Index]
GCC itself aims to be portable to any machine where int is at least
a 32-bit type. It aims to target machines with a flat (non-segmented) byte
addressed data address space (the code address space can be separate).
Target ABIs may have 8, 16, 32 or 64-bit int type. char
can be wider than 8 bits.
GCC gets most of the information about the target machine from a machine description which gives an algebraic formula for each of the machine’s instructions. This is a very clean way to describe the target. But when the compiler needs information that is difficult to express in this fashion, ad-hoc parameters have been defined for machine descriptions. The purpose of portability is to reduce the total work needed on the compiler; it was not of interest for its own sake.
GCC does not contain machine dependent code, but it does contain code
that depends on machine parameters such as endianness (whether the most
significant byte has the highest or lowest address of the bytes in a word)
and the availability of autoincrement addressing. In the RTL-generation
pass, it is often necessary to have multiple strategies for generating code
for a particular kind of syntax tree, strategies that are usable for different
combinations of parameters. Often, not all possible cases have been
addressed, but only the common ones or only the ones that have been
encountered. As a result, a new target may require additional
strategies. You will know
if this happens because the compiler will call abort. Fortunately,
the new strategies can be added in a machine-independent fashion, and will
affect only the target machines that need them.
Next: Libgcc, Previous: Portability, Up: Top [Contents][Index]
GCC is normally configured to use the same function calling convention normally in use on the target system. This is done with the machine-description macros described (see Target Macros).
However, returning of structure and union values is done differently on some target machines. As a result, functions compiled with PCC returning such types cannot be called from code compiled with GCC, and vice versa. This does not cause trouble often because few Unix library routines return structures or unions.
GCC code returns structures and unions that are 1, 2, 4 or 8 bytes
long in the same registers used for int or double return
values. (GCC typically allocates variables of such types in
registers also.) Structures and unions of other sizes are returned by
storing them into an address passed by the caller (usually in a
register). The target hook TARGET_STRUCT_VALUE_RTX
tells GCC where to pass this address.
By contrast, PCC on most target machines returns structures and unions of any size by copying the data into an area of static storage, and then returning the address of that storage as if it were a pointer value. The caller must copy the data from that memory area to the place where the value is wanted. This is slower than the method used by GCC, and fails to be reentrant.
On some target machines, such as RISC machines and the 80386, the standard system convention is to pass to the subroutine the address of where to return the value. On these machines, GCC has been configured to be compatible with the standard compiler, when this method is used. It may not be compatible for structures of 1, 2, 4 or 8 bytes.
GCC uses the system’s standard convention for passing arguments. On some machines, the first few arguments are passed in registers; in others, all are passed on the stack. It would be possible to use registers for argument passing on any machine, and this would probably result in a significant speedup. But the result would be complete incompatibility with code that follows the standard convention. So this change is practical only if you are switching to GCC as the sole C compiler for the system. We may implement register argument passing on certain machines once we have a complete GNU system so that we can compile the libraries with GCC.
On some machines (particularly the SPARC), certain types of arguments are passed “by invisible reference”. This means that the value is stored in memory, and the address of the memory location is passed to the subroutine.
If you use longjmp, beware of automatic variables. ISO C says that
automatic variables that are not declared volatile have undefined
values after a longjmp. And this is all GCC promises to do,
because it is very difficult to restore register variables correctly, and
one of GCC’s features is that it can put variables in registers without
your asking it to.
GCC provides a low-level runtime library, libgcc.a or libgcc_s.so.1 on some platforms. GCC generates calls to routines in this library automatically, whenever it needs to perform some operation that is too complicated to emit inline code for.
Most of the routines in libgcc handle arithmetic operations
that the target processor cannot perform directly. This includes
integer multiply and divide on some machines, and all floating-point
and fixed-point operations on other machines. libgcc also includes
routines for exception handling, and a handful of miscellaneous operations.
Some of these routines can be defined in mostly machine-independent C. Others must be hand-written in assembly language for each processor that needs them.
GCC will also generate calls to C library routines, such as
memcpy and memset, in some cases. The set of routines
that GCC may possibly use is documented in ‘Other
Builtins’ in Using the GNU Compiler Collection (GCC).
These routines take arguments and return values of a specific machine
mode, not a specific C type. See Machine Modes, for an explanation
of this concept. For illustrative purposes, in this chapter the
floating point type float is assumed to correspond to SFmode;
double to DFmode; and long double to both
TFmode and XFmode. Similarly, the integer types int
and unsigned int correspond to SImode; long and
unsigned long to DImode; and long long and
unsigned long long to TImode.
| • Integer library routines: | ||
| • Soft float library routines: | ||
| • Decimal float library routines: | ||
| • Fixed-point fractional library routines: | ||
| • Exception handling routines: | ||
| • Miscellaneous routines: |
Next: Soft float library routines, Up: Libgcc [Contents][Index]
The integer arithmetic routines are used on platforms that don’t provide hardware support for arithmetic operations on some modes.
These functions return the result of shifting a left by b bits.
These functions return the result of arithmetically shifting a right by b bits.
These functions return the quotient of the signed division of a and b.
These functions return the result of logically shifting a right by b bits.
These functions return the remainder of the signed division of a and b.
These functions return the product of a and b.
These functions return the negation of a.
These functions return the quotient of the unsigned division of a and b.
These functions calculate both the quotient and remainder of the unsigned division of a and b. The return value is the quotient, and the remainder is placed in variable pointed to by c.
These functions return the remainder of the unsigned division of a and b.
The following functions implement integral comparisons. These functions implement a low-level compare, upon which the higher level comparison operators (such as less than and greater than or equal to) can be constructed. The returned values lie in the range zero to two, to allow the high-level operators to be implemented by testing the returned result using either signed or unsigned comparison.
These functions perform a signed comparison of a and b. If a is less than b, they return 0; if a is greater than b, they return 2; and if a and b are equal they return 1.
These functions perform an unsigned comparison of a and b. If a is less than b, they return 0; if a is greater than b, they return 2; and if a and b are equal they return 1.
The following functions implement trapping arithmetic. These functions
call the libc function abort upon signed arithmetic overflow.
These functions return the absolute value of a.
These functions return the sum of a and b; that is
a + b.
The functions return the product of a and b; that is
a * b.
These functions return the negation of a; that is -a.
These functions return the difference between b and a;
that is a - b.
These functions return the number of leading 0-bits in a, starting at the most significant bit position. If a is zero, the result is undefined.
These functions return the number of trailing 0-bits in a, starting at the least significant bit position. If a is zero, the result is undefined.
These functions return the index of the least significant 1-bit in a, or the value zero if a is zero. The least significant bit is index one.
These functions return the value zero if the number of bits set in a is even, and the value one otherwise.
These functions return the number of bits set in a.
These functions return the a byteswapped.
Next: Decimal float library routines, Previous: Integer library routines, Up: Libgcc [Contents][Index]
The software floating point library is used on machines which do not have hardware support for floating point. It is also used whenever -msoft-float is used to disable generation of floating point instructions. (Not all targets support this switch.)
For compatibility with other compilers, the floating point emulation
routines can be renamed with the DECLARE_LIBRARY_RENAMES macro
(see Library Calls). In this section, the default names are used.
Presently the library does not support XFmode, which is used
for long double on some architectures.
These functions return the sum of a and b.
These functions return the difference between b and a; that is, a - b.
These functions return the product of a and b.
These functions return the quotient of a and b; that is, a / b.
These functions return the negation of a. They simply flip the sign bit, so they can produce negative zero and negative NaN.
These functions extend a to the wider mode of their return type.
These functions truncate a to the narrower mode of their return type, rounding toward zero.
These functions convert a to a signed integer, rounding toward zero.
These functions convert a to a signed long, rounding toward zero.
These functions convert a to a signed long long, rounding toward zero.
These functions convert a to an unsigned integer, rounding toward zero. Negative values all become zero.
These functions convert a to an unsigned long, rounding toward zero. Negative values all become zero.
These functions convert a to an unsigned long long, rounding toward zero. Negative values all become zero.
These functions convert i, a signed integer, to floating point.
These functions convert i, a signed long, to floating point.
These functions convert i, a signed long long, to floating point.
These functions convert i, an unsigned integer, to floating point.
These functions convert i, an unsigned long, to floating point.
These functions convert i, an unsigned long long, to floating point.
There are two sets of basic comparison functions.
These functions calculate a <=> b. That is, if a is less than b, they return -1; if a is greater than b, they return 1; and if a and b are equal they return 0. If either argument is NaN they return 1, but you should not rely on this; if NaN is a possibility, use one of the higher-level comparison functions.
These functions return a nonzero value if either argument is NaN, otherwise 0.
There is also a complete group of higher level functions which correspond directly to comparison operators. They implement the ISO C semantics for floating-point comparisons, taking NaN into account. Pay careful attention to the return values defined for each set. Under the hood, all of these routines are implemented as
if (__unordXf2 (a, b))
return E;
return __cmpXf2 (a, b);
where E is a constant chosen to give the proper behavior for NaN. Thus, the meaning of the return value is different for each set. Do not rely on this implementation; only the semantics documented below are guaranteed.
These functions return zero if neither argument is NaN, and a and b are equal.
These functions return a nonzero value if either argument is NaN, or if a and b are unequal.
These functions return a value greater than or equal to zero if neither argument is NaN, and a is greater than or equal to b.
These functions return a value less than zero if neither argument is NaN, and a is strictly less than b.
These functions return a value less than or equal to zero if neither argument is NaN, and a is less than or equal to b.
These functions return a value greater than zero if neither argument is NaN, and a is strictly greater than b.
These functions convert raise a to the power b.
These functions return the product of a + ib and c + id, following the rules of C99 Annex G.
These functions return the quotient of a + ib and c + id (i.e., (a + ib) / (c + id)), following the rules of C99 Annex G.
Next: Fixed-point fractional library routines, Previous: Soft float library routines, Up: Libgcc [Contents][Index]
The software decimal floating point library implements IEEE 754-2008 decimal floating point arithmetic and is only activated on selected targets.
The software decimal floating point library supports either DPD (Densely Packed Decimal) or BID (Binary Integer Decimal) encoding as selected at configure time.
These functions return the sum of a and b.
These functions return the difference between b and a; that is, a - b.
These functions return the product of a and b.
These functions return the quotient of a and b; that is, a / b.
These functions return the negation of a. They simply flip the sign bit, so they can produce negative zero and negative NaN.
These functions convert the value a from one decimal floating type to another.
These functions convert the value of a from a binary floating type to a decimal floating type of a different size.
These functions convert the value of a from a decimal floating type to a binary floating type of a different size.
These functions convert the value of a between decimal and binary floating types of the same size.
These functions convert a to a signed integer.
These functions convert a to a signed long.
These functions convert a to an unsigned integer. Negative values all become zero.
These functions convert a to an unsigned long. Negative values all become zero.
These functions convert i, a signed integer, to decimal floating point.
These functions convert i, a signed long, to decimal floating point.
These functions convert i, an unsigned integer, to decimal floating point.
These functions convert i, an unsigned long, to decimal floating point.
These functions return a nonzero value if either argument is NaN, otherwise 0.
There is also a complete group of higher level functions which correspond directly to comparison operators. They implement the ISO C semantics for floating-point comparisons, taking NaN into account. Pay careful attention to the return values defined for each set. Under the hood, all of these routines are implemented as
if (__bid_unordXd2 (a, b))
return E;
return __bid_cmpXd2 (a, b);
where E is a constant chosen to give the proper behavior for NaN. Thus, the meaning of the return value is different for each set. Do not rely on this implementation; only the semantics documented below are guaranteed.
These functions return zero if neither argument is NaN, and a and b are equal.
These functions return a nonzero value if either argument is NaN, or if a and b are unequal.
These functions return a value greater than or equal to zero if neither argument is NaN, and a is greater than or equal to b.
These functions return a value less than zero if neither argument is NaN, and a is strictly less than b.
These functions return a value less than or equal to zero if neither argument is NaN, and a is less than or equal to b.
These functions return a value greater than zero if neither argument is NaN, and a is strictly greater than b.
Next: Exception handling routines, Previous: Decimal float library routines, Up: Libgcc [Contents][Index]
The software fixed-point library implements fixed-point fractional arithmetic, and is only activated on selected targets.
For ease of comprehension fract is an alias for the
_Fract type, accum an alias for _Accum, and
sat an alias for _Sat.
For illustrative purposes, in this section the fixed-point fractional type
short fract is assumed to correspond to machine mode QQmode;
unsigned short fract to UQQmode;
fract to HQmode;
unsigned fract to UHQmode;
long fract to SQmode;
unsigned long fract to USQmode;
long long fract to DQmode;
and unsigned long long fract to UDQmode.
Similarly the fixed-point accumulator type
short accum corresponds to HAmode;
unsigned short accum to UHAmode;
accum to SAmode;
unsigned accum to USAmode;
long accum to DAmode;
unsigned long accum to UDAmode;
long long accum to TAmode;
and unsigned long long accum to UTAmode.
These functions return the sum of a and b.
These functions return the sum of a and b with signed saturation.
These functions return the sum of a and b with unsigned saturation.
These functions return the difference of a and b;
that is, a - b.
These functions return the difference of a and b with signed
saturation; that is, a - b.
These functions return the difference of a and b with unsigned
saturation; that is, a - b.
These functions return the product of a and b.
These functions return the product of a and b with signed saturation.
These functions return the product of a and b with unsigned saturation.
These functions return the quotient of the signed division of a and b.
These functions return the quotient of the unsigned division of a and b.
These functions return the quotient of the signed division of a and b with signed saturation.
These functions return the quotient of the unsigned division of a and b with unsigned saturation.
These functions return the negation of a.
These functions return the negation of a with signed saturation.
These functions return the negation of a with unsigned saturation.
These functions return the result of shifting a left by b bits.
These functions return the result of arithmetically shifting a right by b bits.
These functions return the result of logically shifting a right by b bits.
These functions return the result of shifting a left by b bits with signed saturation.
These functions return the result of shifting a left by b bits with unsigned saturation.
The following functions implement fixed-point comparisons. These functions implement a low-level compare, upon which the higher level comparison operators (such as less than and greater than or equal to) can be constructed. The returned values lie in the range zero to two, to allow the high-level operators to be implemented by testing the returned result using either signed or unsigned comparison.
These functions perform a signed or unsigned comparison of a and b (depending on the selected machine mode). If a is less than b, they return 0; if a is greater than b, they return 2; and if a and b are equal they return 1.
These functions convert from fractional and signed non-fractionals to fractionals and signed non-fractionals, without saturation.
The functions convert from fractional and signed non-fractionals to fractionals, with saturation.
These functions convert from fractionals to unsigned non-fractionals; and from unsigned non-fractionals to fractionals, without saturation.
These functions convert from unsigned non-fractionals to fractionals, with saturation.
Next: Miscellaneous routines, Previous: Fixed-point fractional library routines, Up: Libgcc [Contents][Index]
document me!
_Unwind_DeleteException _Unwind_Find_FDE _Unwind_ForcedUnwind _Unwind_GetGR _Unwind_GetIP _Unwind_GetLanguageSpecificData _Unwind_GetRegionStart _Unwind_GetTextRelBase _Unwind_GetDataRelBase _Unwind_RaiseException _Unwind_Resume _Unwind_SetGR _Unwind_SetIP _Unwind_FindEnclosingFunction _Unwind_SjLj_Register _Unwind_SjLj_Unregister _Unwind_SjLj_RaiseException _Unwind_SjLj_ForcedUnwind _Unwind_SjLj_Resume __deregister_frame __deregister_frame_info __deregister_frame_info_bases __register_frame __register_frame_info __register_frame_info_bases __register_frame_info_table __register_frame_info_table_bases __register_frame_table
Previous: Exception handling routines, Up: Libgcc [Contents][Index]
This function clears the instruction cache between beg and end.
When using -fsplit-stack, this call may be used to iterate over the stack segments. It may be called like this:
void *next_segment = NULL;
void *next_sp = NULL;
void *initial_sp = NULL;
void *stack;
size_t stack_size;
while ((stack = __splitstack_find (next_segment, next_sp,
&stack_size, &next_segment,
&next_sp, &initial_sp))
!= NULL)
{
/* Stack segment starts at stack and is
stack_size bytes long. */
}
There is no way to iterate over the stack segments of a different
thread. However, what is permitted is for one thread to call this
with the segment_arg and sp arguments NULL, to pass
next_segment, next_sp, and initial_sp to a different
thread, and then to suspend one way or another. A different thread
may run the subsequent __splitstack_find iterations. Of
course, this will only work if the first thread is suspended while the
second thread is calling __splitstack_find. If not, the second
thread could be looking at the stack while it is changing, and
anything could happen.
Internal variables used by the -fsplit-stack implementation.
Next: Source Tree, Previous: Libgcc, Up: Top [Contents][Index]
The interface to front ends for languages in GCC, and in particular
the tree structure (see GENERIC), was initially designed for
C, and many aspects of it are still somewhat biased towards C and
C-like languages. It is, however, reasonably well suited to other
procedural languages, and front ends for many such languages have been
written for GCC.
Writing a compiler as a front end for GCC, rather than compiling directly to assembler or generating C code which is then compiled by GCC, has several advantages:
Because of the advantages of writing a compiler as a GCC front end, GCC front ends have also been created for languages very different from those for which GCC was designed, such as the declarative logic/functional language Mercury. For these reasons, it may also be useful to implement compilers created for specialized purposes (for example, as part of a research project) as GCC front ends.
Next: Testsuites, Previous: Languages, Up: Top [Contents][Index]
This chapter describes the structure of the GCC source tree, and how GCC is built. The user documentation for building and installing GCC is in a separate manual (http://gcc.gnu.org/install/), with which it is presumed that you are familiar.
| • Configure Terms: | Configuration terminology and history. | |
| • Top Level: | The top level source directory. | |
| • gcc Directory: | The gcc subdirectory. |
Next: Top Level, Up: Source Tree [Contents][Index]
The configure and build process has a long and colorful history, and can be confusing to anyone who doesn’t know why things are the way they are. While there are other documents which describe the configuration process in detail, here are a few things that everyone working on GCC should know.
There are three system names that the build knows about: the machine you are building on (build), the machine that you are building for (host), and the machine that GCC will produce code for (target). When you configure GCC, you specify these with --build=, --host=, and --target=.
Specifying the host without specifying the build should be avoided, as
configure may (and once did) assume that the host you specify
is also the build, which may not be true.
If build, host, and target are all the same, this is called a native. If build and host are the same but target is different, this is called a cross. If build, host, and target are all different this is called a canadian (for obscure reasons dealing with Canada’s political party and the background of the person working on the build at that time). If host and target are the same, but build is different, you are using a cross-compiler to build a native for a different system. Some people call this a host-x-host, crossed native, or cross-built native. If build and target are the same, but host is different, you are using a cross compiler to build a cross compiler that produces code for the machine you’re building on. This is rare, so there is no common way of describing it. There is a proposal to call this a crossback.
If build and host are the same, the GCC you are building will also be
used to build the target libraries (like libstdc++). If build and host
are different, you must have already built and installed a cross
compiler that will be used to build the target libraries (if you
configured with --target=foo-bar, this compiler will be called
foo-bar-gcc).
In the case of target libraries, the machine you’re building for is the
machine you specified with --target. So, build is the machine
you’re building on (no change there), host is the machine you’re
building for (the target libraries are built for the target, so host is
the target you specified), and target doesn’t apply (because you’re not
building a compiler, you’re building libraries). The configure/make
process will adjust these variables as needed. It also sets
$with_cross_host to the original --host value in case you
need it.
The libiberty support library is built up to three times: once
for the host, once for the target (even if they are the same), and once
for the build if build and host are different. This allows it to be
used by all programs which are generated in the course of the build
process.
Next: gcc Directory, Previous: Configure Terms, Up: Source Tree [Contents][Index]
The top level source directory in a GCC distribution contains several files and directories that are shared with other software distributions such as that of GNU Binutils. It also contains several subdirectories that contain parts of GCC and its runtime libraries:
The Boehm conservative garbage collector, used as part of the Java runtime library.
Autoconf macros and Makefile fragments used throughout the tree.
Contributed scripts that may be found useful in conjunction with GCC. One of these, contrib/texi2pod.pl, is used to generate man pages from Texinfo manuals as part of the GCC build process.
The support for fixing system headers to work with GCC. See fixincludes/README for more information. The headers fixed by this mechanism are installed in libsubdir/include-fixed. Along with those headers, README-fixinc is also installed, as libsubdir/include-fixed/README.
The main sources of GCC itself (except for runtime libraries), including optimizers, support for different target architectures, language front ends, and testsuites. See The gcc Subdirectory, for details.
Support tools for GNAT.
Headers for the libiberty library.
GNU libintl, from GNU gettext, for systems which do not
include it in libc.
The Ada runtime library.
The C preprocessor library.
The Decimal Float support library.
The libffi library, used as part of the Java runtime library.
The GCC runtime library.
The Fortran runtime library.
The Go runtime library. The bulk of this library is mirrored from the master Go repository.
The GNU OpenMP runtime library.
The libiberty library, used for portability and for some
generally useful data structures and algorithms. See Introduction in GNU libiberty, for more information
about this library.
The Java runtime library.
The libmudflap library, used for instrumenting pointer and array
dereferencing operations.
The Objective-C and Objective-C++ runtime library.
The Stack protector runtime library.
The C++ runtime library.
Plugin used by gold if link-time optimizations are enabled.
Scripts used by the gccadmin account on gcc.gnu.org.
The zlib compression library, used by the Java front end, as
part of the Java runtime library, and for compressing and uncompressing
GCC’s intermediate language in LTO object files.
The build system in the top level directory, including how recursion into subdirectories works and how building runtime libraries for multilibs is handled, is documented in a separate manual, included with GNU Binutils. See GNU configure and build system in The GNU configure and build system, for details.
Previous: Top Level, Up: Source Tree [Contents][Index]
The gcc directory contains many files that are part of the C sources of GCC, other files used as part of the configuration and build process, and subdirectories including documentation and a testsuite. The files that are sources of GCC are documented in a separate chapter. See Passes and Files of the Compiler.
| • Subdirectories: | Subdirectories of gcc. | |
| • Configuration: | The configuration process, and the files it uses. | |
| • Build: | The build system in the gcc directory. | |
| • Makefile: | Targets in gcc/Makefile. | |
| • Library Files: | Library source files and headers under gcc/. | |
| • Headers: | Headers installed by GCC. | |
| • Documentation: | Building documentation in GCC. | |
| • Front End: | Anatomy of a language front end. | |
| • Back End: | Anatomy of a target back end. |
Next: Configuration, Up: gcc Directory [Contents][Index]
The gcc directory contains the following subdirectories:
Subdirectories for various languages. Directories containing a file config-lang.in are language subdirectories. The contents of the subdirectories cp (for C++), lto (for LTO), objc (for Objective-C) and objcp (for Objective-C++) are documented in this manual (see Passes and Files of the Compiler); those for other languages are not. See Anatomy of a Language Front End, for details of the files in these directories.
Configuration files for supported architectures and operating systems. See Anatomy of a Target Back End, for details of the files in this directory.
Texinfo documentation for GCC, together with automatically generated man pages and support for converting the installation manual to HTML. See Documentation.
System headers installed by GCC, mainly those required by the C standard of freestanding implementations. See Headers Installed by GCC, for details of when these and other headers are installed.
Message catalogs with translations of messages produced by GCC into
various languages, language.po. This directory also
contains gcc.pot, the template for these message catalogues,
exgettext, a wrapper around gettext to extract the
messages from the GCC sources and create gcc.pot, which is run
by ‘make gcc.pot’, and EXCLUDES, a list of files from
which messages should not be extracted.
The GCC testsuites (except for those for runtime libraries). See Testsuites.
Next: Build, Previous: Subdirectories, Up: gcc Directory [Contents][Index]
The gcc directory is configured with an Autoconf-generated script configure. The configure script is generated from configure.ac and aclocal.m4. From the files configure.ac and acconfig.h, Autoheader generates the file config.in. The file cstamp-h.in is used as a timestamp.
| • Config Fragments: | Scripts used by configure. | |
| • System Config: | The config.build, config.host, and config.gcc files. | |
| • Configuration Files: | Files created by running configure. |
Next: System Config, Up: Configuration [Contents][Index]
configure uses some other scripts to help in its work:
Next: Configuration Files, Previous: Config Fragments, Up: Configuration [Contents][Index]
The config.build file contains specific rules for particular systems which GCC is built on. This should be used as rarely as possible, as the behavior of the build system can always be detected by autoconf.
The config.host file contains specific rules for particular systems which GCC will run on. This is rarely needed.
The config.gcc file contains specific rules for particular systems which GCC will generate code for. This is usually needed.
Each file has a list of the shell variables it sets, with descriptions, at the top of the file.
FIXME: document the contents of these files, and what variables should be set to control build, host and target configuration.
Previous: System Config, Up: Configuration [Contents][Index]
configureHere we spell out what files will be set up by configure in the gcc directory. Some other files are created as temporary files in the configuration process, and are not used in the subsequent build; these are not documented.
outputs, then
the files listed in outputs there are also generated.
The following configuration headers are created from the Makefile,
using mkconfig.sh, rather than directly by configure.
config.h, bconfig.h and tconfig.h all contain the
xm-machine.h header, if any, appropriate to the host,
build and target machines respectively, the configuration headers for
the target, and some definitions; for the host and build machines,
these include the autoconfigured headers generated by
configure. The other configuration headers are determined by
config.gcc. They also contain the typedefs for rtx,
rtvec and tree.
Next: Makefile, Previous: Configuration, Up: gcc Directory [Contents][Index]
FIXME: describe the build system, including what is built in what stages. Also list the various source files that are used in the build process but aren’t source files of GCC itself and so aren’t documented below (see Passes).
Next: Library Files, Previous: Build, Up: gcc Directory [Contents][Index]
These targets are available from the ‘gcc’ directory:
allThis is the default target. Depending on what your build/host/target configuration is, it coordinates all the things that need to be built.
docProduce info-formatted documentation and man pages. Essentially it calls ‘make man’ and ‘make info’.
dviProduce DVI-formatted documentation.
pdfProduce PDF-formatted documentation.
htmlProduce HTML-formatted documentation.
manGenerate man pages.
infoGenerate info-formatted pages.
mostlycleanDelete the files made while building the compiler.
cleanThat, and all the other files built by ‘make all’.
distcleanThat, and all the files created by configure.
maintainer-cleanDistclean plus any file that can be generated from other files. Note that additional tools may be required beyond what is normally needed to build GCC.
srcextraGenerates files in the source directory that are not version-controlled but should go into a release tarball.
srcinfosrcmanCopies the info-formatted and manpage documentation into the source directory usually for the purpose of generating a release tarball.
installInstalls GCC.
uninstallDeletes installed files, though this is not supported.
checkRun the testsuite. This creates a testsuite subdirectory that
has various .sum and .log files containing the results of
the testing. You can run subsets with, for example, ‘make check-gcc’.
You can specify specific tests by setting RUNTESTFLAGS to be the name
of the .exp file, optionally followed by (for some tests) an equals
and a file wildcard, like:
make check-gcc RUNTESTFLAGS="execute.exp=19980413-*"
Note that running the testsuite may require additional tools be installed, such as Tcl or DejaGnu.
The toplevel tree from which you start GCC compilation is not the GCC directory, but rather a complex Makefile that coordinates the various steps of the build, including bootstrapping the compiler and using the new compiler to build target libraries.
When GCC is configured for a native configuration, the default action
for make is to do a full three-stage bootstrap. This means
that GCC is built three times—once with the native compiler, once with
the native-built compiler it just built, and once with the compiler it
built the second time. In theory, the last two should produce the same
results, which ‘make compare’ can check. Each stage is configured
separately and compiled into a separate directory, to minimize problems
due to ABI incompatibilities between the native compiler and GCC.
If you do a change, rebuilding will also start from the first stage and “bubble” up the change through the three stages. Each stage is taken from its build directory (if it had been built previously), rebuilt, and copied to its subdirectory. This will allow you to, for example, continue a bootstrap after fixing a bug which causes the stage2 build to crash. It does not provide as good coverage of the compiler as bootstrapping from scratch, but it ensures that the new code is syntactically correct (e.g., that you did not use GCC extensions by mistake), and avoids spurious bootstrap comparison failures1.
Other targets available from the top level include:
bootstrap-leanLike bootstrap, except that the various stages are removed once
they’re no longer needed. This saves disk space.
bootstrap2bootstrap2-leanPerforms only the first two stages of bootstrap. Unlike a three-stage bootstrap, this does not perform a comparison to test that the compiler is running properly. Note that the disk space required by a “lean” bootstrap is approximately independent of the number of stages.
stageN-bubble (N = 1…4, profile, feedback)Rebuild all the stages up to N, with the appropriate flags, “bubbling” the changes as described above.
all-stageN (N = 1…4, profile, feedback)Assuming that stage N has already been built, rebuild it with the appropriate flags. This is rarely needed.
cleanstrapRemove everything (‘make clean’) and rebuilds (‘make bootstrap’).
compareCompares the results of stages 2 and 3. This ensures that the compiler is running properly, since it should produce the same object files regardless of how it itself was compiled.
profiledbootstrapBuilds a compiler with profiling feedback information. In this case, the second and third stages are named ‘profile’ and ‘feedback’, respectively. For more information, see ‘Building with profile feedback’ in Installing GCC.
restrapRestart a bootstrap, so that everything that was not built with the system compiler is rebuilt.
stageN-start (N = 1…4, profile, feedback)For each package that is bootstrapped, rename directories so that, for example, gcc points to the stageN GCC, compiled with the stageN-1 GCC2.
You will invoke this target if you need to test or debug the stageN GCC. If you only need to execute GCC (but you need not run ‘make’ either to rebuild it or to run test suites), you should be able to work directly in the stageN-gcc directory. This makes it easier to debug multiple stages in parallel.
stageFor each package that is bootstrapped, relocate its build directory to indicate its stage. For example, if the gcc directory points to the stage2 GCC, after invoking this target it will be renamed to stage2-gcc.
If you wish to use non-default GCC flags when compiling the stage2 and
stage3 compilers, set BOOT_CFLAGS on the command line when doing
‘make’.
Usually, the first stage only builds the languages that the compiler
is written in: typically, C and maybe Ada. If you are debugging a
miscompilation of a different stage2 front-end (for example, of the
Fortran front-end), you may want to have front-ends for other languages
in the first stage as well. To do so, set STAGE1_LANGUAGES
on the command line when doing ‘make’.
For example, in the aforementioned scenario of debugging a Fortran front-end miscompilation caused by the stage1 compiler, you may need a command like
make stage2-bubble STAGE1_LANGUAGES=c,fortran
Alternatively, you can use per-language targets to build and test
languages that are not enabled by default in stage1. For example,
make f951 will build a Fortran compiler even in the stage1
build directory.
Next: Headers, Previous: Makefile, Up: gcc Directory [Contents][Index]
FIXME: list here, with explanation, all the C source files and headers under the gcc directory that aren’t built into the GCC executable but rather are part of runtime libraries and object files, such as crtstuff.c and unwind-dw2.c. See Headers Installed by GCC, for more information about the ginclude directory.
Next: Documentation, Previous: Library Files, Up: gcc Directory [Contents][Index]
In general, GCC expects the system C library to provide most of the headers to be used with it. However, GCC will fix those headers if necessary to make them work with GCC, and will install some headers required of freestanding implementations. These headers are installed in libsubdir/include. Headers for non-C runtime libraries are also installed by GCC; these are not documented here. (FIXME: document them somewhere.)
Several of the headers GCC installs are in the ginclude
directory. These headers, iso646.h,
stdarg.h, stdbool.h, and stddef.h,
are installed in libsubdir/include,
unless the target Makefile fragment (see Target Fragment)
overrides this by setting USER_H.
In addition to these headers and those generated by fixing system
headers to work with GCC, some other headers may also be installed in
libsubdir/include. config.gcc may set
extra_headers; this specifies additional headers under
config to be installed on some systems.
GCC installs its own version of <float.h>, from ginclude/float.h.
This is done to cope with command-line options that change the
representation of floating point numbers.
GCC also installs its own version of <limits.h>; this is generated
from glimits.h, together with limitx.h and
limity.h if the system also has its own version of
<limits.h>. (GCC provides its own header because it is
required of ISO C freestanding implementations, but needs to include
the system header from its own header as well because other standards
such as POSIX specify additional values to be defined in
<limits.h>.) The system’s <limits.h> header is used via
libsubdir/include/syslimits.h, which is copied from
gsyslimits.h if it does not need fixing to work with GCC; if it
needs fixing, syslimits.h is the fixed copy.
GCC can also install <tgmath.h>. It will do this when
config.gcc sets use_gcc_tgmath to yes.
Next: Front End, Previous: Headers, Up: gcc Directory [Contents][Index]
The main GCC documentation is in the form of manuals in Texinfo format. These are installed in Info format; DVI versions may be generated by ‘make dvi’, PDF versions by ‘make pdf’, and HTML versions by ‘make html’. In addition, some man pages are generated from the Texinfo manuals, there are some other text files with miscellaneous documentation, and runtime libraries have their own documentation outside the gcc directory. FIXME: document the documentation for runtime libraries somewhere.
| • Texinfo Manuals: | GCC manuals in Texinfo format. | |
| • Man Page Generation: | Generating man pages from Texinfo manuals. | |
| • Miscellaneous Docs: | Miscellaneous text files with documentation. |
Next: Man Page Generation, Up: Documentation [Contents][Index]
The manuals for GCC as a whole, and the C and C++ front ends, are in files doc/*.texi. Other front ends have their own manuals in files language/*.texi. Common files doc/include/*.texi are provided which may be included in multiple manuals; the following files are in doc/include:
The GNU Free Documentation License.
The section “Funding Free Software”.
Common definitions for manuals.
The GNU General Public License.
A copy of texinfo.tex known to work with the GCC manuals.
DVI-formatted manuals are generated by ‘make dvi’, which uses
texi2dvi (via the Makefile macro $(TEXI2DVI)).
PDF-formatted manuals are generated by ‘make pdf’, which uses
texi2pdf (via the Makefile macro $(TEXI2PDF)). HTML
formatted manuals are generated by ‘make html’. Info
manuals are generated by ‘make info’ (which is run as part of
a bootstrap); this generates the manuals in the source directory,
using makeinfo via the Makefile macro $(MAKEINFO),
and they are included in release distributions.
Manuals are also provided on the GCC web site, in both HTML and
PostScript forms. This is done via the script
maintainer-scripts/update_web_docs_svn. Each manual to be
provided online must be listed in the definition of MANUALS in
that file; a file name.texi must only appear once in the
source tree, and the output manual must have the same name as the
source file. (However, other Texinfo files, included in manuals but
not themselves the root files of manuals, may have names that appear
more than once in the source tree.) The manual file
name.texi should only include other files in its own
directory or in doc/include. HTML manuals will be generated by
‘makeinfo --html’, PostScript manuals by texi2dvi
and dvips, and PDF manuals by texi2pdf.
All Texinfo files that are parts of manuals must
be version-controlled, even if they are generated files, for the
generation of online manuals to work.
The installation manual, doc/install.texi, is also provided on the GCC web site. The HTML version is generated by the script doc/install.texi2html.
Next: Miscellaneous Docs, Previous: Texinfo Manuals, Up: Documentation [Contents][Index]
Because of user demand, in addition to full Texinfo manuals, man pages
are provided which contain extracts from those manuals. These man
pages are generated from the Texinfo manuals using
contrib/texi2pod.pl and pod2man. (The man page for
g++, cp/g++.1, just contains a ‘.so’ reference
to gcc.1, but all the other man pages are generated from
Texinfo manuals.)
Because many systems may not have the necessary tools installed to generate the man pages, they are only generated if the configure script detects that recent enough tools are installed, and the Makefiles allow generating man pages to fail without aborting the build. Man pages are also included in release distributions. They are generated in the source directory.
Magic comments in Texinfo files starting ‘@c man’ control what parts of a Texinfo file go into a man page. Only a subset of Texinfo is supported by texi2pod.pl, and it may be necessary to add support for more Texinfo features to this script when generating new man pages. To improve the man page output, some special Texinfo macros are provided in doc/include/gcc-common.texi which texi2pod.pl understands:
@gcctaboptUse in the form ‘@table @gcctabopt’ for tables of options, where for printed output the effect of ‘@code’ is better than that of ‘@option’ but for man page output a different effect is wanted.
@gccoptlistUse for summary lists of options in manuals.
@golUse at the end of each line inside ‘@gccoptlist’. This is necessary to avoid problems with differences in how the ‘@gccoptlist’ macro is handled by different Texinfo formatters.
FIXME: describe the texi2pod.pl input language and magic comments in more detail.
Previous: Man Page Generation, Up: Documentation [Contents][Index]
In addition to the formal documentation that is installed by GCC, there are several other text files in the gcc subdirectory with miscellaneous documentation:
Notes on GCC’s Native Language Support. FIXME: this should be part of this manual rather than a separate file.
Notes on the Free Translation Project.
The GNU General Public License, Versions 2 and 3.
The GNU Lesser General Public License, Versions 2.1 and 3.
Change log files for various parts of GCC.
Details of a few changes to the GCC front-end interface. FIXME: the information in this file should be part of general documentation of the front-end interface in this manual.
Information about new features in old versions of GCC. (For recent versions, the information is on the GCC web site.)
Information about portability issues when writing code in GCC. FIXME: why isn’t this part of this manual or of the GCC Coding Conventions?
FIXME: document such files in subdirectories, at least config, cp, objc, testsuite.
Next: Back End, Previous: Documentation, Up: gcc Directory [Contents][Index]
A front end for a language in GCC has the following parts:
default_compilers in gcc.c for source file
suffixes for that language.
If the front end is added to the official GCC source repository, the following are also necessary:
| • Front End Directory: | The front end language directory. | |
| • Front End Config: | The front end config-lang.in file. | |
| • Front End Makefile: | The front end Make-lang.in file. |
Next: Front End Config, Up: Front End [Contents][Index]
A front end language directory contains the source files of that front end (but not of any runtime libraries, which should be outside the gcc directory). This includes documentation, and possibly some subsidiary programs built alongside the front end. Certain files are special and other parts of the compiler depend on their names:
This file is required in all language subdirectories. See The Front End config-lang.in File, for details of its contents
This file is required in all language subdirectories. See The Front End Make-lang.in File, for details of its contents.
This file registers the set of switches that the front end accepts on the command line, and their --help text. See Options.
This file provides entries for default_compilers in
gcc.c which override the default of giving an error that a
compiler for that language is not installed.
This file, which need not exist, defines any language-specific tree codes.
Next: Front End Makefile, Previous: Front End Directory, Up: Front End [Contents][Index]
Each language subdirectory contains a config-lang.in file. In addition the main directory contains c-config-lang.in, which contains limited information for the C language. This file is a shell script that may define some variables describing the language:
languageThis definition must be present, and gives the name of the language for some purposes such as arguments to --enable-languages.
lang_requiresIf defined, this variable lists (space-separated) language front ends
other than C that this front end requires to be enabled (with the
names given being their language settings). For example, the
Java front end depends on the C++ front end, so sets
‘lang_requires=c++’.
subdir_requiresIf defined, this variable lists (space-separated) front end directories other than C that this front end requires to be present. For example, the Objective-C++ front end uses source files from the C++ and Objective-C front ends, so sets ‘subdir_requires="cp objc"’.
target_libsIf defined, this variable lists (space-separated) targets in the top
level Makefile to build the runtime libraries for this
language, such as target-libobjc.
lang_dirsIf defined, this variable lists (space-separated) top level directories (parallel to gcc), apart from the runtime libraries, that should not be configured if this front end is not built.
build_by_defaultIf defined to ‘no’, this language front end is not built unless enabled in a --enable-languages argument. Otherwise, front ends are built by default, subject to any special logic in configure.ac (as is present to disable the Ada front end if the Ada compiler is not already installed).
boot_languageIf defined to ‘yes’, this front end is built in stage1 of the bootstrap. This is only relevant to front ends written in their own languages.
compilersIf defined, a space-separated list of compiler executables that will be run by the driver. The names here will each end with ‘\$(exeext)’.
outputsIf defined, a space-separated list of files that should be generated by configure substituting values in them. This mechanism can be used to create a file language/Makefile from language/Makefile.in, but this is deprecated, building everything from the single gcc/Makefile is preferred.
gtfilesIf defined, a space-separated list of files that should be scanned by gengtype.c to generate the garbage collection tables and routines for this language. This excludes the files that are common to all front ends. See Type Information.
Previous: Front End Config, Up: Front End [Contents][Index]
Each language subdirectory contains a Make-lang.in file. It contains
targets lang.hook (where lang is the
setting of language in config-lang.in) for the following
values of hook, and any other Makefile rules required to
build those targets (which may if necessary use other Makefiles
specified in outputs in config-lang.in, although this is
deprecated). It also adds any testsuite targets that can use the
standard rule in gcc/Makefile.in to the variable
lang_checks.
all.crossstart.encaprest.encapFIXME: exactly what goes in each of these targets?
tagsBuild an etags TAGS file in the language subdirectory
in the source tree.
infoBuild info documentation for the front end, in the build directory.
This target is only called by ‘make bootstrap’ if a suitable
version of makeinfo is available, so does not need to check
for this, and should fail if an error occurs.
dviBuild DVI documentation for the front end, in the build directory.
This should be done using $(TEXI2DVI), with appropriate
-I arguments pointing to directories of included files.
pdfBuild PDF documentation for the front end, in the build directory.
This should be done using $(TEXI2PDF), with appropriate
-I arguments pointing to directories of included files.
htmlBuild HTML documentation for the front end, in the build directory.
manBuild generated man pages for the front end from Texinfo manuals (see Man Page Generation), in the build directory. This target is only called if the necessary tools are available, but should ignore errors so as not to stop the build if errors occur; man pages are optional and the tools involved may be installed in a broken way.
install-commonInstall everything that is part of the front end, apart from the
compiler executables listed in compilers in
config-lang.in.
install-infoInstall info documentation for the front end, if it is present in the source directory. This target should have dependencies on info files that should be installed.
install-manInstall man pages for the front end. This target should ignore errors.
install-pluginInstall headers needed for plugins.
srcextraCopies its dependencies into the source directory. This generally should be used for generated files such as Bison output files which are not version-controlled, but should be included in any release tarballs. This target will be executed during a bootstrap if ‘--enable-generated-files-in-srcdir’ was specified as a configure option.
srcinfosrcmanCopies its dependencies into the source directory. These targets will be executed during a bootstrap if ‘--enable-generated-files-in-srcdir’ was specified as a configure option.
uninstallUninstall files installed by installing the compiler. This is currently documented not to be supported, so the hook need not do anything.
mostlycleancleandistcleanmaintainer-cleanThe language parts of the standard GNU
‘*clean’ targets. See Standard Targets for
Users in GNU Coding Standards, for details of the standard
targets. For GCC, maintainer-clean should delete
all generated files in the source directory that are not version-controlled,
but should not delete anything that is.
Make-lang.in must also define a variable lang_OBJS
to a list of host object files that are used by that language.
Previous: Front End, Up: gcc Directory [Contents][Index]
A back end for a target architecture in GCC has the following parts:
extra_options variable in
config.gcc. See Options.
__attribute__), including where the
same attribute is already supported on some targets, which are
enumerated in the manual.
libstdc++ porting
manual needs to be installed as info for this to work, or to be a
chapter of this manual.
If the back end is added to the official GCC source repository, the following are also necessary:
Next: Options, Previous: Source Tree, Up: Top [Contents][Index]
GCC contains several testsuites to help maintain compiler quality. Most of the runtime libraries and language front ends in GCC have testsuites. Currently only the C language testsuites are documented here; FIXME: document the others.
| • Test Idioms: | Idioms used in testsuite code. | |
| • Test Directives: | Directives used within DejaGnu tests. | |
| • Ada Tests: | The Ada language testsuites. | |
| • C Tests: | The C language testsuites. | |
| • libgcj Tests: | The Java library testsuites. | |
| • LTO Testing: | Support for testing link-time optimizations. | |
| • gcov Testing: | Support for testing gcov. | |
| • profopt Testing: | Support for testing profile-directed optimizations. | |
| • compat Testing: | Support for testing binary compatibility. | |
| • Torture Tests: | Support for torture testing using multiple options. |
Next: Test Directives, Up: Testsuites [Contents][Index]
In general, C testcases have a trailing -n.c, starting with -1.c, in case other testcases with similar names are added later. If the test is a test of some well-defined feature, it should have a name referring to that feature such as feature-1.c. If it does not test a well-defined feature but just happens to exercise a bug somewhere in the compiler, and a bug report has been filed for this bug in the GCC bug database, prbug-number-1.c is the appropriate form of name. Otherwise (for miscellaneous bugs not filed in the GCC bug database), and previously more generally, test cases are named after the date on which they were added. This allows people to tell at a glance whether a test failure is because of a recently found bug that has not yet been fixed, or whether it may be a regression, but does not give any other information about the bug or where discussion of it may be found. Some other language testsuites follow similar conventions.
In the gcc.dg testsuite, it is often necessary to test that an error is indeed a hard error and not just a warning—for example, where it is a constraint violation in the C standard, which must become an error with -pedantic-errors. The following idiom, where the first line shown is line line of the file and the line that generates the error, is used for this:
/* { dg-bogus "warning" "warning in place of error" } */
/* { dg-error "regexp" "message" { target *-*-* } line } */
It may be necessary to check that an expression is an integer constant
expression and has a certain value. To check that E has
value V, an idiom similar to the following is used:
char x[((E) == (V) ? 1 : -1)];
In gcc.dg tests, __typeof__ is sometimes used to make
assertions about the types of expressions. See, for example,
gcc.dg/c99-condexpr-1.c. The more subtle uses depend on the
exact rules for the types of conditional expressions in the C
standard; see, for example, gcc.dg/c99-intconst-1.c.
It is useful to be able to test that optimizations are being made
properly. This cannot be done in all cases, but it can be done where
the optimization will lead to code being optimized away (for example,
where flow analysis or alias analysis should show that certain code
cannot be called) or to functions not being called because they have
been expanded as built-in functions. Such tests go in
gcc.c-torture/execute. Where code should be optimized away, a
call to a nonexistent function such as link_failure () may be
inserted; a definition
#ifndef __OPTIMIZE__
void
link_failure (void)
{
abort ();
}
#endif
will also be needed so that linking still succeeds when the test is
run without optimization. When all calls to a built-in function
should have been optimized and no calls to the non-built-in version of
the function should remain, that function may be defined as
static to call abort () (although redeclaring a function
as static may not work on all targets).
All testcases must be portable. Target-specific testcases must have appropriate code to avoid causing failures on unsupported systems; unfortunately, the mechanisms for this differ by directory.
FIXME: discuss non-C testsuites here.
Next: Ada Tests, Previous: Test Idioms, Up: Testsuites [Contents][Index]
| • Directives: | Syntax and descriptions of test directives. | |
| • Selectors: | Selecting targets to which a test applies. | |
| • Effective-Target Keywords: | Keywords describing target attributes. | |
| • Add Options: | Features for dg-add-options
| |
| • Require Support: | Variants of dg-require-support
| |
| • Final Actions: | Commands for use in dg-final
|
Next: Selectors, Up: Test Directives [Contents][Index]
Test directives appear within comments in a test source file and begin
with dg-. Some of these are defined within DejaGnu and others
are local to the GCC testsuite.
The order in which test directives appear in a test can be important: directives local to GCC sometimes override information used by the DejaGnu directives, which know nothing about the GCC directives, so the DejaGnu directives must precede GCC directives.
Several test directives include selectors (see Selectors)
which are usually preceded by the keyword target or xfail.
{ dg-do do-what-keyword [{ target/xfail selector }] }do-what-keyword specifies how the test is compiled and whether it is executed. It is one of:
preprocessCompile with -E to run only the preprocessor.
compileCompile with -S to produce an assembly code file.
assembleCompile with -c to produce a relocatable object file.
linkCompile, assemble, and link to produce an executable file.
runProduce and run an executable file, which is expected to return an exit code of 0.
The default is compile. That can be overridden for a set of
tests by redefining dg-do-what-default within the .exp
file for those tests.
If the directive includes the optional ‘{ target selector }’ then the test is skipped unless the target system matches the selector.
If do-what-keyword is run and the directive includes
the optional ‘{ xfail selector }’ and the selector is met
then the test is expected to fail. The xfail clause is ignored
for other values of do-what-keyword; those tests can use
directive dg-xfail-if.
{ dg-options options [{ target selector }] }This DejaGnu directive provides a list of compiler options, to be used if the target system matches selector, that replace the default options used for this set of tests.
{ dg-add-options feature … }Add any compiler options that are needed to access certain features.
This directive does nothing on targets that enable the features by
default, or that don’t provide them at all. It must come after
all dg-options directives.
For supported values of feature see Add Options.
The normal timeout limit, in seconds, is found by searching the following in order:
dg-timeout directive in
the test
{ dg-timeout n [{target selector }] }Set the time limit for the compilation and for the execution of the test to the specified number of seconds.
{ dg-timeout-factor x [{ target selector }] }Multiply the normal time limit for compilation and execution of the test by the specified floating-point factor.
{ dg-skip-if comment { selector } [{ include-opts } [{ exclude-opts }]] }Arguments include-opts and exclude-opts are lists in which each element is a string of zero or more GCC options. Skip the test if all of the following conditions are met:
For example, to skip a test if option -Os is present:
/* { dg-skip-if "" { *-*-* } { "-Os" } { "" } } */
To skip a test if both options -O2 and -g are present:
/* { dg-skip-if "" { *-*-* } { "-O2 -g" } { "" } } */
To skip a test if either -O2 or -O3 is present:
/* { dg-skip-if "" { *-*-* } { "-O2" "-O3" } { "" } } */
To skip a test unless option -Os is present:
/* { dg-skip-if "" { *-*-* } { "*" } { "-Os" } } */
To skip a test if either -O2 or -O3 is used with -g
but not if -fpic is also present:
/* { dg-skip-if "" { *-*-* } { "-O2 -g" "-O3 -g" } { "-fpic" } } */
{ dg-require-effective-target keyword [{ selector }] }Skip the test if the test target, including current multilib flags,
is not covered by the effective-target keyword.
If the directive includes the optional ‘{ selector }’
then the effective-target test is only performed if the target system
matches the selector.
This directive must appear after any dg-do directive in the test
and before any dg-additional-sources directive.
See Effective-Target Keywords.
{ dg-require-support args }Skip the test if the target does not provide the required support.
These directives must appear after any dg-do directive in the test
and before any dg-additional-sources directive.
They require at least one argument, which can be an empty string if the
specific procedure does not examine the argument.
See Require Support, for a complete list of these directives.
{ dg-xfail-if comment { selector } [{ include-opts } [{ exclude-opts }]] }Expect the test to fail if the conditions (which are the same as for
dg-skip-if) are met. This does not affect the execute step.
{ dg-xfail-run-if comment { selector } [{ include-opts } [{ exclude-opts }]] }Expect the execute step of a test to fail if the conditions (which are
the same as for dg-skip-if) are met.
{ dg-shouldfail comment [{ selector } [{ include-opts } [{ exclude-opts }]]] }Expect the test executable to return a nonzero exit status if the
conditions (which are the same as for dg-skip-if) are met.
{ dg-error regexp [comment [{ target/xfail selector } [line] }]] }This DejaGnu directive appears on a source line that is expected to get
an error message, or else specifies the source line associated with the
message. If there is no message for that line or if the text of that
message is not matched by regexp then the check fails and
comment is included in the FAIL message. The check does
not look for the string ‘error’ unless it is part of regexp.
{ dg-warning regexp [comment [{ target/xfail selector } [line] }]] }This DejaGnu directive appears on a source line that is expected to get
a warning message, or else specifies the source line associated with the
message. If there is no message for that line or if the text of that
message is not matched by regexp then the check fails and
comment is included in the FAIL message. The check does
not look for the string ‘warning’ unless it is part of regexp.
{ dg-message regexp [comment [{ target/xfail selector } [line] }]] }The line is expected to get a message other than an error or warning.
If there is no message for that line or if the text of that message is
not matched by regexp then the check fails and comment is
included in the FAIL message.
{ dg-bogus regexp [comment [{ target/xfail selector } [line] }]] }This DejaGnu directive appears on a source line that should not get a message matching regexp, or else specifies the source line associated with the bogus message. It is usually used with ‘xfail’ to indicate that the message is a known problem for a particular set of targets.
{ dg-excess-errors comment [{ target/xfail selector }] }This DejaGnu directive indicates that the test is expected to fail due to compiler messages that are not handled by ‘dg-error’, ‘dg-warning’ or ‘dg-bogus’. For this directive ‘xfail’ has the same effect as ‘target’.
{ dg-prune-output regexp }Prune messages matching regexp from the test output.
{ dg-output regexp [{ target/xfail selector }] }This DejaGnu directive compares regexp to the combined output that the test executable writes to stdout and stderr.
{ dg-additional-files "filelist" }Specify additional files, other than source files, that must be copied to the system where the compiler runs.
{ dg-additional-sources "filelist" }Specify additional source files to appear in the compile line following the main test file.
{ dg-final { local-directive } }This DejaGnu directive is placed within a comment anywhere in the
source file and is processed after the test has been compiled and run.
Multiple ‘dg-final’ commands are processed in the order in which
they appear in the source file. See Final Actions, for a list
of directives that can be used within dg-final.
Next: Effective-Target Keywords, Previous: Directives, Up: Test Directives [Contents][Index]
Several test directives include selectors to limit the targets for which a test is run or to declare that a test is expected to fail on particular targets.
A selector is:
Depending on the context, the selector specifies whether a test is skipped and reported as unsupported or is expected to fail. Use ‘*-*-*’ to match any target.
A selector expression appears within curly braces and uses a single logical operator: one of ‘!’, ‘&&’, or ‘||’. An operand is another selector expression, an effective-target keyword, a single target triplet, or a list of target triplets within quotes or curly braces. For example:
{ target { ! "hppa*-*-* ia64*-*-*" } }
{ target { powerpc*-*-* && lp64 } }
{ xfail { lp64 || vect_no_align } }
Next: Add Options, Previous: Selectors, Up: Test Directives [Contents][Index]
Effective-target keywords identify sets of targets that support particular functionality. They are used to limit tests to be run only for particular targets, or to specify that particular sets of targets are expected to fail some tests.
Effective-target keywords are defined in lib/target-supports.exp in the GCC testsuite, with the exception of those that are documented as being local to a particular test directory.
The ‘effective target’ takes into account all of the compiler options
with which the test will be compiled, including the multilib options.
By convention, keywords ending in _nocache can also include options
specified for the particular test in an earlier dg-options or
dg-add-options directive.
ilp32Target has 32-bit int, long, and pointers.
lp64Target has 32-bit int, 64-bit long and pointers.
llp64Target has 32-bit int and long, 64-bit long long
and pointers.
double64Target has 64-bit double.
double64plusTarget has double that is 64 bits or longer.
int32plusTarget has int that is at 32 bits or longer.
int16Target has int that is 16 bits or shorter.
large_doubleTarget supports double that is longer than float.
large_long_doubleTarget supports long double that is longer than double.
ptr32plusTarget has pointers that are 32 bits or longer.
size32plusTarget supports array and structure sizes that are 32 bits or longer.
4byte_wchar_tTarget has wchar_t that is at least 4 bytes.
fortran_integer_16Target supports Fortran integer that is 16 bytes or longer.
fortran_large_intTarget supports Fortran integer kinds larger than integer(8).
fortran_large_realTarget supports Fortran real kinds larger than real(8).
vect_conditionTarget supports vector conditional operations.
vect_doubleTarget supports hardware vectors of double.
vect_floatTarget supports hardware vectors of float.
vect_intTarget supports hardware vectors of int.
vect_longTarget supports hardware vectors of long.
vect_long_longTarget supports hardware vectors of long long.
vect_aligned_arraysTarget aligns arrays to vector alignment boundary.
vect_hw_misalignTarget supports a vector misalign access.
vect_no_alignTarget does not support a vector alignment mechanism.
vect_no_int_maxTarget does not support a vector max instruction on int.
vect_no_int_addTarget does not support a vector add instruction on int.
vect_no_bitwiseTarget does not support vector bitwise instructions.
vect_char_multTarget supports vector char multiplication.
vect_short_multTarget supports vector short multiplication.
vect_int_multTarget supports vector int multiplication.
vect_extract_even_oddTarget supports vector even/odd element extraction.
vect_extract_even_odd_wideTarget supports vector even/odd element extraction of vectors with elements
SImode or larger.
vect_interleaveTarget supports vector interleaving.
vect_stridedTarget supports vector interleaving and extract even/odd.
vect_strided_wideTarget supports vector interleaving and extract even/odd for wide element types.
vect_permTarget supports vector permutation.
vect_shiftTarget supports a hardware vector shift operation.
vect_widen_sum_hi_to_siTarget supports a vector widening summation of short operands
into int results, or can promote (unpack) from short
to int.
vect_widen_sum_qi_to_hiTarget supports a vector widening summation of char operands
into short results, or can promote (unpack) from char
to short.
vect_widen_sum_qi_to_siTarget supports a vector widening summation of char operands
into int results.
vect_widen_mult_qi_to_hiTarget supports a vector widening multiplication of char operands
into short results, or can promote (unpack) from char to
short and perform non-widening multiplication of short.
vect_widen_mult_hi_to_siTarget supports a vector widening multiplication of short operands
into int results, or can promote (unpack) from short to
int and perform non-widening multiplication of int.
vect_sdot_qiTarget supports a vector dot-product of signed char.
vect_udot_qiTarget supports a vector dot-product of unsigned char.
vect_sdot_hiTarget supports a vector dot-product of signed short.
vect_udot_hiTarget supports a vector dot-product of unsigned short.
vect_pack_truncTarget supports a vector demotion (packing) of short to char
and from int to short using modulo arithmetic.
vect_unpackTarget supports a vector promotion (unpacking) of char to short
and from char to int.
vect_intfloat_cvtTarget supports conversion from signed int to float.
vect_uintfloat_cvtTarget supports conversion from unsigned int to float.
vect_floatint_cvtTarget supports conversion from float to signed int.
vect_floatuint_cvtTarget supports conversion from float to unsigned int.
tlsTarget supports thread-local storage.
tls_nativeTarget supports native (rather than emulated) thread-local storage.
tls_runtimeTest system supports executing TLS executables.
dfpTargets supports compiling decimal floating point extension to C.
dfp_nocacheIncluding the options used to compile this particular test, the target supports compiling decimal floating point extension to C.
dfprtTest system can execute decimal floating point tests.
dfprt_nocacheIncluding the options used to compile this particular test, the test system can execute decimal floating point tests.
hard_dfpTarget generates decimal floating point instructions with current options.
arm32ARM target generates 32-bit code.
arm_eabiARM target adheres to the ABI for the ARM Architecture.
arm_hard_vfp_okARM target supports -mfpu=vfp -mfloat-abi=hard.
Some multilibs may be incompatible with these options.
arm_iwmmxt_okARM target supports -mcpu=iwmmxt.
Some multilibs may be incompatible with this option.
arm_neonARM target supports generating NEON instructions.
arm_neon_hwTest system supports executing NEON instructions.
arm_neon_okARM Target supports -mfpu=neon -mfloat-abi=softfp or compatible
options. Some multilibs may be incompatible with these options.
arm_neon_fp16_okARM Target supports -mfpu=neon-fp16 -mfloat-abi=softfp or compatible
options. Some multilibs may be incompatible with these options.
arm_thumb1_okARM target generates Thumb-1 code for -mthumb.
arm_thumb2_okARM target generates Thumb-2 code for -mthumb.
arm_vfp_okARM target supports -mfpu=vfp -mfloat-abi=softfp.
Some multilibs may be incompatible with these options.
mips64MIPS target supports 64-bit instructions.
nomips16MIPS target does not produce MIPS16 code.
mips16_attributeMIPS target can generate MIPS16 code.
mips_loongsonMIPS target is a Loongson-2E or -2F target using an ABI that supports the Loongson vector modes.
mips_newabi_large_long_doubleMIPS target supports long double larger than double
when using the new ABI.
mpaired_singleMIPS target supports -mpaired-single.
powerpc64Test system supports executing 64-bit instructions.
powerpc_altivecPowerPC target supports AltiVec.
powerpc_altivec_okPowerPC target supports -maltivec.
powerpc_fprsPowerPC target supports floating-point registers.
powerpc_hard_doublePowerPC target supports hardware double-precision floating-point.
powerpc_ppu_okPowerPC target supports -mcpu=cell.
powerpc_spePowerPC target supports PowerPC SPE.
powerpc_spe_nocacheIncluding the options used to compile this particular test, the PowerPC target supports PowerPC SPE.
powerpc_spuPowerPC target supports PowerPC SPU.
spu_auto_overlaySPU target has toolchain that supports automatic overlay generation.
powerpc_vsx_okPowerPC target supports -mvsx.
powerpc_405_nocacheIncluding the options used to compile this particular test, the PowerPC target supports PowerPC 405.
vmx_hwPowerPC target supports executing AltiVec instructions.
avxTarget supports compiling avx instructions.
avx_runtimeTarget supports the execution of avx instructions.
cell_hwTest system can execute AltiVec and Cell PPU instructions.
coldfire_fpuTarget uses a ColdFire FPU.
hard_floatTarget supports FPU instructions.
sseTarget supports compiling sse instructions.
sse_runtimeTarget supports the execution of sse instructions.
sse2Target supports compiling sse2 instructions.
sse2_runtimeTarget supports the execution of sse2 instructions.
sync_char_shortTarget supports atomic operations on char and short.
sync_int_longTarget supports atomic operations on int and long.
ultrasparc_hwTest environment appears to run executables on a simulator that
accepts only EM_SPARC executables and chokes on EM_SPARC32PLUS
or EM_SPARCV9 executables.
vect_cmdline_neededTarget requires a command line argument to enable a SIMD instruction set.
cThe language for the compiler under test is C.
c++The language for the compiler under test is C++.
c99_runtimeTarget provides a full C99 runtime.
correct_iso_cpp_string_wchar_protosTarget string.h and wchar.h headers provide C++ required
overloads for strchr etc. functions.
dummy_wcsftimeTarget uses a dummy wcsftime function that always returns zero.
fd_truncateTarget can truncate a file from a file descriptor, as used by
libgfortran/io/unix.c:fd_truncate; i.e. ftruncate or
chsize.
freestandingTarget is ‘freestanding’ as defined in section 4 of the C99 standard. Effectively, it is a target which supports no extra headers or libraries other than what is considered essential.
init_priorityTarget supports constructors with initialization priority arguments.
inttypes_typesTarget has the basic signed and unsigned types in inttypes.h.
This is for tests that GCC’s notions of these types agree with those
in the header, as some systems have only inttypes.h.
lax_strtofpTarget might have errors of a few ULP in string to floating-point conversion functions and overflow is not always detected correctly by those functions.
newlibTarget supports Newlib.
pow10Target provides pow10 function.
pthreadTarget can compile using pthread.h with no errors or warnings.
pthread_hTarget has pthread.h.
run_expensive_testsExpensive testcases (usually those that consume excessive amounts of CPU
time) should be run on this target. This can be enabled by setting the
GCC_TEST_RUN_EXPENSIVE environment variable to a non-empty string.
simulatorTest system runs executables on a simulator (i.e. slowly) rather than hardware (i.e. fast).
stdint_typesTarget has the basic signed and unsigned C types in stdint.h.
This will be obsolete when GCC ensures a working stdint.h for
all targets.
trampolinesTarget supports trampolines.
uclibcTarget supports uClibc.
unwrappedTarget does not use a status wrapper.
vxworks_kernelTarget is a VxWorks kernel.
vxworks_rtpTarget is a VxWorks RTP.
wcharTarget supports wide characters.
automatic_stack_alignmentTarget supports automatic stack alignment.
cxa_atexitTarget uses __cxa_atexit.
default_packedTarget has packed layout of structure members by default.
fgraphiteTarget supports Graphite optimizations.
fixed_pointTarget supports fixed-point extension to C.
fopenmpTarget supports OpenMP via -fopenmp.
fpicTarget supports -fpic and -fPIC.
freorderTarget supports -freorder-blocks-and-partition.
fstack_protectorTarget supports -fstack-protector.
gasTarget uses GNU as.
gc_sectionsTarget supports --gc-sections.
keeps_null_pointer_checksTarget keeps null pointer checks, either due to the use of -fno-delete-null-pointer-checks or hardwired into the target.
ltoCompiler has been configured to support link-time optimization (LTO).
named_sectionsTarget supports named sections.
natural_alignment_32Target uses natural alignment (aligned to type size) for types of 32 bits or less.
target_natural_alignment_64Target uses natural alignment (aligned to type size) for types of 64 bits or less.
nonpicTarget does not generate PIC by default.
pcc_bitfield_type_mattersTarget defines PCC_BITFIELD_TYPE_MATTERS.
pe_aligned_commonsTarget supports -mpe-aligned-commons.
section_anchorsTarget supports section anchors.
short_enumsTarget defaults to short enums.
staticTarget supports -static.
static_libgfortranTarget supports statically linking ‘libgfortran’.
string_mergingTarget supports merging string constants at link time.
ucnTarget supports compiling and assembling UCN.
ucn_nocacheIncluding the options used to compile this particular test, the target supports compiling and assembling UCN.
unaligned_stackTarget does not guarantee that its STACK_BOUNDARY is greater than
or equal to the required vector alignment.
vector_alignment_reachableVector alignment is reachable for types of 32 bits or less.
vector_alignment_reachable_for_64bitVector alignment is reachable for types of 64 bits or less.
wchar_t_char16_t_compatibleTarget supports wchar_t that is compatible with char16_t.
wchar_t_char32_t_compatibleTarget supports wchar_t that is compatible with char32_t.
gcc.target/i3863dnowTarget supports compiling 3dnow instructions.
aesTarget supports compiling aes instructions.
fma4Target supports compiling fma4 instructions.
ms_hook_prologueTarget supports attribute ms_hook_prologue.
pclmulTarget supports compiling pclmul instructions.
sse3Target supports compiling sse3 instructions.
sse4Target supports compiling sse4 instructions.
sse4aTarget supports compiling sse4a instructions.
ssse3Target supports compiling ssse3 instructions.
vaesTarget supports compiling vaes instructions.
vpclmulTarget supports compiling vpclmul instructions.
xopTarget supports compiling xop instructions.
gcc.target/spu/eaealibTarget __ea library functions are available.
gcc.test-frameworknoAlways returns 0.
yesAlways returns 1.
Next: Require Support, Previous: Effective-Target Keywords, Up: Test Directives [Contents][Index]
dg-add-optionsThe supported values of feature for directive dg-add-options
are:
arm_neonNEON support. Only ARM targets support this feature, and only then in certain modes; see the arm_neon_ok effective target keyword.
arm_neon_fp16NEON and half-precision floating point support. Only ARM targets support this feature, and only then in certain modes; see the arm_neon_fp16_ok effective target keyword.
bind_pic_locallyAdd the target-specific flags needed to enable functions to bind locally when using pic/PIC passes in the testsuite.
c99_runtimeAdd the target-specific flags needed to access the C99 runtime.
ieeeAdd the target-specific flags needed to enable full IEEE compliance mode.
mips16_attributemips16 function attributes.
Only MIPS targets support this feature, and only then in certain modes.
tlsAdd the target-specific flags needed to use thread-local storage.
Next: Final Actions, Previous: Add Options, Up: Test Directives [Contents][Index]
dg-require-supportA few of the dg-require directives take arguments.
dg-require-iconv codesetSkip the test if the target does not support iconv. codeset is the codeset to convert to.
dg-require-profiling profoptSkip the test if the target does not support profiling with option profopt.
dg-require-visibility visSkip the test if the target does not support the visibility attribute.
If vis is "", support for visibility("hidden") is
checked, for visibility("vis") otherwise.
The original dg-require directives were defined before there
was support for effective-target keywords. The directives that do not
take arguments could be replaced with effective-target keywords.
dg-require-alias ""Skip the test if the target does not support the ‘alias’ attribute.
dg-require-ascii-locale ""Skip the test if the host does not support an ASCII locale.
dg-require-compat-dfp ""Skip this test unless both compilers in a compat testsuite support decimal floating point.
dg-require-cxa-atexit ""Skip the test if the target does not support __cxa_atexit.
This is equivalent to dg-require-effective-target cxa_atexit.
dg-require-dll ""Skip the test if the target does not support DLL attributes.
dg-require-fork ""Skip the test if the target does not support fork.
dg-require-gc-sections ""Skip the test if the target’s linker does not support the
--gc-sections flags.
This is equivalent to dg-require-effective-target gc-sections.
dg-require-host-local ""Skip the test if the host is remote, rather than the same as the build
system. Some tests are incompatible with DejaGnu’s handling of remote
hosts, which involves copying the source file to the host and compiling
it with a relative path and "-o a.out".
dg-require-mkfifo ""Skip the test if the target does not support mkfifo.
dg-require-named-sections ""Skip the test is the target does not support named sections.
This is equivalent to dg-require-effective-target named_sections.
dg-require-weak ""Skip the test if the target does not support weak symbols.
dg-require-weak-override ""Skip the test if the target does not support overriding weak symbols.
Previous: Require Support, Up: Test Directives [Contents][Index]
dg-finalThe GCC testsuite defines the following directives to be used within
dg-final.
scan-file filename regexp [{ target/xfail selector }]Passes if regexp matches text in filename.
scan-file-not filename regexp [{ target/xfail selector }]Passes if regexp does not match text in filename.
scan-module module regexp [{ target/xfail selector }]Passes if regexp matches in Fortran module module.
scan-assembler regex [{ target/xfail selector }]Passes if regex matches text in the test’s assembler output.
scan-assembler-not regex [{ target/xfail selector }]Passes if regex does not match text in the test’s assembler output.
scan-assembler-times regex num [{ target/xfail selector }]Passes if regex is matched exactly num times in the test’s assembler output.
scan-assembler-dem regex [{ target/xfail selector }]Passes if regex matches text in the test’s demangled assembler output.
scan-assembler-dem-not regex [{ target/xfail selector }]Passes if regex does not match text in the test’s demangled assembler output.
scan-hidden symbol [{ target/xfail selector }]Passes if symbol is defined as a hidden symbol in the test’s assembly output.
scan-not-hidden symbol [{ target/xfail selector }]Passes if symbol is not defined as a hidden symbol in the test’s assembly output.
These commands are available for kind of tree, rtl,
and ipa.
scan-kind-dump regex suffix [{ target/xfail selector }]Passes if regex matches text in the dump file with suffix suffix.
scan-kind-dump-not regex suffix [{ target/xfail selector }]Passes if regex does not match text in the dump file with suffix suffix.
scan-kind-dump-times regex num suffix [{ target/xfail selector }]Passes if regex is found exactly num times in the dump file with suffix suffix.
scan-kind-dump-dem regex suffix [{ target/xfail selector }]Passes if regex matches demangled text in the dump file with suffix suffix.
scan-kind-dump-dem-not regex suffix [{ target/xfail selector }]Passes if regex does not match demangled text in the dump file with suffix suffix.
output-exists [{ target/xfail selector }]Passes if compiler output file exists.
output-exists-not [{ target/xfail selector }]Passes if compiler output file does not exist.
scan-symbol regexp [{ target/xfail selector }]Passes if the pattern is present in the final executable.
gcov testsrun-gcov sourcefileCheck line counts in gcov tests.
run-gcov [branches] [calls] { opts sourcefile }Check branch and/or call counts, in addition to line counts, in
gcov tests.
cleanup-coverage-filesRemoves coverage data files generated for this test.
cleanup-ipa-dump suffixRemoves IPA dump files generated for this test.
cleanup-modulesRemoves Fortran module files generated for this test.
cleanup-profile-fileRemoves profiling files generated for this test.
cleanup-repo-filesRemoves files generated for this test for -frepo.
cleanup-rtl-dump suffixRemoves RTL dump files generated for this test.
cleanup-saved-tempsRemoves files for the current test which were kept for -save-temps.
cleanup-tree-dump suffixRemoves tree dump files matching suffix which were generated for this test.
Next: C Tests, Previous: Test Directives, Up: Testsuites [Contents][Index]
The Ada testsuite includes executable tests from the ACATS 2.5 testsuite, publicly available at http://www.adaic.org/compilers/acats/2.5.
These tests are integrated in the GCC testsuite in the
ada/acats directory, and
enabled automatically when running make check, assuming
the Ada language has been enabled when configuring GCC.
You can also run the Ada testsuite independently, using
make check-ada, or run a subset of the tests by specifying which
chapter to run, e.g.:
$ make check-ada CHAPTERS="c3 c9"
The tests are organized by directory, each directory corresponding to a chapter of the Ada Reference Manual. So for example, c9 corresponds to chapter 9, which deals with tasking features of the language.
There is also an extra chapter called gcc containing a template for creating new executable tests, although this is deprecated in favor of the gnat.dg testsuite.
The tests are run using two sh scripts: run_acats and
run_all.sh. To run the tests using a simulator or a cross
target, see the small
customization section at the top of run_all.sh.
These tests are run using the build tree: they can be run without doing
a make install.
Next: libgcj Tests, Previous: Ada Tests, Up: Testsuites [Contents][Index]
GCC contains the following C language testsuites, in the gcc/testsuite directory:
This contains tests of particular features of the C compiler, using the more modern ‘dg’ harness. Correctness tests for various compiler features should go here if possible.
Magic comments determine whether the file is preprocessed, compiled, linked or run. In these tests, error and warning message texts are compared against expected texts or regular expressions given in comments. These tests are run with the options ‘-ansi -pedantic’ unless other options are given in the test. Except as noted below they are not run with multiple optimization options.
This subdirectory contains tests for binary compatibility using lib/compat.exp, which in turn uses the language-independent support (see Support for testing binary compatibility).
This subdirectory contains tests of the preprocessor.
This subdirectory contains tests for debug formats. Tests in this subdirectory are run for each debug format that the compiler supports.
This subdirectory contains tests of the -Wformat format checking. Tests in this directory are run with and without -DWIDE.
This subdirectory contains tests of code that should not compile and does not need any special compilation options. They are run with multiple optimization options, since sometimes invalid code crashes the compiler with optimization.
FIXME: describe this.
This contains particular code fragments which have historically broken easily. These tests are run with multiple optimization options, so tests for features which only break at some optimization levels belong here. This also contains tests to check that certain optimizations occur. It might be worthwhile to separate the correctness tests cleanly from the code quality tests, but it hasn’t been done yet.
FIXME: describe this.
This directory should probably not be used for new tests.
This testsuite contains test cases that should compile, but do not
need to link or run. These test cases are compiled with several
different combinations of optimization options. All warnings are
disabled for these test cases, so this directory is not suitable if
you wish to test for the presence or absence of compiler warnings.
While special options can be set, and tests disabled on specific
platforms, by the use of .x files, mostly these test cases
should not contain platform dependencies. FIXME: discuss how defines
such as NO_LABEL_VALUES and STACK_SIZE are used.
This testsuite contains test cases that should compile, link and run; otherwise the same comments as for gcc.c-torture/compile apply.
This contains tests which are specific to IEEE floating point.
FIXME: describe this.
This directory should probably not be used for new tests.
This directory contains C tests that require special handling. Some of these tests have individual expect files, and others share special-purpose expect files:
bprob*.cTest -fbranch-probabilities using gcc.misc-tests/bprob.exp, which in turn uses the generic, language-independent framework (see Support for testing profile-directed optimizations).
gcov*.cTest gcov output using gcov.exp, which in turn uses the
language-independent support (see Support for testing gcov).
i386-pf-*.cTest i386-specific support for data prefetch using i386-prefetch.exp.
dg-*.cTest the testsuite itself using gcc.test-framework/test-framework.exp.
FIXME: merge in testsuite/README.gcc and discuss the format of test cases and magic comments more.
Next: LTO Testing, Previous: C Tests, Up: Testsuites [Contents][Index]
Runtime tests are executed via ‘make check’ in the target/libjava/testsuite directory in the build tree. Additional runtime tests can be checked into this testsuite.
Regression testing of the core packages in libgcj is also covered by the Mauve testsuite. The Mauve Project develops tests for the Java Class Libraries. These tests are run as part of libgcj testing by placing the Mauve tree within the libjava testsuite sources at libjava/testsuite/libjava.mauve/mauve, or by specifying the location of that tree when invoking ‘make’, as in ‘make MAUVEDIR=~/mauve check’.
To detect regressions, a mechanism in mauve.exp compares the failures for a test run against the list of expected failures in libjava/testsuite/libjava.mauve/xfails from the source hierarchy. Update this file when adding new failing tests to Mauve, or when fixing bugs in libgcj that had caused Mauve test failures.
We encourage developers to contribute test cases to Mauve.
Next: gcov Testing, Previous: libgcj Tests, Up: Testsuites [Contents][Index]
Tests for link-time optimizations usually require multiple source files that are compiled separately, perhaps with different sets of options. There are several special-purpose test directives used for these tests.
{ dg-lto-do do-what-keyword }do-what-keyword specifies how the test is compiled and whether it is executed. It is one of:
assembleCompile with -c to produce a relocatable object file.
linkCompile, assemble, and link to produce an executable file.
runProduce and run an executable file, which is expected to return an exit code of 0.
The default is assemble. That can be overridden for a set of
tests by redefining dg-do-what-default within the .exp
file for those tests.
Unlike dg-do, dg-lto-do does not support an optional
‘target’ or ‘xfail’ list. Use dg-skip-if,
dg-xfail-if, or dg-xfail-run-if.
{ dg-lto-options { { options } [{ options }] } [{ target selector }]}This directive provides a list of one or more sets of compiler options to override LTO_OPTIONS. Each test will be compiled and run with each of these sets of options.
{ dg-extra-ld-options options [{ target selector }]}This directive adds options to the linker options used.
{ dg-suppress-ld-options options [{ target selector }]}This directive removes options from the set of linker options used.
Next: profopt Testing, Previous: LTO Testing, Up: Testsuites [Contents][Index]
gcovLanguage-independent support for testing gcov, and for checking
that branch profiling produces expected values, is provided by the
expect file lib/gcov.exp. gcov tests also rely on procedures
in lib/gcc-dg.exp to compile and run the test program. A typical
gcov test contains the following DejaGnu commands within comments:
{ dg-options "-fprofile-arcs -ftest-coverage" }
{ dg-do run { target native } }
{ dg-final { run-gcov sourcefile } }
Checks of gcov output can include line counts, branch percentages,
and call return percentages. All of these checks are requested via
commands that appear in comments in the test’s source file.
Commands to check line counts are processed by default.
Commands to check branch percentages and call return percentages are
processed if the run-gcov command has arguments branches
or calls, respectively. For example, the following specifies
checking both, as well as passing -b to gcov:
{ dg-final { run-gcov branches calls { -b sourcefile } } }
A line count command appears within a comment on the source line
that is expected to get the specified count and has the form
count(cnt). A test should only check line counts for
lines that will get the same count for any architecture.
Commands to check branch percentages (branch) and call
return percentages (returns) are very similar to each other.
A beginning command appears on or before the first of a range of
lines that will report the percentage, and the ending command
follows that range of lines. The beginning command can include a
list of percentages, all of which are expected to be found within
the range. A range is terminated by the next command of the same
kind. A command branch(end) or returns(end) marks
the end of a range without starting a new one. For example:
if (i > 10 && j > i && j < 20) /* branch(27 50 75) */ /* branch(end) */ foo (i, j);
For a call return percentage, the value specified is the percentage of calls reported to return. For a branch percentage, the value is either the expected percentage or 100 minus that value, since the direction of a branch can differ depending on the target or the optimization level.
Not all branches and calls need to be checked. A test should not check for branches that might be optimized away or replaced with predicated instructions. Don’t check for calls inserted by the compiler or ones that might be inlined or optimized away.
A single test can check for combinations of line counts, branch percentages, and call return percentages. The command to check a line count must appear on the line that will report that count, but commands to check branch percentages and call return percentages can bracket the lines that report them.
Next: compat Testing, Previous: gcov Testing, Up: Testsuites [Contents][Index]
The file profopt.exp provides language-independent support for checking correct execution of a test built with profile-directed optimization. This testing requires that a test program be built and executed twice. The first time it is compiled to generate profile data, and the second time it is compiled to use the data that was generated during the first execution. The second execution is to verify that the test produces the expected results.
To check that the optimization actually generated better code, a test can be built and run a third time with normal optimizations to verify that the performance is better with the profile-directed optimizations. profopt.exp has the beginnings of this kind of support.
profopt.exp provides generic support for profile-directed optimizations. Each set of tests that uses it provides information about a specific optimization:
tooltool being tested, e.g., gcc
profile_optionoptions used to generate profile data
feedback_optionoptions used to optimize using that profile data
prof_extsuffix of profile data files
PROFOPT_OPTIONSlist of options with which to run each test, similar to the lists for torture tests
{ dg-final-generate { local-directive } }This directive is similar to dg-final, but the
local-directive is run after the generation of profile data.
{ dg-final-use { local-directive } }The local-directive is run after the profile data have been used.
Next: Torture Tests, Previous: profopt Testing, Up: Testsuites [Contents][Index]
The file compat.exp provides language-independent support for binary compatibility testing. It supports testing interoperability of two compilers that follow the same ABI, or of multiple sets of compiler options that should not affect binary compatibility. It is intended to be used for testsuites that complement ABI testsuites.
A test supported by this framework has three parts, each in a separate source file: a main program and two pieces that interact with each other to split up the functionality being tested.
Contains the main program, which calls a function in file testname_x.suffix.
Contains at least one call to a function in testname_y.suffix.
Shares data with, or gets arguments from, testname_x.suffix.
Within each test, the main program and one functional piece are compiled by the GCC under test. The other piece can be compiled by an alternate compiler. If no alternate compiler is specified, then all three source files are all compiled by the GCC under test. You can specify pairs of sets of compiler options. The first element of such a pair specifies options used with the GCC under test, and the second element of the pair specifies options used with the alternate compiler. Each test is compiled with each pair of options.
compat.exp defines default pairs of compiler options.
These can be overridden by defining the environment variable
COMPAT_OPTIONS as:
COMPAT_OPTIONS="[list [list {tst1} {alt1}]
…[list {tstn} {altn}]]"
where tsti and alti are lists of options, with tsti
used by the compiler under test and alti used by the alternate
compiler. For example, with
[list [list {-g -O0} {-O3}] [list {-fpic} {-fPIC -O2}]],
the test is first built with -g -O0 by the compiler under
test and with -O3 by the alternate compiler. The test is
built a second time using -fpic by the compiler under test
and -fPIC -O2 by the alternate compiler.
An alternate compiler is specified by defining an environment
variable to be the full pathname of an installed compiler; for C
define ALT_CC_UNDER_TEST, and for C++ define
ALT_CXX_UNDER_TEST. These will be written to the
site.exp file used by DejaGnu. The default is to build each
test with the compiler under test using the first of each pair of
compiler options from COMPAT_OPTIONS. When
ALT_CC_UNDER_TEST or
ALT_CXX_UNDER_TEST is same, each test is built using
the compiler under test but with combinations of the options from
COMPAT_OPTIONS.
To run only the C++ compatibility suite using the compiler under test and another version of GCC using specific compiler options, do the following from objdir/gcc:
rm site.exp
make -k \
ALT_CXX_UNDER_TEST=${alt_prefix}/bin/g++ \
COMPAT_OPTIONS="lists as shown above" \
check-c++ \
RUNTESTFLAGS="compat.exp"
A test that fails when the source files are compiled with different compilers, but passes when the files are compiled with the same compiler, demonstrates incompatibility of the generated code or runtime support. A test that fails for the alternate compiler but passes for the compiler under test probably tests for a bug that was fixed in the compiler under test but is present in the alternate compiler.
The binary compatibility tests support a small number of test framework commands that appear within comments in a test file.
dg-require-*These commands can be used in testname_main.suffix to skip the test if specific support is not available on the target.
dg-optionsThe specified options are used for compiling this particular source
file, appended to the options from COMPAT_OPTIONS. When this
command appears in testname_main.suffix the options
are also used to link the test program.
dg-xfail-ifThis command can be used in a secondary source file to specify that compilation is expected to fail for particular options on particular targets.
Previous: compat Testing, Up: Testsuites [Contents][Index]
Throughout the compiler testsuite there are several directories whose tests are run multiple times, each with a different set of options. These are known as torture tests. lib/torture-options.exp defines procedures to set up these lists:
torture-initInitialize use of torture lists.
set-torture-optionsSet lists of torture options to use for tests with and without loops. Optionally combine a set of torture options with a set of other options, as is done with Objective-C runtime options.
torture-finishFinalize use of torture lists.
The .exp file for a set of tests that use torture options must include calls to these three procedures if:
gcc-dg-runtest and overrides DG_TORTURE_OPTIONS.
-torture or
${tool}-torture-execute, where tool is c,
fortran, or objc.
dg-pch.
It is not necessary for a .exp file that calls gcc-dg-runtest
to call the torture procedures if the tests should use the list in
DG_TORTURE_OPTIONS defined in gcc-dg.exp.
Most uses of torture options can override the default lists by defining TORTURE_OPTIONS or add to the default list by defining ADDITIONAL_TORTURE_OPTIONS. Define these in a .dejagnurc file or add them to the site.exp file; for example
set ADDITIONAL_TORTURE_OPTIONS [list \
{ -O2 -ftree-loop-linear } \
{ -O2 -fpeel-loops } ]
Next: Passes, Previous: Testsuites, Up: Top [Contents][Index]
Most GCC command-line options are described by special option
definition files, the names of which conventionally end in
.opt. This chapter describes the format of these files.
| • Option file format: | The general layout of the files | |
| • Option properties: | Supported option properties |
Next: Option properties, Up: Options [Contents][Index]
Option files are a simple list of records in which each field occupies its own line and in which the records themselves are separated by blank lines. Comments may appear on their own line anywhere within the file and are preceded by semicolons. Whitespace is allowed before the semicolon.
The files can contain the following types of record:
cl_target_option structure.
Var properties.
gcc_options structure, but these variables are also stored in
the cl_target_option structure. The variables are saved in the
target save code and restored in the target restore code.
#ifdef
sequences to properly set up the initialization. These records have
two fields: the string ‘SourceInclude’ and the name of the
include file.
Name(name)This property is required; name must be a name (suitable for use
in C identifiers) used to identify the set of strings in Enum
option properties.
Type(type)This property is required; type is the C type for variables set
by options using this enumeration together with Var.
UnknownError(message)The message message will be used as an error message if the
argument is invalid; for enumerations without UnknownError, a
generic error message is used. message should contain a single
‘%qs’ format, which will be used to format the invalid argument.
Enum(name)This property is required; name says which ‘Enum’ record this ‘EnumValue’ record corresponds to.
String(string)This property is required; string is the string option argument being described by this record.
Value(value)This property is required; it says what value (representable as
int) should be used for the given string.
CanonicalThis property is optional. If present, it says the present string is the canonical one among all those with the given value. Other strings yielding that value will be mapped to this one so specs do not need to handle them.
DriverOnlyThis property is optional. If present, the present string will only be accepted by the driver. This is used for cases such as -march=native that are processed by the driver so that ‘gcc -v’ shows how the options chosen depended on the system on which the compiler was run.
Undocumented property).
By default, all options beginning with “f”, “W” or “m” are
implicitly assumed to take a “no-” form. This form should not be
listed separately. If an option beginning with one of these letters
does not have a “no-” form, you can use the RejectNegative
property to reject it.
The help text is automatically line-wrapped before being displayed. Normally the name of the option is printed on the left-hand side of the output and the help text is printed on the right. However, if the help text contains a tab character, the text to the left of the tab is used instead of the option’s name and the text to the right of the tab forms the help text. This allows you to elaborate on what type of argument the option takes.
target_flags (see Run-time Target) for
each mask name x and set the macro MASK_x to the
appropriate bitmask. It will also declare a TARGET_x
macro that has the value 1 when bit MASK_x is set and
0 otherwise.
They are primarily intended to declare target masks that are not associated with user options, either because these masks represent internal switches or because the options are not available on all configurations and yet the masks always need to be defined.
Previous: Option file format, Up: Options [Contents][Index]
The second field of an option record can specify any of the following properties. When an option takes an argument, it is enclosed in parentheses following the option property name. The parser that handles option files is quite simplistic, and will be tricked by any nested parentheses within the argument text itself; in this case, the entire option argument can be wrapped in curly braces within the parentheses to demarcate it, e.g.:
Condition({defined (USE_CYGWIN_LIBSTDCXX_WRAPPERS)})
CommonThe option is available for all languages and targets.
TargetThe option is available for all languages but is target-specific.
DriverThe option is handled by the compiler driver using code not shared with the compilers proper (cc1 etc.).
languageThe option is available when compiling for the given language.
It is possible to specify several different languages for the same
option. Each language must have been declared by an earlier
Language record. See Option file format.
RejectDriverThe option is only handled by the compilers proper (cc1 etc.) and should not be accepted by the driver.
RejectNegativeThe option does not have a “no-” form. All options beginning with “f”, “W” or “m” are assumed to have a “no-” form unless this property is used.
Negative(othername)The option will turn off another option othername, which is
the option name with the leading “-” removed. This chain action will
propagate through the Negative property of the option to be
turned off.
JoinedSeparateThe option takes a mandatory argument. Joined indicates
that the option and argument can be included in the same argv
entry (as with -mflush-func=name, for example).
Separate indicates that the option and argument can be
separate argv entries (as with -o). An option is
allowed to have both of these properties.
JoinedOrMissingThe option takes an optional argument. If the argument is given,
it will be part of the same argv entry as the option itself.
This property cannot be used alongside Joined or Separate.
MissingArgError(message)For an option marked Joined or Separate, the message
message will be used as an error message if the mandatory
argument is missing; for options without MissingArgError, a
generic error message is used. message should contain a single
‘%qs’ format, which will be used to format the name of the option
passed.
Args(n)For an option marked Separate, indicate that it takes n
arguments. The default is 1.
UIntegerThe option’s argument is a non-negative integer. The option parser
will check and convert the argument before passing it to the relevant
option handler. UInteger should also be used on options like
-falign-loops where both -falign-loops and
-falign-loops=n are supported to make sure the saved
options are given a full integer.
NoDriverArgFor an option marked Separate, the option only takes an
argument in the compiler proper, not in the driver. This is for
compatibility with existing options that are used both directly and
via -Wp,; new options should not have this property.
Var(var)The state of this option should be stored in variable var
(actually a macro for global_options.x_var).
The way that the state is stored depends on the type of option:
Mask or InverseMask properties,
var is the integer variable that contains the mask.
UInteger property,
var is an integer variable that stores the value of the argument.
Enum property,
var is a variable (type given in the Type property of the
‘Enum’ record whose Name property has the same argument as
the Enum property of this option) that stores the value of the
argument.
Defer property, var is a pointer to
a VEC(cl_deferred_option,heap) that stores the option for later
processing. (var is declared with type void * and needs
to be cast to VEC(cl_deferred_option,heap) before use.)
The option-processing script will usually zero-initialize var.
You can modify this behavior using Init.
Var(var, set)The option controls an integer variable var and is active when
var equals set. The option parser will set var to
set when the positive form of the option is used and !set
when the “no-” form is used.
var is declared in the same way as for the single-argument form described above.
Init(value)The variable specified by the Var property should be statically
initialized to value. If more than one option using the same
variable specifies Init, all must specify the same initializer.
Mask(name)The option is associated with a bit in the target_flags
variable (see Run-time Target) and is active when that bit is set.
You may also specify Var to select a variable other than
target_flags.
The options-processing script will automatically allocate a unique bit
for the option. If the option is attached to ‘target_flags’,
the script will set the macro MASK_name to the appropriate
bitmask. It will also declare a TARGET_name macro that has
the value 1 when the option is active and 0 otherwise. If you use Var
to attach the option to a different variable, the associated macros are
called OPTION_MASK_name and OPTION_name respectively.
You can disable automatic bit allocation using MaskExists.
InverseMask(othername)InverseMask(othername, thisname)The option is the inverse of another option that has the
Mask(othername) property. If thisname is given,
the options-processing script will declare a TARGET_thisname
macro that is 1 when the option is active and 0 otherwise.
MaskExistsThe mask specified by the Mask property already exists.
No MASK or TARGET definitions should be added to
options.h in response to this option record.
The main purpose of this property is to support synonymous options. The first option should use ‘Mask(name)’ and the others should use ‘Mask(name) MaskExists’.
Enum(name)The option’s argument is a string from the set of strings associated with the corresponding ‘Enum’ record. The string is checked and converted to the integer specified in the corresponding ‘EnumValue’ record before being passed to option handlers.
DeferThe option should be stored in a vector, specified with Var,
for later processing.
Alias(opt)Alias(opt, arg)Alias(opt, posarg, negarg)The option is an alias for -opt. In the first form, any argument passed to the alias is considered to be passed to -opt, and -opt is considered to be negated if the alias is used in negated form. In the second form, the alias may not be negated or have an argument, and posarg is considered to be passed as an argument to -opt. In the third form, the alias may not have an argument, if the alias is used in the positive form then posarg is considered to be passed to -opt, and if the alias is used in the negative form then negarg is considered to be passed to -opt.
Aliases should not specify Var or Mask or
UInteger. Aliases should normally specify the same languages
as the target of the alias; the flags on the target will be used to
determine any diagnostic for use of an option for the wrong language,
while those on the alias will be used to identify what command-line
text is the option and what text is any argument to that option.
When an Alias definition is used for an option, driver specs do
not need to handle it and no ‘OPT_’ enumeration value is defined
for it; only the canonical form of the option will be seen in those
places.
IgnoreThis option is ignored apart from printing any warning specified using
Warn. The option will not be seen by specs and no ‘OPT_’
enumeration value is defined for it.
SeparateAliasFor an option marked with Joined, Separate and
Alias, the option only acts as an alias when passed a separate
argument; with a joined argument it acts as a normal option, with an
‘OPT_’ enumeration value. This is for compatibility with the
Java -d option and should not be used for new options.
Warn(message)If this option is used, output the warning message.
message is a format string, either taking a single operand with
a ‘%qs’ format which is the option name, or not taking any
operands, which is passed to the ‘warning’ function. If an alias
is marked Warn, the target of the alias must not also be marked
Warn.
ReportThe state of the option should be printed by -fverbose-asm.
WarningThis is a warning option and should be shown as such in --help output. This flag does not currently affect anything other than --help.
OptimizationThis is an optimization option. It should be shown as such in
--help output, and any associated variable named using
Var should be saved and restored when the optimization level is
changed with optimize attributes.
UndocumentedThe option is deliberately missing documentation and should not be included in the --help output.
Condition(cond)The option should only be accepted if preprocessor condition cond is true. Note that any C declarations associated with the option will be present even if cond is false; cond simply controls whether the option is accepted and whether it is printed in the --help output.
SaveBuild the cl_target_option structure to hold a copy of the
option, add the functions cl_target_option_save and
cl_target_option_restore to save and restore the options.
SetByCombinedThe option may also be set by a combined option such as
-ffast-math. This causes the gcc_options struct to
have a field frontend_set_name, where name
is the name of the field holding the value of this option (without the
leading x_). This gives the front end a way to indicate that
the value has been set explicitly and should not be changed by the
combined option. For example, some front ends use this to prevent
-ffast-math and -fno-fast-math from changing the
value of -fmath-errno for languages that do not use
errno.
This chapter is dedicated to giving an overview of the optimization and code generation passes of the compiler. In the process, it describes some of the language front end interface, though this description is no where near complete.
| • Parsing pass: | The language front end turns text into bits. | |
| • Gimplification pass: | The bits are turned into something we can optimize. | |
| • Pass manager: | Sequencing the optimization passes. | |
| • Tree SSA passes: | Optimizations on a high-level representation. | |
| • RTL passes: | Optimizations on a low-level representation. |
Next: Gimplification pass, Up: Passes [Contents][Index]
The language front end is invoked only once, via
lang_hooks.parse_file, to parse the entire input. The language
front end may use any intermediate language representation deemed
appropriate. The C front end uses GENERIC trees (see GENERIC), plus
a double handful of language specific tree codes defined in
c-common.def. The Fortran front end uses a completely different
private representation.
At some point the front end must translate the representation used in the front end to a representation understood by the language-independent portions of the compiler. Current practice takes one of two forms. The C front end manually invokes the gimplifier (see GIMPLE) on each function, and uses the gimplifier callbacks to convert the language-specific tree nodes directly to GIMPLE before passing the function off to be compiled. The Fortran front end converts from a private representation to GENERIC, which is later lowered to GIMPLE when the function is compiled. Which route to choose probably depends on how well GENERIC (plus extensions) can be made to match up with the source language and necessary parsing data structures.
BUG: Gimplification must occur before nested function lowering, and nested function lowering must be done by the front end before passing the data off to cgraph.
TODO: Cgraph should control nested function lowering. It would only be invoked when it is certain that the outer-most function is used.
TODO: Cgraph needs a gimplify_function callback. It should be invoked when (1) it is certain that the function is used, (2) warning flags specified by the user require some amount of compilation in order to honor, (3) the language indicates that semantic analysis is not complete until gimplification occurs. Hum… this sounds overly complicated. Perhaps we should just have the front end gimplify always; in most cases it’s only one function call.
The front end needs to pass all function definitions and top level declarations off to the middle-end so that they can be compiled and emitted to the object file. For a simple procedural language, it is usually most convenient to do this as each top level declaration or definition is seen. There is also a distinction to be made between generating functional code and generating complete debug information. The only thing that is absolutely required for functional code is that function and data definitions be passed to the middle-end. For complete debug information, function, data and type declarations should all be passed as well.
In any case, the front end needs each complete top-level function or
data declaration, and each data definition should be passed to
rest_of_decl_compilation. Each complete type definition should
be passed to rest_of_type_compilation. Each function definition
should be passed to cgraph_finalize_function.
TODO: I know rest_of_compilation currently has all sorts of RTL generation semantics. I plan to move all code generation bits (both Tree and RTL) to compile_function. Should we hide cgraph from the front ends and move back to rest_of_compilation as the official interface? Possibly we should rename all three interfaces such that the names match in some meaningful way and that is more descriptive than "rest_of".
The middle-end will, at its option, emit the function and data definitions immediately or queue them for later processing.
Next: Pass manager, Previous: Parsing pass, Up: Passes [Contents][Index]
Gimplification is a whimsical term for the process of converting the intermediate representation of a function into the GIMPLE language (see GIMPLE). The term stuck, and so words like “gimplification”, “gimplify”, “gimplifier” and the like are sprinkled throughout this section of code.
While a front end may certainly choose to generate GIMPLE directly if it chooses, this can be a moderately complex process unless the intermediate language used by the front end is already fairly simple. Usually it is easier to generate GENERIC trees plus extensions and let the language-independent gimplifier do most of the work.
The main entry point to this pass is gimplify_function_tree
located in gimplify.c. From here we process the entire
function gimplifying each statement in turn. The main workhorse
for this pass is gimplify_expr. Approximately everything
passes through here at least once, and it is from here that we
invoke the lang_hooks.gimplify_expr callback.
The callback should examine the expression in question and return
GS_UNHANDLED if the expression is not a language specific
construct that requires attention. Otherwise it should alter the
expression in some way to such that forward progress is made toward
producing valid GIMPLE. If the callback is certain that the
transformation is complete and the expression is valid GIMPLE, it
should return GS_ALL_DONE. Otherwise it should return
GS_OK, which will cause the expression to be processed again.
If the callback encounters an error during the transformation (because
the front end is relying on the gimplification process to finish
semantic checks), it should return GS_ERROR.
Next: Tree SSA passes, Previous: Gimplification pass, Up: Passes [Contents][Index]
The pass manager is located in passes.c, tree-optimize.c and tree-pass.h. Its job is to run all of the individual passes in the correct order, and take care of standard bookkeeping that applies to every pass.
The theory of operation is that each pass defines a structure that represents everything we need to know about that pass—when it should be run, how it should be run, what intermediate language form or on-the-side data structures it needs. We register the pass to be run in some particular order, and the pass manager arranges for everything to happen in the correct order.
The actuality doesn’t completely live up to the theory at present.
Command-line switches and timevar_id_t enumerations must still
be defined elsewhere. The pass manager validates constraints but does
not attempt to (re-)generate data structures or lower intermediate
language form based on the requirements of the next pass. Nevertheless,
what is present is useful, and a far sight better than nothing at all.
Each pass should have a unique name. Each pass may have its own dump file (for GCC debugging purposes). Passes with a name starting with a star do not dump anything. Sometimes passes are supposed to share a dump file / option name. To still give these unique names, you can use a prefix that is delimited by a space from the part that is used for the dump file / option name. E.g. When the pass name is "ud dce", the name used for dump file/options is "dce".
TODO: describe the global variables set up by the pass manager, and a brief description of how a new pass should use it. I need to look at what info RTL passes use first...
Next: RTL passes, Previous: Pass manager, Up: Passes [Contents][Index]
The following briefly describes the Tree optimization passes that are run after gimplification and what source files they are located in.
This pass is an extremely simple sweep across the gimple code in which
we identify obviously dead code and remove it. Here we do things like
simplify if statements with constant conditions, remove
exception handling constructs surrounding code that obviously cannot
throw, remove lexical bindings that contain no variables, and other
assorted simplistic cleanups. The idea is to get rid of the obvious
stuff quickly rather than wait until later when it’s more work to get
rid of it. This pass is located in tree-cfg.c and described by
pass_remove_useless_stmts.
If mudflap (see ‘-fmudflap -fmudflapth
-fmudflapir’ in Using the GNU Compiler Collection (GCC)) is
enabled, we generate code to register some variable declarations with
the mudflap runtime. Specifically, the runtime tracks the lifetimes of
those variable declarations that have their addresses taken, or whose
bounds are unknown at compile time (extern). This pass generates
new exception handling constructs (try/finally), and so
must run before those are lowered. In addition, the pass enqueues
declarations of static variables whose lifetimes extend to the entire
program. The pass is located in tree-mudflap.c and is described
by pass_mudflap_1.
If OpenMP generation (-fopenmp) is enabled, this pass lowers OpenMP constructs into GIMPLE.
Lowering of OpenMP constructs involves creating replacement
expressions for local variables that have been mapped using data
sharing clauses, exposing the control flow of most synchronization
directives and adding region markers to facilitate the creation of the
control flow graph. The pass is located in omp-low.c and is
described by pass_lower_omp.
If OpenMP generation (-fopenmp) is enabled, this pass expands
parallel regions into their own functions to be invoked by the thread
library. The pass is located in omp-low.c and is described by
pass_expand_omp.
This pass flattens if statements (COND_EXPR)
and moves lexical bindings (BIND_EXPR) out of line. After
this pass, all if statements will have exactly two goto
statements in its then and else arms. Lexical binding
information for each statement will be found in TREE_BLOCK rather
than being inferred from its position under a BIND_EXPR. This
pass is found in gimple-low.c and is described by
pass_lower_cf.
This pass decomposes high-level exception handling constructs
(TRY_FINALLY_EXPR and TRY_CATCH_EXPR) into a form
that explicitly represents the control flow involved. After this
pass, lookup_stmt_eh_region will return a non-negative
number for any statement that may have EH control flow semantics;
examine tree_can_throw_internal or tree_can_throw_external
for exact semantics. Exact control flow may be extracted from
foreach_reachable_handler. The EH region nesting tree is defined
in except.h and built in except.c. The lowering pass
itself is in tree-eh.c and is described by pass_lower_eh.
This pass decomposes a function into basic blocks and creates all of
the edges that connect them. It is located in tree-cfg.c and
is described by pass_build_cfg.
This pass walks the entire function and collects an array of all
variables referenced in the function, referenced_vars. The
index at which a variable is found in the array is used as a UID
for the variable within this function. This data is needed by the
SSA rewriting routines. The pass is located in tree-dfa.c
and is described by pass_referenced_vars.
This pass rewrites the function such that it is in SSA form. After
this pass, all is_gimple_reg variables will be referenced by
SSA_NAME, and all occurrences of other variables will be
annotated with VDEFS and VUSES; PHI nodes will have
been inserted as necessary for each basic block. This pass is
located in tree-ssa.c and is described by pass_build_ssa.
This pass scans the function for uses of SSA_NAMEs that
are fed by default definition. For non-parameter variables, such
uses are uninitialized. The pass is run twice, before and after
optimization (if turned on). In the first pass we only warn for uses that are
positively uninitialized; in the second pass we warn for uses that
are possibly uninitialized. The pass is located in tree-ssa.c
and is defined by pass_early_warn_uninitialized and
pass_late_warn_uninitialized.
This pass scans the function for statements without side effects whose
result is unused. It does not do memory life analysis, so any value
that is stored in memory is considered used. The pass is run multiple
times throughout the optimization process. It is located in
tree-ssa-dce.c and is described by pass_dce.
This pass performs trivial dominator-based copy and constant propagation,
expression simplification, and jump threading. It is run multiple times
throughout the optimization process. It is located in tree-ssa-dom.c
and is described by pass_dominator.
This pass attempts to remove redundant computation by substituting
variables that are used once into the expression that uses them and
seeing if the result can be simplified. It is located in
tree-ssa-forwprop.c and is described by pass_forwprop.
This pass attempts to change the name of compiler temporaries involved in
copy operations such that SSA->normal can coalesce the copy away. When compiler
temporaries are copies of user variables, it also renames the compiler
temporary to the user variable resulting in better use of user symbols. It is
located in tree-ssa-copyrename.c and is described by
pass_copyrename.
This pass recognizes forms of PHI inputs that can be represented as
conditional expressions and rewrites them into straight line code.
It is located in tree-ssa-phiopt.c and is described by
pass_phiopt.
This pass performs a flow sensitive SSA-based points-to analysis.
The resulting may-alias, must-alias, and escape analysis information
is used to promote variables from in-memory addressable objects to
non-aliased variables that can be renamed into SSA form. We also
update the VDEF/VUSE memory tags for non-renameable
aggregates so that we get fewer false kills. The pass is located
in tree-ssa-alias.c and is described by pass_may_alias.
Interprocedural points-to information is located in
tree-ssa-structalias.c and described by pass_ipa_pta.
This pass rewrites the function in order to collect runtime block
and value profiling data. Such data may be fed back into the compiler
on a subsequent run so as to allow optimization based on expected
execution frequencies. The pass is located in predict.c and
is described by pass_profile.
This pass rewrites complex arithmetic operations into their component
scalar arithmetic operations. The pass is located in tree-complex.c
and is described by pass_lower_complex.
This pass rewrites suitable non-aliased local aggregate variables into
a set of scalar variables. The resulting scalar variables are
rewritten into SSA form, which allows subsequent optimization passes
to do a significantly better job with them. The pass is located in
tree-sra.c and is described by pass_sra.
This pass eliminates stores to memory that are subsequently overwritten
by another store, without any intervening loads. The pass is located
in tree-ssa-dse.c and is described by pass_dse.
This pass transforms tail recursion into a loop. It is located in
tree-tailcall.c and is described by pass_tail_recursion.
This pass sinks stores and assignments down the flowgraph closer to their
use point. The pass is located in tree-ssa-sink.c and is
described by pass_sink_code.
This pass eliminates partially redundant computations, as well as
performing load motion. The pass is located in tree-ssa-pre.c
and is described by pass_pre.
Just before partial redundancy elimination, if
-funsafe-math-optimizations is on, GCC tries to convert
divisions to multiplications by the reciprocal. The pass is located
in tree-ssa-math-opts.c and is described by
pass_cse_reciprocal.
This is a simpler form of PRE that only eliminates redundancies that
occur an all paths. It is located in tree-ssa-pre.c and
described by pass_fre.
The main driver of the pass is placed in tree-ssa-loop.c
and described by pass_loop.
The optimizations performed by this pass are:
Loop invariant motion. This pass moves only invariants that would be hard to handle on RTL level (function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion. The pass is implemented in tree-ssa-loop-im.c.
Canonical induction variable creation. This pass creates a simple counter for number of iterations of the loop and replaces the exit condition of the loop using it, in case when a complicated analysis is necessary to determine the number of iterations. Later optimizations then may determine the number easily. The pass is implemented in tree-ssa-loop-ivcanon.c.
Induction variable optimizations. This pass performs standard induction variable optimizations, including strength reduction, induction variable merging and induction variable elimination. The pass is implemented in tree-ssa-loop-ivopts.c.
Loop unswitching. This pass moves the conditional jumps that are invariant out of the loops. To achieve this, a duplicate of the loop is created for each possible outcome of conditional jump(s). The pass is implemented in tree-ssa-loop-unswitch.c. This pass should eventually replace the RTL level loop unswitching in loop-unswitch.c, but currently the RTL level pass is not completely redundant yet due to deficiencies in tree level alias analysis.
The optimizations also use various utility functions contained in tree-ssa-loop-manip.c, cfgloop.c, cfgloopanal.c and cfgloopmanip.c.
Vectorization. This pass transforms loops to operate on vector types
instead of scalar types. Data parallelism across loop iterations is exploited
to group data elements from consecutive iterations into a vector and operate
on them in parallel. Depending on available target support the loop is
conceptually unrolled by a factor VF (vectorization factor), which is
the number of elements operated upon in parallel in each iteration, and the
VF copies of each scalar operation are fused to form a vector operation.
Additional loop transformations such as peeling and versioning may take place
to align the number of iterations, and to align the memory accesses in the
loop.
The pass is implemented in tree-vectorizer.c (the main driver),
tree-vect-loop.c and tree-vect-loop-manip.c (loop specific parts
and general loop utilities), tree-vect-slp (loop-aware SLP
functionality), tree-vect-stmts.c and tree-vect-data-refs.c.
Analysis of data references is in tree-data-ref.c.
SLP Vectorization. This pass performs vectorization of straight-line code. The pass is implemented in tree-vectorizer.c (the main driver), tree-vect-slp.c, tree-vect-stmts.c and tree-vect-data-refs.c.
Autoparallelization. This pass splits the loop iteration space to run into several threads. The pass is implemented in tree-parloops.c.
Graphite is a loop transformation framework based on the polyhedral model. Graphite stands for Gimple Represented as Polyhedra. The internals of this infrastructure are documented in http://gcc.gnu.org/wiki/Graphite. The passes working on this representation are implemented in the various graphite-* files.
This pass applies if-conversion to simple loops to help vectorizer.
We identify if convertible loops, if-convert statements and merge
basic blocks in one big block. The idea is to present loop in such
form so that vectorizer can have one to one mapping between statements
and available vector operations. This pass is located in
tree-if-conv.c and is described by pass_if_conversion.
This pass relaxes a lattice of values in order to identify those
that must be constant even in the presence of conditional branches.
The pass is located in tree-ssa-ccp.c and is described
by pass_ccp.
A related pass that works on memory loads and stores, and not just
register values, is located in tree-ssa-ccp.c and described by
pass_store_ccp.
This is similar to constant propagation but the lattice of values is
the “copy-of” relation. It eliminates redundant copies from the
code. The pass is located in tree-ssa-copy.c and described by
pass_copy_prop.
A related pass that works on memory copies, and not just register
copies, is located in tree-ssa-copy.c and described by
pass_store_copy_prop.
This transformation is similar to constant propagation but
instead of propagating single constant values, it propagates
known value ranges. The implementation is based on Patterson’s
range propagation algorithm (Accurate Static Branch Prediction by
Value Range Propagation, J. R. C. Patterson, PLDI ’95). In
contrast to Patterson’s algorithm, this implementation does not
propagate branch probabilities nor it uses more than a single
range per SSA name. This means that the current implementation
cannot be used for branch prediction (though adapting it would
not be difficult). The pass is located in tree-vrp.c and is
described by pass_vrp.
This pass simplifies built-in functions, as applicable, with constant
arguments or with inferable string lengths. It is located in
tree-ssa-ccp.c and is described by pass_fold_builtins.
This pass identifies critical edges and inserts empty basic blocks
such that the edge is no longer critical. The pass is located in
tree-cfg.c and is described by pass_split_crit_edges.
This pass is a stronger form of dead code elimination that can
eliminate unnecessary control flow statements. It is located
in tree-ssa-dce.c and is described by pass_cd_dce.
This pass identifies function calls that may be rewritten into
jumps. No code transformation is actually applied here, but the
data and control flow problem is solved. The code transformation
requires target support, and so is delayed until RTL. In the
meantime CALL_EXPR_TAILCALL is set indicating the possibility.
The pass is located in tree-tailcall.c and is described by
pass_tail_calls. The RTL transformation is handled by
fixup_tail_calls in calls.c.
For non-void functions, this pass locates return statements that do
not specify a value and issues a warning. Such a statement may have
been injected by falling off the end of the function. This pass is
run last so that we have as much time as possible to prove that the
statement is not reachable. It is located in tree-cfg.c and
is described by pass_warn_function_return.
If mudflap is enabled, we rewrite some memory accesses with code to
validate that the memory access is correct. In particular, expressions
involving pointer dereferences (INDIRECT_REF, ARRAY_REF,
etc.) are replaced by code that checks the selected address range
against the mudflap runtime’s database of valid regions. This check
includes an inline lookup into a direct-mapped cache, based on
shift/mask operations of the pointer value, with a fallback function
call into the runtime. The pass is located in tree-mudflap.c and
is described by pass_mudflap_2.
This pass rewrites the function such that it is in normal form. At
the same time, we eliminate as many single-use temporaries as possible,
so the intermediate language is no longer GIMPLE, but GENERIC. The
pass is located in tree-outof-ssa.c and is described by
pass_del_ssa.
This is part of the CFG cleanup passes. It attempts to join PHI nodes
from a forwarder CFG block into another block with PHI nodes. The
pass is located in tree-cfgcleanup.c and is described by
pass_merge_phi.
If a function always returns the same local variable, and that local
variable is an aggregate type, then the variable is replaced with the
return value for the function (i.e., the function’s DECL_RESULT). This
is equivalent to the C++ named return value optimization applied to
GIMPLE. The pass is located in tree-nrv.c and is described by
pass_nrv.
If a function returns a memory object and is called as var =
foo(), this pass tries to change the call so that the address of
var is sent to the caller to avoid an extra memory copy. This
pass is located in tree-nrv.c and is described by
pass_return_slot.
__builtin_object_size
This is a propagation pass similar to CCP that tries to remove calls
to __builtin_object_size when the size of the object can be
computed at compile-time. This pass is located in
tree-object-size.c and is described by
pass_object_sizes.
This pass removes expensive loop-invariant computations out of loops.
The pass is located in tree-ssa-loop.c and described by
pass_lim.
This is a family of loop transformations that works on loop nests. It
includes loop interchange, scaling, skewing and reversal and they are
all geared to the optimization of data locality in array traversals
and the removal of dependencies that hamper optimizations such as loop
parallelization and vectorization. The pass is located in
tree-loop-linear.c and described by
pass_linear_transform.
This pass removes loops with no code in them. The pass is located in
tree-ssa-loop-ivcanon.c and described by
pass_empty_loop.
This pass completely unrolls loops with few iterations. The pass
is located in tree-ssa-loop-ivcanon.c and described by
pass_complete_unroll.
This pass makes the code reuse the computations from the previous
iterations of the loops, especially loads and stores to memory.
It does so by storing the values of these computations to a bank
of temporary variables that are rotated at the end of loop. To avoid
the need for this rotation, the loop is then unrolled and the copies
of the loop body are rewritten to use the appropriate version of
the temporary variable. This pass is located in tree-predcom.c
and described by pass_predcom.
This pass issues prefetch instructions for array references inside
loops. The pass is located in tree-ssa-loop-prefetch.c and
described by pass_loop_prefetch.
This pass rewrites arithmetic expressions to enable optimizations that
operate on them, like redundancy elimination and vectorization. The
pass is located in tree-ssa-reassoc.c and described by
pass_reassoc.
stdarg functions
This pass tries to avoid the saving of register arguments into the
stack on entry to stdarg functions. If the function doesn’t
use any va_start macros, no registers need to be saved. If
va_start macros are used, the va_list variables don’t
escape the function, it is only necessary to save registers that will
be used in va_arg macros. For instance, if va_arg is
only used with integral types in the function, floating point
registers don’t need to be saved. This pass is located in
tree-stdarg.c and described by pass_stdarg.
Previous: Tree SSA passes, Up: Passes [Contents][Index]
The following briefly describes the RTL generation and optimization passes that are run after the Tree optimization passes.
The source files for RTL generation include
stmt.c,
calls.c,
expr.c,
explow.c,
expmed.c,
function.c,
optabs.c
and emit-rtl.c.
Also, the file
insn-emit.c, generated from the machine description by the
program genemit, is used in this pass. The header file
expr.h is used for communication within this pass.
The header files insn-flags.h and insn-codes.h,
generated from the machine description by the programs genflags
and gencodes, tell this pass which standard names are available
for use and which patterns correspond to them.
This pass generates the glue that handles communication between the exception handling library routines and the exception handlers within the function. Entry points in the function that are invoked by the exception handling library are called landing pads. The code for this pass is located in except.c.
This pass removes unreachable code, simplifies jumps to next, jumps to jump, jumps across jumps, etc. The pass is run multiple times. For historical reasons, it is occasionally referred to as the “jump optimization pass”. The bulk of the code for this pass is in cfgcleanup.c, and there are support routines in cfgrtl.c and jump.c.
This pass attempts to remove redundant computation by substituting variables that come from a single definition, and seeing if the result can be simplified. It performs copy propagation and addressing mode selection. The pass is run twice, with values being propagated into loops only on the second run. The code is located in fwprop.c.
This pass removes redundant computation within basic blocks, and optimizes addressing modes based on cost. The pass is run twice. The code for this pass is located in cse.c.
This pass performs two different types of GCSE depending on whether you are optimizing for size or not (LCM based GCSE tends to increase code size for a gain in speed, while Morel-Renvoise based GCSE does not). When optimizing for size, GCSE is done using Morel-Renvoise Partial Redundancy Elimination, with the exception that it does not try to move invariants out of loops—that is left to the loop optimization pass. If MR PRE GCSE is done, code hoisting (aka unification) is also done, as well as load motion. If you are optimizing for speed, LCM (lazy code motion) based GCSE is done. LCM is based on the work of Knoop, Ruthing, and Steffen. LCM based GCSE also does loop invariant code motion. We also perform load and store motion when optimizing for speed. Regardless of which type of GCSE is used, the GCSE pass also performs global constant and copy propagation. The source file for this pass is gcse.c, and the LCM routines are in lcm.c.
This pass performs several loop related optimizations. The source files cfgloopanal.c and cfgloopmanip.c contain generic loop analysis and manipulation code. Initialization and finalization of loop structures is handled by loop-init.c. A loop invariant motion pass is implemented in loop-invariant.c. Basic block level optimizations—unrolling, peeling and unswitching loops— are implemented in loop-unswitch.c and loop-unroll.c. Replacing of the exit condition of loops by special machine-dependent instructions is handled by loop-doloop.c.
This pass is an aggressive form of GCSE that transforms the control flow graph of a function by propagating constants into conditional branch instructions. The source file for this pass is gcse.c.
This pass attempts to replace conditional branches and surrounding assignments with arithmetic, boolean value producing comparison instructions, and conditional move instructions. In the very last invocation after reload, it will generate predicated instructions when supported by the target. The code is located in ifcvt.c.
This pass splits independent uses of each pseudo-register. This can improve effect of the other transformation, such as CSE or register allocation. The code for this pass is located in web.c.
This pass attempts to combine groups of two or three instructions that are related by data flow into single instructions. It combines the RTL expressions for the instructions by substitution, simplifies the result using algebra, and then attempts to match the result against the machine description. The code is located in combine.c.
This pass looks for cases where matching constraints would force an instruction to need a reload, and this reload would be a register-to-register move. It then attempts to change the registers used by the instruction to avoid the move instruction. The code is located in regmove.c.
This pass looks for instructions that require the processor to be in a specific “mode” and minimizes the number of mode changes required to satisfy all users. What these modes are, and what they apply to are completely target-specific. The code for this pass is located in mode-switching.c.
This pass looks at innermost loops and reorders their instructions by overlapping different iterations. Modulo scheduling is performed immediately before instruction scheduling. The code for this pass is located in modulo-sched.c.
This pass looks for instructions whose output will not be available by the time that it is used in subsequent instructions. Memory loads and floating point instructions often have this behavior on RISC machines. It re-orders instructions within a basic block to try to separate the definition and use of items that otherwise would cause pipeline stalls. This pass is performed twice, before and after register allocation. The code for this pass is located in haifa-sched.c, sched-deps.c, sched-ebb.c, sched-rgn.c and sched-vis.c.
These passes make sure that all occurrences of pseudo registers are eliminated, either by allocating them to a hard register, replacing them by an equivalent expression (e.g. a constant) or by placing them on the stack. This is done in several subpasses:
Source files of the allocator are ira.c, ira-build.c, ira-costs.c, ira-conflicts.c, ira-color.c, ira-emit.c, ira-lives, plus header files ira.h and ira-int.h used for the communication between the allocator and the rest of the compiler and between the IRA files.
The reload pass also optionally eliminates the frame pointer and inserts instructions to save and restore call-clobbered registers around calls.
Source files are reload.c and reload1.c, plus the header reload.h used for communication between them.
This pass implements profile guided code positioning. If profile information is not available, various types of static analysis are performed to make the predictions normally coming from the profile feedback (IE execution frequency, branch probability, etc). It is implemented in the file bb-reorder.c, and the various prediction routines are in predict.c.
This pass computes where the variables are stored at each position in code and generates notes describing the variable locations to RTL code. The location lists are then generated according to these notes to debug information if the debugging information format supports location lists. The code is located in var-tracking.c.
This optional pass attempts to find instructions that can go into the delay slots of other instructions, usually jumps and calls. The code for this pass is located in reorg.c.
On many RISC machines, branch instructions have a limited range. Thus, longer sequences of instructions must be used for long branches. In this pass, the compiler figures out what how far each instruction will be from each other instruction, and therefore whether the usual instructions, or the longer sequences, must be used for each branch. The code for this pass is located in final.c.
Conversion from usage of some hard registers to usage of a register stack may be done at this point. Currently, this is supported only for the floating-point registers of the Intel 80387 coprocessor. The code for this pass is located in reg-stack.c.
This pass outputs the assembler code for the function. The source files
are final.c plus insn-output.c; the latter is generated
automatically from the machine description by the tool genoutput.
The header file conditions.h is used for communication between
these files. If mudflap is enabled, the queue of deferred declarations
and any addressed constants (e.g., string literals) is processed by
mudflap_finish_file into a synthetic constructor function
containing calls into the mudflap runtime.
This is run after final because it must output the stack slot offsets for pseudo registers that did not get hard registers. Source files are dbxout.c for DBX symbol table format, sdbout.c for SDB symbol table format, dwarfout.c for DWARF symbol table format, files dwarf2out.c and dwarf2asm.c for DWARF2 symbol table format, and vmsdbgout.c for VMS debug symbol table format.
The last part of the compiler work is done on a low-level intermediate representation called Register Transfer Language. In this language, the instructions to be output are described, pretty much one by one, in an algebraic form that describes what the instruction does.
RTL is inspired by Lisp lists. It has both an internal form, made up of structures that point at other structures, and a textual form that is used in the machine description and in printed debugging dumps. The textual form uses nested parentheses to indicate the pointers in the internal form.
| • RTL Objects: | Expressions vs vectors vs strings vs integers. | |
| • RTL Classes: | Categories of RTL expression objects, and their structure. | |
| • Accessors: | Macros to access expression operands or vector elts. | |
| • Special Accessors: | Macros to access specific annotations on RTL. | |
| • Flags: | Other flags in an RTL expression. | |
| • Machine Modes: | Describing the size and format of a datum. | |
| • Constants: | Expressions with constant values. | |
| • Regs and Memory: | Expressions representing register contents or memory. | |
| • Arithmetic: | Expressions representing arithmetic on other expressions. | |
| • Comparisons: | Expressions representing comparison of expressions. | |
| • Bit-Fields: | Expressions representing bit-fields in memory or reg. | |
| • Vector Operations: | Expressions involving vector datatypes. | |
| • Conversions: | Extending, truncating, floating or fixing. | |
| • RTL Declarations: | Declaring volatility, constancy, etc. | |
| • Side Effects: | Expressions for storing in registers, etc. | |
| • Incdec: | Embedded side-effects for autoincrement addressing. | |
| • Assembler: | Representing asm with operands.
| |
| • Debug Information: | Expressions representing debugging information. | |
| • Insns: | Expression types for entire insns. | |
| • Calls: | RTL representation of function call insns. | |
| • Sharing: | Some expressions are unique; others *must* be copied. | |
| • Reading RTL: | Reading textual RTL from a file. |
Next: RTL Classes, Up: RTL [Contents][Index]
RTL uses five kinds of objects: expressions, integers, wide integers,
strings and vectors. Expressions are the most important ones. An RTL
expression (“RTX”, for short) is a C structure, but it is usually
referred to with a pointer; a type that is given the typedef name
rtx.
An integer is simply an int; their written form uses decimal
digits. A wide integer is an integral object whose type is
HOST_WIDE_INT; their written form uses decimal digits.
A string is a sequence of characters. In core it is represented as a
char * in usual C fashion, and it is written in C syntax as well.
However, strings in RTL may never be null. If you write an empty string in
a machine description, it is represented in core as a null pointer rather
than as a pointer to a null character. In certain contexts, these null
pointers instead of strings are valid. Within RTL code, strings are most
commonly found inside symbol_ref expressions, but they appear in
other contexts in the RTL expressions that make up machine descriptions.
In a machine description, strings are normally written with double quotes, as you would in C. However, strings in machine descriptions may extend over many lines, which is invalid C, and adjacent string constants are not concatenated as they are in C. Any string constant may be surrounded with a single set of parentheses. Sometimes this makes the machine description easier to read.
There is also a special syntax for strings, which can be useful when C code is embedded in a machine description. Wherever a string can appear, it is also valid to write a C-style brace block. The entire brace block, including the outermost pair of braces, is considered to be the string constant. Double quote characters inside the braces are not special. Therefore, if you write string constants in the C code, you need not escape each quote character with a backslash.
A vector contains an arbitrary number of pointers to expressions. The number of elements in the vector is explicitly present in the vector. The written form of a vector consists of square brackets (‘[…]’) surrounding the elements, in sequence and with whitespace separating them. Vectors of length zero are not created; null pointers are used instead.
Expressions are classified by expression codes (also called RTX
codes). The expression code is a name defined in rtl.def, which is
also (in uppercase) a C enumeration constant. The possible expression
codes and their meanings are machine-independent. The code of an RTX can
be extracted with the macro GET_CODE (x) and altered with
PUT_CODE (x, newcode).
The expression code determines how many operands the expression contains,
and what kinds of objects they are. In RTL, unlike Lisp, you cannot tell
by looking at an operand what kind of object it is. Instead, you must know
from its context—from the expression code of the containing expression.
For example, in an expression of code subreg, the first operand is
to be regarded as an expression and the second operand as an integer. In
an expression of code plus, there are two operands, both of which
are to be regarded as expressions. In a symbol_ref expression,
there is one operand, which is to be regarded as a string.
Expressions are written as parentheses containing the name of the expression type, its flags and machine mode if any, and then the operands of the expression (separated by spaces).
Expression code names in the ‘md’ file are written in lowercase,
but when they appear in C code they are written in uppercase. In this
manual, they are shown as follows: const_int.
In a few contexts a null pointer is valid where an expression is normally
wanted. The written form of this is (nil).
Next: Accessors, Previous: RTL Objects, Up: RTL [Contents][Index]
The various expression codes are divided into several classes,
which are represented by single characters. You can determine the class
of an RTX code with the macro GET_RTX_CLASS (code).
Currently, rtl.def defines these classes:
RTX_OBJAn RTX code that represents an actual object, such as a register
(REG) or a memory location (MEM, SYMBOL_REF).
LO_SUM) is also included; instead, SUBREG and
STRICT_LOW_PART are not in this class, but in class x.
RTX_CONST_OBJAn RTX code that represents a constant object. HIGH is also
included in this class.
RTX_COMPAREAn RTX code for a non-symmetric comparison, such as GEU or
LT.
RTX_COMM_COMPAREAn RTX code for a symmetric (commutative) comparison, such as EQ
or ORDERED.
RTX_UNARYAn RTX code for a unary arithmetic operation, such as NEG,
NOT, or ABS. This category also includes value extension
(sign or zero) and conversions between integer and floating point.
RTX_COMM_ARITHAn RTX code for a commutative binary operation, such as PLUS or
AND. NE and EQ are comparisons, so they have class
<.
RTX_BIN_ARITHAn RTX code for a non-commutative binary operation, such as MINUS,
DIV, or ASHIFTRT.
RTX_BITFIELD_OPSAn RTX code for a bit-field operation. Currently only
ZERO_EXTRACT and SIGN_EXTRACT. These have three inputs
and are lvalues (so they can be used for insertion as well).
See Bit-Fields.
RTX_TERNARYAn RTX code for other three input operations. Currently only
IF_THEN_ELSE, VEC_MERGE, SIGN_EXTRACT,
ZERO_EXTRACT, and FMA.
RTX_INSNAn RTX code for an entire instruction: INSN, JUMP_INSN, and
CALL_INSN. See Insns.
RTX_MATCHAn RTX code for something that matches in insns, such as
MATCH_DUP. These only occur in machine descriptions.
RTX_AUTOINCAn RTX code for an auto-increment addressing mode, such as
POST_INC.
RTX_EXTRAAll other RTX codes. This category includes the remaining codes used
only in machine descriptions (DEFINE_*, etc.). It also includes
all the codes describing side effects (SET, USE,
CLOBBER, etc.) and the non-insns that may appear on an insn
chain, such as NOTE, BARRIER, and CODE_LABEL.
SUBREG is also part of this class.
For each expression code, rtl.def specifies the number of
contained objects and their kinds using a sequence of characters
called the format of the expression code. For example,
the format of subreg is ‘ei’.
These are the most commonly used format characters:
eAn expression (actually a pointer to an expression).
iAn integer.
wA wide integer.
sA string.
EA vector of expressions.
A few other format characters are used occasionally:
u‘u’ is equivalent to ‘e’ except that it is printed differently in debugging dumps. It is used for pointers to insns.
n‘n’ is equivalent to ‘i’ except that it is printed differently
in debugging dumps. It is used for the line number or code number of a
note insn.
S‘S’ indicates a string which is optional. In the RTL objects in core, ‘S’ is equivalent to ‘s’, but when the object is read, from an ‘md’ file, the string value of this operand may be omitted. An omitted string is taken to be the null string.
V‘V’ indicates a vector which is optional. In the RTL objects in core, ‘V’ is equivalent to ‘E’, but when the object is read from an ‘md’ file, the vector value of this operand may be omitted. An omitted vector is effectively the same as a vector of no elements.
B‘B’ indicates a pointer to basic block structure.
0‘0’ means a slot whose contents do not fit any normal category. ‘0’ slots are not printed at all in dumps, and are often used in special ways by small parts of the compiler.
There are macros to get the number of operands and the format of an expression code:
GET_RTX_LENGTH (code)Number of operands of an RTX of code code.
GET_RTX_FORMAT (code)The format of an RTX of code code, as a C string.
Some classes of RTX codes always have the same format. For example, it
is safe to assume that all comparison operations have format ee.
1All codes of this class have format e.
<c2All codes of these classes have format ee.
b3All codes of these classes have format eee.
iAll codes of this class have formats that begin with iuueiee.
See Insns. Note that not all RTL objects linked onto an insn chain
are of class i.
omxYou can make no assumptions about the format of these codes.
Next: Special Accessors, Previous: RTL Classes, Up: RTL [Contents][Index]
Operands of expressions are accessed using the macros XEXP,
XINT, XWINT and XSTR. Each of these macros takes
two arguments: an expression-pointer (RTX) and an operand number
(counting from zero). Thus,
XEXP (x, 2)
accesses operand 2 of expression x, as an expression.
XINT (x, 2)
accesses the same operand as an integer. XSTR, used in the same
fashion, would access it as a string.
Any operand can be accessed as an integer, as an expression or as a string. You must choose the correct method of access for the kind of value actually stored in the operand. You would do this based on the expression code of the containing expression. That is also how you would know how many operands there are.
For example, if x is a subreg expression, you know that it has
two operands which can be correctly accessed as XEXP (x, 0)
and XINT (x, 1). If you did XINT (x, 0), you
would get the address of the expression operand but cast as an integer;
that might occasionally be useful, but it would be cleaner to write
(int) XEXP (x, 0). XEXP (x, 1) would also
compile without error, and would return the second, integer operand cast as
an expression pointer, which would probably result in a crash when
accessed. Nothing stops you from writing XEXP (x, 28) either,
but this will access memory past the end of the expression with
unpredictable results.
Access to operands which are vectors is more complicated. You can use the
macro XVEC to get the vector-pointer itself, or the macros
XVECEXP and XVECLEN to access the elements and length of a
vector.
XVEC (exp, idx)Access the vector-pointer which is operand number idx in exp.
XVECLEN (exp, idx)Access the length (number of elements) in the vector which is
in operand number idx in exp. This value is an int.
XVECEXP (exp, idx, eltnum)Access element number eltnum in the vector which is in operand number idx in exp. This value is an RTX.
It is up to you to make sure that eltnum is not negative
and is less than XVECLEN (exp, idx).
All the macros defined in this section expand into lvalues and therefore can be used to assign the operands, lengths and vector elements as well as to access them.
Some RTL nodes have special annotations associated with them.
MEMMEM_ALIAS_SET (x)If 0, x is not in any alias set, and may alias anything. Otherwise,
x can only alias MEMs in a conflicting alias set. This value
is set in a language-dependent manner in the front-end, and should not be
altered in the back-end. In some front-ends, these numbers may correspond
in some way to types, or other language-level entities, but they need not,
and the back-end makes no such assumptions.
These set numbers are tested with alias_sets_conflict_p.
MEM_EXPR (x)If this register is known to hold the value of some user-level
declaration, this is that tree node. It may also be a
COMPONENT_REF, in which case this is some field reference,
and TREE_OPERAND (x, 0) contains the declaration,
or another COMPONENT_REF, or null if there is no compile-time
object associated with the reference.
MEM_OFFSET (x)The offset from the start of MEM_EXPR as a CONST_INT rtx.
MEM_SIZE (x)The size in bytes of the memory reference as a CONST_INT rtx.
This is mostly relevant for BLKmode references as otherwise
the size is implied by the mode.
MEM_ALIGN (x)The known alignment in bits of the memory reference.
MEM_ADDR_SPACE (x)The address space of the memory reference. This will commonly be zero for the generic address space.
REGORIGINAL_REGNO (x)This field holds the number the register “originally” had; for a pseudo register turned into a hard reg this will hold the old pseudo register number.
REG_EXPR (x)If this register is known to hold the value of some user-level declaration, this is that tree node.
REG_OFFSET (x)If this register is known to hold the value of some user-level declaration, this is the offset into that logical storage.
SYMBOL_REFSYMBOL_REF_DECL (x)If the symbol_ref x was created for a VAR_DECL or
a FUNCTION_DECL, that tree is recorded here. If this value is
null, then x was created by back end code generation routines,
and there is no associated front end symbol table entry.
SYMBOL_REF_DECL may also point to a tree of class 'c',
that is, some sort of constant. In this case, the symbol_ref
is an entry in the per-file constant pool; again, there is no associated
front end symbol table entry.
SYMBOL_REF_CONSTANT (x)If ‘CONSTANT_POOL_ADDRESS_P (x)’ is true, this is the constant pool entry for x. It is null otherwise.
SYMBOL_REF_DATA (x)A field of opaque type used to store SYMBOL_REF_DECL or
SYMBOL_REF_CONSTANT.
SYMBOL_REF_FLAGS (x)In a symbol_ref, this is used to communicate various predicates
about the symbol. Some of these are common enough to be computed by
common code, some are specific to the target. The common bits are:
SYMBOL_FLAG_FUNCTIONSet if the symbol refers to a function.
SYMBOL_FLAG_LOCALSet if the symbol is local to this “module”.
See TARGET_BINDS_LOCAL_P.
SYMBOL_FLAG_EXTERNALSet if this symbol is not defined in this translation unit.
Note that this is not the inverse of SYMBOL_FLAG_LOCAL.
SYMBOL_FLAG_SMALLSet if the symbol is located in the small data section.
See TARGET_IN_SMALL_DATA_P.
SYMBOL_REF_TLS_MODEL (x)This is a multi-bit field accessor that returns the tls_model
to be used for a thread-local storage symbol. It returns zero for
non-thread-local symbols.
SYMBOL_FLAG_HAS_BLOCK_INFOSet if the symbol has SYMBOL_REF_BLOCK and
SYMBOL_REF_BLOCK_OFFSET fields.
SYMBOL_FLAG_ANCHORSet if the symbol is used as a section anchor. “Section anchors”
are symbols that have a known position within an object_block
and that can be used to access nearby members of that block.
They are used to implement -fsection-anchors.
If this flag is set, then SYMBOL_FLAG_HAS_BLOCK_INFO will be too.
Bits beginning with SYMBOL_FLAG_MACH_DEP are available for
the target’s use.
SYMBOL_REF_BLOCK (x)If ‘SYMBOL_REF_HAS_BLOCK_INFO_P (x)’, this is the
‘object_block’ structure to which the symbol belongs,
or NULL if it has not been assigned a block.
SYMBOL_REF_BLOCK_OFFSET (x)If ‘SYMBOL_REF_HAS_BLOCK_INFO_P (x)’, this is the offset of x from the first object in ‘SYMBOL_REF_BLOCK (x)’. The value is negative if x has not yet been assigned to a block, or it has not been given an offset within that block.
Next: Machine Modes, Previous: Special Accessors, Up: RTL [Contents][Index]
RTL expressions contain several flags (one-bit bit-fields) that are used in certain types of expression. Most often they are accessed with the following macros, which expand into lvalues.
CONSTANT_POOL_ADDRESS_P (x)Nonzero in a symbol_ref if it refers to part of the current
function’s constant pool. For most targets these addresses are in a
.rodata section entirely separate from the function, but for
some targets the addresses are close to the beginning of the function.
In either case GCC assumes these addresses can be addressed directly,
perhaps with the help of base registers.
Stored in the unchanging field and printed as ‘/u’.
RTL_CONST_CALL_P (x)In a call_insn indicates that the insn represents a call to a
const function. Stored in the unchanging field and printed as
‘/u’.
RTL_PURE_CALL_P (x)In a call_insn indicates that the insn represents a call to a
pure function. Stored in the return_val field and printed as
‘/i’.
RTL_CONST_OR_PURE_CALL_P (x)In a call_insn, true if RTL_CONST_CALL_P or
RTL_PURE_CALL_P is true.
RTL_LOOPING_CONST_OR_PURE_CALL_P (x)In a call_insn indicates that the insn represents a possibly
infinite looping call to a const or pure function. Stored in the
call field and printed as ‘/c’. Only true if one of
RTL_CONST_CALL_P or RTL_PURE_CALL_P is true.
INSN_ANNULLED_BRANCH_P (x)In a jump_insn, call_insn, or insn indicates
that the branch is an annulling one. See the discussion under
sequence below. Stored in the unchanging field and
printed as ‘/u’.
INSN_DELETED_P (x)In an insn, call_insn, jump_insn, code_label,
barrier, or note,
nonzero if the insn has been deleted. Stored in the
volatil field and printed as ‘/v’.
INSN_FROM_TARGET_P (x)In an insn or jump_insn or call_insn in a delay
slot of a branch, indicates that the insn
is from the target of the branch. If the branch insn has
INSN_ANNULLED_BRANCH_P set, this insn will only be executed if
the branch is taken. For annulled branches with
INSN_FROM_TARGET_P clear, the insn will be executed only if the
branch is not taken. When INSN_ANNULLED_BRANCH_P is not set,
this insn will always be executed. Stored in the in_struct
field and printed as ‘/s’.
LABEL_PRESERVE_P (x)In a code_label or note, indicates that the label is referenced by
code or data not visible to the RTL of a given function.
Labels referenced by a non-local goto will have this bit set. Stored
in the in_struct field and printed as ‘/s’.
LABEL_REF_NONLOCAL_P (x)In label_ref and reg_label expressions, nonzero if this is
a reference to a non-local label.
Stored in the volatil field and printed as ‘/v’.
MEM_IN_STRUCT_P (x)In mem expressions, nonzero for reference to an entire structure,
union or array, or to a component of one. Zero for references to a
scalar variable or through a pointer to a scalar. If both this flag and
MEM_SCALAR_P are clear, then we don’t know whether this mem
is in a structure or not. Both flags should never be simultaneously set.
Stored in the in_struct field and printed as ‘/s’.
MEM_KEEP_ALIAS_SET_P (x)In mem expressions, 1 if we should keep the alias set for this
mem unchanged when we access a component. Set to 1, for example, when we
are already in a non-addressable component of an aggregate.
Stored in the jump field and printed as ‘/j’.
MEM_SCALAR_P (x)In mem expressions, nonzero for reference to a scalar known not
to be a member of a structure, union, or array. Zero for such
references and for indirections through pointers, even pointers pointing
to scalar types. If both this flag and MEM_IN_STRUCT_P are clear,
then we don’t know whether this mem is in a structure or not.
Both flags should never be simultaneously set.
Stored in the return_val field and printed as ‘/i’.
MEM_VOLATILE_P (x)In mem, asm_operands, and asm_input expressions,
nonzero for volatile memory references.
Stored in the volatil field and printed as ‘/v’.
MEM_NOTRAP_P (x)In mem, nonzero for memory references that will not trap.
Stored in the call field and printed as ‘/c’.
MEM_POINTER (x)Nonzero in a mem if the memory reference holds a pointer.
Stored in the frame_related field and printed as ‘/f’.
REG_FUNCTION_VALUE_P (x)Nonzero in a reg if it is the place in which this function’s
value is going to be returned. (This happens only in a hard
register.) Stored in the return_val field and printed as
‘/i’.
REG_POINTER (x)Nonzero in a reg if the register holds a pointer. Stored in the
frame_related field and printed as ‘/f’.
REG_USERVAR_P (x)In a reg, nonzero if it corresponds to a variable present in
the user’s source code. Zero for temporaries generated internally by
the compiler. Stored in the volatil field and printed as
‘/v’.
The same hard register may be used also for collecting the values of
functions called by this one, but REG_FUNCTION_VALUE_P is zero
in this kind of use.
RTX_FRAME_RELATED_P (x)Nonzero in an insn, call_insn, jump_insn,
barrier, or set which is part of a function prologue
and sets the stack pointer, sets the frame pointer, or saves a register.
This flag should also be set on an instruction that sets up a temporary
register to use in place of the frame pointer.
Stored in the frame_related field and printed as ‘/f’.
In particular, on RISC targets where there are limits on the sizes of
immediate constants, it is sometimes impossible to reach the register
save area directly from the stack pointer. In that case, a temporary
register is used that is near enough to the register save area, and the
Canonical Frame Address, i.e., DWARF2’s logical frame pointer, register
must (temporarily) be changed to be this temporary register. So, the
instruction that sets this temporary register must be marked as
RTX_FRAME_RELATED_P.
If the marked instruction is overly complex (defined in terms of what
dwarf2out_frame_debug_expr can handle), you will also have to
create a REG_FRAME_RELATED_EXPR note and attach it to the
instruction. This note should contain a simple expression of the
computation performed by this instruction, i.e., one that
dwarf2out_frame_debug_expr can handle.
This flag is required for exception handling support on targets with RTL prologues.
MEM_READONLY_P (x)Nonzero in a mem, if the memory is statically allocated and read-only.
Read-only in this context means never modified during the lifetime of the program, not necessarily in ROM or in write-disabled pages. A common example of the later is a shared library’s global offset table. This table is initialized by the runtime loader, so the memory is technically writable, but after control is transfered from the runtime loader to the application, this memory will never be subsequently modified.
Stored in the unchanging field and printed as ‘/u’.
SCHED_GROUP_P (x)During instruction scheduling, in an insn, call_insn or
jump_insn, indicates that the
previous insn must be scheduled together with this insn. This is used to
ensure that certain groups of instructions will not be split up by the
instruction scheduling pass, for example, use insns before
a call_insn may not be separated from the call_insn.
Stored in the in_struct field and printed as ‘/s’.
SET_IS_RETURN_P (x)For a set, nonzero if it is for a return.
Stored in the jump field and printed as ‘/j’.
SIBLING_CALL_P (x)For a call_insn, nonzero if the insn is a sibling call.
Stored in the jump field and printed as ‘/j’.
STRING_POOL_ADDRESS_P (x)For a symbol_ref expression, nonzero if it addresses this function’s
string constant pool.
Stored in the frame_related field and printed as ‘/f’.
SUBREG_PROMOTED_UNSIGNED_P (x)Returns a value greater then zero for a subreg that has
SUBREG_PROMOTED_VAR_P nonzero if the object being referenced is kept
zero-extended, zero if it is kept sign-extended, and less then zero if it is
extended some other way via the ptr_extend instruction.
Stored in the unchanging
field and volatil field, printed as ‘/u’ and ‘/v’.
This macro may only be used to get the value it may not be used to change
the value. Use SUBREG_PROMOTED_UNSIGNED_SET to change the value.
SUBREG_PROMOTED_UNSIGNED_SET (x)Set the unchanging and volatil fields in a subreg
to reflect zero, sign, or other extension. If volatil is
zero, then unchanging as nonzero means zero extension and as
zero means sign extension. If volatil is nonzero then some
other type of extension was done via the ptr_extend instruction.
SUBREG_PROMOTED_VAR_P (x)Nonzero in a subreg if it was made when accessing an object that
was promoted to a wider mode in accord with the PROMOTED_MODE machine
description macro (see Storage Layout). In this case, the mode of
the subreg is the declared mode of the object and the mode of
SUBREG_REG is the mode of the register that holds the object.
Promoted variables are always either sign- or zero-extended to the wider
mode on every assignment. Stored in the in_struct field and
printed as ‘/s’.
SYMBOL_REF_USED (x)In a symbol_ref, indicates that x has been used. This is
normally only used to ensure that x is only declared external
once. Stored in the used field.
SYMBOL_REF_WEAK (x)In a symbol_ref, indicates that x has been declared weak.
Stored in the return_val field and printed as ‘/i’.
SYMBOL_REF_FLAG (x)In a symbol_ref, this is used as a flag for machine-specific purposes.
Stored in the volatil field and printed as ‘/v’.
Most uses of SYMBOL_REF_FLAG are historic and may be subsumed
by SYMBOL_REF_FLAGS. Certainly use of SYMBOL_REF_FLAGS
is mandatory if the target requires more than one bit of storage.
PREFETCH_SCHEDULE_BARRIER_P (x)In a prefetch, indicates that the prefetch is a scheduling barrier.
No other INSNs will be moved over it.
Stored in the volatil field and printed as ‘/v’.
These are the fields to which the above macros refer:
callIn a mem, 1 means that the memory reference will not trap.
In a call, 1 means that this pure or const call may possibly
infinite loop.
In an RTL dump, this flag is represented as ‘/c’.
frame_relatedIn an insn or set expression, 1 means that it is part of
a function prologue and sets the stack pointer, sets the frame pointer,
saves a register, or sets up a temporary register to use in place of the
frame pointer.
In reg expressions, 1 means that the register holds a pointer.
In mem expressions, 1 means that the memory reference holds a pointer.
In symbol_ref expressions, 1 means that the reference addresses
this function’s string constant pool.
In an RTL dump, this flag is represented as ‘/f’.
in_structIn mem expressions, it is 1 if the memory datum referred to is
all or part of a structure or array; 0 if it is (or might be) a scalar
variable. A reference through a C pointer has 0 because the pointer
might point to a scalar variable. This information allows the compiler
to determine something about possible cases of aliasing.
In reg expressions, it is 1 if the register has its entire life
contained within the test expression of some loop.
In subreg expressions, 1 means that the subreg is accessing
an object that has had its mode promoted from a wider mode.
In label_ref expressions, 1 means that the referenced label is
outside the innermost loop containing the insn in which the label_ref
was found.
In code_label expressions, it is 1 if the label may never be deleted.
This is used for labels which are the target of non-local gotos. Such a
label that would have been deleted is replaced with a note of type
NOTE_INSN_DELETED_LABEL.
In an insn during dead-code elimination, 1 means that the insn is
dead code.
In an insn or jump_insn during reorg for an insn in the
delay slot of a branch,
1 means that this insn is from the target of the branch.
In an insn during instruction scheduling, 1 means that this insn
must be scheduled as part of a group together with the previous insn.
In an RTL dump, this flag is represented as ‘/s’.
return_valIn reg expressions, 1 means the register contains
the value to be returned by the current function. On
machines that pass parameters in registers, the same register number
may be used for parameters as well, but this flag is not set on such
uses.
In mem expressions, 1 means the memory reference is to a scalar
known not to be a member of a structure, union, or array.
In symbol_ref expressions, 1 means the referenced symbol is weak.
In call expressions, 1 means the call is pure.
In an RTL dump, this flag is represented as ‘/i’.
jumpIn a mem expression, 1 means we should keep the alias set for this
mem unchanged when we access a component.
In a set, 1 means it is for a return.
In a call_insn, 1 means it is a sibling call.
In an RTL dump, this flag is represented as ‘/j’.
unchangingIn reg and mem expressions, 1 means
that the value of the expression never changes.
In subreg expressions, it is 1 if the subreg references an
unsigned object whose mode has been promoted to a wider mode.
In an insn or jump_insn in the delay slot of a branch
instruction, 1 means an annulling branch should be used.
In a symbol_ref expression, 1 means that this symbol addresses
something in the per-function constant pool.
In a call_insn 1 means that this instruction is a call to a const
function.
In an RTL dump, this flag is represented as ‘/u’.
usedThis flag is used directly (without an access macro) at the end of RTL generation for a function, to count the number of times an expression appears in insns. Expressions that appear more than once are copied, according to the rules for shared structure (see Sharing).
For a reg, it is used directly (without an access macro) by the
leaf register renumbering code to ensure that each register is only
renumbered once.
In a symbol_ref, it indicates that an external declaration for
the symbol has already been written.
volatilIn a mem, asm_operands, or asm_input
expression, it is 1 if the memory
reference is volatile. Volatile memory references may not be deleted,
reordered or combined.
In a symbol_ref expression, it is used for machine-specific
purposes.
In a reg expression, it is 1 if the value is a user-level variable.
0 indicates an internal compiler temporary.
In an insn, 1 means the insn has been deleted.
In label_ref and reg_label expressions, 1 means a reference
to a non-local label.
In prefetch expressions, 1 means that the containing insn is a
scheduling barrier.
In an RTL dump, this flag is represented as ‘/v’.
A machine mode describes a size of data object and the representation used
for it. In the C code, machine modes are represented by an enumeration
type, enum machine_mode, defined in machmode.def. Each RTL
expression has room for a machine mode and so do certain kinds of tree
expressions (declarations and types, to be precise).
In debugging dumps and machine descriptions, the machine mode of an RTL
expression is written after the expression code with a colon to separate
them. The letters ‘mode’ which appear at the end of each machine mode
name are omitted. For example, (reg:SI 38) is a reg
expression with machine mode SImode. If the mode is
VOIDmode, it is not written at all.
Here is a table of machine modes. The term “byte” below refers to an
object of BITS_PER_UNIT bits (see Storage Layout).
BImode“Bit” mode represents a single bit, for predicate registers.
QImode“Quarter-Integer” mode represents a single byte treated as an integer.
HImode“Half-Integer” mode represents a two-byte integer.
PSImode“Partial Single Integer” mode represents an integer which occupies four bytes but which doesn’t really use all four. On some machines, this is the right mode to use for pointers.
SImode“Single Integer” mode represents a four-byte integer.
PDImode“Partial Double Integer” mode represents an integer which occupies eight bytes but which doesn’t really use all eight. On some machines, this is the right mode to use for certain pointers.
DImode“Double Integer” mode represents an eight-byte integer.
TImode“Tetra Integer” (?) mode represents a sixteen-byte integer.
OImode“Octa Integer” (?) mode represents a thirty-two-byte integer.
QFmode“Quarter-Floating” mode represents a quarter-precision (single byte) floating point number.
HFmode“Half-Floating” mode represents a half-precision (two byte) floating point number.
TQFmode“Three-Quarter-Floating” (?) mode represents a three-quarter-precision (three byte) floating point number.
SFmode“Single Floating” mode represents a four byte floating point number. In the common case, of a processor with IEEE arithmetic and 8-bit bytes, this is a single-precision IEEE floating point number; it can also be used for double-precision (on processors with 16-bit bytes) and single-precision VAX and IBM types.
DFmode“Double Floating” mode represents an eight byte floating point number. In the common case, of a processor with IEEE arithmetic and 8-bit bytes, this is a double-precision IEEE floating point number.
XFmode“Extended Floating” mode represents an IEEE extended floating point number. This mode only has 80 meaningful bits (ten bytes). Some processors require such numbers to be padded to twelve bytes, others to sixteen; this mode is used for either.
SDmode“Single Decimal Floating” mode represents a four byte decimal floating point number (as distinct from conventional binary floating point).
DDmode“Double Decimal Floating” mode represents an eight byte decimal floating point number.
TDmode“Tetra Decimal Floating” mode represents a sixteen byte decimal floating point number all 128 of whose bits are meaningful.
TFmode“Tetra Floating” mode represents a sixteen byte floating point number all 128 of whose bits are meaningful. One common use is the IEEE quad-precision format.
QQmode“Quarter-Fractional” mode represents a single byte treated as a signed fractional number. The default format is “s.7”.
HQmode“Half-Fractional” mode represents a two-byte signed fractional number. The default format is “s.15”.
SQmode“Single Fractional” mode represents a four-byte signed fractional number. The default format is “s.31”.
DQmode“Double Fractional” mode represents an eight-byte signed fractional number. The default format is “s.63”.
TQmode“Tetra Fractional” mode represents a sixteen-byte signed fractional number. The default format is “s.127”.
UQQmode“Unsigned Quarter-Fractional” mode represents a single byte treated as an unsigned fractional number. The default format is “.8”.
UHQmode“Unsigned Half-Fractional” mode represents a two-byte unsigned fractional number. The default format is “.16”.
USQmode“Unsigned Single Fractional” mode represents a four-byte unsigned fractional number. The default format is “.32”.
UDQmode“Unsigned Double Fractional” mode represents an eight-byte unsigned fractional number. The default format is “.64”.
UTQmode“Unsigned Tetra Fractional” mode represents a sixteen-byte unsigned fractional number. The default format is “.128”.
HAmode“Half-Accumulator” mode represents a two-byte signed accumulator. The default format is “s8.7”.
SAmode“Single Accumulator” mode represents a four-byte signed accumulator. The default format is “s16.15”.
DAmode“Double Accumulator” mode represents an eight-byte signed accumulator. The default format is “s32.31”.
TAmode“Tetra Accumulator” mode represents a sixteen-byte signed accumulator. The default format is “s64.63”.
UHAmode“Unsigned Half-Accumulator” mode represents a two-byte unsigned accumulator. The default format is “8.8”.
USAmode“Unsigned Single Accumulator” mode represents a four-byte unsigned accumulator. The default format is “16.16”.
UDAmode“Unsigned Double Accumulator” mode represents an eight-byte unsigned accumulator. The default format is “32.32”.
UTAmode“Unsigned Tetra Accumulator” mode represents a sixteen-byte unsigned accumulator. The default format is “64.64”.
CCmode“Condition Code” mode represents the value of a condition code, which
is a machine-specific set of bits used to represent the result of a
comparison operation. Other machine-specific modes may also be used for
the condition code. These modes are not used on machines that use
cc0 (see Condition Code).
BLKmode“Block” mode represents values that are aggregates to which none of
the other modes apply. In RTL, only memory references can have this mode,
and only if they appear in string-move or vector instructions. On machines
which have no such instructions, BLKmode will not appear in RTL.
VOIDmodeVoid mode means the absence of a mode or an unspecified mode.
For example, RTL expressions of code const_int have mode
VOIDmode because they can be taken to have whatever mode the context
requires. In debugging dumps of RTL, VOIDmode is expressed by
the absence of any mode.
QCmode, HCmode, SCmode, DCmode, XCmode, TCmodeThese modes stand for a complex number represented as a pair of floating
point values. The floating point values are in QFmode,
HFmode, SFmode, DFmode, XFmode, and
TFmode, respectively.
CQImode, CHImode, CSImode, CDImode, CTImode, COImodeThese modes stand for a complex number represented as a pair of integer
values. The integer values are in QImode, HImode,
SImode, DImode, TImode, and OImode,
respectively.
The machine description defines Pmode as a C macro which expands
into the machine mode used for addresses. Normally this is the mode
whose size is BITS_PER_WORD, SImode on 32-bit machines.
The only modes which a machine description must support are
QImode, and the modes corresponding to BITS_PER_WORD,
FLOAT_TYPE_SIZE and DOUBLE_TYPE_SIZE.
The compiler will attempt to use DImode for 8-byte structures and
unions, but this can be prevented by overriding the definition of
MAX_FIXED_MODE_SIZE. Alternatively, you can have the compiler
use TImode for 16-byte structures and unions. Likewise, you can
arrange for the C type short int to avoid using HImode.
Very few explicit references to machine modes remain in the compiler and
these few references will soon be removed. Instead, the machine modes
are divided into mode classes. These are represented by the enumeration
type enum mode_class defined in machmode.h. The possible
mode classes are:
MODE_INTInteger modes. By default these are BImode, QImode,
HImode, SImode, DImode, TImode, and
OImode.
MODE_PARTIAL_INTThe “partial integer” modes, PQImode, PHImode,
PSImode and PDImode.
MODE_FLOATFloating point modes. By default these are QFmode,
HFmode, TQFmode, SFmode, DFmode,
XFmode and TFmode.
MODE_DECIMAL_FLOATDecimal floating point modes. By default these are SDmode,
DDmode and TDmode.
MODE_FRACTSigned fractional modes. By default these are QQmode, HQmode,
SQmode, DQmode and TQmode.
MODE_UFRACTUnsigned fractional modes. By default these are UQQmode, UHQmode,
USQmode, UDQmode and UTQmode.
MODE_ACCUMSigned accumulator modes. By default these are HAmode,
SAmode, DAmode and TAmode.
MODE_UACCUMUnsigned accumulator modes. By default these are UHAmode,
USAmode, UDAmode and UTAmode.
MODE_COMPLEX_INTComplex integer modes. (These are not currently implemented).
MODE_COMPLEX_FLOATComplex floating point modes. By default these are QCmode,
HCmode, SCmode, DCmode, XCmode, and
TCmode.
MODE_FUNCTIONAlgol or Pascal function variables including a static chain. (These are not currently implemented).
MODE_CCModes representing condition code values. These are CCmode plus
any CC_MODE modes listed in the machine-modes.def.
See Jump Patterns,
also see Condition Code.
MODE_RANDOMThis is a catchall mode class for modes which don’t fit into the above
classes. Currently VOIDmode and BLKmode are in
MODE_RANDOM.
Here are some C macros that relate to machine modes:
GET_MODE (x)Returns the machine mode of the RTX x.
PUT_MODE (x, newmode)Alters the machine mode of the RTX x to be newmode.
NUM_MACHINE_MODESStands for the number of machine modes available on the target machine. This is one greater than the largest numeric value of any machine mode.
GET_MODE_NAME (m)Returns the name of mode m as a string.
GET_MODE_CLASS (m)Returns the mode class of mode m.
GET_MODE_WIDER_MODE (m)Returns the next wider natural mode. For example, the expression
GET_MODE_WIDER_MODE (QImode) returns HImode.
GET_MODE_SIZE (m)Returns the size in bytes of a datum of mode m.
GET_MODE_BITSIZE (m)Returns the size in bits of a datum of mode m.
GET_MODE_IBIT (m)Returns the number of integral bits of a datum of fixed-point mode m.
GET_MODE_FBIT (m)Returns the number of fractional bits of a datum of fixed-point mode m.
GET_MODE_MASK (m)Returns a bitmask containing 1 for all bits in a word that fit within
mode m. This macro can only be used for modes whose bitsize is
less than or equal to HOST_BITS_PER_INT.
GET_MODE_ALIGNMENT (m)Return the required alignment, in bits, for an object of mode m.
GET_MODE_UNIT_SIZE (m)Returns the size in bytes of the subunits of a datum of mode m.
This is the same as GET_MODE_SIZE except in the case of complex
modes. For them, the unit size is the size of the real or imaginary
part.
GET_MODE_NUNITS (m)Returns the number of units contained in a mode, i.e.,
GET_MODE_SIZE divided by GET_MODE_UNIT_SIZE.
GET_CLASS_NARROWEST_MODE (c)Returns the narrowest mode in mode class c.
The global variables byte_mode and word_mode contain modes
whose classes are MODE_INT and whose bitsizes are either
BITS_PER_UNIT or BITS_PER_WORD, respectively. On 32-bit
machines, these are QImode and SImode, respectively.
Next: Regs and Memory, Previous: Machine Modes, Up: RTL [Contents][Index]
The simplest RTL expressions are those that represent constant values.
(const_int i)This type of expression represents the integer value i. i
is customarily accessed with the macro INTVAL as in
INTVAL (exp), which is equivalent to XWINT (exp, 0).
Constants generated for modes with fewer bits than HOST_WIDE_INT
must be sign extended to full width (e.g., with gen_int_mode).
There is only one expression object for the integer value zero; it is
the value of the variable const0_rtx. Likewise, the only
expression for integer value one is found in const1_rtx, the only
expression for integer value two is found in const2_rtx, and the
only expression for integer value negative one is found in
constm1_rtx. Any attempt to create an expression of code
const_int and value zero, one, two or negative one will return
const0_rtx, const1_rtx, const2_rtx or
constm1_rtx as appropriate.
Similarly, there is only one object for the integer whose value is
STORE_FLAG_VALUE. It is found in const_true_rtx. If
STORE_FLAG_VALUE is one, const_true_rtx and
const1_rtx will point to the same object. If
STORE_FLAG_VALUE is -1, const_true_rtx and
constm1_rtx will point to the same object.
(const_double:m i0 i1 …)Represents either a floating-point constant of mode m or an
integer constant too large to fit into HOST_BITS_PER_WIDE_INT
bits but small enough to fit within twice that number of bits (GCC
does not provide a mechanism to represent even larger constants). In
the latter case, m will be VOIDmode.
If m is VOIDmode, the bits of the value are stored in
i0 and i1. i0 is customarily accessed with the macro
CONST_DOUBLE_LOW and i1 with CONST_DOUBLE_HIGH.
If the constant is floating point (regardless of its precision), then
the number of integers used to store the value depends on the size of
REAL_VALUE_TYPE (see Floating Point). The integers
represent a floating point number, but not precisely in the target
machine’s or host machine’s floating point format. To convert them to
the precise bit pattern used by the target machine, use the macro
REAL_VALUE_TO_TARGET_DOUBLE and friends (see Data Output).
(const_fixed:m …)Represents a fixed-point constant of mode m.
The operand is a data structure of type struct fixed_value and
is accessed with the macro CONST_FIXED_VALUE. The high part of
data is accessed with CONST_FIXED_VALUE_HIGH; the low part is
accessed with CONST_FIXED_VALUE_LOW.
(const_vector:m [x0 x1 …])Represents a vector constant. The square brackets stand for the vector
containing the constant elements. x0, x1 and so on are
the const_int, const_double or const_fixed elements.
The number of units in a const_vector is obtained with the macro
CONST_VECTOR_NUNITS as in CONST_VECTOR_NUNITS (v).
Individual elements in a vector constant are accessed with the macro
CONST_VECTOR_ELT as in CONST_VECTOR_ELT (v, n)
where v is the vector constant and n is the element
desired.
(const_string str)Represents a constant string with value str. Currently this is used only for insn attributes (see Insn Attributes) since constant strings in C are placed in memory.
(symbol_ref:mode symbol)Represents the value of an assembler label for data. symbol is a string that describes the name of the assembler label. If it starts with a ‘*’, the label is the rest of symbol not including the ‘*’. Otherwise, the label is symbol, usually prefixed with ‘_’.
The symbol_ref contains a mode, which is usually Pmode.
Usually that is the only mode for which a symbol is directly valid.
(label_ref:mode label)Represents the value of an assembler label for code. It contains one
operand, an expression, which must be a code_label or a note
of type NOTE_INSN_DELETED_LABEL that appears in the instruction
sequence to identify the place where the label should go.
The reason for using a distinct expression type for code label references is so that jump optimization can distinguish them.
The label_ref contains a mode, which is usually Pmode.
Usually that is the only mode for which a label is directly valid.
(const:m exp)Represents a constant that is the result of an assembly-time
arithmetic computation. The operand, exp, is an expression that
contains only constants (const_int, symbol_ref and
label_ref expressions) combined with plus and
minus. However, not all combinations are valid, since the
assembler cannot do arbitrary arithmetic on relocatable symbols.
m should be Pmode.
(high:m exp)Represents the high-order bits of exp, usually a
symbol_ref. The number of bits is machine-dependent and is
normally the number of bits specified in an instruction that initializes
the high order bits of a register. It is used with lo_sum to
represent the typical two-instruction sequence used in RISC machines to
reference a global memory location.
m should be Pmode.
The macro CONST0_RTX (mode) refers to an expression with
value 0 in mode mode. If mode mode is of mode class
MODE_INT, it returns const0_rtx. If mode mode is of
mode class MODE_FLOAT, it returns a CONST_DOUBLE
expression in mode mode. Otherwise, it returns a
CONST_VECTOR expression in mode mode. Similarly, the macro
CONST1_RTX (mode) refers to an expression with value 1 in
mode mode and similarly for CONST2_RTX. The
CONST1_RTX and CONST2_RTX macros are undefined
for vector modes.
Next: Arithmetic, Previous: Constants, Up: RTL [Contents][Index]
Here are the RTL expression types for describing access to machine registers and to main memory.
(reg:m n)For small values of the integer n (those that are less than
FIRST_PSEUDO_REGISTER), this stands for a reference to machine
register number n: a hard register. For larger values of
n, it stands for a temporary value or pseudo register.
The compiler’s strategy is to generate code assuming an unlimited
number of such pseudo registers, and later convert them into hard
registers or into memory references.
m is the machine mode of the reference. It is necessary because machines can generally refer to each register in more than one mode. For example, a register may contain a full word but there may be instructions to refer to it as a half word or as a single byte, as well as instructions to refer to it as a floating point number of various precisions.
Even for a register that the machine can access in only one mode, the mode must always be specified.
The symbol FIRST_PSEUDO_REGISTER is defined by the machine
description, since the number of hard registers on the machine is an
invariant characteristic of the machine. Note, however, that not
all of the machine registers must be general registers. All the
machine registers that can be used for storage of data are given
hard register numbers, even those that can be used only in certain
instructions or can hold only certain types of data.
A hard register may be accessed in various modes throughout one
function, but each pseudo register is given a natural mode
and is accessed only in that mode. When it is necessary to describe
an access to a pseudo register using a nonnatural mode, a subreg
expression is used.
A reg expression with a machine mode that specifies more than
one word of data may actually stand for several consecutive registers.
If in addition the register number specifies a hardware register, then
it actually represents several consecutive hardware registers starting
with the specified one.
Each pseudo register number used in a function’s RTL code is
represented by a unique reg expression.
Some pseudo register numbers, those within the range of
FIRST_VIRTUAL_REGISTER to LAST_VIRTUAL_REGISTER only
appear during the RTL generation phase and are eliminated before the
optimization phases. These represent locations in the stack frame that
cannot be determined until RTL generation for the function has been
completed. The following virtual register numbers are defined:
VIRTUAL_INCOMING_ARGS_REGNUMThis points to the first word of the incoming arguments passed on the stack. Normally these arguments are placed there by the caller, but the callee may have pushed some arguments that were previously passed in registers.
When RTL generation is complete, this virtual register is replaced
by the sum of the register given by ARG_POINTER_REGNUM and the
value of FIRST_PARM_OFFSET.
VIRTUAL_STACK_VARS_REGNUMIf FRAME_GROWS_DOWNWARD is defined to a nonzero value, this points
to immediately above the first variable on the stack. Otherwise, it points
to the first variable on the stack.
VIRTUAL_STACK_VARS_REGNUM is replaced with the sum of the
register given by FRAME_POINTER_REGNUM and the value
STARTING_FRAME_OFFSET.
VIRTUAL_STACK_DYNAMIC_REGNUMThis points to the location of dynamically allocated memory on the stack immediately after the stack pointer has been adjusted by the amount of memory desired.
This virtual register is replaced by the sum of the register given by
STACK_POINTER_REGNUM and the value STACK_DYNAMIC_OFFSET.
VIRTUAL_OUTGOING_ARGS_REGNUMThis points to the location in the stack at which outgoing arguments
should be written when the stack is pre-pushed (arguments pushed using
push insns should always use STACK_POINTER_REGNUM).
This virtual register is replaced by the sum of the register given by
STACK_POINTER_REGNUM and the value STACK_POINTER_OFFSET.
(subreg:m1 reg:m2 bytenum)subreg expressions are used to refer to a register in a machine
mode other than its natural one, or to refer to one register of
a multi-part reg that actually refers to several registers.
Each pseudo register has a natural mode. If it is necessary to
operate on it in a different mode, the register must be
enclosed in a subreg.
There are currently three supported types for the first operand of a
subreg:
subregs have pseudo
regs as their first operand.
subregs of mem were common in earlier versions of GCC and
are still supported. During the reload pass these are replaced by plain
mems. On machines that do not do instruction scheduling, use of
subregs of mem are still used, but this is no longer
recommended. Such subregs are considered to be
register_operands rather than memory_operands before and
during reload. Because of this, the scheduling passes cannot properly
schedule instructions with subregs of mem, so for machines
that do scheduling, subregs of mem should never be used.
To support this, the combine and recog passes have explicit code to
inhibit the creation of subregs of mem when
INSN_SCHEDULING is defined.
The use of subregs of mem after the reload pass is an area
that is not well understood and should be avoided. There is still some
code in the compiler to support this, but this code has possibly rotted.
This use of subregs is discouraged and will most likely not be
supported in the future.
subregs; such
registers would normally reduce to a single reg rtx. This use of
subregs is discouraged and may not be supported in the future.
subregs of subregs are not supported. Using
simplify_gen_subreg is the recommended way to avoid this problem.
subregs come in two distinct flavors, each having its own
usage and rules:
When m1 is strictly wider than m2, the subreg
expression is called paradoxical. The canonical test for this
class of subreg is:
GET_MODE_SIZE (m1) > GET_MODE_SIZE (m2)
Paradoxical subregs can be used as both lvalues and rvalues.
When used as an lvalue, the low-order bits of the source value
are stored in reg and the high-order bits are discarded.
When used as an rvalue, the low-order bits of the subreg are
taken from reg while the high-order bits may or may not be
defined.
The high-order bits of rvalues are in the following circumstances:
subregs of mem
When m2 is smaller than a word, the macro LOAD_EXTEND_OP,
can control how the high-order bits are defined.
subreg of regs
The upper bits are defined when SUBREG_PROMOTED_VAR_P is true.
SUBREG_PROMOTED_UNSIGNED_P describes what the upper bits hold.
Such subregs usually represent local variables, register variables
and parameter pseudo variables that have been promoted to a wider mode.
bytenum is always zero for a paradoxical subreg, even on
big-endian targets.
For example, the paradoxical subreg:
(set (subreg:SI (reg:HI x) 0) y)
stores the lower 2 bytes of y in x and discards the upper 2 bytes. A subsequent:
(set z (subreg:SI (reg:HI x) 0))
would set the lower two bytes of z to y and set the upper
two bytes to an unknown value assuming SUBREG_PROMOTED_VAR_P is
false.
When m1 is at least as narrow as m2 the subreg
expression is called normal.
Normal subregs restrict consideration to certain bits of
reg. There are two cases. If m1 is smaller than a word,
the subreg refers to the least-significant part (or
lowpart) of one word of reg. If m1 is word-sized or
greater, the subreg refers to one or more complete words.
When used as an lvalue, subreg is a word-based accessor.
Storing to a subreg modifies all the words of reg that
overlap the subreg, but it leaves the other words of reg
alone.
When storing to a normal subreg that is smaller than a word,
the other bits of the referenced word are usually left in an undefined
state. This laxity makes it easier to generate efficient code for
such instructions. To represent an instruction that preserves all the
bits outside of those in the subreg, use strict_low_part
or zero_extract around the subreg.
bytenum must identify the offset of the first byte of the
subreg from the start of reg, assuming that reg is
laid out in memory order. The memory order of bytes is defined by
two target macros, WORDS_BIG_ENDIAN and BYTES_BIG_ENDIAN:
WORDS_BIG_ENDIAN, if set to 1, says that byte number zero is
part of the most significant word; otherwise, it is part of the least
significant word.
BYTES_BIG_ENDIAN, if set to 1, says that byte number zero is
the most significant byte within a word; otherwise, it is the least
significant byte within a word.
On a few targets, FLOAT_WORDS_BIG_ENDIAN disagrees with
WORDS_BIG_ENDIAN. However, most parts of the compiler treat
floating point values as if they had the same endianness as integer
values. This works because they handle them solely as a collection of
integer values, with no particular numerical value. Only real.c and
the runtime libraries care about FLOAT_WORDS_BIG_ENDIAN.
Thus,
(subreg:HI (reg:SI x) 2)
on a BYTES_BIG_ENDIAN, ‘UNITS_PER_WORD == 4’ target is the same as
(subreg:HI (reg:SI x) 0)
on a little-endian, ‘UNITS_PER_WORD == 4’ target. Both
subregs access the lower two bytes of register x.
A MODE_PARTIAL_INT mode behaves as if it were as wide as the
corresponding MODE_INT mode, except that it has an unknown
number of undefined bits. For example:
(subreg:PSI (reg:SI 0) 0)
accesses the whole of ‘(reg:SI 0)’, but the exact relationship
between the PSImode value and the SImode value is not
defined. If we assume ‘UNITS_PER_WORD <= 4’, then the following
two subregs:
(subreg:PSI (reg:DI 0) 0) (subreg:PSI (reg:DI 0) 4)
represent independent 4-byte accesses to the two halves of
‘(reg:DI 0)’. Both subregs have an unknown number
of undefined bits.
If ‘UNITS_PER_WORD <= 2’ then these two subregs:
(subreg:HI (reg:PSI 0) 0) (subreg:HI (reg:PSI 0) 2)
represent independent 2-byte accesses that together span the whole
of ‘(reg:PSI 0)’. Storing to the first subreg does not
affect the value of the second, and vice versa. ‘(reg:PSI 0)’
has an unknown number of undefined bits, so the assignment:
(set (subreg:HI (reg:PSI 0) 0) (reg:HI 4))
does not guarantee that ‘(subreg:HI (reg:PSI 0) 0)’ has the value ‘(reg:HI 4)’.
The rules above apply to both pseudo regs and hard regs. If the semantics are not correct for particular combinations of m1, m2 and hard reg, the target-specific code must ensure that those combinations are never used. For example:
CANNOT_CHANGE_MODE_CLASS (m2, m1, class)
must be true for every class class that includes reg.
The first operand of a subreg expression is customarily accessed
with the SUBREG_REG macro and the second operand is customarily
accessed with the SUBREG_BYTE macro.
It has been several years since a platform in which
BYTES_BIG_ENDIAN not equal to WORDS_BIG_ENDIAN has
been tested. Anyone wishing to support such a platform in the future
may be confronted with code rot.
(scratch:m)This represents a scratch register that will be required for the
execution of a single instruction and not used subsequently. It is
converted into a reg by either the local register allocator or
the reload pass.
scratch is usually present inside a clobber operation
(see Side Effects).
(cc0)This refers to the machine’s condition code register. It has no operands and may not have a machine mode. There are two ways to use it:
With this technique, (cc0) may be validly used in only two
contexts: as the destination of an assignment (in test and compare
instructions) and in comparison operators comparing against zero
(const_int with value zero; that is to say, const0_rtx).
With this technique, (cc0) may be validly used in only two
contexts: as the destination of an assignment (in test and compare
instructions) where the source is a comparison operator, and as the
first operand of if_then_else (in a conditional branch).
There is only one expression object of code cc0; it is the
value of the variable cc0_rtx. Any attempt to create an
expression of code cc0 will return cc0_rtx.
Instructions can set the condition code implicitly. On many machines,
nearly all instructions set the condition code based on the value that
they compute or store. It is not necessary to record these actions
explicitly in the RTL because the machine description includes a
prescription for recognizing the instructions that do so (by means of
the macro NOTICE_UPDATE_CC). See Condition Code. Only
instructions whose sole purpose is to set the condition code, and
instructions that use the condition code, need mention (cc0).
On some machines, the condition code register is given a register number
and a reg is used instead of (cc0). This is usually the
preferable approach if only a small subset of instructions modify the
condition code. Other machines store condition codes in general
registers; in such cases a pseudo register should be used.
Some machines, such as the SPARC and RS/6000, have two sets of
arithmetic instructions, one that sets and one that does not set the
condition code. This is best handled by normally generating the
instruction that does not set the condition code, and making a pattern
that both performs the arithmetic and sets the condition code register
(which would not be (cc0) in this case). For examples, search
for ‘addcc’ and ‘andcc’ in sparc.md.
(pc)This represents the machine’s program counter. It has no operands and
may not have a machine mode. (pc) may be validly used only in
certain specific contexts in jump instructions.
There is only one expression object of code pc; it is the value
of the variable pc_rtx. Any attempt to create an expression of
code pc will return pc_rtx.
All instructions that do not jump alter the program counter implicitly by incrementing it, but there is no need to mention this in the RTL.
(mem:m addr alias)This RTX represents a reference to main memory at an address represented by the expression addr. m specifies how large a unit of memory is accessed. alias specifies an alias set for the reference. In general two items are in different alias sets if they cannot reference the same memory address.
The construct (mem:BLK (scratch)) is considered to alias all
other memories. Thus it may be used as a memory barrier in epilogue
stack deallocation patterns.
(concatm rtx rtx)This RTX represents the concatenation of two other RTXs. This is used for complex values. It should only appear in the RTL attached to declarations and during RTL generation. It should not appear in the ordinary insn chain.
(concatnm [rtx …])This RTX represents the concatenation of all the rtx to make a
single value. Like concat, this should only appear in
declarations, and not in the insn chain.
Next: Comparisons, Previous: Regs and Memory, Up: RTL [Contents][Index]
Unless otherwise specified, all the operands of arithmetic expressions
must be valid for mode m. An operand is valid for mode m
if it has mode m, or if it is a const_int or
const_double and m is a mode of class MODE_INT.
For commutative binary operations, constants should be placed in the second operand.
(plus:m x y)(ss_plus:m x y)(us_plus:m x y)These three expressions all represent the sum of the values
represented by x and y carried out in machine mode
m. They differ in their behavior on overflow of integer modes.
plus wraps round modulo the width of m; ss_plus
saturates at the maximum signed value representable in m;
us_plus saturates at the maximum unsigned value.
(lo_sum:m x y)This expression represents the sum of x and the low-order bits
of y. It is used with high (see Constants) to
represent the typical two-instruction sequence used in RISC machines
to reference a global memory location.
The number of low order bits is machine-dependent but is
normally the number of bits in a Pmode item minus the number of
bits set by high.
m should be Pmode.
(minus:m x y)(ss_minus:m x y)(us_minus:m x y)These three expressions represent the result of subtracting y
from x, carried out in mode M. Behavior on overflow is
the same as for the three variants of plus (see above).
(compare:m x y)Represents the result of subtracting y from x for purposes of comparison. The result is computed without overflow, as if with infinite precision.
Of course, machines can’t really subtract with infinite precision.
However, they can pretend to do so when only the sign of the result will
be used, which is the case when the result is stored in the condition
code. And that is the only way this kind of expression may
validly be used: as a value to be stored in the condition codes, either
(cc0) or a register. See Comparisons.
The mode m is not related to the modes of x and y, but
instead is the mode of the condition code value. If (cc0) is
used, it is VOIDmode. Otherwise it is some mode in class
MODE_CC, often CCmode. See Condition Code. If m
is VOIDmode or CCmode, the operation returns sufficient
information (in an unspecified format) so that any comparison operator
can be applied to the result of the COMPARE operation. For other
modes in class MODE_CC, the operation only returns a subset of
this information.
Normally, x and y must have the same mode. Otherwise,
compare is valid only if the mode of x is in class
MODE_INT and y is a const_int or
const_double with mode VOIDmode. The mode of x
determines what mode the comparison is to be done in; thus it must not
be VOIDmode.
If one of the operands is a constant, it should be placed in the second operand and the comparison code adjusted as appropriate.
A compare specifying two VOIDmode constants is not valid
since there is no way to know in what mode the comparison is to be
performed; the comparison must either be folded during the compilation
or the first operand must be loaded into a register while its mode is
still known.
(neg:m x)(ss_neg:m x)(us_neg:m x)These two expressions represent the negation (subtraction from zero) of
the value represented by x, carried out in mode m. They
differ in the behavior on overflow of integer modes. In the case of
neg, the negation of the operand may be a number not representable
in mode m, in which case it is truncated to m. ss_neg
and us_neg ensure that an out-of-bounds result saturates to the
maximum or minimum signed or unsigned value.
(mult:m x y)(ss_mult:m x y)(us_mult:m x y)Represents the signed product of the values represented by x and
y carried out in machine mode m.
ss_mult and us_mult ensure that an out-of-bounds result
saturates to the maximum or minimum signed or unsigned value.
Some machines support a multiplication that generates a product wider than the operands. Write the pattern for this as
(mult:m (sign_extend:m x) (sign_extend:m y))
where m is wider than the modes of x and y, which need not be the same.
For unsigned widening multiplication, use the same idiom, but with
zero_extend instead of sign_extend.
(fma:m x y z)Represents the fma, fmaf, and fmal builtin
functions that do a combined multiply of x and y and then
adding toz without doing an intermediate rounding step.
(div:m x y)(ss_div:m x y)Represents the quotient in signed division of x by y,
carried out in machine mode m. If m is a floating point
mode, it represents the exact quotient; otherwise, the integerized
quotient.
ss_div ensures that an out-of-bounds result saturates to the maximum
or minimum signed value.
Some machines have division instructions in which the operands and
quotient widths are not all the same; you should represent
such instructions using truncate and sign_extend as in,
(truncate:m1 (div:m2 x (sign_extend:m2 y)))
(udiv:m x y)(us_div:m x y)Like div but represents unsigned division.
us_div ensures that an out-of-bounds result saturates to the maximum
or minimum unsigned value.
(mod:m x y)(umod:m x y)Like div and udiv but represent the remainder instead of
the quotient.
(smin:m x y)(smax:m x y)Represents the smaller (for smin) or larger (for smax) of
x and y, interpreted as signed values in mode m.
When used with floating point, if both operands are zeros, or if either
operand is NaN, then it is unspecified which of the two operands
is returned as the result.
(umin:m x y)(umax:m x y)Like smin and smax, but the values are interpreted as unsigned
integers.
(not:m x)Represents the bitwise complement of the value represented by x, carried out in mode m, which must be a fixed-point machine mode.
(and:m x y)Represents the bitwise logical-and of the values represented by x and y, carried out in machine mode m, which must be a fixed-point machine mode.
(ior:m x y)Represents the bitwise inclusive-or of the values represented by x and y, carried out in machine mode m, which must be a fixed-point mode.
(xor:m x y)Represents the bitwise exclusive-or of the values represented by x and y, carried out in machine mode m, which must be a fixed-point mode.
(ashift:m x c)(ss_ashift:m x c)(us_ashift:m x c)These three expressions represent the result of arithmetically shifting x
left by c places. They differ in their behavior on overflow of integer
modes. An ashift operation is a plain shift with no special behavior
in case of a change in the sign bit; ss_ashift and us_ashift
saturates to the minimum or maximum representable value if any of the bits
shifted out differs from the final sign bit.
x have mode m, a fixed-point machine mode. c
be a fixed-point mode or be a constant with mode VOIDmode; which
mode is determined by the mode called for in the machine description
entry for the left-shift instruction. For example, on the VAX, the mode
of c is QImode regardless of m.
(lshiftrt:m x c)(ashiftrt:m x c)Like ashift but for right shift. Unlike the case for left shift,
these two operations are distinct.
(rotate:m x c)(rotatert:m x c)Similar but represent left and right rotate. If c is a constant,
use rotate.
(abs:m x)(ss_abs:m x)Represents the absolute value of x, computed in mode m.
ss_abs ensures that an out-of-bounds result saturates to the
maximum signed value.
(sqrt:m x)Represents the square root of x, computed in mode m. Most often m will be a floating point mode.
(ffs:m x)Represents one plus the index of the least significant 1-bit in x, represented as an integer of mode m. (The value is zero if x is zero.) The mode of x need not be m; depending on the target machine, various mode combinations may be valid.
(clz:m x)Represents the number of leading 0-bits in x, represented as an
integer of mode m, starting at the most significant bit position.
If x is zero, the value is determined by
CLZ_DEFINED_VALUE_AT_ZERO (see Misc). Note that this is one of
the few expressions that is not invariant under widening. The mode of
x will usually be an integer mode.
(ctz:m x)Represents the number of trailing 0-bits in x, represented as an
integer of mode m, starting at the least significant bit position.
If x is zero, the value is determined by
CTZ_DEFINED_VALUE_AT_ZERO (see Misc). Except for this case,
ctz(x) is equivalent to ffs(x) - 1. The mode of
x will usually be an integer mode.
(popcount:m x)Represents the number of 1-bits in x, represented as an integer of mode m. The mode of x will usually be an integer mode.
(parity:m x)Represents the number of 1-bits modulo 2 in x, represented as an integer of mode m. The mode of x will usually be an integer mode.
(bswap:m x)Represents the value x with the order of bytes reversed, carried out in mode m, which must be a fixed-point machine mode.
Next: Bit-Fields, Previous: Arithmetic, Up: RTL [Contents][Index]
Comparison operators test a relation on two operands and are considered
to represent a machine-dependent nonzero value described by, but not
necessarily equal to, STORE_FLAG_VALUE (see Misc)
if the relation holds, or zero if it does not, for comparison operators
whose results have a ‘MODE_INT’ mode,
FLOAT_STORE_FLAG_VALUE (see Misc) if the relation holds, or
zero if it does not, for comparison operators that return floating-point
values, and a vector of either VECTOR_STORE_FLAG_VALUE (see Misc)
if the relation holds, or of zeros if it does not, for comparison operators
that return vector results.
The mode of the comparison operation is independent of the mode
of the data being compared. If the comparison operation is being tested
(e.g., the first operand of an if_then_else), the mode must be
VOIDmode.
There are two ways that comparison operations may be used. The
comparison operators may be used to compare the condition codes
(cc0) against zero, as in (eq (cc0) (const_int 0)). Such
a construct actually refers to the result of the preceding instruction
in which the condition codes were set. The instruction setting the
condition code must be adjacent to the instruction using the condition
code; only note insns may separate them.
Alternatively, a comparison operation may directly compare two data objects. The mode of the comparison is determined by the operands; they must both be valid for a common machine mode. A comparison with both operands constant would be invalid as the machine mode could not be deduced from it, but such a comparison should never exist in RTL due to constant folding.
In the example above, if (cc0) were last set to
(compare x y), the comparison operation is
identical to (eq x y). Usually only one style
of comparisons is supported on a particular machine, but the combine
pass will try to merge the operations to produce the eq shown
in case it exists in the context of the particular insn involved.
Inequality comparisons come in two flavors, signed and unsigned. Thus,
there are distinct expression codes gt and gtu for signed and
unsigned greater-than. These can produce different results for the same
pair of integer values: for example, 1 is signed greater-than -1 but not
unsigned greater-than, because -1 when regarded as unsigned is actually
0xffffffff which is greater than 1.
The signed comparisons are also used for floating point values. Floating point comparisons are distinguished by the machine modes of the operands.
(eq:m x y)STORE_FLAG_VALUE if the values represented by x and y
are equal, otherwise 0.
(ne:m x y)STORE_FLAG_VALUE if the values represented by x and y
are not equal, otherwise 0.
(gt:m x y)STORE_FLAG_VALUE if the x is greater than y. If they
are fixed-point, the comparison is done in a signed sense.
(gtu:m x y)Like gt but does unsigned comparison, on fixed-point numbers only.
(lt:m x y)(ltu:m x y)Like gt and gtu but test for “less than”.
(ge:m x y)(geu:m x y)Like gt and gtu but test for “greater than or equal”.
(le:m x y)(leu:m x y)Like gt and gtu but test for “less than or equal”.
(if_then_else cond then else)This is not a comparison operation but is listed here because it is always used in conjunction with a comparison operation. To be precise, cond is a comparison expression. This expression represents a choice, according to cond, between the value represented by then and the one represented by else.
On most machines, if_then_else expressions are valid only
to express conditional jumps.
(cond [test1 value1 test2 value2 …] default)Similar to if_then_else, but more general. Each of test1,
test2, … is performed in turn. The result of this expression is
the value corresponding to the first nonzero test, or default if
none of the tests are nonzero expressions.
This is currently not valid for instruction patterns and is supported only for insn attributes. See Insn Attributes.
Next: Vector Operations, Previous: Comparisons, Up: RTL [Contents][Index]
Special expression codes exist to represent bit-field instructions.
(sign_extract:m loc size pos)This represents a reference to a sign-extended bit-field contained or
starting in loc (a memory or register reference). The bit-field
is size bits wide and starts at bit pos. The compilation
option BITS_BIG_ENDIAN says which end of the memory unit
pos counts from.
If loc is in memory, its mode must be a single-byte integer mode.
If loc is in a register, the mode to use is specified by the
operand of the insv or extv pattern
(see Standard Names) and is usually a full-word integer mode,
which is the default if none is specified.
The mode of pos is machine-specific and is also specified
in the insv or extv pattern.
The mode m is the same as the mode that would be used for loc if it were a register.
A sign_extract can not appear as an lvalue, or part thereof,
in RTL.
(zero_extract:m loc size pos)Like sign_extract but refers to an unsigned or zero-extended
bit-field. The same sequence of bits are extracted, but they
are filled to an entire word with zeros instead of by sign-extension.
Unlike sign_extract, this type of expressions can be lvalues
in RTL; they may appear on the left side of an assignment, indicating
insertion of a value into the specified bit-field.
Next: Conversions, Previous: Bit-Fields, Up: RTL [Contents][Index]
All normal RTL expressions can be used with vector modes; they are interpreted as operating on each part of the vector independently. Additionally, there are a few new expressions to describe specific vector operations.
(vec_merge:m vec1 vec2 items)This describes a merge operation between two vectors. The result is a vector
of mode m; its elements are selected from either vec1 or
vec2. Which elements are selected is described by items, which
is a bit mask represented by a const_int; a zero bit indicates the
corresponding element in the result vector is taken from vec2 while
a set bit indicates it is taken from vec1.
(vec_select:m vec1 selection)This describes an operation that selects parts of a vector. vec1 is
the source vector, and selection is a parallel that contains a
const_int for each of the subparts of the result vector, giving the
number of the source subpart that should be stored into it.
The result mode m is either the submode for a single element of
vec1 (if only one subpart is selected), or another vector mode
with that element submode (if multiple subparts are selected).
(vec_concat:m vec1 vec2)Describes a vector concat operation. The result is a concatenation of the vectors vec1 and vec2; its length is the sum of the lengths of the two inputs.
(vec_duplicate:m vec)This operation converts a small vector into a larger one by duplicating the input values. The output vector mode must have the same submodes as the input vector mode, and the number of output parts must be an integer multiple of the number of input parts.
Next: RTL Declarations, Previous: Vector Operations, Up: RTL [Contents][Index]
All conversions between machine modes must be represented by
explicit conversion operations. For example, an expression
which is the sum of a byte and a full word cannot be written as
(plus:SI (reg:QI 34) (reg:SI 80)) because the plus
operation requires two operands of the same machine mode.
Therefore, the byte-sized operand is enclosed in a conversion
operation, as in
(plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80))
The conversion operation is not a mere placeholder, because there may be more than one way of converting from a given starting mode to the desired final mode. The conversion operation code says how to do it.
For all conversion operations, x must not be VOIDmode
because the mode in which to do the conversion would not be known.
The conversion must either be done at compile-time or x
must be placed into a register.
(sign_extend:m x)Represents the result of sign-extending the value x to machine mode m. m must be a fixed-point mode and x a fixed-point value of a mode narrower than m.
(zero_extend:m x)Represents the result of zero-extending the value x to machine mode m. m must be a fixed-point mode and x a fixed-point value of a mode narrower than m.
(float_extend:m x)Represents the result of extending the value x to machine mode m. m must be a floating point mode and x a floating point value of a mode narrower than m.
(truncate:m x)Represents the result of truncating the value x to machine mode m. m must be a fixed-point mode and x a fixed-point value of a mode wider than m.
(ss_truncate:m x)Represents the result of truncating the value x to machine mode m, using signed saturation in the case of overflow. Both m and the mode of x must be fixed-point modes.
(us_truncate:m x)Represents the result of truncating the value x to machine mode m, using unsigned saturation in the case of overflow. Both m and the mode of x must be fixed-point modes.
(float_truncate:m x)Represents the result of truncating the value x to machine mode m. m must be a floating point mode and x a floating point value of a mode wider than m.
(float:m x)Represents the result of converting fixed point value x, regarded as signed, to floating point mode m.
(unsigned_float:m x)Represents the result of converting fixed point value x, regarded as unsigned, to floating point mode m.
(fix:m x)When m is a floating-point mode, represents the result of converting floating point value x (valid for mode m) to an integer, still represented in floating point mode m, by rounding towards zero.
When m is a fixed-point mode, represents the result of converting floating point value x to mode m, regarded as signed. How rounding is done is not specified, so this operation may be used validly in compiling C code only for integer-valued operands.
(unsigned_fix:m x)Represents the result of converting floating point value x to fixed point mode m, regarded as unsigned. How rounding is done is not specified.
(fract_convert:m x)Represents the result of converting fixed-point value x to fixed-point mode m, signed integer value x to fixed-point mode m, floating-point value x to fixed-point mode m, fixed-point value x to integer mode m regarded as signed, or fixed-point value x to floating-point mode m. When overflows or underflows happen, the results are undefined.
(sat_fract:m x)Represents the result of converting fixed-point value x to fixed-point mode m, signed integer value x to fixed-point mode m, or floating-point value x to fixed-point mode m. When overflows or underflows happen, the results are saturated to the maximum or the minimum.
(unsigned_fract_convert:m x)Represents the result of converting fixed-point value x to integer mode m regarded as unsigned, or unsigned integer value x to fixed-point mode m. When overflows or underflows happen, the results are undefined.
(unsigned_sat_fract:m x)Represents the result of converting unsigned integer value x to fixed-point mode m. When overflows or underflows happen, the results are saturated to the maximum or the minimum.
Next: Side Effects, Previous: Conversions, Up: RTL [Contents][Index]
Declaration expression codes do not represent arithmetic operations but rather state assertions about their operands.
(strict_low_part (subreg:m (reg:n r) 0))This expression code is used in only one context: as the destination operand of a
set expression. In addition, the operand of this expression
must be a non-paradoxical subreg expression.
The presence of strict_low_part says that the part of the
register which is meaningful in mode n, but is not part of
mode m, is not to be altered. Normally, an assignment to such
a subreg is allowed to have undefined effects on the rest of the
register when m is less than a word.
Next: Incdec, Previous: RTL Declarations, Up: RTL [Contents][Index]
The expression codes described so far represent values, not actions. But machine instructions never produce values; they are meaningful only for their side effects on the state of the machine. Special expression codes are used to represent side effects.
The body of an instruction is always one of these side effect codes; the codes described above, which represent values, appear only as the operands of these.
(set lval x)Represents the action of storing the value of x into the place
represented by lval. lval must be an expression
representing a place that can be stored in: reg (or subreg,
strict_low_part or zero_extract), mem, pc,
parallel, or cc0.
If lval is a reg, subreg or mem, it has a
machine mode; then x must be valid for that mode.
If lval is a reg whose machine mode is less than the full
width of the register, then it means that the part of the register
specified by the machine mode is given the specified value and the
rest of the register receives an undefined value. Likewise, if
lval is a subreg whose machine mode is narrower than
the mode of the register, the rest of the register can be changed in
an undefined way.
If lval is a strict_low_part of a subreg, then the part
of the register specified by the machine mode of the subreg is
given the value x and the rest of the register is not changed.
If lval is a zero_extract, then the referenced part of
the bit-field (a memory or register reference) specified by the
zero_extract is given the value x and the rest of the
bit-field is not changed. Note that sign_extract can not
appear in lval.
If lval is (cc0), it has no machine mode, and x may
be either a compare expression or a value that may have any mode.
The latter case represents a “test” instruction. The expression
(set (cc0) (reg:m n)) is equivalent to
(set (cc0) (compare (reg:m n) (const_int 0))).
Use the former expression to save space during the compilation.
If lval is a parallel, it is used to represent the case of
a function returning a structure in multiple registers. Each element
of the parallel is an expr_list whose first operand is a
reg and whose second operand is a const_int representing the
offset (in bytes) into the structure at which the data in that register
corresponds. The first element may be null to indicate that the structure
is also passed partly in memory.
If lval is (pc), we have a jump instruction, and the
possibilities for x are very limited. It may be a
label_ref expression (unconditional jump). It may be an
if_then_else (conditional jump), in which case either the
second or the third operand must be (pc) (for the case which
does not jump) and the other of the two must be a label_ref
(for the case which does jump). x may also be a mem or
(plus:SI (pc) y), where y may be a reg or a
mem; these unusual patterns are used to represent jumps through
branch tables.
If lval is neither (cc0) nor (pc), the mode of
lval must not be VOIDmode and the mode of x must be
valid for the mode of lval.
lval is customarily accessed with the SET_DEST macro and
x with the SET_SRC macro.
(return)As the sole expression in a pattern, represents a return from the
current function, on machines where this can be done with one
instruction, such as VAXen. On machines where a multi-instruction
“epilogue” must be executed in order to return from the function,
returning is done by jumping to a label which precedes the epilogue, and
the return expression code is never used.
Inside an if_then_else expression, represents the value to be
placed in pc to return to the caller.
Note that an insn pattern of (return) is logically equivalent to
(set (pc) (return)), but the latter form is never used.
(call function nargs)Represents a function call. function is a mem expression
whose address is the address of the function to be called.
nargs is an expression which can be used for two purposes: on
some machines it represents the number of bytes of stack argument; on
others, it represents the number of argument registers.
Each machine has a standard machine mode which function must
have. The machine description defines macro FUNCTION_MODE to
expand into the requisite mode name. The purpose of this mode is to
specify what kind of addressing is allowed, on machines where the
allowed kinds of addressing depend on the machine mode being
addressed.
(clobber x)Represents the storing or possible storing of an unpredictable,
undescribed value into x, which must be a reg,
scratch, parallel or mem expression.
One place this is used is in string instructions that store standard values into particular hard registers. It may not be worth the trouble to describe the values that are stored, but it is essential to inform the compiler that the registers will be altered, lest it attempt to keep data in them across the string instruction.
If x is (mem:BLK (const_int 0)) or
(mem:BLK (scratch)), it means that all memory
locations must be presumed clobbered. If x is a parallel,
it has the same meaning as a parallel in a set expression.
Note that the machine description classifies certain hard registers as
“call-clobbered”. All function call instructions are assumed by
default to clobber these registers, so there is no need to use
clobber expressions to indicate this fact. Also, each function
call is assumed to have the potential to alter any memory location,
unless the function is declared const.
If the last group of expressions in a parallel are each a
clobber expression whose arguments are reg or
match_scratch (see RTL Template) expressions, the combiner
phase can add the appropriate clobber expressions to an insn it
has constructed when doing so will cause a pattern to be matched.
This feature can be used, for example, on a machine that whose multiply and add instructions don’t use an MQ register but which has an add-accumulate instruction that does clobber the MQ register. Similarly, a combined instruction might require a temporary register while the constituent instructions might not.
When a clobber expression for a register appears inside a
parallel with other side effects, the register allocator
guarantees that the register is unoccupied both before and after that
insn if it is a hard register clobber. For pseudo-register clobber,
the register allocator and the reload pass do not assign the same hard
register to the clobber and the input operands if there is an insn
alternative containing the ‘&’ constraint (see Modifiers) for
the clobber and the hard register is in register classes of the
clobber in the alternative. You can clobber either a specific hard
register, a pseudo register, or a scratch expression; in the
latter two cases, GCC will allocate a hard register that is available
there for use as a temporary.
For instructions that require a temporary register, you should use
scratch instead of a pseudo-register because this will allow the
combiner phase to add the clobber when required. You do this by
coding (clobber (match_scratch …)). If you do
clobber a pseudo register, use one which appears nowhere else—generate
a new one each time. Otherwise, you may confuse CSE.
There is one other known use for clobbering a pseudo register in a
parallel: when one of the input operands of the insn is also
clobbered by the insn. In this case, using the same pseudo register in
the clobber and elsewhere in the insn produces the expected results.
(use x)Represents the use of the value of x. It indicates that the
value in x at this point in the program is needed, even though
it may not be apparent why this is so. Therefore, the compiler will
not attempt to delete previous instructions whose only effect is to
store a value in x. x must be a reg expression.
In some situations, it may be tempting to add a use of a
register in a parallel to describe a situation where the value
of a special register will modify the behavior of the instruction.
A hypothetical example might be a pattern for an addition that can
either wrap around or use saturating addition depending on the value
of a special control register:
(parallel [(set (reg:SI 2) (unspec:SI [(reg:SI 3)
(reg:SI 4)] 0))
(use (reg:SI 1))])
This will not work, several of the optimizers only look at expressions
locally; it is very likely that if you have multiple insns with
identical inputs to the unspec, they will be optimized away even
if register 1 changes in between.
This means that use can only be used to describe
that the register is live. You should think twice before adding
use statements, more often you will want to use unspec
instead. The use RTX is most commonly useful to describe that
a fixed register is implicitly used in an insn. It is also safe to use
in patterns where the compiler knows for other reasons that the result
of the whole pattern is variable, such as ‘movmemm’ or
‘call’ patterns.
During the reload phase, an insn that has a use as pattern
can carry a reg_equal note. These use insns will be deleted
before the reload phase exits.
During the delayed branch scheduling phase, x may be an insn.
This indicates that x previously was located at this place in the
code and its data dependencies need to be taken into account. These
use insns will be deleted before the delayed branch scheduling
phase exits.
(parallel [x0 x1 …])Represents several side effects performed in parallel. The square
brackets stand for a vector; the operand of parallel is a
vector of expressions. x0, x1 and so on are individual
side effect expressions—expressions of code set, call,
return, clobber or use.
“In parallel” means that first all the values used in the individual side-effects are computed, and second all the actual side-effects are performed. For example,
(parallel [(set (reg:SI 1) (mem:SI (reg:SI 1)))
(set (mem:SI (reg:SI 1)) (reg:SI 1))])
says unambiguously that the values of hard register 1 and the memory
location addressed by it are interchanged. In both places where
(reg:SI 1) appears as a memory address it refers to the value
in register 1 before the execution of the insn.
It follows that it is incorrect to use parallel and
expect the result of one set to be available for the next one.
For example, people sometimes attempt to represent a jump-if-zero
instruction this way:
(parallel [(set (cc0) (reg:SI 34))
(set (pc) (if_then_else
(eq (cc0) (const_int 0))
(label_ref …)
(pc)))])
But this is incorrect, because it says that the jump condition depends on the condition code value before this instruction, not on the new value that is set by this instruction.
Peephole optimization, which takes place together with final assembly
code output, can produce insns whose patterns consist of a parallel
whose elements are the operands needed to output the resulting
assembler code—often reg, mem or constant expressions.
This would not be well-formed RTL at any other stage in compilation,
but it is ok then because no further optimization remains to be done.
However, the definition of the macro NOTICE_UPDATE_CC, if
any, must deal with such insns if you define any peephole optimizations.
(cond_exec [cond expr])Represents a conditionally executed expression. The expr is executed only if the cond is nonzero. The cond expression must not have side-effects, but the expr may very well have side-effects.
(sequence [insns …])Represents a sequence of insns. Each of the insns that appears
in the vector is suitable for appearing in the chain of insns, so it
must be an insn, jump_insn, call_insn,
code_label, barrier or note.
A sequence RTX is never placed in an actual insn during RTL
generation. It represents the sequence of insns that result from a
define_expand before those insns are passed to
emit_insn to insert them in the chain of insns. When actually
inserted, the individual sub-insns are separated out and the
sequence is forgotten.
After delay-slot scheduling is completed, an insn and all the insns that
reside in its delay slots are grouped together into a sequence.
The insn requiring the delay slot is the first insn in the vector;
subsequent insns are to be placed in the delay slot.
INSN_ANNULLED_BRANCH_P is set on an insn in a delay slot to
indicate that a branch insn should be used that will conditionally annul
the effect of the insns in the delay slots. In such a case,
INSN_FROM_TARGET_P indicates that the insn is from the target of
the branch and should be executed only if the branch is taken; otherwise
the insn should be executed only if the branch is not taken.
See Delay Slots.
These expression codes appear in place of a side effect, as the body of an insn, though strictly speaking they do not always describe side effects as such:
(asm_input s)Represents literal assembler code as described by the string s.
(unspec [operands …] index)(unspec_volatile [operands …] index)Represents a machine-specific operation on operands. index
selects between multiple machine-specific operations.
unspec_volatile is used for volatile operations and operations
that may trap; unspec is used for other operations.
These codes may appear inside a pattern of an
insn, inside a parallel, or inside an expression.
(addr_vec:m [lr0 lr1 …])Represents a table of jump addresses. The vector elements lr0,
etc., are label_ref expressions. The mode m specifies
how much space is given to each address; normally m would be
Pmode.
(addr_diff_vec:m base [lr0 lr1 …] min max flags)Represents a table of jump addresses expressed as offsets from
base. The vector elements lr0, etc., are label_ref
expressions and so is base. The mode m specifies how much
space is given to each address-difference. min and max
are set up by branch shortening and hold a label with a minimum and a
maximum address, respectively. flags indicates the relative
position of base, min and max to the containing insn
and of min and max to base. See rtl.def for details.
(prefetch:m addr rw locality)Represents prefetch of memory at address addr. Operand rw is 1 if the prefetch is for data to be written, 0 otherwise; targets that do not support write prefetches should treat this as a normal prefetch. Operand locality specifies the amount of temporal locality; 0 if there is none or 1, 2, or 3 for increasing levels of temporal locality; targets that do not support locality hints should ignore this.
This insn is used to minimize cache-miss latency by moving data into a cache before it is accessed. It should use only non-faulting data prefetch instructions.
Next: Assembler, Previous: Side Effects, Up: RTL [Contents][Index]
Six special side-effect expression codes appear as memory addresses.
(pre_dec:m x)Represents the side effect of decrementing x by a standard
amount and represents also the value that x has after being
decremented. x must be a reg or mem, but most
machines allow only a reg. m must be the machine mode
for pointers on the machine in use. The amount x is decremented
by is the length in bytes of the machine mode of the containing memory
reference of which this expression serves as the address. Here is an
example of its use:
(mem:DF (pre_dec:SI (reg:SI 39)))
This says to decrement pseudo register 39 by the length of a DFmode
value and use the result to address a DFmode value.
(pre_inc:m x)Similar, but specifies incrementing x instead of decrementing it.
(post_dec:m x)Represents the same side effect as pre_dec but a different
value. The value represented here is the value x has before
being decremented.
(post_inc:m x)Similar, but specifies incrementing x instead of decrementing it.
(post_modify:m x y)Represents the side effect of setting x to y and
represents x before x is modified. x must be a
reg or mem, but most machines allow only a reg.
m must be the machine mode for pointers on the machine in use.
The expression y must be one of three forms:
(plus:m x z),
(minus:m x z), or
(plus:m x i),
where z is an index register and i is a constant.
Here is an example of its use:
(mem:SF (post_modify:SI (reg:SI 42) (plus (reg:SI 42)
(reg:SI 48))))
This says to modify pseudo register 42 by adding the contents of pseudo register 48 to it, after the use of what ever 42 points to.
(pre_modify:m x expr)Similar except side effects happen before the use.
These embedded side effect expressions must be used with care. Instruction patterns may not use them. Until the ‘flow’ pass of the compiler, they may occur only to represent pushes onto the stack. The ‘flow’ pass finds cases where registers are incremented or decremented in one instruction and used as an address shortly before or after; these cases are then transformed to use pre- or post-increment or -decrement.
If a register used as the operand of these expressions is used in another address in an insn, the original value of the register is used. Uses of the register outside of an address are not permitted within the same insn as a use in an embedded side effect expression because such insns behave differently on different machines and hence must be treated as ambiguous and disallowed.
An instruction that can be represented with an embedded side effect
could also be represented using parallel containing an additional
set to describe how the address register is altered. This is not
done because machines that allow these operations at all typically
allow them wherever a memory address is called for. Describing them as
additional parallel stores would require doubling the number of entries
in the machine description.
Next: Debug Information, Previous: Incdec, Up: RTL [Contents][Index]
The RTX code asm_operands represents a value produced by a
user-specified assembler instruction. It is used to represent
an asm statement with arguments. An asm statement with
a single output operand, like this:
asm ("foo %1,%2,%0" : "=a" (outputvar) : "g" (x + y), "di" (*z));
is represented using a single asm_operands RTX which represents
the value that is stored in outputvar:
(set rtx-for-outputvar
(asm_operands "foo %1,%2,%0" "a" 0
[rtx-for-addition-result rtx-for-*z]
[(asm_input:m1 "g")
(asm_input:m2 "di")]))
Here the operands of the asm_operands RTX are the assembler
template string, the output-operand’s constraint, the index-number of the
output operand among the output operands specified, a vector of input
operand RTX’s, and a vector of input-operand modes and constraints. The
mode m1 is the mode of the sum x+y; m2 is that of
*z.
When an asm statement has multiple output values, its insn has
several such set RTX’s inside of a parallel. Each set
contains an asm_operands; all of these share the same assembler
template and vectors, but each contains the constraint for the respective
output operand. They are also distinguished by the output-operand index
number, which is 0, 1, … for successive output operands.
Variable tracking relies on MEM_EXPR and REG_EXPR
annotations to determine what user variables memory and register
references refer to.
Variable tracking at assignments uses these notes only when they refer to variables that live at fixed locations (e.g., addressable variables, global non-automatic variables). For variables whose location may vary, it relies on the following types of notes.
(var_location:mode var exp stat)Binds variable var, a tree, to value exp, an RTL
expression. It appears only in NOTE_INSN_VAR_LOCATION and
DEBUG_INSNs, with slightly different meanings. mode, if
present, represents the mode of exp, which is useful if it is a
modeless expression. stat is only meaningful in notes,
indicating whether the variable is known to be initialized or
uninitialized.
(debug_expr:mode decl)Stands for the value bound to the DEBUG_EXPR_DECL decl,
that points back to it, within value expressions in
VAR_LOCATION nodes.
Next: Calls, Previous: Debug Information, Up: RTL [Contents][Index]
The RTL representation of the code for a function is a doubly-linked
chain of objects called insns. Insns are expressions with
special codes that are used for no other purpose. Some insns are
actual instructions; others represent dispatch tables for switch
statements; others represent labels to jump to or various sorts of
declarative information.
In addition to its own specific data, each insn must have a unique
id-number that distinguishes it from all other insns in the current
function (after delayed branch scheduling, copies of an insn with the
same id-number may be present in multiple places in a function, but
these copies will always be identical and will only appear inside a
sequence), and chain pointers to the preceding and following
insns. These three fields occupy the same position in every insn,
independent of the expression code of the insn. They could be accessed
with XEXP and XINT, but instead three special macros are
always used:
INSN_UID (i)Accesses the unique id of insn i.
PREV_INSN (i)Accesses the chain pointer to the insn preceding i. If i is the first insn, this is a null pointer.
NEXT_INSN (i)Accesses the chain pointer to the insn following i. If i is the last insn, this is a null pointer.
The first insn in the chain is obtained by calling get_insns; the
last insn is the result of calling get_last_insn. Within the
chain delimited by these insns, the NEXT_INSN and
PREV_INSN pointers must always correspond: if insn is not
the first insn,
NEXT_INSN (PREV_INSN (insn)) == insn
is always true and if insn is not the last insn,
PREV_INSN (NEXT_INSN (insn)) == insn
is always true.
After delay slot scheduling, some of the insns in the chain might be
sequence expressions, which contain a vector of insns. The value
of NEXT_INSN in all but the last of these insns is the next insn
in the vector; the value of NEXT_INSN of the last insn in the vector
is the same as the value of NEXT_INSN for the sequence in
which it is contained. Similar rules apply for PREV_INSN.
This means that the above invariants are not necessarily true for insns
inside sequence expressions. Specifically, if insn is the
first insn in a sequence, NEXT_INSN (PREV_INSN (insn))
is the insn containing the sequence expression, as is the value
of PREV_INSN (NEXT_INSN (insn)) if insn is the last
insn in the sequence expression. You can use these expressions
to find the containing sequence expression.
Every insn has one of the following expression codes:
insnThe expression code insn is used for instructions that do not jump
and do not do function calls. sequence expressions are always
contained in insns with code insn even if one of those insns
should jump or do function calls.
Insns with code insn have four additional fields beyond the three
mandatory ones listed above. These four are described in a table below.
jump_insnThe expression code jump_insn is used for instructions that may
jump (or, more generally, may contain label_ref expressions to
which pc can be set in that instruction). If there is an
instruction to return from the current function, it is recorded as a
jump_insn.
jump_insn insns have the same extra fields as insn insns,
accessed in the same way and in addition contain a field
JUMP_LABEL which is defined once jump optimization has completed.
For simple conditional and unconditional jumps, this field contains
the code_label to which this insn will (possibly conditionally)
branch. In a more complex jump, JUMP_LABEL records one of the
labels that the insn refers to; other jump target labels are recorded
as REG_LABEL_TARGET notes. The exception is addr_vec
and addr_diff_vec, where JUMP_LABEL is NULL_RTX
and the only way to find the labels is to scan the entire body of the
insn.
Return insns count as jumps, but since they do not refer to any
labels, their JUMP_LABEL is NULL_RTX.
call_insnThe expression code call_insn is used for instructions that may do
function calls. It is important to distinguish these instructions because
they imply that certain registers and memory locations may be altered
unpredictably.
call_insn insns have the same extra fields as insn insns,
accessed in the same way and in addition contain a field
CALL_INSN_FUNCTION_USAGE, which contains a list (chain of
expr_list expressions) containing use and clobber
expressions that denote hard registers and MEMs used or
clobbered by the called function.
A MEM generally points to a stack slots in which arguments passed
to the libcall by reference (see TARGET_PASS_BY_REFERENCE) are stored. If the argument is
caller-copied (see TARGET_CALLEE_COPIES),
the stack slot will be mentioned in CLOBBER and USE
entries; if it’s callee-copied, only a USE will appear, and the
MEM may point to addresses that are not stack slots.
CLOBBERed registers in this list augment registers specified in
CALL_USED_REGISTERS (see Register Basics).
code_labelA code_label insn represents a label that a jump insn can jump
to. It contains two special fields of data in addition to the three
standard ones. CODE_LABEL_NUMBER is used to hold the label
number, a number that identifies this label uniquely among all the
labels in the compilation (not just in the current function).
Ultimately, the label is represented in the assembler output as an
assembler label, usually of the form ‘Ln’ where n is
the label number.
When a code_label appears in an RTL expression, it normally
appears within a label_ref which represents the address of
the label, as a number.
Besides as a code_label, a label can also be represented as a
note of type NOTE_INSN_DELETED_LABEL.
The field LABEL_NUSES is only defined once the jump optimization
phase is completed. It contains the number of times this label is
referenced in the current function.
The field LABEL_KIND differentiates four different types of
labels: LABEL_NORMAL, LABEL_STATIC_ENTRY,
LABEL_GLOBAL_ENTRY, and LABEL_WEAK_ENTRY. The only labels
that do not have type LABEL_NORMAL are alternate entry
points to the current function. These may be static (visible only in
the containing translation unit), global (exposed to all translation
units), or weak (global, but can be overridden by another symbol with the
same name).
Much of the compiler treats all four kinds of label identically. Some
of it needs to know whether or not a label is an alternate entry point;
for this purpose, the macro LABEL_ALT_ENTRY_P is provided. It is
equivalent to testing whether ‘LABEL_KIND (label) == LABEL_NORMAL’.
The only place that cares about the distinction between static, global,
and weak alternate entry points, besides the front-end code that creates
them, is the function output_alternate_entry_point, in
final.c.
To set the kind of a label, use the SET_LABEL_KIND macro.
barrierBarriers are placed in the instruction stream when control cannot flow
past them. They are placed after unconditional jump instructions to
indicate that the jumps are unconditional and after calls to
volatile functions, which do not return (e.g., exit).
They contain no information beyond the three standard fields.
notenote insns are used to represent additional debugging and
declarative information. They contain two nonstandard fields, an
integer which is accessed with the macro NOTE_LINE_NUMBER and a
string accessed with NOTE_SOURCE_FILE.
If NOTE_LINE_NUMBER is positive, the note represents the
position of a source line and NOTE_SOURCE_FILE is the source file name
that the line came from. These notes control generation of line
number data in the assembler output.
Otherwise, NOTE_LINE_NUMBER is not really a line number but a
code with one of the following values (and NOTE_SOURCE_FILE
must contain a null pointer):
NOTE_INSN_DELETEDSuch a note is completely ignorable. Some passes of the compiler delete insns by altering them into notes of this kind.
NOTE_INSN_DELETED_LABELThis marks what used to be a code_label, but was not used for other
purposes than taking its address and was transformed to mark that no
code jumps to it.
NOTE_INSN_BLOCK_BEGNOTE_INSN_BLOCK_ENDThese types of notes indicate the position of the beginning and end of a level of scoping of variable names. They control the output of debugging information.
NOTE_INSN_EH_REGION_BEGNOTE_INSN_EH_REGION_ENDThese types of notes indicate the position of the beginning and end of a
level of scoping for exception handling. NOTE_BLOCK_NUMBER
identifies which CODE_LABEL or note of type
NOTE_INSN_DELETED_LABEL is associated with the given region.
NOTE_INSN_LOOP_BEGNOTE_INSN_LOOP_ENDThese types of notes indicate the position of the beginning and end
of a while or for loop. They enable the loop optimizer
to find loops quickly.
NOTE_INSN_LOOP_CONTAppears at the place in a loop that continue statements jump to.
NOTE_INSN_LOOP_VTOPThis note indicates the place in a loop where the exit test begins for those loops in which the exit test has been duplicated. This position becomes another virtual start of the loop when considering loop invariants.
NOTE_INSN_FUNCTION_BEGAppears at the start of the function body, after the function prologue.
NOTE_INSN_VAR_LOCATIONThis note is used to generate variable location debugging information.
It indicates that the user variable in its VAR_LOCATION operand
is at the location given in the RTL expression, or holds a value that
can be computed by evaluating the RTL expression from that static
point in the program up to the next such note for the same user
variable.
These codes are printed symbolically when they appear in debugging dumps.
debug_insnThe expression code debug_insn is used for pseudo-instructions
that hold debugging information for variable tracking at assignments
(see -fvar-tracking-assignments option). They are the RTL
representation of GIMPLE_DEBUG statements
(GIMPLE_DEBUG), with a VAR_LOCATION operand that
binds a user variable tree to an RTL representation of the
value in the corresponding statement. A DEBUG_EXPR in
it stands for the value bound to the corresponding
DEBUG_EXPR_DECL.
Throughout optimization passes, binding information is kept in pseudo-instruction form, so that, unlike notes, it gets the same treatment and adjustments that regular instructions would. It is the variable tracking pass that turns these pseudo-instructions into var location notes, analyzing control flow, value equivalences and changes to registers and memory referenced in value expressions, propagating the values of debug temporaries and determining expressions that can be used to compute the value of each user variable at as many points (ranges, actually) in the program as possible.
Unlike NOTE_INSN_VAR_LOCATION, the value expression in an
INSN_VAR_LOCATION denotes a value at that specific point in the
program, rather than an expression that can be evaluated at any later
point before an overriding VAR_LOCATION is encountered. E.g.,
if a user variable is bound to a REG and then a subsequent insn
modifies the REG, the note location would keep mapping the user
variable to the register across the insn, whereas the insn location
would keep the variable bound to the value, so that the variable
tracking pass would emit another location note for the variable at the
point in which the register is modified.
The machine mode of an insn is normally VOIDmode, but some
phases use the mode for various purposes.
The common subexpression elimination pass sets the mode of an insn to
QImode when it is the first insn in a block that has already
been processed.
The second Haifa scheduling pass, for targets that can multiple issue,
sets the mode of an insn to TImode when it is believed that the
instruction begins an issue group. That is, when the instruction
cannot issue simultaneously with the previous. This may be relied on
by later passes, in particular machine-dependent reorg.
Here is a table of the extra fields of insn, jump_insn
and call_insn insns:
PATTERN (i)An expression for the side effect performed by this insn. This must be
one of the following codes: set, call, use,
clobber, return, asm_input, asm_output,
addr_vec, addr_diff_vec, trap_if, unspec,
unspec_volatile, parallel, cond_exec, or sequence. If it is a parallel,
each element of the parallel must be one these codes, except that
parallel expressions cannot be nested and addr_vec and
addr_diff_vec are not permitted inside a parallel expression.
INSN_CODE (i)An integer that says which pattern in the machine description matches this insn, or -1 if the matching has not yet been attempted.
Such matching is never attempted and this field remains -1 on an insn
whose pattern consists of a single use, clobber,
asm_input, addr_vec or addr_diff_vec expression.
Matching is also never attempted on insns that result from an asm
statement. These contain at least one asm_operands expression.
The function asm_noperands returns a non-negative value for
such insns.
In the debugging output, this field is printed as a number followed by a symbolic representation that locates the pattern in the md file as some small positive or negative offset from a named pattern.
LOG_LINKS (i)A list (chain of insn_list expressions) giving information about
dependencies between instructions within a basic block. Neither a jump
nor a label may come between the related insns. These are only used by
the schedulers and by combine. This is a deprecated data structure.
Def-use and use-def chains are now preferred.
REG_NOTES (i)A list (chain of expr_list and insn_list expressions)
giving miscellaneous information about the insn. It is often
information pertaining to the registers used in this insn.
The LOG_LINKS field of an insn is a chain of insn_list
expressions. Each of these has two operands: the first is an insn,
and the second is another insn_list expression (the next one in
the chain). The last insn_list in the chain has a null pointer
as second operand. The significant thing about the chain is which
insns appear in it (as first operands of insn_list
expressions). Their order is not significant.
This list is originally set up by the flow analysis pass; it is a null pointer until then. Flow only adds links for those data dependencies which can be used for instruction combination. For each insn, the flow analysis pass adds a link to insns which store into registers values that are used for the first time in this insn.
The REG_NOTES field of an insn is a chain similar to the
LOG_LINKS field but it includes expr_list expressions in
addition to insn_list expressions. There are several kinds of
register notes, which are distinguished by the machine mode, which in a
register note is really understood as being an enum reg_note.
The first operand op of the note is data whose meaning depends on
the kind of note.
The macro REG_NOTE_KIND (x) returns the kind of
register note. Its counterpart, the macro PUT_REG_NOTE_KIND
(x, newkind) sets the register note type of x to be
newkind.
Register notes are of three classes: They may say something about an
input to an insn, they may say something about an output of an insn, or
they may create a linkage between two insns. There are also a set
of values that are only used in LOG_LINKS.
These register notes annotate inputs to an insn:
REG_DEADThe value in op dies in this insn; that is to say, altering the value immediately after this insn would not affect the future behavior of the program.
It does not follow that the register op has no useful value after this insn since op is not necessarily modified by this insn. Rather, no subsequent instruction uses the contents of op.
REG_UNUSEDThe register op being set by this insn will not be used in a
subsequent insn. This differs from a REG_DEAD note, which
indicates that the value in an input will not be used subsequently.
These two notes are independent; both may be present for the same
register.
REG_INCThe register op is incremented (or decremented; at this level
there is no distinction) by an embedded side effect inside this insn.
This means it appears in a post_inc, pre_inc,
post_dec or pre_dec expression.
REG_NONNEGThe register op is known to have a nonnegative value when this insn is reached. This is used so that decrement and branch until zero instructions, such as the m68k dbra, can be matched.
The REG_NONNEG note is added to insns only if the machine
description has a ‘decrement_and_branch_until_zero’ pattern.
REG_LABEL_OPERANDThis insn uses op, a code_label or a note of type
NOTE_INSN_DELETED_LABEL, but is not a jump_insn, or it
is a jump_insn that refers to the operand as an ordinary
operand. The label may still eventually be a jump target, but if so
in an indirect jump in a subsequent insn. The presence of this note
allows jump optimization to be aware that op is, in fact, being
used, and flow optimization to build an accurate flow graph.
REG_LABEL_TARGETThis insn is a jump_insn but not an addr_vec or
addr_diff_vec. It uses op, a code_label as a
direct or indirect jump target. Its purpose is similar to that of
REG_LABEL_OPERAND. This note is only present if the insn has
multiple targets; the last label in the insn (in the highest numbered
insn-field) goes into the JUMP_LABEL field and does not have a
REG_LABEL_TARGET note. See JUMP_LABEL.
REG_CROSSING_JUMPThis insn is a branching instruction (either an unconditional jump or an indirect jump) which crosses between hot and cold sections, which could potentially be very far apart in the executable. The presence of this note indicates to other optimizations that this branching instruction should not be “collapsed” into a simpler branching construct. It is used when the optimization to partition basic blocks into hot and cold sections is turned on.
REG_SETJMPAppears attached to each CALL_INSN to setjmp or a
related function.
The following notes describe attributes of outputs of an insn:
REG_EQUIVREG_EQUALThis note is only valid on an insn that sets only one register and
indicates that that register will be equal to op at run time; the
scope of this equivalence differs between the two types of notes. The
value which the insn explicitly copies into the register may look
different from op, but they will be equal at run time. If the
output of the single set is a strict_low_part expression,
the note refers to the register that is contained in SUBREG_REG
of the subreg expression.
For REG_EQUIV, the register is equivalent to op throughout
the entire function, and could validly be replaced in all its
occurrences by op. (“Validly” here refers to the data flow of
the program; simple replacement may make some insns invalid.) For
example, when a constant is loaded into a register that is never
assigned any other value, this kind of note is used.
When a parameter is copied into a pseudo-register at entry to a function, a note of this kind records that the register is equivalent to the stack slot where the parameter was passed. Although in this case the register may be set by other insns, it is still valid to replace the register by the stack slot throughout the function.
A REG_EQUIV note is also used on an instruction which copies a
register parameter into a pseudo-register at entry to a function, if
there is a stack slot where that parameter could be stored. Although
other insns may set the pseudo-register, it is valid for the compiler to
replace the pseudo-register by stack slot throughout the function,
provided the compiler ensures that the stack slot is properly
initialized by making the replacement in the initial copy instruction as
well. This is used on machines for which the calling convention
allocates stack space for register parameters. See
REG_PARM_STACK_SPACE in Stack Arguments.
In the case of REG_EQUAL, the register that is set by this insn
will be equal to op at run time at the end of this insn but not
necessarily elsewhere in the function. In this case, op
is typically an arithmetic expression. For example, when a sequence of
insns such as a library call is used to perform an arithmetic operation,
this kind of note is attached to the insn that produces or copies the
final value.
These two notes are used in different ways by the compiler passes.
REG_EQUAL is used by passes prior to register allocation (such as
common subexpression elimination and loop optimization) to tell them how
to think of that value. REG_EQUIV notes are used by register
allocation to indicate that there is an available substitute expression
(either a constant or a mem expression for the location of a
parameter on the stack) that may be used in place of a register if
insufficient registers are available.
Except for stack homes for parameters, which are indicated by a
REG_EQUIV note and are not useful to the early optimization
passes and pseudo registers that are equivalent to a memory location
throughout their entire life, which is not detected until later in
the compilation, all equivalences are initially indicated by an attached
REG_EQUAL note. In the early stages of register allocation, a
REG_EQUAL note is changed into a REG_EQUIV note if
op is a constant and the insn represents the only set of its
destination register.
Thus, compiler passes prior to register allocation need only check for
REG_EQUAL notes and passes subsequent to register allocation
need only check for REG_EQUIV notes.
These notes describe linkages between insns. They occur in pairs: one insn has one of a pair of notes that points to a second insn, which has the inverse note pointing back to the first insn.
REG_CC_SETTERREG_CC_USEROn machines that use cc0, the insns which set and use cc0
set and use cc0 are adjacent. However, when branch delay slot
filling is done, this may no longer be true. In this case a
REG_CC_USER note will be placed on the insn setting cc0 to
point to the insn using cc0 and a REG_CC_SETTER note will
be placed on the insn using cc0 to point to the insn setting
cc0.
These values are only used in the LOG_LINKS field, and indicate
the type of dependency that each link represents. Links which indicate
a data dependence (a read after write dependence) do not use any code,
they simply have mode VOIDmode, and are printed without any
descriptive text.
REG_DEP_TRUEThis indicates a true dependence (a read after write dependence).
REG_DEP_OUTPUTThis indicates an output dependence (a write after write dependence).
REG_DEP_ANTIThis indicates an anti dependence (a write after read dependence).
These notes describe information gathered from gcov profile data. They
are stored in the REG_NOTES field of an insn as an
expr_list.
REG_BR_PROBThis is used to specify the ratio of branches to non-branches of a branch insn according to the profile data. The value is stored as a value between 0 and REG_BR_PROB_BASE; larger values indicate a higher probability that the branch will be taken.
REG_BR_PREDThese notes are found in JUMP insns after delayed branch scheduling has taken place. They indicate both the direction and the likelihood of the JUMP. The format is a bitmask of ATTR_FLAG_* values.
REG_FRAME_RELATED_EXPRThis is used on an RTX_FRAME_RELATED_P insn wherein the attached expression is used in place of the actual insn pattern. This is done in cases where the pattern is either complex or misleading.
For convenience, the machine mode in an insn_list or
expr_list is printed using these symbolic codes in debugging dumps.
The only difference between the expression codes insn_list and
expr_list is that the first operand of an insn_list is
assumed to be an insn and is printed in debugging dumps as the insn’s
unique id; the first operand of an expr_list is printed in the
ordinary way as an expression.
Insns that call subroutines have the RTL expression code call_insn.
These insns must satisfy special rules, and their bodies must use a special
RTL expression code, call.
A call expression has two operands, as follows:
(call (mem:fm addr) nbytes)
Here nbytes is an operand that represents the number of bytes of
argument data being passed to the subroutine, fm is a machine mode
(which must equal as the definition of the FUNCTION_MODE macro in
the machine description) and addr represents the address of the
subroutine.
For a subroutine that returns no value, the call expression as
shown above is the entire body of the insn, except that the insn might
also contain use or clobber expressions.
For a subroutine that returns a value whose mode is not BLKmode,
the value is returned in a hard register. If this register’s number is
r, then the body of the call insn looks like this:
(set (reg:m r)
(call (mem:fm addr) nbytes))
This RTL expression makes it clear (to the optimizer passes) that the appropriate register receives a useful value in this insn.
When a subroutine returns a BLKmode value, it is handled by
passing to the subroutine the address of a place to store the value.
So the call insn itself does not “return” any value, and it has the
same RTL form as a call that returns nothing.
On some machines, the call instruction itself clobbers some register,
for example to contain the return address. call_insn insns
on these machines should have a body which is a parallel
that contains both the call expression and clobber
expressions that indicate which registers are destroyed. Similarly,
if the call instruction requires some register other than the stack
pointer that is not explicitly mentioned in its RTL, a use
subexpression should mention that register.
Functions that are called are assumed to modify all registers listed in
the configuration macro CALL_USED_REGISTERS (see Register Basics) and, with the exception of const functions and library
calls, to modify all of memory.
Insns containing just use expressions directly precede the
call_insn insn to indicate which registers contain inputs to the
function. Similarly, if registers other than those in
CALL_USED_REGISTERS are clobbered by the called function, insns
containing a single clobber follow immediately after the call to
indicate which registers.
Next: Reading RTL, Previous: Calls, Up: RTL [Contents][Index]
The compiler assumes that certain kinds of RTL expressions are unique; there do not exist two distinct objects representing the same value. In other cases, it makes an opposite assumption: that no RTL expression object of a certain kind appears in more than one place in the containing structure.
These assumptions refer to a single function; except for the RTL objects that describe global variables and external functions, and a few standard objects such as small integer constants, no RTL objects are common to two functions.
reg object to represent it,
and therefore only a single machine mode.
symbol_ref object
referring to it.
const_int expressions with equal values are shared.
pc expression.
cc0 expression.
const_double expression with value 0 for
each floating point mode. Likewise for values 1 and 2.
const_vector expression with value 0 for
each vector mode, be it an integer or a double constant vector.
label_ref or scratch appears in more than one place in
the RTL structure; in other words, it is safe to do a tree-walk of all
the insns in the function and assume that each time a label_ref
or scratch is seen it is distinct from all others that are seen.
mem object is normally created for each static
variable or stack slot, so these objects are frequently shared in all
the places they appear. However, separate but equal objects for these
variables are occasionally made.
asm statement has multiple output operands, a
distinct asm_operands expression is made for each output operand.
However, these all share the vector which contains the sequence of input
operands. This sharing is used later on to test whether two
asm_operands expressions come from the same statement, so all
optimizations must carefully preserve the sharing if they copy the
vector at all.
unshare_all_rtl in emit-rtl.c,
after which the above rules are guaranteed to be followed.
copy_rtx_if_shared, which is a subroutine of
unshare_all_rtl.
To read an RTL object from a file, call read_rtx. It takes one
argument, a stdio stream, and returns a single RTL object. This routine
is defined in read-rtl.c. It is not available in the compiler
itself, only the various programs that generate the compiler back end
from the machine description.
People frequently have the idea of using RTL stored as text in a file as an interface between a language front end and the bulk of GCC. This idea is not feasible.
GCC was designed to use RTL internally only. Correct RTL for a given program is very dependent on the particular target machine. And the RTL does not contain all the information about the program.
The proper way to interface GCC to a new language front end is with the “tree” data structure, described in the files tree.h and tree.def. The documentation for this structure (see GENERIC) is incomplete.
The purpose of GENERIC is simply to provide a
language-independent way of representing an entire function in
trees. To this end, it was necessary to add a few new tree codes
to the back end, but most everything was already there. If you
can express it with the codes in gcc/tree.def, it’s
GENERIC.
Early on, there was a great deal of debate about how to think
about statements in a tree IL. In GENERIC, a statement is
defined as any expression whose value, if any, is ignored. A
statement will always have TREE_SIDE_EFFECTS set (or it
will be discarded), but a non-statement expression may also have
side effects. A CALL_EXPR, for instance.
It would be possible for some local optimizations to work on the
GENERIC form of a function; indeed, the adapted tree inliner
works fine on GENERIC, but the current compiler performs inlining
after lowering to GIMPLE (a restricted form described in the next
section). Indeed, currently the frontends perform this lowering
before handing off to tree_rest_of_compilation, but this
seems inelegant.
| • Deficiencies: | Topics net yet covered in this document. | |
| • Tree overview: | All about trees.
| |
| • Types: | Fundamental and aggregate types. | |
| • Declarations: | Type declarations and variables. | |
| • Attributes: | Declaration and type attributes. | |
| • Expressions: | Operating on data. | |
| • Statements: | Control flow and related trees. | |
| • Functions: | Function bodies, linkage, and other aspects. | |
| • Language-dependent trees: | Topics and trees specific to language front ends. | |
| • C and C++ Trees: | Trees specific to C and C++. | |
| • Java Trees: | Trees specific to Java. |
Next: Tree overview, Up: GENERIC [Contents][Index]
There are many places in which this document is incomplet and incorrekt. It is, as of yet, only preliminary documentation.
Next: Types, Previous: Deficiencies, Up: GENERIC [Contents][Index]
The central data structure used by the internal representation is the
tree. These nodes, while all of the C type tree, are of
many varieties. A tree is a pointer type, but the object to
which it points may be of a variety of types. From this point forward,
we will refer to trees in ordinary type, rather than in this
font, except when talking about the actual C type tree.
You can tell what kind of node a particular tree is by using the
TREE_CODE macro. Many, many macros take trees as input and
return trees as output. However, most macros require a certain kind of
tree node as input. In other words, there is a type-system for trees,
but it is not reflected in the C type-system.
For safety, it is useful to configure GCC with --enable-checking. Although this results in a significant performance penalty (since all tree types are checked at run-time), and is therefore inappropriate in a release version, it is extremely helpful during the development process.
Many macros behave as predicates. Many, although not all, of these
predicates end in ‘_P’. Do not rely on the result type of these
macros being of any particular type. You may, however, rely on the fact
that the type can be compared to 0, so that statements like
if (TEST_P (t) && !TEST_P (y)) x = 1;
and
int i = (TEST_P (t) != 0);
are legal. Macros that return int values now may be changed to
return tree values, or other pointers in the future. Even those
that continue to return int may return multiple nonzero codes
where previously they returned only zero and one. Therefore, you should
not write code like
if (TEST_P (t) == 1)
as this code is not guaranteed to work correctly in the future.
You should not take the address of values returned by the macros or functions described here. In particular, no guarantee is given that the values are lvalues.
In general, the names of macros are all in uppercase, while the names of functions are entirely in lowercase. There are rare exceptions to this rule. You should assume that any macro or function whose name is made up entirely of uppercase letters may evaluate its arguments more than once. You may assume that a macro or function whose name is made up entirely of lowercase letters will evaluate its arguments only once.
The error_mark_node is a special tree. Its tree code is
ERROR_MARK, but since there is only ever one node with that code,
the usual practice is to compare the tree against
error_mark_node. (This test is just a test for pointer
equality.) If an error has occurred during front-end processing the
flag errorcount will be set. If the front end has encountered
code it cannot handle, it will issue a message to the user and set
sorrycount. When these flags are set, any macro or function
which normally returns a tree of a particular kind may instead return
the error_mark_node. Thus, if you intend to do any processing of
erroneous code, you must be prepared to deal with the
error_mark_node.
Occasionally, a particular tree slot (like an operand to an expression, or a particular field in a declaration) will be referred to as “reserved for the back end”. These slots are used to store RTL when the tree is converted to RTL for use by the GCC back end. However, if that process is not taking place (e.g., if the front end is being hooked up to an intelligent editor), then those slots may be used by the back end presently in use.
If you encounter situations that do not match this documentation, such as tree nodes of types not mentioned here, or macros documented to return entities of a particular kind that instead return entities of some different kind, you have found a bug, either in the front end or in the documentation. Please report these bugs as you would any other bug.
| • Macros and Functions: | Macros and functions that can be used with all trees. | |
| • Identifiers: | The names of things. | |
| • Containers: | Lists and vectors. |
Next: Identifiers, Up: Tree overview [Contents][Index]
All GENERIC trees have two fields in common. First, TREE_CHAIN
is a pointer that can be used as a singly-linked list to other trees.
The other is TREE_TYPE. Many trees store the type of an
expression or declaration in this field.
These are some other functions for handling trees:
tree_size
Return the number of bytes a tree takes.
build0
build1
build2
build3
build4
build5
build6
These functions build a tree and supply values to put in each
parameter. The basic signature is ‘code, type, [operands]’.
code is the TREE_CODE, and type is a tree
representing the TREE_TYPE. These are followed by the
operands, each of which is also a tree.
Next: Containers, Previous: Macros and Functions, Up: Tree overview [Contents][Index]
An IDENTIFIER_NODE represents a slightly more general concept
that the standard C or C++ concept of identifier. In particular, an
IDENTIFIER_NODE may contain a ‘$’, or other extraordinary
characters.
There are never two distinct IDENTIFIER_NODEs representing the
same identifier. Therefore, you may use pointer equality to compare
IDENTIFIER_NODEs, rather than using a routine like
strcmp. Use get_identifier to obtain the unique
IDENTIFIER_NODE for a supplied string.
You can use the following macros to access identifiers:
IDENTIFIER_POINTER
The string represented by the identifier, represented as a
char*. This string is always NUL-terminated, and contains
no embedded NUL characters.
IDENTIFIER_LENGTH
The length of the string returned by IDENTIFIER_POINTER, not
including the trailing NUL. This value of
IDENTIFIER_LENGTH (x) is always the same as strlen
(IDENTIFIER_POINTER (x)).
IDENTIFIER_OPNAME_P
This predicate holds if the identifier represents the name of an
overloaded operator. In this case, you should not depend on the
contents of either the IDENTIFIER_POINTER or the
IDENTIFIER_LENGTH.
IDENTIFIER_TYPENAME_P
This predicate holds if the identifier represents the name of a
user-defined conversion operator. In this case, the TREE_TYPE of
the IDENTIFIER_NODE holds the type to which the conversion
operator converts.
Previous: Identifiers, Up: Tree overview [Contents][Index]
Two common container data structures can be represented directly with
tree nodes. A TREE_LIST is a singly linked list containing two
trees per node. These are the TREE_PURPOSE and TREE_VALUE
of each node. (Often, the TREE_PURPOSE contains some kind of
tag, or additional information, while the TREE_VALUE contains the
majority of the payload. In other cases, the TREE_PURPOSE is
simply NULL_TREE, while in still others both the
TREE_PURPOSE and TREE_VALUE are of equal stature.) Given
one TREE_LIST node, the next node is found by following the
TREE_CHAIN. If the TREE_CHAIN is NULL_TREE, then
you have reached the end of the list.
A TREE_VEC is a simple vector. The TREE_VEC_LENGTH is an
integer (not a tree) giving the number of nodes in the vector. The
nodes themselves are accessed using the TREE_VEC_ELT macro, which
takes two arguments. The first is the TREE_VEC in question; the
second is an integer indicating which element in the vector is desired.
The elements are indexed from zero.
Next: Declarations, Previous: Tree overview, Up: GENERIC [Contents][Index]
All types have corresponding tree nodes. However, you should not assume that there is exactly one tree node corresponding to each type. There are often multiple nodes corresponding to the same type.
For the most part, different kinds of types have different tree codes.
(For example, pointer types use a POINTER_TYPE code while arrays
use an ARRAY_TYPE code.) However, pointers to member functions
use the RECORD_TYPE code. Therefore, when writing a
switch statement that depends on the code associated with a
particular type, you should take care to handle pointers to member
functions under the RECORD_TYPE case label.
The following functions and macros deal with cv-qualification of types:
TYPE_MAIN_VARIANT
This macro returns the unqualified version of a type. It may be applied to an unqualified type, but it is not always the identity function in that case.
A few other macros and functions are usable with all types:
TYPE_SIZE
The number of bits required to represent the type, represented as an
INTEGER_CST. For an incomplete type, TYPE_SIZE will be
NULL_TREE.
TYPE_ALIGN
The alignment of the type, in bits, represented as an int.
TYPE_NAME
This macro returns a declaration (in the form of a TYPE_DECL) for
the type. (Note this macro does not return an
IDENTIFIER_NODE, as you might expect, given its name!) You can
look at the DECL_NAME of the TYPE_DECL to obtain the
actual name of the type. The TYPE_NAME will be NULL_TREE
for a type that is not a built-in type, the result of a typedef, or a
named class type.
TYPE_CANONICAL
This macro returns the “canonical” type for the given type
node. Canonical types are used to improve performance in the C++ and
Objective-C++ front ends by allowing efficient comparison between two
type nodes in same_type_p: if the TYPE_CANONICAL values
of the types are equal, the types are equivalent; otherwise, the types
are not equivalent. The notion of equivalence for canonical types is
the same as the notion of type equivalence in the language itself. For
instance,
When TYPE_CANONICAL is NULL_TREE, there is no canonical
type for the given type node. In this case, comparison between this
type and any other type requires the compiler to perform a deep,
“structural” comparison to see if the two type nodes have the same
form and properties.
The canonical type for a node is always the most fundamental type in
the equivalence class of types. For instance, int is its own
canonical type. A typedef I of int will have int
as its canonical type. Similarly, I* and a typedef IP (defined to I*) will has int* as their canonical
type. When building a new type node, be sure to set
TYPE_CANONICAL to the appropriate canonical type. If the new
type is a compound type (built from other types), and any of those
other types require structural equality, use
SET_TYPE_STRUCTURAL_EQUALITY to ensure that the new type also
requires structural equality. Finally, if for some reason you cannot
guarantee that TYPE_CANONICAL will point to the canonical type,
use SET_TYPE_STRUCTURAL_EQUALITY to make sure that the new
type–and any type constructed based on it–requires structural
equality. If you suspect that the canonical type system is
miscomparing types, pass --param verify-canonical-types=1 to
the compiler or configure with --enable-checking to force the
compiler to verify its canonical-type comparisons against the
structural comparisons; the compiler will then print any warnings if
the canonical types miscompare.
TYPE_STRUCTURAL_EQUALITY_P
This predicate holds when the node requires structural equality
checks, e.g., when TYPE_CANONICAL is NULL_TREE.
SET_TYPE_STRUCTURAL_EQUALITY
This macro states that the type node it is given requires structural
equality checks, e.g., it sets TYPE_CANONICAL to
NULL_TREE.
same_type_p
This predicate takes two types as input, and holds if they are the same
type. For example, if one type is a typedef for the other, or
both are typedefs for the same type. This predicate also holds if
the two trees given as input are simply copies of one another; i.e.,
there is no difference between them at the source level, but, for
whatever reason, a duplicate has been made in the representation. You
should never use == (pointer equality) to compare types; always
use same_type_p instead.
Detailed below are the various kinds of types, and the macros that can be used to access them. Although other kinds of types are used elsewhere in G++, the types described here are the only ones that you will encounter while examining the intermediate representation.
VOID_TYPEUsed to represent the void type.
INTEGER_TYPEUsed to represent the various integral types, including char,
short, int, long, and long long. This code
is not used for enumeration types, nor for the bool type.
The TYPE_PRECISION is the number of bits used in
the representation, represented as an unsigned int. (Note that
in the general case this is not the same value as TYPE_SIZE;
suppose that there were a 24-bit integer type, but that alignment
requirements for the ABI required 32-bit alignment. Then,
TYPE_SIZE would be an INTEGER_CST for 32, while
TYPE_PRECISION would be 24.) The integer type is unsigned if
TYPE_UNSIGNED holds; otherwise, it is signed.
The TYPE_MIN_VALUE is an INTEGER_CST for the smallest
integer that may be represented by this type. Similarly, the
TYPE_MAX_VALUE is an INTEGER_CST for the largest integer
that may be represented by this type.
REAL_TYPEUsed to represent the float, double, and long
double types. The number of bits in the floating-point representation
is given by TYPE_PRECISION, as in the INTEGER_TYPE case.
FIXED_POINT_TYPEUsed to represent the short _Fract, _Fract, long
_Fract, long long _Fract, short _Accum, _Accum,
long _Accum, and long long _Accum types. The number of bits
in the fixed-point representation is given by TYPE_PRECISION,
as in the INTEGER_TYPE case. There may be padding bits, fractional
bits and integral bits. The number of fractional bits is given by
TYPE_FBIT, and the number of integral bits is given by TYPE_IBIT.
The fixed-point type is unsigned if TYPE_UNSIGNED holds; otherwise,
it is signed.
The fixed-point type is saturating if TYPE_SATURATING holds; otherwise,
it is not saturating.
COMPLEX_TYPEUsed to represent GCC built-in __complex__ data types. The
TREE_TYPE is the type of the real and imaginary parts.
ENUMERAL_TYPEUsed to represent an enumeration type. The TYPE_PRECISION gives
(as an int), the number of bits used to represent the type. If
there are no negative enumeration constants, TYPE_UNSIGNED will
hold. The minimum and maximum enumeration constants may be obtained
with TYPE_MIN_VALUE and TYPE_MAX_VALUE, respectively; each
of these macros returns an INTEGER_CST.
The actual enumeration constants themselves may be obtained by looking
at the TYPE_VALUES. This macro will return a TREE_LIST,
containing the constants. The TREE_PURPOSE of each node will be
an IDENTIFIER_NODE giving the name of the constant; the
TREE_VALUE will be an INTEGER_CST giving the value
assigned to that constant. These constants will appear in the order in
which they were declared. The TREE_TYPE of each of these
constants will be the type of enumeration type itself.
BOOLEAN_TYPEUsed to represent the bool type.
POINTER_TYPEUsed to represent pointer types, and pointer to data member types. The
TREE_TYPE gives the type to which this type points.
REFERENCE_TYPEUsed to represent reference types. The TREE_TYPE gives the type
to which this type refers.
FUNCTION_TYPEUsed to represent the type of non-member functions and of static member
functions. The TREE_TYPE gives the return type of the function.
The TYPE_ARG_TYPES are a TREE_LIST of the argument types.
The TREE_VALUE of each node in this list is the type of the
corresponding argument; the TREE_PURPOSE is an expression for the
default argument value, if any. If the last node in the list is
void_list_node (a TREE_LIST node whose TREE_VALUE
is the void_type_node), then functions of this type do not take
variable arguments. Otherwise, they do take a variable number of
arguments.
Note that in C (but not in C++) a function declared like void f()
is an unprototyped function taking a variable number of arguments; the
TYPE_ARG_TYPES of such a function will be NULL.
METHOD_TYPEUsed to represent the type of a non-static member function. Like a
FUNCTION_TYPE, the return type is given by the TREE_TYPE.
The type of *this, i.e., the class of which functions of this
type are a member, is given by the TYPE_METHOD_BASETYPE. The
TYPE_ARG_TYPES is the parameter list, as for a
FUNCTION_TYPE, and includes the this argument.
ARRAY_TYPEUsed to represent array types. The TREE_TYPE gives the type of
the elements in the array. If the array-bound is present in the type,
the TYPE_DOMAIN is an INTEGER_TYPE whose
TYPE_MIN_VALUE and TYPE_MAX_VALUE will be the lower and
upper bounds of the array, respectively. The TYPE_MIN_VALUE will
always be an INTEGER_CST for zero, while the
TYPE_MAX_VALUE will be one less than the number of elements in
the array, i.e., the highest value which may be used to index an element
in the array.
RECORD_TYPEUsed to represent struct and class types, as well as
pointers to member functions and similar constructs in other languages.
TYPE_FIELDS contains the items contained in this type, each of
which can be a FIELD_DECL, VAR_DECL, CONST_DECL, or
TYPE_DECL. You may not make any assumptions about the ordering
of the fields in the type or whether one or more of them overlap.
UNION_TYPEUsed to represent union types. Similar to RECORD_TYPE
except that all FIELD_DECL nodes in TYPE_FIELD start at
bit position zero.
QUAL_UNION_TYPEUsed to represent part of a variant record in Ada. Similar to
UNION_TYPE except that each FIELD_DECL has a
DECL_QUALIFIER field, which contains a boolean expression that
indicates whether the field is present in the object. The type will only
have one field, so each field’s DECL_QUALIFIER is only evaluated
if none of the expressions in the previous fields in TYPE_FIELDS
are nonzero. Normally these expressions will reference a field in the
outer object using a PLACEHOLDER_EXPR.
LANG_TYPEThis node is used to represent a language-specific type. The front end must handle it.
OFFSET_TYPEThis node is used to represent a pointer-to-data member. For a data
member X::m the TYPE_OFFSET_BASETYPE is X and the
TREE_TYPE is the type of m.
There are variables whose values represent some of the basic types. These include:
void_type_nodeA node for void.
integer_type_nodeA node for int.
unsigned_type_node.A node for unsigned int.
char_type_node.A node for char.
It may sometimes be useful to compare one of these variables with a type
in hand, using same_type_p.
Next: Attributes, Previous: Types, Up: GENERIC [Contents][Index]
This section covers the various kinds of declarations that appear in the
internal representation, except for declarations of functions
(represented by FUNCTION_DECL nodes), which are described in
Functions.
| • Working with declarations: | Macros and functions that work on declarations. | |
| • Internal structure: | How declaration nodes are represented. |
Next: Internal structure, Up: Declarations [Contents][Index]
Some macros can be used with any kind of declaration. These include:
DECL_NAME
This macro returns an IDENTIFIER_NODE giving the name of the
entity.
TREE_TYPE
This macro returns the type of the entity declared.
EXPR_FILENAME
This macro returns the name of the file in which the entity was
declared, as a char*. For an entity declared implicitly by the
compiler (like __builtin_memcpy), this will be the string
"<internal>".
EXPR_LINENO
This macro returns the line number at which the entity was declared, as
an int.
DECL_ARTIFICIAL
This predicate holds if the declaration was implicitly generated by the
compiler. For example, this predicate will hold of an implicitly
declared member function, or of the TYPE_DECL implicitly
generated for a class type. Recall that in C++ code like:
struct S {};
is roughly equivalent to C code like:
struct S {};
typedef struct S S;
The implicitly generated typedef declaration is represented by a
TYPE_DECL for which DECL_ARTIFICIAL holds.
The various kinds of declarations include:
LABEL_DECLThese nodes are used to represent labels in function bodies. For more information, see Functions. These nodes only appear in block scopes.
CONST_DECLThese nodes are used to represent enumeration constants. The value of
the constant is given by DECL_INITIAL which will be an
INTEGER_CST with the same type as the TREE_TYPE of the
CONST_DECL, i.e., an ENUMERAL_TYPE.
RESULT_DECLThese nodes represent the value returned by a function. When a value is
assigned to a RESULT_DECL, that indicates that the value should
be returned, via bitwise copy, by the function. You can use
DECL_SIZE and DECL_ALIGN on a RESULT_DECL, just as
with a VAR_DECL.
TYPE_DECLThese nodes represent typedef declarations. The TREE_TYPE
is the type declared to have the name given by DECL_NAME. In
some cases, there is no associated name.
VAR_DECLThese nodes represent variables with namespace or block scope, as well
as static data members. The DECL_SIZE and DECL_ALIGN are
analogous to TYPE_SIZE and TYPE_ALIGN. For a declaration,
you should always use the DECL_SIZE and DECL_ALIGN rather
than the TYPE_SIZE and TYPE_ALIGN given by the
TREE_TYPE, since special attributes may have been applied to the
variable to give it a particular size and alignment. You may use the
predicates DECL_THIS_STATIC or DECL_THIS_EXTERN to test
whether the storage class specifiers static or extern were
used to declare a variable.
If this variable is initialized (but does not require a constructor),
the DECL_INITIAL will be an expression for the initializer. The
initializer should be evaluated, and a bitwise copy into the variable
performed. If the DECL_INITIAL is the error_mark_node,
there is an initializer, but it is given by an explicit statement later
in the code; no bitwise copy is required.
GCC provides an extension that allows either automatic variables, or
global variables, to be placed in particular registers. This extension
is being used for a particular VAR_DECL if DECL_REGISTER
holds for the VAR_DECL, and if DECL_ASSEMBLER_NAME is not
equal to DECL_NAME. In that case, DECL_ASSEMBLER_NAME is
the name of the register into which the variable will be placed.
PARM_DECLUsed to represent a parameter to a function. Treat these nodes
similarly to VAR_DECL nodes. These nodes only appear in the
DECL_ARGUMENTS for a FUNCTION_DECL.
The DECL_ARG_TYPE for a PARM_DECL is the type that will
actually be used when a value is passed to this function. It may be a
wider type than the TREE_TYPE of the parameter; for example, the
ordinary type might be short while the DECL_ARG_TYPE is
int.
DEBUG_EXPR_DECLUsed to represent an anonymous debug-information temporary created to hold an expression as it is optimized away, so that its value can be referenced in debug bind statements.
FIELD_DECLThese nodes represent non-static data members. The DECL_SIZE and
DECL_ALIGN behave as for VAR_DECL nodes.
The position of the field within the parent record is specified by a
combination of three attributes. DECL_FIELD_OFFSET is the position,
counting in bytes, of the DECL_OFFSET_ALIGN-bit sized word containing
the bit of the field closest to the beginning of the structure.
DECL_FIELD_BIT_OFFSET is the bit offset of the first bit of the field
within this word; this may be nonzero even for fields that are not bit-fields,
since DECL_OFFSET_ALIGN may be greater than the natural alignment
of the field’s type.
If DECL_C_BIT_FIELD holds, this field is a bit-field. In a bit-field,
DECL_BIT_FIELD_TYPE also contains the type that was originally
specified for it, while DECL_TYPE may be a modified type with lesser precision,
according to the size of the bit field.
NAMESPACE_DECLNamespaces provide a name hierarchy for other declarations. They
appear in the DECL_CONTEXT of other _DECL nodes.
Previous: Working with declarations, Up: Declarations [Contents][Index]
DECL nodes are represented internally as a hierarchy of
structures.
| • Current structure hierarchy: | The current DECL node structure hierarchy. | |
| • Adding new DECL node types: | How to add a new DECL node to a frontend. |
Next: Adding new DECL node types, Up: Internal structure [Contents][Index]
struct tree_decl_minimalThis is the minimal structure to inherit from in order for common
DECL macros to work. The fields it contains are a unique ID,
source location, context, and name.
struct tree_decl_commonThis structure inherits from struct tree_decl_minimal. It
contains fields that most DECL nodes need, such as a field to
store alignment, machine mode, size, and attributes.
struct tree_field_declThis structure inherits from struct tree_decl_common. It is
used to represent FIELD_DECL.
struct tree_label_declThis structure inherits from struct tree_decl_common. It is
used to represent LABEL_DECL.
struct tree_translation_unit_declThis structure inherits from struct tree_decl_common. It is
used to represent TRANSLATION_UNIT_DECL.
struct tree_decl_with_rtlThis structure inherits from struct tree_decl_common. It
contains a field to store the low-level RTL associated with a
DECL node.
struct tree_result_declThis structure inherits from struct tree_decl_with_rtl. It is
used to represent RESULT_DECL.
struct tree_const_declThis structure inherits from struct tree_decl_with_rtl. It is
used to represent CONST_DECL.
struct tree_parm_declThis structure inherits from struct tree_decl_with_rtl. It is
used to represent PARM_DECL.
struct tree_decl_with_visThis structure inherits from struct tree_decl_with_rtl. It
contains fields necessary to store visibility information, as well as
a section name and assembler name.
struct tree_var_declThis structure inherits from struct tree_decl_with_vis. It is
used to represent VAR_DECL.
struct tree_function_declThis structure inherits from struct tree_decl_with_vis. It is
used to represent FUNCTION_DECL.
Previous: Current structure hierarchy, Up: Internal structure [Contents][Index]
Adding a new DECL tree consists of the following steps
DECL nodeFor language specific DECL nodes, there is a .def file
in each frontend directory where the tree code should be added.
For DECL nodes that are part of the middle-end, the code should
be added to tree.def.
DECL nodeThese structures should inherit from one of the existing structures in the language hierarchy by using that structure as the first member.
struct tree_foo_decl
{
struct tree_decl_with_vis common;
}
Would create a structure name tree_foo_decl that inherits from
struct tree_decl_with_vis.
For language specific DECL nodes, this new structure type
should go in the appropriate .h file.
For DECL nodes that are part of the middle-end, the structure
type should go in tree.h.
For garbage collection and dynamic checking purposes, each DECL
node structure type is required to have a unique enumerator value
specified with it.
For language specific DECL nodes, this new enumerator value
should go in the appropriate .def file.
For DECL nodes that are part of the middle-end, the enumerator
values are specified in treestruct.def.
union tree_nodeIn order to make your new structure type usable, it must be added to
union tree_node.
For language specific DECL nodes, a new entry should be added
to the appropriate .h file of the form
struct tree_foo_decl GTY ((tag ("TS_VAR_DECL"))) foo_decl;
For DECL nodes that are part of the middle-end, the additional
member goes directly into union tree_node in tree.h.
In order to be able to check whether accessing a named portion of
union tree_node is legal, and whether a certain DECL node
contains one of the enumerated DECL node structures in the
hierarchy, a simple lookup table is used.
This lookup table needs to be kept up to date with the tree structure
hierarchy, or else checking and containment macros will fail
inappropriately.
For language specific DECL nodes, their is an init_ts
function in an appropriate .c file, which initializes the lookup
table.
Code setting up the table for new DECL nodes should be added
there.
For each DECL tree code and enumerator value representing a
member of the inheritance hierarchy, the table should contain 1 if
that tree code inherits (directly or indirectly) from that member.
Thus, a FOO_DECL node derived from struct decl_with_rtl,
and enumerator value TS_FOO_DECL, would be set up as follows
tree_contains_struct[FOO_DECL][TS_FOO_DECL] = 1; tree_contains_struct[FOO_DECL][TS_DECL_WRTL] = 1; tree_contains_struct[FOO_DECL][TS_DECL_COMMON] = 1; tree_contains_struct[FOO_DECL][TS_DECL_MINIMAL] = 1;
For DECL nodes that are part of the middle-end, the setup code
goes into tree.c.
Each added field or flag should have a macro that is used to access
it, that performs appropriate checking to ensure only the right type of
DECL nodes access the field.
These macros generally take the following form
#define FOO_DECL_FIELDNAME(NODE) FOO_DECL_CHECK(NODE)->foo_decl.fieldname
However, if the structure is simply a base class for further structures, something like the following should be used
#define BASE_STRUCT_CHECK(T) CONTAINS_STRUCT_CHECK(T, TS_BASE_STRUCT) #define BASE_STRUCT_FIELDNAME(NODE) \ (BASE_STRUCT_CHECK(NODE)->base_struct.fieldname
Next: Expression trees, Previous: Declarations, Up: GENERIC [Contents][Index]
Attributes, as specified using the __attribute__ keyword, are
represented internally as a TREE_LIST. The TREE_PURPOSE
is the name of the attribute, as an IDENTIFIER_NODE. The
TREE_VALUE is a TREE_LIST of the arguments of the
attribute, if any, or NULL_TREE if there are no arguments; the
arguments are stored as the TREE_VALUE of successive entries in
the list, and may be identifiers or expressions. The TREE_CHAIN
of the attribute is the next attribute in a list of attributes applying
to the same declaration or type, or NULL_TREE if there are no
further attributes in the list.
Attributes may be attached to declarations and to types; these attributes may be accessed with the following macros. All attributes are stored in this way, and many also cause other changes to the declaration or type or to other internal compiler data structures.
This macro returns the attributes on the declaration decl.
This macro returns the attributes on the type type.
Next: Statements, Previous: Attributes, Up: GENERIC [Contents][Index]
The internal representation for expressions is for the most part quite straightforward. However, there are a few facts that one must bear in mind. In particular, the expression “tree” is actually a directed acyclic graph. (For example there may be many references to the integer constant zero throughout the source program; many of these will be represented by the same expression node.) You should not rely on certain kinds of node being shared, nor should you rely on certain kinds of nodes being unshared.
The following macros can be used with all expression nodes:
TREE_TYPE
Returns the type of the expression. This value may not be precisely the same type that would be given the expression in the original program.
In what follows, some nodes that one might expect to always have type
bool are documented to have either integral or boolean type. At
some point in the future, the C front end may also make use of this same
intermediate representation, and at this point these nodes will
certainly have integral type. The previous sentence is not meant to
imply that the C++ front end does not or will not give these nodes
integral type.
Below, we list the various kinds of expression nodes. Except where
noted otherwise, the operands to an expression are accessed using the
TREE_OPERAND macro. For example, to access the first operand to
a binary plus expression expr, use:
TREE_OPERAND (expr, 0)
As this example indicates, the operands are zero-indexed.
| • Constants: | ||
| • Storage References: | ||
| • Unary and Binary Expressions: | ||
| • Vectors: |
Next: Storage References, Up: Expression trees [Contents][Index]
The table below begins with constants, moves on to unary expressions, then proceeds to binary expressions, and concludes with various other kinds of expressions:
INTEGER_CSTThese nodes represent integer constants. Note that the type of these
constants is obtained with TREE_TYPE; they are not always of type
int. In particular, char constants are represented with
INTEGER_CST nodes. The value of the integer constant e is
given by
((TREE_INT_CST_HIGH (e) << HOST_BITS_PER_WIDE_INT) + TREE_INST_CST_LOW (e))
HOST_BITS_PER_WIDE_INT is at least thirty-two on all platforms. Both
TREE_INT_CST_HIGH and TREE_INT_CST_LOW return a
HOST_WIDE_INT. The value of an INTEGER_CST is interpreted
as a signed or unsigned quantity depending on the type of the constant.
In general, the expression given above will overflow, so it should not
be used to calculate the value of the constant.
The variable integer_zero_node is an integer constant with value
zero. Similarly, integer_one_node is an integer constant with
value one. The size_zero_node and size_one_node variables
are analogous, but have type size_t rather than int.
The function tree_int_cst_lt is a predicate which holds if its
first argument is less than its second. Both constants are assumed to
have the same signedness (i.e., either both should be signed or both
should be unsigned.) The full width of the constant is used when doing
the comparison; the usual rules about promotions and conversions are
ignored. Similarly, tree_int_cst_equal holds if the two
constants are equal. The tree_int_cst_sgn function returns the
sign of a constant. The value is 1, 0, or -1
according on whether the constant is greater than, equal to, or less
than zero. Again, the signedness of the constant’s type is taken into
account; an unsigned constant is never less than zero, no matter what
its bit-pattern.
REAL_CSTFIXME: Talk about how to obtain representations of this constant, do comparisons, and so forth.
FIXED_CSTThese nodes represent fixed-point constants. The type of these constants
is obtained with TREE_TYPE. TREE_FIXED_CST_PTR points to
a struct fixed_value; TREE_FIXED_CST returns the structure
itself. struct fixed_value contains data with the size of two
HOST_BITS_PER_WIDE_INT and mode as the associated fixed-point
machine mode for data.
COMPLEX_CSTThese nodes are used to represent complex number constants, that is a
__complex__ whose parts are constant nodes. The
TREE_REALPART and TREE_IMAGPART return the real and the
imaginary parts respectively.
VECTOR_CSTThese nodes are used to represent vector constants, whose parts are
constant nodes. Each individual constant node is either an integer or a
double constant node. The first operand is a TREE_LIST of the
constant nodes and is accessed through TREE_VECTOR_CST_ELTS.
STRING_CSTThese nodes represent string-constants. The TREE_STRING_LENGTH
returns the length of the string, as an int. The
TREE_STRING_POINTER is a char* containing the string
itself. The string may not be NUL-terminated, and it may contain
embedded NUL characters. Therefore, the
TREE_STRING_LENGTH includes the trailing NUL if it is
present.
For wide string constants, the TREE_STRING_LENGTH is the number
of bytes in the string, and the TREE_STRING_POINTER
points to an array of the bytes of the string, as represented on the
target system (that is, as integers in the target endianness). Wide and
non-wide string constants are distinguished only by the TREE_TYPE
of the STRING_CST.
FIXME: The formats of string constants are not well-defined when the target system bytes are not the same width as host system bytes.
Next: Unary and Binary Expressions, Previous: Constant expressions, Up: Expression trees [Contents][Index]
ARRAY_REFThese nodes represent array accesses. The first operand is the array;
the second is the index. To calculate the address of the memory
accessed, you must scale the index by the size of the type of the array
elements. The type of these expressions must be the type of a component of
the array. The third and fourth operands are used after gimplification
to represent the lower bound and component size but should not be used
directly; call array_ref_low_bound and array_ref_element_size
instead.
ARRAY_RANGE_REFThese nodes represent access to a range (or “slice”) of an array. The
operands are the same as that for ARRAY_REF and have the same
meanings. The type of these expressions must be an array whose component
type is the same as that of the first operand. The range of that array
type determines the amount of data these expressions access.
TARGET_MEM_REFThese nodes represent memory accesses whose address directly map to
an addressing mode of the target architecture. The first argument
is TMR_SYMBOL and must be a VAR_DECL of an object with
a fixed address. The second argument is TMR_BASE and the
third one is TMR_INDEX. The fourth argument is
TMR_STEP and must be an INTEGER_CST. The fifth
argument is TMR_OFFSET and must be an INTEGER_CST.
Any of the arguments may be NULL if the appropriate component
does not appear in the address. Address of the TARGET_MEM_REF
is determined in the following way.
&TMR_SYMBOL + TMR_BASE + TMR_INDEX * TMR_STEP + TMR_OFFSET
The sixth argument is the reference to the original memory access, which is preserved for the purposes of the RTL alias analysis. The seventh argument is a tag representing the results of tree level alias analysis.
ADDR_EXPRThese nodes are used to represent the address of an object. (These expressions will always have pointer or reference type.) The operand may be another expression, or it may be a declaration.
As an extension, GCC allows users to take the address of a label. In
this case, the operand of the ADDR_EXPR will be a
LABEL_DECL. The type of such an expression is void*.
If the object addressed is not an lvalue, a temporary is created, and the address of the temporary is used.
INDIRECT_REFThese nodes are used to represent the object pointed to by a pointer. The operand is the pointer being dereferenced; it will always have pointer or reference type.
MEM_REFThese nodes are used to represent the object pointed to by a pointer offset by a constant. The first operand is the pointer being dereferenced; it will always have pointer or reference type. The second operand is a pointer constant. Its type is specifying the type to be used for type-based alias analysis.
COMPONENT_REFThese nodes represent non-static data member accesses. The first
operand is the object (rather than a pointer to it); the second operand
is the FIELD_DECL for the data member. The third operand represents
the byte offset of the field, but should not be used directly; call
component_ref_field_offset instead.
Next: Vectors, Previous: Storage References, Up: Expression trees [Contents][Index]
NEGATE_EXPRThese nodes represent unary negation of the single operand, for both integer and floating-point types. The type of negation can be determined by looking at the type of the expression.
The behavior of this operation on signed arithmetic overflow is
controlled by the flag_wrapv and flag_trapv variables.
ABS_EXPRThese nodes represent the absolute value of the single operand, for
both integer and floating-point types. This is typically used to
implement the abs, labs and llabs builtins for
integer types, and the fabs, fabsf and fabsl
builtins for floating point types. The type of abs operation can
be determined by looking at the type of the expression.
This node is not used for complex types. To represent the modulus
or complex abs of a complex value, use the BUILT_IN_CABS,
BUILT_IN_CABSF or BUILT_IN_CABSL builtins, as used
to implement the C99 cabs, cabsf and cabsl
built-in functions.
BIT_NOT_EXPRThese nodes represent bitwise complement, and will always have integral type. The only operand is the value to be complemented.
TRUTH_NOT_EXPRThese nodes represent logical negation, and will always have integral
(or boolean) type. The operand is the value being negated. The type
of the operand and that of the result are always of BOOLEAN_TYPE
or INTEGER_TYPE.
PREDECREMENT_EXPRPREINCREMENT_EXPRPOSTDECREMENT_EXPRPOSTINCREMENT_EXPRThese nodes represent increment and decrement expressions. The value of
the single operand is computed, and the operand incremented or
decremented. In the case of PREDECREMENT_EXPR and
PREINCREMENT_EXPR, the value of the expression is the value
resulting after the increment or decrement; in the case of
POSTDECREMENT_EXPR and POSTINCREMENT_EXPR is the value
before the increment or decrement occurs. The type of the operand, like
that of the result, will be either integral, boolean, or floating-point.
FIX_TRUNC_EXPRThese nodes represent conversion of a floating-point value to an integer. The single operand will have a floating-point type, while the complete expression will have an integral (or boolean) type. The operand is rounded towards zero.
FLOAT_EXPRThese nodes represent conversion of an integral (or boolean) value to a floating-point value. The single operand will have integral type, while the complete expression will have a floating-point type.
FIXME: How is the operand supposed to be rounded? Is this dependent on -mieee?
COMPLEX_EXPRThese nodes are used to represent complex numbers constructed from two expressions of the same (integer or real) type. The first operand is the real part and the second operand is the imaginary part.
CONJ_EXPRThese nodes represent the conjugate of their operand.
REALPART_EXPRIMAGPART_EXPRThese nodes represent respectively the real and the imaginary parts of complex numbers (their sole argument).
NON_LVALUE_EXPRThese nodes indicate that their one and only operand is not an lvalue. A back end can treat these identically to the single operand.
NOP_EXPRThese nodes are used to represent conversions that do not require any
code-generation. For example, conversion of a char* to an
int* does not require any code be generated; such a conversion is
represented by a NOP_EXPR. The single operand is the expression
to be converted. The conversion from a pointer to a reference is also
represented with a NOP_EXPR.
CONVERT_EXPRThese nodes are similar to NOP_EXPRs, but are used in those
situations where code may need to be generated. For example, if an
int* is converted to an int code may need to be generated
on some platforms. These nodes are never used for C++-specific
conversions, like conversions between pointers to different classes in
an inheritance hierarchy. Any adjustments that need to be made in such
cases are always indicated explicitly. Similarly, a user-defined
conversion is never represented by a CONVERT_EXPR; instead, the
function calls are made explicit.
FIXED_CONVERT_EXPRThese nodes are used to represent conversions that involve fixed-point values. For example, from a fixed-point value to another fixed-point value, from an integer to a fixed-point value, from a fixed-point value to an integer, from a floating-point value to a fixed-point value, or from a fixed-point value to a floating-point value.
LSHIFT_EXPRRSHIFT_EXPRThese nodes represent left and right shifts, respectively. The first operand is the value to shift; it will always be of integral type. The second operand is an expression for the number of bits by which to shift. Right shift should be treated as arithmetic, i.e., the high-order bits should be zero-filled when the expression has unsigned type and filled with the sign bit when the expression has signed type. Note that the result is undefined if the second operand is larger than or equal to the first operand’s type size.
BIT_IOR_EXPRBIT_XOR_EXPRBIT_AND_EXPRThese nodes represent bitwise inclusive or, bitwise exclusive or, and bitwise and, respectively. Both operands will always have integral type.
TRUTH_ANDIF_EXPRTRUTH_ORIF_EXPRThese nodes represent logical “and” and logical “or”, respectively.
These operators are not strict; i.e., the second operand is evaluated
only if the value of the expression is not determined by evaluation of
the first operand. The type of the operands and that of the result are
always of BOOLEAN_TYPE or INTEGER_TYPE.
TRUTH_AND_EXPRTRUTH_OR_EXPRTRUTH_XOR_EXPRThese nodes represent logical and, logical or, and logical exclusive or.
They are strict; both arguments are always evaluated. There are no
corresponding operators in C or C++, but the front end will sometimes
generate these expressions anyhow, if it can tell that strictness does
not matter. The type of the operands and that of the result are
always of BOOLEAN_TYPE or INTEGER_TYPE.
POINTER_PLUS_EXPRThis node represents pointer arithmetic. The first operand is always a pointer/reference type. The second operand is always an unsigned integer type compatible with sizetype. This is the only binary arithmetic operand that can operate on pointer types.
PLUS_EXPRMINUS_EXPRMULT_EXPRThese nodes represent various binary arithmetic operations. Respectively, these operations are addition, subtraction (of the second operand from the first) and multiplication. Their operands may have either integral or floating type, but there will never be case in which one operand is of floating type and the other is of integral type.
The behavior of these operations on signed arithmetic overflow is
controlled by the flag_wrapv and flag_trapv variables.
RDIV_EXPRThis node represents a floating point division operation.
TRUNC_DIV_EXPRFLOOR_DIV_EXPRCEIL_DIV_EXPRROUND_DIV_EXPRThese nodes represent integer division operations that return an integer
result. TRUNC_DIV_EXPR rounds towards zero, FLOOR_DIV_EXPR
rounds towards negative infinity, CEIL_DIV_EXPR rounds towards
positive infinity and ROUND_DIV_EXPR rounds to the closest integer.
Integer division in C and C++ is truncating, i.e. TRUNC_DIV_EXPR.
The behavior of these operations on signed arithmetic overflow, when
dividing the minimum signed integer by minus one, is controlled by the
flag_wrapv and flag_trapv variables.
TRUNC_MOD_EXPRFLOOR_MOD_EXPRCEIL_MOD_EXPRROUND_MOD_EXPRThese nodes represent the integer remainder or modulus operation.
The integer modulus of two operands a and b is
defined as a - (a/b)*b where the division calculated using
the corresponding division operator. Hence for TRUNC_MOD_EXPR
this definition assumes division using truncation towards zero, i.e.
TRUNC_DIV_EXPR. Integer remainder in C and C++ uses truncating
division, i.e. TRUNC_MOD_EXPR.
EXACT_DIV_EXPRThe EXACT_DIV_EXPR code is used to represent integer divisions where
the numerator is known to be an exact multiple of the denominator. This
allows the backend to choose between the faster of TRUNC_DIV_EXPR,
CEIL_DIV_EXPR and FLOOR_DIV_EXPR for the current target.
LT_EXPRLE_EXPRGT_EXPRGE_EXPREQ_EXPRNE_EXPRThese nodes represent the less than, less than or equal to, greater than, greater than or equal to, equal, and not equal comparison operators. The first and second operand with either be both of integral type or both of floating type. The result type of these expressions will always be of integral or boolean type. These operations return the result type’s zero value for false, and the result type’s one value for true.
For floating point comparisons, if we honor IEEE NaNs and either operand
is NaN, then NE_EXPR always returns true and the remaining operators
always return false. On some targets, comparisons against an IEEE NaN,
other than equality and inequality, may generate a floating point exception.
ORDERED_EXPRUNORDERED_EXPRThese nodes represent non-trapping ordered and unordered comparison operators. These operations take two floating point operands and determine whether they are ordered or unordered relative to each other. If either operand is an IEEE NaN, their comparison is defined to be unordered, otherwise the comparison is defined to be ordered. The result type of these expressions will always be of integral or boolean type. These operations return the result type’s zero value for false, and the result type’s one value for true.
UNLT_EXPRUNLE_EXPRUNGT_EXPRUNGE_EXPRUNEQ_EXPRLTGT_EXPRThese nodes represent the unordered comparison operators.
These operations take two floating point operands and determine whether
the operands are unordered or are less than, less than or equal to,
greater than, greater than or equal to, or equal respectively. For
example, UNLT_EXPR returns true if either operand is an IEEE
NaN or the first operand is less than the second. With the possible
exception of LTGT_EXPR, all of these operations are guaranteed
not to generate a floating point exception. The result
type of these expressions will always be of integral or boolean type.
These operations return the result type’s zero value for false,
and the result type’s one value for true.
MODIFY_EXPRThese nodes represent assignment. The left-hand side is the first
operand; the right-hand side is the second operand. The left-hand side
will be a VAR_DECL, INDIRECT_REF, COMPONENT_REF, or
other lvalue.
These nodes are used to represent not only assignment with ‘=’ but also compound assignments (like ‘+=’), by reduction to ‘=’ assignment. In other words, the representation for ‘i += 3’ looks just like that for ‘i = i + 3’.
INIT_EXPRThese nodes are just like MODIFY_EXPR, but are used only when a
variable is initialized, rather than assigned to subsequently. This
means that we can assume that the target of the initialization is not
used in computing its own value; any reference to the lhs in computing
the rhs is undefined.
COMPOUND_EXPRThese nodes represent comma-expressions. The first operand is an expression whose value is computed and thrown away prior to the evaluation of the second operand. The value of the entire expression is the value of the second operand.
COND_EXPRThese nodes represent ?: expressions. The first operand
is of boolean or integral type. If it evaluates to a nonzero value,
the second operand should be evaluated, and returned as the value of the
expression. Otherwise, the third operand is evaluated, and returned as
the value of the expression.
The second operand must have the same type as the entire expression,
unless it unconditionally throws an exception or calls a noreturn
function, in which case it should have void type. The same constraints
apply to the third operand. This allows array bounds checks to be
represented conveniently as (i >= 0 && i < 10) ? i : abort().
As a GNU extension, the C language front-ends allow the second
operand of the ?: operator may be omitted in the source.
For example, x ? : 3 is equivalent to x ? x : 3,
assuming that x is an expression without side-effects.
In the tree representation, however, the second operand is always
present, possibly protected by SAVE_EXPR if the first
argument does cause side-effects.
CALL_EXPRThese nodes are used to represent calls to functions, including
non-static member functions. CALL_EXPRs are implemented as
expression nodes with a variable number of operands. Rather than using
TREE_OPERAND to extract them, it is preferable to use the
specialized accessor macros and functions that operate specifically on
CALL_EXPR nodes.
CALL_EXPR_FN returns a pointer to the
function to call; it is always an expression whose type is a
POINTER_TYPE.
The number of arguments to the call is returned by call_expr_nargs,
while the arguments themselves can be accessed with the CALL_EXPR_ARG
macro. The arguments are zero-indexed and numbered left-to-right.
You can iterate over the arguments using FOR_EACH_CALL_EXPR_ARG, as in:
tree call, arg; call_expr_arg_iterator iter; FOR_EACH_CALL_EXPR_ARG (arg, iter, call) /* arg is bound to successive arguments of call. */ …;
For non-static
member functions, there will be an operand corresponding to the
this pointer. There will always be expressions corresponding to
all of the arguments, even if the function is declared with default
arguments and some arguments are not explicitly provided at the call
sites.
CALL_EXPRs also have a CALL_EXPR_STATIC_CHAIN operand that
is used to implement nested functions. This operand is otherwise null.
CLEANUP_POINT_EXPRThese nodes represent full-expressions. The single operand is an expression to evaluate. Any destructor calls engendered by the creation of temporaries during the evaluation of that expression should be performed immediately after the expression is evaluated.
CONSTRUCTORThese nodes represent the brace-enclosed initializers for a structure or
array. The first operand is reserved for use by the back end. The
second operand is a TREE_LIST. If the TREE_TYPE of the
CONSTRUCTOR is a RECORD_TYPE or UNION_TYPE, then
the TREE_PURPOSE of each node in the TREE_LIST will be a
FIELD_DECL and the TREE_VALUE of each node will be the
expression used to initialize that field.
If the TREE_TYPE of the CONSTRUCTOR is an
ARRAY_TYPE, then the TREE_PURPOSE of each element in the
TREE_LIST will be an INTEGER_CST or a RANGE_EXPR of
two INTEGER_CSTs. A single INTEGER_CST indicates which
element of the array (indexed from zero) is being assigned to. A
RANGE_EXPR indicates an inclusive range of elements to
initialize. In both cases the TREE_VALUE is the corresponding
initializer. It is re-evaluated for each element of a
RANGE_EXPR. If the TREE_PURPOSE is NULL_TREE, then
the initializer is for the next available array element.
In the front end, you should not depend on the fields appearing in any particular order. However, in the middle end, fields must appear in declaration order. You should not assume that all fields will be represented. Unrepresented fields will be set to zero.
COMPOUND_LITERAL_EXPRThese nodes represent ISO C99 compound literals. The
COMPOUND_LITERAL_EXPR_DECL_EXPR is a DECL_EXPR
containing an anonymous VAR_DECL for
the unnamed object represented by the compound literal; the
DECL_INITIAL of that VAR_DECL is a CONSTRUCTOR
representing the brace-enclosed list of initializers in the compound
literal. That anonymous VAR_DECL can also be accessed directly
by the COMPOUND_LITERAL_EXPR_DECL macro.
SAVE_EXPRA SAVE_EXPR represents an expression (possibly involving
side-effects) that is used more than once. The side-effects should
occur only the first time the expression is evaluated. Subsequent uses
should just reuse the computed value. The first operand to the
SAVE_EXPR is the expression to evaluate. The side-effects should
be executed where the SAVE_EXPR is first encountered in a
depth-first preorder traversal of the expression tree.
TARGET_EXPRA TARGET_EXPR represents a temporary object. The first operand
is a VAR_DECL for the temporary variable. The second operand is
the initializer for the temporary. The initializer is evaluated and,
if non-void, copied (bitwise) into the temporary. If the initializer
is void, that means that it will perform the initialization itself.
Often, a TARGET_EXPR occurs on the right-hand side of an
assignment, or as the second operand to a comma-expression which is
itself the right-hand side of an assignment, etc. In this case, we say
that the TARGET_EXPR is “normal”; otherwise, we say it is
“orphaned”. For a normal TARGET_EXPR the temporary variable
should be treated as an alias for the left-hand side of the assignment,
rather than as a new temporary variable.
The third operand to the TARGET_EXPR, if present, is a
cleanup-expression (i.e., destructor call) for the temporary. If this
expression is orphaned, then this expression must be executed when the
statement containing this expression is complete. These cleanups must
always be executed in the order opposite to that in which they were
encountered. Note that if a temporary is created on one branch of a
conditional operator (i.e., in the second or third operand to a
COND_EXPR), the cleanup must be run only if that branch is
actually executed.
VA_ARG_EXPRThis node is used to implement support for the C/C++ variable argument-list
mechanism. It represents expressions like va_arg (ap, type).
Its TREE_TYPE yields the tree representation for type and
its sole argument yields the representation for ap.
Previous: Unary and Binary Expressions, Up: Expression trees [Contents][Index]
VEC_LSHIFT_EXPRVEC_RSHIFT_EXPRThese nodes represent whole vector left and right shifts, respectively. The first operand is the vector to shift; it will always be of vector type. The second operand is an expression for the number of bits by which to shift. Note that the result is undefined if the second operand is larger than or equal to the first operand’s type size.
VEC_WIDEN_MULT_HI_EXPRVEC_WIDEN_MULT_LO_EXPRThese nodes represent widening vector multiplication of the high and low
parts of the two input vectors, respectively. Their operands are vectors
that contain the same number of elements (N) of the same integral type.
The result is a vector that contains half as many elements, of an integral type
whose size is twice as wide. In the case of VEC_WIDEN_MULT_HI_EXPR the
high N/2 elements of the two vector are multiplied to produce the
vector of N/2 products. In the case of VEC_WIDEN_MULT_LO_EXPR the
low N/2 elements of the two vector are multiplied to produce the
vector of N/2 products.
VEC_UNPACK_HI_EXPRVEC_UNPACK_LO_EXPRThese nodes represent unpacking of the high and low parts of the input vector,
respectively. The single operand is a vector that contains N elements
of the same integral or floating point type. The result is a vector
that contains half as many elements, of an integral or floating point type
whose size is twice as wide. In the case of VEC_UNPACK_HI_EXPR the
high N/2 elements of the vector are extracted and widened (promoted).
In the case of VEC_UNPACK_LO_EXPR the low N/2 elements of the
vector are extracted and widened (promoted).
VEC_UNPACK_FLOAT_HI_EXPRVEC_UNPACK_FLOAT_LO_EXPRThese nodes represent unpacking of the high and low parts of the input vector,
where the values are converted from fixed point to floating point. The
single operand is a vector that contains N elements of the same
integral type. The result is a vector that contains half as many elements
of a floating point type whose size is twice as wide. In the case of
VEC_UNPACK_HI_EXPR the high N/2 elements of the vector are
extracted, converted and widened. In the case of VEC_UNPACK_LO_EXPR
the low N/2 elements of the vector are extracted, converted and widened.
VEC_PACK_TRUNC_EXPRThis node represents packing of truncated elements of the two input vectors into the output vector. Input operands are vectors that contain the same number of elements of the same integral or floating point type. The result is a vector that contains twice as many elements of an integral or floating point type whose size is half as wide. The elements of the two vectors are demoted and merged (concatenated) to form the output vector.
VEC_PACK_SAT_EXPRThis node represents packing of elements of the two input vectors into the output vector using saturation. Input operands are vectors that contain the same number of elements of the same integral type. The result is a vector that contains twice as many elements of an integral type whose size is half as wide. The elements of the two vectors are demoted and merged (concatenated) to form the output vector.
VEC_PACK_FIX_TRUNC_EXPRThis node represents packing of elements of the two input vectors into the output vector, where the values are converted from floating point to fixed point. Input operands are vectors that contain the same number of elements of a floating point type. The result is a vector that contains twice as many elements of an integral type whose size is half as wide. The elements of the two vectors are merged (concatenated) to form the output vector.
VEC_EXTRACT_EVEN_EXPRVEC_EXTRACT_ODD_EXPRThese nodes represent extracting of the even/odd elements of the two input vectors, respectively. Their operands and result are vectors that contain the same number of elements of the same type.
VEC_INTERLEAVE_HIGH_EXPRVEC_INTERLEAVE_LOW_EXPRThese nodes represent merging and interleaving of the high/low elements of the
two input vectors, respectively. The operands and the result are vectors that
contain the same number of elements (N) of the same type.
In the case of VEC_INTERLEAVE_HIGH_EXPR, the high N/2 elements of
the first input vector are interleaved with the high N/2 elements of the
second input vector. In the case of VEC_INTERLEAVE_LOW_EXPR, the low
N/2 elements of the first input vector are interleaved with the low
N/2 elements of the second input vector.
Next: Functions, Previous: Expression trees, Up: GENERIC [Contents][Index]
Most statements in GIMPLE are assignment statements, represented by
GIMPLE_ASSIGN. No other C expressions can appear at statement level;
a reference to a volatile object is converted into a
GIMPLE_ASSIGN.
There are also several varieties of complex statements.
| • Basic Statements: | ||
| • Blocks: | ||
| • Statement Sequences: | ||
| • Empty Statements: | ||
| • Jumps: | ||
| • Cleanups: | ||
| • OpenMP: |
Next: Blocks, Up: Statements [Contents][Index]
ASM_EXPRUsed to represent an inline assembly statement. For an inline assembly statement like:
asm ("mov x, y");
The ASM_STRING macro will return a STRING_CST node for
"mov x, y". If the original statement made use of the
extended-assembly syntax, then ASM_OUTPUTS,
ASM_INPUTS, and ASM_CLOBBERS will be the outputs, inputs,
and clobbers for the statement, represented as STRING_CST nodes.
The extended-assembly syntax looks like:
asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));
The first string is the ASM_STRING, containing the instruction
template. The next two strings are the output and inputs, respectively;
this statement has no clobbers. As this example indicates, “plain”
assembly statements are merely a special case of extended assembly
statements; they have no cv-qualifiers, outputs, inputs, or clobbers.
All of the strings will be NUL-terminated, and will contain no
embedded NUL-characters.
If the assembly statement is declared volatile, or if the
statement was not an extended assembly statement, and is therefore
implicitly volatile, then the predicate ASM_VOLATILE_P will hold
of the ASM_EXPR.
DECL_EXPRUsed to represent a local declaration. The DECL_EXPR_DECL macro
can be used to obtain the entity declared. This declaration may be a
LABEL_DECL, indicating that the label declared is a local label.
(As an extension, GCC allows the declaration of labels with scope.) In
C, this declaration may be a FUNCTION_DECL, indicating the
use of the GCC nested function extension. For more information,
see Functions.
LABEL_EXPRUsed to represent a label. The LABEL_DECL declared by this
statement can be obtained with the LABEL_EXPR_LABEL macro. The
IDENTIFIER_NODE giving the name of the label can be obtained from
the LABEL_DECL with DECL_NAME.
GOTO_EXPRUsed to represent a goto statement. The GOTO_DESTINATION will
usually be a LABEL_DECL. However, if the “computed goto” extension
has been used, the GOTO_DESTINATION will be an arbitrary expression
indicating the destination. This expression will always have pointer type.
RETURN_EXPRUsed to represent a return statement. Operand 0 represents the
value to return. It should either be the RESULT_DECL for the
containing function, or a MODIFY_EXPR or INIT_EXPR
setting the function’s RESULT_DECL. It will be
NULL_TREE if the statement was just
return;
LOOP_EXPRThese nodes represent “infinite” loops. The LOOP_EXPR_BODY
represents the body of the loop. It should be executed forever, unless
an EXIT_EXPR is encountered.
EXIT_EXPRThese nodes represent conditional exits from the nearest enclosing
LOOP_EXPR. The single operand is the condition; if it is
nonzero, then the loop should be exited. An EXIT_EXPR will only
appear within a LOOP_EXPR.
SWITCH_STMTUsed to represent a switch statement. The SWITCH_STMT_COND
is the expression on which the switch is occurring. See the documentation
for an IF_STMT for more information on the representation used
for the condition. The SWITCH_STMT_BODY is the body of the switch
statement. The SWITCH_STMT_TYPE is the original type of switch
expression as given in the source, before any compiler conversions.
CASE_LABEL_EXPRUse to represent a case label, range of case labels, or a
default label. If CASE_LOW is NULL_TREE, then this is a
default label. Otherwise, if CASE_HIGH is NULL_TREE, then
this is an ordinary case label. In this case, CASE_LOW is
an expression giving the value of the label. Both CASE_LOW and
CASE_HIGH are INTEGER_CST nodes. These values will have
the same type as the condition expression in the switch statement.
Otherwise, if both CASE_LOW and CASE_HIGH are defined, the
statement is a range of case labels. Such statements originate with the
extension that allows users to write things of the form:
case 2 ... 5:
The first value will be CASE_LOW, while the second will be
CASE_HIGH.
Next: Statement Sequences, Previous: Basic Statements, Up: Statements [Contents][Index]
Block scopes and the variables they declare in GENERIC are
expressed using the BIND_EXPR code, which in previous
versions of GCC was primarily used for the C statement-expression
extension.
Variables in a block are collected into BIND_EXPR_VARS in
declaration order through their TREE_CHAIN field. Any runtime
initialization is moved out of DECL_INITIAL and into a
statement in the controlled block. When gimplifying from C or C++,
this initialization replaces the DECL_STMT. These variables
will never require cleanups. The scope of these variables is just the
body
Variable-length arrays (VLAs) complicate this process, as their
size often refers to variables initialized earlier in the block.
To handle this, we currently split the block at that point, and
move the VLA into a new, inner BIND_EXPR. This strategy
may change in the future.
A C++ program will usually contain more BIND_EXPRs than
there are syntactic blocks in the source code, since several C++
constructs have implicit scopes associated with them. On the
other hand, although the C++ front end uses pseudo-scopes to
handle cleanups for objects with destructors, these don’t
translate into the GIMPLE form; multiple declarations at the same
level use the same BIND_EXPR.
Next: Empty Statements, Previous: Blocks, Up: Statements [Contents][Index]
Multiple statements at the same nesting level are collected into
a STATEMENT_LIST. Statement lists are modified and
traversed using the interface in ‘tree-iterator.h’.
Next: Jumps, Previous: Statement Sequences, Up: Statements [Contents][Index]
Whenever possible, statements with no effect are discarded. But
if they are nested within another construct which cannot be
discarded for some reason, they are instead replaced with an
empty statement, generated by build_empty_stmt.
Initially, all empty statements were shared, after the pattern of
the Java front end, but this caused a lot of trouble in practice.
An empty statement is represented as (void)0.
Next: Cleanups, Previous: Empty Statements, Up: Statements [Contents][Index]
Other jumps are expressed by either GOTO_EXPR or
RETURN_EXPR.
The operand of a GOTO_EXPR must be either a label or a
variable containing the address to jump to.
The operand of a RETURN_EXPR is either NULL_TREE,
RESULT_DECL, or a MODIFY_EXPR which sets the return
value. It would be nice to move the MODIFY_EXPR into a
separate statement, but the special return semantics in
expand_return make that difficult. It may still happen in
the future, perhaps by moving most of that logic into
expand_assignment.
Next: OpenMP, Previous: Jumps, Up: Statements [Contents][Index]
Destructors for local C++ objects and similar dynamic cleanups are
represented in GIMPLE by a TRY_FINALLY_EXPR.
TRY_FINALLY_EXPR has two operands, both of which are a sequence
of statements to execute. The first sequence is executed. When it
completes the second sequence is executed.
The first sequence may complete in the following ways:
GOTO_EXPR) to an ordinary
label outside the sequence.
RETURN_EXPR).
The second sequence is not executed if the first sequence completes by
calling setjmp or exit or any other function that does
not return. The second sequence is also not executed if the first
sequence completes via a non-local goto or a computed goto (in general
the compiler does not know whether such a goto statement exits the
first sequence or not, so we assume that it doesn’t).
After the second sequence is executed, if it completes normally by falling off the end, execution continues wherever the first sequence would have continued, by falling off the end, or doing a goto, etc.
TRY_FINALLY_EXPR complicates the flow graph, since the cleanup
needs to appear on every edge out of the controlled block; this
reduces the freedom to move code across these edges. Therefore, the
EH lowering pass which runs before most of the optimization passes
eliminates these expressions by explicitly adding the cleanup to each
edge. Rethrowing the exception is represented using RESX_EXPR.
Previous: Cleanups, Up: Statements [Contents][Index]
All the statements starting with OMP_ represent directives and
clauses used by the OpenMP API http://www.openmp.org/.
OMP_PARALLELRepresents #pragma omp parallel [clause1 … clauseN]. It
has four operands:
Operand OMP_PARALLEL_BODY is valid while in GENERIC and
High GIMPLE forms. It contains the body of code to be executed
by all the threads. During GIMPLE lowering, this operand becomes
NULL and the body is emitted linearly after
OMP_PARALLEL.
Operand OMP_PARALLEL_CLAUSES is the list of clauses
associated with the directive.
Operand OMP_PARALLEL_FN is created by
pass_lower_omp, it contains the FUNCTION_DECL
for the function that will contain the body of the parallel
region.
Operand OMP_PARALLEL_DATA_ARG is also created by
pass_lower_omp. If there are shared variables to be
communicated to the children threads, this operand will contain
the VAR_DECL that contains all the shared values and
variables.
OMP_FORRepresents #pragma omp for [clause1 … clauseN]. It
has 5 operands:
Operand OMP_FOR_BODY contains the loop body.
Operand OMP_FOR_CLAUSES is the list of clauses
associated with the directive.
Operand OMP_FOR_INIT is the loop initialization code of
the form VAR = N1.
Operand OMP_FOR_COND is the loop conditional expression
of the form VAR {<,>,<=,>=} N2.
Operand OMP_FOR_INCR is the loop index increment of the
form VAR {+=,-=} INCR.
Operand OMP_FOR_PRE_BODY contains side-effect code from
operands OMP_FOR_INIT, OMP_FOR_COND and
OMP_FOR_INC. These side-effects are part of the
OMP_FOR block but must be evaluated before the start of
loop body.
The loop index variable VAR must be a signed integer variable,
which is implicitly private to each thread. Bounds
N1 and N2 and the increment expression
INCR are required to be loop invariant integer
expressions that are evaluated without any synchronization. The
evaluation order, frequency of evaluation and side-effects are
unspecified by the standard.
OMP_SECTIONSRepresents #pragma omp sections [clause1 … clauseN].
Operand OMP_SECTIONS_BODY contains the sections body,
which in turn contains a set of OMP_SECTION nodes for
each of the concurrent sections delimited by #pragma omp
section.
Operand OMP_SECTIONS_CLAUSES is the list of clauses
associated with the directive.
OMP_SECTIONSection delimiter for OMP_SECTIONS.
OMP_SINGLERepresents #pragma omp single.
Operand OMP_SINGLE_BODY contains the body of code to be
executed by a single thread.
Operand OMP_SINGLE_CLAUSES is the list of clauses
associated with the directive.
OMP_MASTERRepresents #pragma omp master.
Operand OMP_MASTER_BODY contains the body of code to be
executed by the master thread.
OMP_ORDEREDRepresents #pragma omp ordered.
Operand OMP_ORDERED_BODY contains the body of code to be
executed in the sequential order dictated by the loop index
variable.
OMP_CRITICALRepresents #pragma omp critical [name].
Operand OMP_CRITICAL_BODY is the critical section.
Operand OMP_CRITICAL_NAME is an optional identifier to
label the critical section.
OMP_RETURNThis does not represent any OpenMP directive, it is an artificial
marker to indicate the end of the body of an OpenMP. It is used
by the flow graph (tree-cfg.c) and OpenMP region
building code (omp-low.c).
OMP_CONTINUESimilarly, this instruction does not represent an OpenMP
directive, it is used by OMP_FOR and
OMP_SECTIONS to mark the place where the code needs to
loop to the next iteration (in the case of OMP_FOR) or
the next section (in the case of OMP_SECTIONS).
In some cases, OMP_CONTINUE is placed right before
OMP_RETURN. But if there are cleanups that need to
occur right after the looping body, it will be emitted between
OMP_CONTINUE and OMP_RETURN.
OMP_ATOMICRepresents #pragma omp atomic.
Operand 0 is the address at which the atomic operation is to be performed.
Operand 1 is the expression to evaluate. The gimplifier tries three alternative code generation strategies. Whenever possible, an atomic update built-in is used. If that fails, a compare-and-swap loop is attempted. If that also fails, a regular critical section around the expression is used.
OMP_CLAUSERepresents clauses associated with one of the OMP_ directives.
Clauses are represented by separate sub-codes defined in
tree.h. Clauses codes can be one of:
OMP_CLAUSE_PRIVATE, OMP_CLAUSE_SHARED,
OMP_CLAUSE_FIRSTPRIVATE,
OMP_CLAUSE_LASTPRIVATE, OMP_CLAUSE_COPYIN,
OMP_CLAUSE_COPYPRIVATE, OMP_CLAUSE_IF,
OMP_CLAUSE_NUM_THREADS, OMP_CLAUSE_SCHEDULE,
OMP_CLAUSE_NOWAIT, OMP_CLAUSE_ORDERED,
OMP_CLAUSE_DEFAULT, and OMP_CLAUSE_REDUCTION. Each code
represents the corresponding OpenMP clause.
Clauses associated with the same directive are chained together
via OMP_CLAUSE_CHAIN. Those clauses that accept a list
of variables are restricted to exactly one, accessed with
OMP_CLAUSE_VAR. Therefore, multiple variables under the
same clause C need to be represented as multiple C clauses
chained together. This facilitates adding new clauses during
compilation.
Next: Language-dependent trees, Previous: Statements, Up: GENERIC [Contents][Index]
A function is represented by a FUNCTION_DECL node. It stores
the basic pieces of the function such as body, parameters, and return
type as well as information on the surrounding context, visibility,
and linkage.
| • Function Basics: | Function names, body, and parameters. | |
| • Function Properties: | Context, linkage, etc. |
Next: Function Properties, Up: Functions [Contents][Index]
A function has four core parts: the name, the parameters, the result,
and the body. The following macros and functions access these parts
of a FUNCTION_DECL as well as other basic features:
DECL_NAME
This macro returns the unqualified name of the function, as an
IDENTIFIER_NODE. For an instantiation of a function template,
the DECL_NAME is the unqualified name of the template, not
something like f<int>. The value of DECL_NAME is
undefined when used on a constructor, destructor, overloaded operator,
or type-conversion operator, or any function that is implicitly
generated by the compiler. See below for macros that can be used to
distinguish these cases.
DECL_ASSEMBLER_NAME
This macro returns the mangled name of the function, also an
IDENTIFIER_NODE. This name does not contain leading underscores
on systems that prefix all identifiers with underscores. The mangled
name is computed in the same way on all platforms; if special processing
is required to deal with the object file format used on a particular
platform, it is the responsibility of the back end to perform those
modifications. (Of course, the back end should not modify
DECL_ASSEMBLER_NAME itself.)
Using DECL_ASSEMBLER_NAME will cause additional memory to be
allocated (for the mangled name of the entity) so it should be used
only when emitting assembly code. It should not be used within the
optimizers to determine whether or not two declarations are the same,
even though some of the existing optimizers do use it in that way.
These uses will be removed over time.
DECL_ARGUMENTS
This macro returns the PARM_DECL for the first argument to the
function. Subsequent PARM_DECL nodes can be obtained by
following the TREE_CHAIN links.
DECL_RESULT
This macro returns the RESULT_DECL for the function.
DECL_SAVED_TREE
This macro returns the complete body of the function.
TREE_TYPE
This macro returns the FUNCTION_TYPE or METHOD_TYPE for
the function.
DECL_INITIAL
A function that has a definition in the current translation unit will
have a non-NULL DECL_INITIAL. However, back ends should not make
use of the particular value given by DECL_INITIAL.
It should contain a tree of BLOCK nodes that mirrors the scopes
that variables are bound in the function. Each block contains a list
of decls declared in a basic block, a pointer to a chain of blocks at
the next lower scope level, then a pointer to the next block at the
same level and a backpointer to the parent BLOCK or
FUNCTION_DECL. So given a function as follows:
void foo()
{
int a;
{
int b;
}
int c;
}
you would get the following:
tree foo = FUNCTION_DECL; tree decl_a = VAR_DECL; tree decl_b = VAR_DECL; tree decl_c = VAR_DECL; tree block_a = BLOCK; tree block_b = BLOCK; tree block_c = BLOCK; BLOCK_VARS(block_a) = decl_a; BLOCK_SUBBLOCKS(block_a) = block_b; BLOCK_CHAIN(block_a) = block_c; BLOCK_SUPERCONTEXT(block_a) = foo; BLOCK_VARS(block_b) = decl_b; BLOCK_SUPERCONTEXT(block_b) = block_a; BLOCK_VARS(block_c) = decl_c; BLOCK_SUPERCONTEXT(block_c) = foo; DECL_INITIAL(foo) = block_a;
Previous: Function Basics, Up: Functions [Contents][Index]
To determine the scope of a function, you can use the
DECL_CONTEXT macro. This macro will return the class
(either a RECORD_TYPE or a UNION_TYPE) or namespace (a
NAMESPACE_DECL) of which the function is a member. For a virtual
function, this macro returns the class in which the function was
actually defined, not the base class in which the virtual declaration
occurred.
In C, the DECL_CONTEXT for a function maybe another function.
This representation indicates that the GNU nested function extension
is in use. For details on the semantics of nested functions, see the
GCC Manual. The nested function can refer to local variables in its
containing function. Such references are not explicitly marked in the
tree structure; back ends must look at the DECL_CONTEXT for the
referenced VAR_DECL. If the DECL_CONTEXT for the
referenced VAR_DECL is not the same as the function currently
being processed, and neither DECL_EXTERNAL nor
TREE_STATIC hold, then the reference is to a local variable in
a containing function, and the back end must take appropriate action.
DECL_EXTERNAL
This predicate holds if the function is undefined.
TREE_PUBLIC
This predicate holds if the function has external linkage.
TREE_STATIC
This predicate holds if the function has been defined.
TREE_THIS_VOLATILE
This predicate holds if the function does not return normally.
TREE_READONLY
This predicate holds if the function can only read its arguments.
DECL_PURE_P
This predicate holds if the function can only read its arguments, but may also read global memory.
DECL_VIRTUAL_P
This predicate holds if the function is virtual.
DECL_ARTIFICIAL
This macro holds if the function was implicitly generated by the compiler, rather than explicitly declared. In addition to implicitly generated class member functions, this macro holds for the special functions created to implement static initialization and destruction, to compute run-time type information, and so forth.
DECL_FUNCTION_SPECIFIC_TARGET
This macro returns a tree node that holds the target options that are
to be used to compile this particular function or NULL_TREE if
the function is to be compiled with the target options specified on
the command line.
DECL_FUNCTION_SPECIFIC_OPTIMIZATION
This macro returns a tree node that holds the optimization options
that are to be used to compile this particular function or
NULL_TREE if the function is to be compiled with the
optimization options specified on the command line.
Next: C and C++ Trees, Previous: Functions, Up: GENERIC [Contents][Index]
Front ends may wish to keep some state associated with various GENERIC
trees while parsing. To support this, trees provide a set of flags
that may be used by the front end. They are accessed using
TREE_LANG_FLAG_n where ‘n’ is currently 0 through 6.
If necessary, a front end can use some language-dependent tree codes in its GENERIC representation, so long as it provides a hook for converting them to GIMPLE and doesn’t expect them to work with any (hypothetical) optimizers that run before the conversion to GIMPLE. The intermediate representation used while parsing C and C++ looks very little like GENERIC, but the C and C++ gimplifier hooks are perfectly happy to take it as input and spit out GIMPLE.
Next: Java Trees, Previous: Language-dependent trees, Up: GENERIC [Contents][Index]
This section documents the internal representation used by GCC to represent C and C++ source programs. When presented with a C or C++ source program, GCC parses the program, performs semantic analysis (including the generation of error messages), and then produces the internal representation described here. This representation contains a complete representation for the entire translation unit provided as input to the front end. This representation is then typically processed by a code-generator in order to produce machine code, but could also be used in the creation of source browsers, intelligent editors, automatic documentation generators, interpreters, and any other programs needing the ability to process C or C++ code.
This section explains the internal representation. In particular, it documents the internal representation for C and C++ source constructs, and the macros, functions, and variables that can be used to access these constructs. The C++ representation is largely a superset of the representation used in the C front end. There is only one construct used in C that does not appear in the C++ front end and that is the GNU “nested function” extension. Many of the macros documented here do not apply in C because the corresponding language constructs do not appear in C.
The C and C++ front ends generate a mix of GENERIC trees and ones specific to C and C++. These language-specific trees are higher-level constructs than the ones in GENERIC to make the parser’s job easier. This section describes those trees that aren’t part of GENERIC as well as aspects of GENERIC trees that are treated in a language-specific manner.
If you are developing a “back end”, be it is a code-generator or some other tool, that uses this representation, you may occasionally find that you need to ask questions not easily answered by the functions and macros available here. If that situation occurs, it is quite likely that GCC already supports the functionality you desire, but that the interface is simply not documented here. In that case, you should ask the GCC maintainers (via mail to gcc@gcc.gnu.org) about documenting the functionality you require. Similarly, if you find yourself writing functions that do not deal directly with your back end, but instead might be useful to other people using the GCC front end, you should submit your patches for inclusion in GCC.
| • Types for C++: | Fundamental and aggregate types. | |
| • Namespaces: | Namespaces. | |
| • Classes: | Classes. | |
| • Functions for C++: | Overloading and accessors for C++. | |
| • Statements for C++: | Statements specific to C and C++. | |
| • C++ Expressions: | From typeid to throw.
|
Next: Namespaces, Up: C and C++ Trees [Contents][Index]
In C++, an array type is not qualified; rather the type of the array
elements is qualified. This situation is reflected in the intermediate
representation. The macros described here will always examine the
qualification of the underlying element type when applied to an array
type. (If the element type is itself an array, then the recursion
continues until a non-array type is found, and the qualification of this
type is examined.) So, for example, CP_TYPE_CONST_P will hold of
the type const int ()[7], denoting an array of seven ints.
The following functions and macros deal with cv-qualification of types:
CP_TYPE_QUALS
This macro returns the set of type qualifiers applied to this type.
This value is TYPE_UNQUALIFIED if no qualifiers have been
applied. The TYPE_QUAL_CONST bit is set if the type is
const-qualified. The TYPE_QUAL_VOLATILE bit is set if the
type is volatile-qualified. The TYPE_QUAL_RESTRICT bit is
set if the type is restrict-qualified.
CP_TYPE_CONST_P
This macro holds if the type is const-qualified.
CP_TYPE_VOLATILE_P
This macro holds if the type is volatile-qualified.
CP_TYPE_RESTRICT_P
This macro holds if the type is restrict-qualified.
CP_TYPE_CONST_NON_VOLATILE_P
This predicate holds for a type that is const-qualified, but
not volatile-qualified; other cv-qualifiers are ignored as
well: only the const-ness is tested.
A few other macros and functions are usable with all types:
TYPE_SIZE
The number of bits required to represent the type, represented as an
INTEGER_CST. For an incomplete type, TYPE_SIZE will be
NULL_TREE.
TYPE_ALIGN
The alignment of the type, in bits, represented as an int.
TYPE_NAME
This macro returns a declaration (in the form of a TYPE_DECL) for
the type. (Note this macro does not return an
IDENTIFIER_NODE, as you might expect, given its name!) You can
look at the DECL_NAME of the TYPE_DECL to obtain the
actual name of the type. The TYPE_NAME will be NULL_TREE
for a type that is not a built-in type, the result of a typedef, or a
named class type.
CP_INTEGRAL_TYPE
This predicate holds if the type is an integral type. Notice that in C++, enumerations are not integral types.
ARITHMETIC_TYPE_P
This predicate holds if the type is an integral type (in the C++ sense) or a floating point type.
CLASS_TYPE_P
This predicate holds for a class-type.
TYPE_BUILT_IN
This predicate holds for a built-in type.
TYPE_PTRMEM_P
This predicate holds if the type is a pointer to data member.
TYPE_PTR_P
This predicate holds if the type is a pointer type, and the pointee is not a data member.
TYPE_PTRFN_P
This predicate holds for a pointer to function type.
TYPE_PTROB_P
This predicate holds for a pointer to object type. Note however that it
does not hold for the generic pointer to object type void *. You
may use TYPE_PTROBV_P to test for a pointer to object type as
well as void *.
The table below describes types specific to C and C++ as well as language-dependent info about GENERIC types.
POINTER_TYPEUsed to represent pointer types, and pointer to data member types. If
TREE_TYPE
is a pointer to data member type, then TYPE_PTRMEM_P will hold.
For a pointer to data member type of the form ‘T X::*’,
TYPE_PTRMEM_CLASS_TYPE will be the type X, while
TYPE_PTRMEM_POINTED_TO_TYPE will be the type T.
RECORD_TYPEUsed to represent struct and class types in C and C++. If
TYPE_PTRMEMFUNC_P holds, then this type is a pointer-to-member
type. In that case, the TYPE_PTRMEMFUNC_FN_TYPE is a
POINTER_TYPE pointing to a METHOD_TYPE. The
METHOD_TYPE is the type of a function pointed to by the
pointer-to-member function. If TYPE_PTRMEMFUNC_P does not hold,
this type is a class type. For more information, see Classes.
UNKNOWN_TYPEThis node is used to represent a type the knowledge of which is insufficient for a sound processing.
TYPENAME_TYPEUsed to represent a construct of the form typename T::A. The
TYPE_CONTEXT is T; the TYPE_NAME is an
IDENTIFIER_NODE for A. If the type is specified via a
template-id, then TYPENAME_TYPE_FULLNAME yields a
TEMPLATE_ID_EXPR. The TREE_TYPE is non-NULL if the
node is implicitly generated in support for the implicit typename
extension; in which case the TREE_TYPE is a type node for the
base-class.
TYPEOF_TYPEUsed to represent the __typeof__ extension. The
TYPE_FIELDS is the expression the type of which is being
represented.
Next: Classes, Previous: Types for C++, Up: C and C++ Trees [Contents][Index]
The root of the entire intermediate representation is the variable
global_namespace. This is the namespace specified with ::
in C++ source code. All other namespaces, types, variables, functions,
and so forth can be found starting with this namespace.
However, except for the fact that it is distinguished as the root of the representation, the global namespace is no different from any other namespace. Thus, in what follows, we describe namespaces generally, rather than the global namespace in particular.
A namespace is represented by a NAMESPACE_DECL node.
The following macros and functions can be used on a NAMESPACE_DECL:
DECL_NAME
This macro is used to obtain the IDENTIFIER_NODE corresponding to
the unqualified name of the name of the namespace (see Identifiers).
The name of the global namespace is ‘::’, even though in C++ the
global namespace is unnamed. However, you should use comparison with
global_namespace, rather than DECL_NAME to determine
whether or not a namespace is the global one. An unnamed namespace
will have a DECL_NAME equal to anonymous_namespace_name.
Within a single translation unit, all unnamed namespaces will have the
same name.
DECL_CONTEXT
This macro returns the enclosing namespace. The DECL_CONTEXT for
the global_namespace is NULL_TREE.
DECL_NAMESPACE_ALIAS
If this declaration is for a namespace alias, then
DECL_NAMESPACE_ALIAS is the namespace for which this one is an
alias.
Do not attempt to use cp_namespace_decls for a namespace which is
an alias. Instead, follow DECL_NAMESPACE_ALIAS links until you
reach an ordinary, non-alias, namespace, and call
cp_namespace_decls there.
DECL_NAMESPACE_STD_P
This predicate holds if the namespace is the special ::std
namespace.
cp_namespace_decls
This function will return the declarations contained in the namespace,
including types, overloaded functions, other namespaces, and so forth.
If there are no declarations, this function will return
NULL_TREE. The declarations are connected through their
TREE_CHAIN fields.
Although most entries on this list will be declarations,
TREE_LIST nodes may also appear. In this case, the
TREE_VALUE will be an OVERLOAD. The value of the
TREE_PURPOSE is unspecified; back ends should ignore this value.
As with the other kinds of declarations returned by
cp_namespace_decls, the TREE_CHAIN will point to the next
declaration in this list.
For more information on the kinds of declarations that can occur on this
list, See Declarations. Some declarations will not appear on this
list. In particular, no FIELD_DECL, LABEL_DECL, or
PARM_DECL nodes will appear here.
This function cannot be used with namespaces that have
DECL_NAMESPACE_ALIAS set.
Next: Functions for C++, Previous: Namespaces, Up: C and C++ Trees [Contents][Index]
Besides namespaces, the other high-level scoping construct in C++ is the
class. (Throughout this manual the term class is used to mean the
types referred to in the ANSI/ISO C++ Standard as classes; these include
types defined with the class, struct, and union
keywords.)
A class type is represented by either a RECORD_TYPE or a
UNION_TYPE. A class declared with the union tag is
represented by a UNION_TYPE, while classes declared with either
the struct or the class tag are represented by
RECORD_TYPEs. You can use the CLASSTYPE_DECLARED_CLASS
macro to discern whether or not a particular type is a class as
opposed to a struct. This macro will be true only for classes
declared with the class tag.
Almost all non-function members are available on the TYPE_FIELDS
list. Given one member, the next can be found by following the
TREE_CHAIN. You should not depend in any way on the order in
which fields appear on this list. All nodes on this list will be
‘DECL’ nodes. A FIELD_DECL is used to represent a non-static
data member, a VAR_DECL is used to represent a static data
member, and a TYPE_DECL is used to represent a type. Note that
the CONST_DECL for an enumeration constant will appear on this
list, if the enumeration type was declared in the class. (Of course,
the TYPE_DECL for the enumeration type will appear here as well.)
There are no entries for base classes on this list. In particular,
there is no FIELD_DECL for the “base-class portion” of an
object.
The TYPE_VFIELD is a compiler-generated field used to point to
virtual function tables. It may or may not appear on the
TYPE_FIELDS list. However, back ends should handle the
TYPE_VFIELD just like all the entries on the TYPE_FIELDS
list.
The function members are available on the TYPE_METHODS list.
Again, subsequent members are found by following the TREE_CHAIN
field. If a function is overloaded, each of the overloaded functions
appears; no OVERLOAD nodes appear on the TYPE_METHODS
list. Implicitly declared functions (including default constructors,
copy constructors, assignment operators, and destructors) will appear on
this list as well.
Every class has an associated binfo, which can be obtained with
TYPE_BINFO. Binfos are used to represent base-classes. The
binfo given by TYPE_BINFO is the degenerate case, whereby every
class is considered to be its own base-class. The base binfos for a
particular binfo are held in a vector, whose length is obtained with
BINFO_N_BASE_BINFOS. The base binfos themselves are obtained
with BINFO_BASE_BINFO and BINFO_BASE_ITERATE. To add a
new binfo, use BINFO_BASE_APPEND. The vector of base binfos can
be obtained with BINFO_BASE_BINFOS, but normally you do not need
to use that. The class type associated with a binfo is given by
BINFO_TYPE. It is not always the case that BINFO_TYPE
(TYPE_BINFO (x)), because of typedefs and qualified types. Neither is
it the case that TYPE_BINFO (BINFO_TYPE (y)) is the same binfo as
y. The reason is that if y is a binfo representing a
base-class B of a derived class D, then BINFO_TYPE
(y) will be B, and TYPE_BINFO (BINFO_TYPE (y)) will be
B as its own base-class, rather than as a base-class of D.
The access to a base type can be found with BINFO_BASE_ACCESS.
This will produce access_public_node, access_private_node
or access_protected_node. If bases are always public,
BINFO_BASE_ACCESSES may be NULL.
BINFO_VIRTUAL_P is used to specify whether the binfo is inherited
virtually or not. The other flags, BINFO_MARKED_P and
BINFO_FLAG_1 to BINFO_FLAG_6 can be used for language
specific use.
The following macros can be used on a tree node representing a class-type.
LOCAL_CLASS_P
This predicate holds if the class is local class i.e. declared inside a function body.
TYPE_POLYMORPHIC_P
This predicate holds if the class has at least one virtual function (declared or inherited).
TYPE_HAS_DEFAULT_CONSTRUCTOR
This predicate holds whenever its argument represents a class-type with default constructor.
CLASSTYPE_HAS_MUTABLE
TYPE_HAS_MUTABLE_P
These predicates hold for a class-type having a mutable data member.
CLASSTYPE_NON_POD_P
This predicate holds only for class-types that are not PODs.
TYPE_HAS_NEW_OPERATOR
This predicate holds for a class-type that defines
operator new.
TYPE_HAS_ARRAY_NEW_OPERATOR
This predicate holds for a class-type for which
operator new[] is defined.
TYPE_OVERLOADS_CALL_EXPR
This predicate holds for class-type for which the function call
operator() is overloaded.
TYPE_OVERLOADS_ARRAY_REF
This predicate holds for a class-type that overloads
operator[]
TYPE_OVERLOADS_ARROW
This predicate holds for a class-type for which operator-> is
overloaded.
Next: Statements for C++, Previous: Classes, Up: C and C++ Trees [Contents][Index]
A function is represented by a FUNCTION_DECL node. A set of
overloaded functions is sometimes represented by an OVERLOAD node.
An OVERLOAD node is not a declaration, so none of the
‘DECL_’ macros should be used on an OVERLOAD. An
OVERLOAD node is similar to a TREE_LIST. Use
OVL_CURRENT to get the function associated with an
OVERLOAD node; use OVL_NEXT to get the next
OVERLOAD node in the list of overloaded functions. The macros
OVL_CURRENT and OVL_NEXT are actually polymorphic; you can
use them to work with FUNCTION_DECL nodes as well as with
overloads. In the case of a FUNCTION_DECL, OVL_CURRENT
will always return the function itself, and OVL_NEXT will always
be NULL_TREE.
To determine the scope of a function, you can use the
DECL_CONTEXT macro. This macro will return the class
(either a RECORD_TYPE or a UNION_TYPE) or namespace (a
NAMESPACE_DECL) of which the function is a member. For a virtual
function, this macro returns the class in which the function was
actually defined, not the base class in which the virtual declaration
occurred.
If a friend function is defined in a class scope, the
DECL_FRIEND_CONTEXT macro can be used to determine the class in
which it was defined. For example, in
class C { friend void f() {} };
the DECL_CONTEXT for f will be the
global_namespace, but the DECL_FRIEND_CONTEXT will be the
RECORD_TYPE for C.
The following macros and functions can be used on a FUNCTION_DECL:
DECL_MAIN_P
This predicate holds for a function that is the program entry point
::code.
DECL_LOCAL_FUNCTION_P
This predicate holds if the function was declared at block scope, even though it has a global scope.
DECL_ANTICIPATED
This predicate holds if the function is a built-in function but its prototype is not yet explicitly declared.
DECL_EXTERN_C_FUNCTION_P
This predicate holds if the function is declared as an
‘extern "C"’ function.
DECL_LINKONCE_P
This macro holds if multiple copies of this function may be emitted in
various translation units. It is the responsibility of the linker to
merge the various copies. Template instantiations are the most common
example of functions for which DECL_LINKONCE_P holds; G++
instantiates needed templates in all translation units which require them,
and then relies on the linker to remove duplicate instantiations.
FIXME: This macro is not yet implemented.
DECL_FUNCTION_MEMBER_P
This macro holds if the function is a member of a class, rather than a member of a namespace.
DECL_STATIC_FUNCTION_P
This predicate holds if the function a static member function.
DECL_NONSTATIC_MEMBER_FUNCTION_P
This macro holds for a non-static member function.
DECL_CONST_MEMFUNC_P
This predicate holds for a const-member function.
DECL_VOLATILE_MEMFUNC_P
This predicate holds for a volatile-member function.
DECL_CONSTRUCTOR_P
This macro holds if the function is a constructor.
DECL_NONCONVERTING_P
This predicate holds if the constructor is a non-converting constructor.
DECL_COMPLETE_CONSTRUCTOR_P
This predicate holds for a function which is a constructor for an object of a complete type.
DECL_BASE_CONSTRUCTOR_P
This predicate holds for a function which is a constructor for a base class sub-object.
DECL_COPY_CONSTRUCTOR_P
This predicate holds for a function which is a copy-constructor.
DECL_DESTRUCTOR_P
This macro holds if the function is a destructor.
DECL_COMPLETE_DESTRUCTOR_P
This predicate holds if the function is the destructor for an object a complete type.
DECL_OVERLOADED_OPERATOR_P
This macro holds if the function is an overloaded operator.
DECL_CONV_FN_P
This macro holds if the function is a type-conversion operator.
DECL_GLOBAL_CTOR_P
This predicate holds if the function is a file-scope initialization function.
DECL_GLOBAL_DTOR_P
This predicate holds if the function is a file-scope finalization function.
DECL_THUNK_P
This predicate holds if the function is a thunk.
These functions represent stub code that adjusts the this pointer
and then jumps to another function. When the jumped-to function
returns, control is transferred directly to the caller, without
returning to the thunk. The first parameter to the thunk is always the
this pointer; the thunk should add THUNK_DELTA to this
value. (The THUNK_DELTA is an int, not an
INTEGER_CST.)
Then, if THUNK_VCALL_OFFSET (an INTEGER_CST) is nonzero
the adjusted this pointer must be adjusted again. The complete
calculation is given by the following pseudo-code:
this += THUNK_DELTA if (THUNK_VCALL_OFFSET) this += (*((ptrdiff_t **) this))[THUNK_VCALL_OFFSET]
Finally, the thunk should jump to the location given
by DECL_INITIAL; this will always be an expression for the
address of a function.
DECL_NON_THUNK_FUNCTION_P
This predicate holds if the function is not a thunk function.
GLOBAL_INIT_PRIORITY
If either DECL_GLOBAL_CTOR_P or DECL_GLOBAL_DTOR_P holds,
then this gives the initialization priority for the function. The
linker will arrange that all functions for which
DECL_GLOBAL_CTOR_P holds are run in increasing order of priority
before main is called. When the program exits, all functions for
which DECL_GLOBAL_DTOR_P holds are run in the reverse order.
TYPE_RAISES_EXCEPTIONS
This macro returns the list of exceptions that a (member-)function can
raise. The returned list, if non NULL, is comprised of nodes
whose TREE_VALUE represents a type.
TYPE_NOTHROW_P
This predicate holds when the exception-specification of its arguments
is of the form ‘()’.
DECL_ARRAY_DELETE_OPERATOR_P
This predicate holds if the function an overloaded
operator delete[].
Next: C++ Expressions, Previous: Functions for C++, Up: C and C++ Trees [Contents][Index]
A function that has a definition in the current translation unit will
have a non-NULL DECL_INITIAL. However, back ends should not make
use of the particular value given by DECL_INITIAL.
The DECL_SAVED_TREE macro will give the complete body of the
function.
There are tree nodes corresponding to all of the source-level statement constructs, used within the C and C++ frontends. These are enumerated here, together with a list of the various macros that can be used to obtain information about them. There are a few macros that can be used with all statements:
STMT_IS_FULL_EXPR_P
In C++, statements normally constitute “full expressions”; temporaries
created during a statement are destroyed when the statement is complete.
However, G++ sometimes represents expressions by statements; these
statements will not have STMT_IS_FULL_EXPR_P set. Temporaries
created during such statements should be destroyed when the innermost
enclosing statement with STMT_IS_FULL_EXPR_P set is exited.
Here is the list of the various statement nodes, and the macros used to access them. This documentation describes the use of these nodes in non-template functions (including instantiations of template functions). In template functions, the same nodes are used, but sometimes in slightly different ways.
Many of the statements have substatements. For example, a while
loop will have a body, which is itself a statement. If the substatement
is NULL_TREE, it is considered equivalent to a statement
consisting of a single ;, i.e., an expression statement in which
the expression has been omitted. A substatement may in fact be a list
of statements, connected via their TREE_CHAINs. So, you should
always process the statement tree by looping over substatements, like
this:
void process_stmt (stmt)
tree stmt;
{
while (stmt)
{
switch (TREE_CODE (stmt))
{
case IF_STMT:
process_stmt (THEN_CLAUSE (stmt));
/* More processing here. */
break;
…
}
stmt = TREE_CHAIN (stmt);
}
}
In other words, while the then clause of an if statement
in C++ can be only one statement (although that one statement may be a
compound statement), the intermediate representation will sometimes use
several statements chained together.
BREAK_STMTUsed to represent a break statement. There are no additional
fields.
CLEANUP_STMTUsed to represent an action that should take place upon exit from the
enclosing scope. Typically, these actions are calls to destructors for
local objects, but back ends cannot rely on this fact. If these nodes
are in fact representing such destructors, CLEANUP_DECL will be
the VAR_DECL destroyed. Otherwise, CLEANUP_DECL will be
NULL_TREE. In any case, the CLEANUP_EXPR is the
expression to execute. The cleanups executed on exit from a scope
should be run in the reverse order of the order in which the associated
CLEANUP_STMTs were encountered.
CONTINUE_STMTUsed to represent a continue statement. There are no additional
fields.
CTOR_STMTUsed to mark the beginning (if CTOR_BEGIN_P holds) or end (if
CTOR_END_P holds of the main body of a constructor. See also
SUBOBJECT for more information on how to use these nodes.
DO_STMTUsed to represent a do loop. The body of the loop is given by
DO_BODY while the termination condition for the loop is given by
DO_COND. The condition for a do-statement is always an
expression.
EMPTY_CLASS_EXPRUsed to represent a temporary object of a class with no data whose
address is never taken. (All such objects are interchangeable.) The
TREE_TYPE represents the type of the object.
EXPR_STMTUsed to represent an expression statement. Use EXPR_STMT_EXPR to
obtain the expression.
FOR_STMTUsed to represent a for statement. The FOR_INIT_STMT is
the initialization statement for the loop. The FOR_COND is the
termination condition. The FOR_EXPR is the expression executed
right before the FOR_COND on each loop iteration; often, this
expression increments a counter. The body of the loop is given by
FOR_BODY. Note that FOR_INIT_STMT and FOR_BODY
return statements, while FOR_COND and FOR_EXPR return
expressions.
HANDLERUsed to represent a C++ catch block. The HANDLER_TYPE
is the type of exception that will be caught by this handler; it is
equal (by pointer equality) to NULL if this handler is for all
types. HANDLER_PARMS is the DECL_STMT for the catch
parameter, and HANDLER_BODY is the code for the block itself.
IF_STMTUsed to represent an if statement. The IF_COND is the
expression.
If the condition is a TREE_LIST, then the TREE_PURPOSE is
a statement (usually a DECL_STMT). Each time the condition is
evaluated, the statement should be executed. Then, the
TREE_VALUE should be used as the conditional expression itself.
This representation is used to handle C++ code like this:
C++ distinguishes between this and COND_EXPR for handling templates.
if (int i = 7) …
where there is a new local variable (or variables) declared within the condition.
The THEN_CLAUSE represents the statement given by the then
condition, while the ELSE_CLAUSE represents the statement given
by the else condition.
SUBOBJECTIn a constructor, these nodes are used to mark the point at which a
subobject of this is fully constructed. If, after this point, an
exception is thrown before a CTOR_STMT with CTOR_END_P set
is encountered, the SUBOBJECT_CLEANUP must be executed. The
cleanups must be executed in the reverse order in which they appear.
SWITCH_STMTUsed to represent a switch statement. The SWITCH_STMT_COND
is the expression on which the switch is occurring. See the documentation
for an IF_STMT for more information on the representation used
for the condition. The SWITCH_STMT_BODY is the body of the switch
statement. The SWITCH_STMT_TYPE is the original type of switch
expression as given in the source, before any compiler conversions.
TRY_BLOCKUsed to represent a try block. The body of the try block is
given by TRY_STMTS. Each of the catch blocks is a HANDLER
node. The first handler is given by TRY_HANDLERS. Subsequent
handlers are obtained by following the TREE_CHAIN link from one
handler to the next. The body of the handler is given by
HANDLER_BODY.
If CLEANUP_P holds of the TRY_BLOCK, then the
TRY_HANDLERS will not be a HANDLER node. Instead, it will
be an expression that should be executed if an exception is thrown in
the try block. It must rethrow the exception after executing that code.
And, if an exception is thrown while the expression is executing,
terminate must be called.
USING_STMTUsed to represent a using directive. The namespace is given by
USING_STMT_NAMESPACE, which will be a NAMESPACE_DECL. This node
is needed inside template functions, to implement using directives
during instantiation.
WHILE_STMTUsed to represent a while loop. The WHILE_COND is the
termination condition for the loop. See the documentation for an
IF_STMT for more information on the representation used for the
condition.
The WHILE_BODY is the body of the loop.
Previous: Statements for C++, Up: C and C++ Trees [Contents][Index]
This section describes expressions specific to the C and C++ front ends.
TYPEID_EXPRUsed to represent a typeid expression.
NEW_EXPRVEC_NEW_EXPRUsed to represent a call to new and new[] respectively.
DELETE_EXPRVEC_DELETE_EXPRUsed to represent a call to delete and delete[] respectively.
MEMBER_REFRepresents a reference to a member of a class.
THROW_EXPRRepresents an instance of throw in the program. Operand 0,
which is the expression to throw, may be NULL_TREE.
AGGR_INIT_EXPRAn AGGR_INIT_EXPR represents the initialization as the return
value of a function call, or as the result of a constructor. An
AGGR_INIT_EXPR will only appear as a full-expression, or as the
second operand of a TARGET_EXPR. AGGR_INIT_EXPRs have
a representation similar to that of CALL_EXPRs. You can use
the AGGR_INIT_EXPR_FN and AGGR_INIT_EXPR_ARG macros to access
the function to call and the arguments to pass.
If AGGR_INIT_VIA_CTOR_P holds of the AGGR_INIT_EXPR, then
the initialization is via a constructor call. The address of the
AGGR_INIT_EXPR_SLOT operand, which is always a VAR_DECL,
is taken, and this value replaces the first argument in the argument
list.
In either case, the expression is void.
Previous: C and C++ Trees, Up: GENERIC [Contents][Index]
GIMPLE is a three-address representation derived from GENERIC by
breaking down GENERIC expressions into tuples of no more than 3
operands (with some exceptions like function calls). GIMPLE was
heavily influenced by the SIMPLE IL used by the McCAT compiler
project at McGill University, though we have made some different
choices. For one thing, SIMPLE doesn’t support goto.
Temporaries are introduced to hold intermediate values needed to compute complex expressions. Additionally, all the control structures used in GENERIC are lowered into conditional jumps, lexical scopes are removed and exception regions are converted into an on the side exception region tree.
The compiler pass which converts GENERIC into GIMPLE is referred to as the ‘gimplifier’. The gimplifier works recursively, generating GIMPLE tuples out of the original GENERIC expressions.
One of the early implementation strategies used for the GIMPLE representation was to use the same internal data structures used by front ends to represent parse trees. This simplified implementation because we could leverage existing functionality and interfaces. However, GIMPLE is a much more restrictive representation than abstract syntax trees (AST), therefore it does not require the full structural complexity provided by the main tree data structure.
The GENERIC representation of a function is stored in the
DECL_SAVED_TREE field of the associated FUNCTION_DECL
tree node. It is converted to GIMPLE by a call to
gimplify_function_tree.
If a front end wants to include language-specific tree codes in the tree
representation which it provides to the back end, it must provide a
definition of LANG_HOOKS_GIMPLIFY_EXPR which knows how to
convert the front end trees to GIMPLE. Usually such a hook will involve
much of the same code for expanding front end trees to RTL. This function
can return fully lowered GIMPLE, or it can return GENERIC trees and let the
main gimplifier lower them the rest of the way; this is often simpler.
GIMPLE that is not fully lowered is known as “High GIMPLE” and
consists of the IL before the pass pass_lower_cf. High GIMPLE
contains some container statements like lexical scopes
(represented by GIMPLE_BIND) and nested expressions (e.g.,
GIMPLE_TRY), while “Low GIMPLE” exposes all of the
implicit jumps for control and exception expressions directly in
the IL and EH region trees.
The C and C++ front ends currently convert directly from front end
trees to GIMPLE, and hand that off to the back end rather than first
converting to GENERIC. Their gimplifier hooks know about all the
_STMT nodes and how to convert them to GENERIC forms. There
was some work done on a genericization pass which would run first, but
the existence of STMT_EXPR meant that in order to convert all
of the C statements into GENERIC equivalents would involve walking the
entire tree anyway, so it was simpler to lower all the way. This
might change in the future if someone writes an optimization pass
which would work better with higher-level trees, but currently the
optimizers all expect GIMPLE.
You can request to dump a C-like representation of the GIMPLE form with the flag -fdump-tree-gimple.
Next: GIMPLE instruction set, Up: GIMPLE [Contents][Index]
GIMPLE instructions are tuples of variable size divided in two groups: a header describing the instruction and its locations, and a variable length body with all the operands. Tuples are organized into a hierarchy with 3 main classes of tuples.
gimple_statement_base (gsbase)This is the root of the hierarchy, it holds basic information needed by most GIMPLE statements. There are some fields that may not be relevant to every GIMPLE statement, but those were moved into the base structure to take advantage of holes left by other fields (thus making the structure more compact). The structure takes 4 words (32 bytes) on 64 bit hosts:
| Field | Size (bits) |
code | 8 |
subcode | 16 |
no_warning | 1 |
visited | 1 |
nontemporal_move | 1 |
plf | 2 |
modified | 1 |
has_volatile_ops | 1 |
references_memory_p | 1 |
uid | 32 |
location | 32 |
num_ops | 32 |
bb | 64 |
block | 63 |
| Total size | 32 bytes |
code
Main identifier for a GIMPLE instruction.
subcode
Used to distinguish different variants of the same basic
instruction or provide flags applicable to a given code. The
subcode flags field has different uses depending on the code of
the instruction, but mostly it distinguishes instructions of the
same family. The most prominent use of this field is in
assignments, where subcode indicates the operation done on the
RHS of the assignment. For example, a = b + c is encoded as
GIMPLE_ASSIGN <PLUS_EXPR, a, b, c>.
no_warning
Bitflag to indicate whether a warning has already been issued on
this statement.
visited
General purpose “visited” marker. Set and cleared by each pass
when needed.
nontemporal_move
Bitflag used in assignments that represent non-temporal moves.
Although this bitflag is only used in assignments, it was moved
into the base to take advantage of the bit holes left by the
previous fields.
plf
Pass Local Flags. This 2-bit mask can be used as general purpose
markers by any pass. Passes are responsible for clearing and
setting these two flags accordingly.
modified
Bitflag to indicate whether the statement has been modified.
Used mainly by the operand scanner to determine when to re-scan a
statement for operands.
has_volatile_ops
Bitflag to indicate whether this statement contains operands that
have been marked volatile.
references_memory_p
Bitflag to indicate whether this statement contains memory
references (i.e., its operands are either global variables, or
pointer dereferences or anything that must reside in memory).
uid
This is an unsigned integer used by passes that want to assign
IDs to every statement. These IDs must be assigned and used by
each pass.
location
This is a location_t identifier to specify source code
location for this statement. It is inherited from the front
end.
num_ops
Number of operands that this statement has. This specifies the
size of the operand vector embedded in the tuple. Only used in
some tuples, but it is declared in the base tuple to take
advantage of the 32-bit hole left by the previous fields.
bb
Basic block holding the instruction.
block
Lexical block holding this statement. Also used for debug
information generation.
gimple_statement_with_opsThis tuple is actually split in two:
gimple_statement_with_ops_base and
gimple_statement_with_ops. This is needed to accommodate the
way the operand vector is allocated. The operand vector is
defined to be an array of 1 element. So, to allocate a dynamic
number of operands, the memory allocator (gimple_alloc) simply
allocates enough memory to hold the structure itself plus N
- 1 operands which run “off the end” of the structure. For
example, to allocate space for a tuple with 3 operands,
gimple_alloc reserves sizeof (struct
gimple_statement_with_ops) + 2 * sizeof (tree) bytes.
On the other hand, several fields in this tuple need to be shared
with the gimple_statement_with_memory_ops tuple. So, these
common fields are placed in gimple_statement_with_ops_base which
is then inherited from the other two tuples.
gsbase | 256 |
def_ops | 64 |
use_ops | 64 |
op | num_ops * 64 |
| Total size | 48 + 8 * num_ops bytes |
gsbase
Inherited from struct gimple_statement_base.
def_ops
Array of pointers into the operand array indicating all the slots that
contain a variable written-to by the statement. This array is
also used for immediate use chaining. Note that it would be
possible to not rely on this array, but the changes required to
implement this are pretty invasive.
use_ops
Similar to def_ops but for variables read by the statement.
op
Array of trees with num_ops slots.
gimple_statement_with_memory_opsThis tuple is essentially identical to gimple_statement_with_ops,
except that it contains 4 additional fields to hold vectors
related memory stores and loads. Similar to the previous case,
the structure is split in two to accommodate for the operand
vector (gimple_statement_with_memory_ops_base and
gimple_statement_with_memory_ops).
| Field | Size (bits) |
gsbase | 256 |
def_ops | 64 |
use_ops | 64 |
vdef_ops | 64 |
vuse_ops | 64 |
stores | 64 |
loads | 64 |
op | num_ops * 64 |
| Total size | 80 + 8 * num_ops bytes |
vdef_ops
Similar to def_ops but for VDEF operators. There is
one entry per memory symbol written by this statement. This is
used to maintain the memory SSA use-def and def-def chains.
vuse_ops
Similar to use_ops but for VUSE operators. There is
one entry per memory symbol loaded by this statement. This is
used to maintain the memory SSA use-def chains.
stores
Bitset with all the UIDs for the symbols written-to by the
statement. This is different than vdef_ops in that all the
affected symbols are mentioned in this set. If memory
partitioning is enabled, the vdef_ops vector will refer to memory
partitions. Furthermore, no SSA information is stored in this
set.
loads
Similar to stores, but for memory loads. (Note that there
is some amount of redundancy here, it should be possible to
reduce memory utilization further by removing these sets).
All the other tuples are defined in terms of these three basic
ones. Each tuple will add some fields. The main gimple type
is defined to be the union of all these structures (GTY markers
elided for clarity):
union gimple_statement_d
{
struct gimple_statement_base gsbase;
struct gimple_statement_with_ops gsops;
struct gimple_statement_with_memory_ops gsmem;
struct gimple_statement_omp omp;
struct gimple_statement_bind gimple_bind;
struct gimple_statement_catch gimple_catch;
struct gimple_statement_eh_filter gimple_eh_filter;
struct gimple_statement_phi gimple_phi;
struct gimple_statement_resx gimple_resx;
struct gimple_statement_try gimple_try;
struct gimple_statement_wce gimple_wce;
struct gimple_statement_asm gimple_asm;
struct gimple_statement_omp_critical gimple_omp_critical;
struct gimple_statement_omp_for gimple_omp_for;
struct gimple_statement_omp_parallel gimple_omp_parallel;
struct gimple_statement_omp_task gimple_omp_task;
struct gimple_statement_omp_sections gimple_omp_sections;
struct gimple_statement_omp_single gimple_omp_single;
struct gimple_statement_omp_continue gimple_omp_continue;
struct gimple_statement_omp_atomic_load gimple_omp_atomic_load;
struct gimple_statement_omp_atomic_store gimple_omp_atomic_store;
};
Next: GIMPLE Exception Handling, Previous: Tuple representation, Up: GIMPLE [Contents][Index]
The following table briefly describes the GIMPLE instruction set.
| Instruction | High GIMPLE | Low GIMPLE |
GIMPLE_ASM | x | x |
GIMPLE_ASSIGN | x | x |
GIMPLE_BIND | x | |
GIMPLE_CALL | x | x |
GIMPLE_CATCH | x | |
GIMPLE_COND | x | x |
GIMPLE_DEBUG | x | x |
GIMPLE_EH_FILTER | x | |
GIMPLE_GOTO | x | x |
GIMPLE_LABEL | x | x |
GIMPLE_NOP | x | x |
GIMPLE_OMP_ATOMIC_LOAD | x | x |
GIMPLE_OMP_ATOMIC_STORE | x | x |
GIMPLE_OMP_CONTINUE | x | x |
GIMPLE_OMP_CRITICAL | x | x |
GIMPLE_OMP_FOR | x | x |
GIMPLE_OMP_MASTER | x | x |
GIMPLE_OMP_ORDERED | x | x |
GIMPLE_OMP_PARALLEL | x | x |
GIMPLE_OMP_RETURN | x | x |
GIMPLE_OMP_SECTION | x | x |
GIMPLE_OMP_SECTIONS | x | x |
GIMPLE_OMP_SECTIONS_SWITCH | x | x |
GIMPLE_OMP_SINGLE | x | x |
GIMPLE_PHI | x | |
GIMPLE_RESX | x | |
GIMPLE_RETURN | x | x |
GIMPLE_SWITCH | x | x |
GIMPLE_TRY | x |
Next: Temporaries, Previous: GIMPLE instruction set, Up: GIMPLE [Contents][Index]
Other exception handling constructs are represented using
GIMPLE_TRY_CATCH. GIMPLE_TRY_CATCH has two operands. The
first operand is a sequence of statements to execute. If executing
these statements does not throw an exception, then the second operand
is ignored. Otherwise, if an exception is thrown, then the second
operand of the GIMPLE_TRY_CATCH is checked. The second
operand may have the following forms:
GIMPLE_CATCH statements. Each
GIMPLE_CATCH has a list of applicable exception types and
handler code. If the thrown exception matches one of the caught
types, the associated handler code is executed. If the handler
code falls off the bottom, execution continues after the original
GIMPLE_TRY_CATCH.
GIMPLE_EH_FILTER statement. This has a list of
permitted exception types, and code to handle a match failure. If the
thrown exception does not match one of the allowed types, the
associated match failure code is executed. If the thrown exception
does match, it continues unwinding the stack looking for the next
handler.
Currently throwing an exception is not directly represented in GIMPLE, since it is implemented by calling a function. At some point in the future we will want to add some way to express that the call will throw an exception of a known type.
Just before running the optimizers, the compiler lowers the
high-level EH constructs above into a set of ‘goto’s, magic
labels, and EH regions. Continuing to unwind at the end of a
cleanup is represented with a GIMPLE_RESX.
Next: Operands, Previous: GIMPLE Exception Handling, Up: GIMPLE [Contents][Index]
When gimplification encounters a subexpression that is too
complex, it creates a new temporary variable to hold the value of
the subexpression, and adds a new statement to initialize it
before the current statement. These special temporaries are known
as ‘expression temporaries’, and are allocated using
get_formal_tmp_var. The compiler tries to always evaluate
identical expressions into the same temporary, to simplify
elimination of redundant calculations.
We can only use expression temporaries when we know that it will
not be reevaluated before its value is used, and that it will not
be otherwise modified3. Other temporaries can be allocated
using get_initialized_tmp_var or create_tmp_var.
Currently, an expression like a = b + 5 is not reduced any
further. We tried converting it to something like
T1 = b + 5; a = T1;
but this bloated the representation for minimal benefit. However, a variable which must live in memory cannot appear in an expression; its value is explicitly loaded into a temporary first. Similarly, storing the value of an expression to a memory variable goes through a temporary.
Next: Manipulating GIMPLE statements, Previous: Temporaries, Up: GIMPLE [Contents][Index]
In general, expressions in GIMPLE consist of an operation and the
appropriate number of simple operands; these operands must either be a
GIMPLE rvalue (is_gimple_val), i.e. a constant or a register
variable. More complex operands are factored out into temporaries, so
that
a = b + c + d
becomes
T1 = b + c; a = T1 + d;
The same rule holds for arguments to a GIMPLE_CALL.
The target of an assignment is usually a variable, but can also be a
MEM_REF or a compound lvalue as described below.
| • Compound Expressions: | ||
| • Compound Lvalues: | ||
| • Conditional Expressions: | ||
| • Logical Operators: |
Next: Compound Lvalues, Up: Operands [Contents][Index]
The left-hand side of a C comma expression is simply moved into a separate statement.
Next: Conditional Expressions, Previous: Compound Expressions, Up: Operands [Contents][Index]
Currently compound lvalues involving array and structure field references
are not broken down; an expression like a.b[2] = 42 is not reduced
any further (though complex array subscripts are). This restriction is a
workaround for limitations in later optimizers; if we were to convert this
to
T1 = &a.b; T1[2] = 42;
alias analysis would not remember that the reference to T1[2] came
by way of a.b, so it would think that the assignment could alias
another member of a; this broke struct-alias-1.c. Future
optimizer improvements may make this limitation unnecessary.
Next: Logical Operators, Previous: Compound Lvalues, Up: Operands [Contents][Index]
A C ?: expression is converted into an if statement with
each branch assigning to the same temporary. So,
a = b ? c : d;
becomes
if (b == 1) T1 = c; else T1 = d; a = T1;
The GIMPLE level if-conversion pass re-introduces ?:
expression, if appropriate. It is used to vectorize loops with
conditions using vector conditional operations.
Note that in GIMPLE, if statements are represented using
GIMPLE_COND, as described below.
Previous: Conditional Expressions, Up: Operands [Contents][Index]
Except when they appear in the condition operand of a
GIMPLE_COND, logical ‘and’ and ‘or’ operators are simplified
as follows: a = b && c becomes
T1 = (bool)b; if (T1 == true) T1 = (bool)c; a = T1;
Note that T1 in this example cannot be an expression temporary,
because it has two different assignments.
All gimple operands are of type tree. But only certain
types of trees are allowed to be used as operand tuples. Basic
validation is controlled by the function
get_gimple_rhs_class, which given a tree code, returns an
enum with the following values of type enum
gimple_rhs_class
GIMPLE_INVALID_RHS
The tree cannot be used as a GIMPLE operand.
GIMPLE_TERNARY_RHS
The tree is a valid GIMPLE ternary operation.
GIMPLE_BINARY_RHS
The tree is a valid GIMPLE binary operation.
GIMPLE_UNARY_RHS
The tree is a valid GIMPLE unary operation.
GIMPLE_SINGLE_RHS
The tree is a single object, that cannot be split into simpler
operands (for instance, SSA_NAME, VAR_DECL, COMPONENT_REF, etc).
This operand class also acts as an escape hatch for tree nodes
that may be flattened out into the operand vector, but would need
more than two slots on the RHS. For instance, a COND_EXPR
expression of the form (a op b) ? x : y could be flattened
out on the operand vector using 4 slots, but it would also
require additional processing to distinguish c = a op b
from c = a op b ? x : y. Something similar occurs with
ASSERT_EXPR. In time, these special case tree
expressions should be flattened into the operand vector.
For tree nodes in the categories GIMPLE_TERNARY_RHS,
GIMPLE_BINARY_RHS and GIMPLE_UNARY_RHS, they cannot be
stored inside tuples directly. They first need to be flattened and
separated into individual components. For instance, given the GENERIC
expression
a = b + c
its tree representation is:
MODIFY_EXPR <VAR_DECL <a>, PLUS_EXPR <VAR_DECL <b>, VAR_DECL <c>>>
In this case, the GIMPLE form for this statement is logically
identical to its GENERIC form but in GIMPLE, the PLUS_EXPR
on the RHS of the assignment is not represented as a tree,
instead the two operands are taken out of the PLUS_EXPR sub-tree
and flattened into the GIMPLE tuple as follows:
GIMPLE_ASSIGN <PLUS_EXPR, VAR_DECL <a>, VAR_DECL <b>, VAR_DECL <c>>
The operand vector is stored at the bottom of the three tuple structures that accept operands. This means, that depending on the code of a given statement, its operand vector will be at different offsets from the base of the structure. To access tuple operands use the following accessors
Returns the number of operands in statement G.
Returns operand I from statement G.
Returns a pointer into the operand vector for statement G. This
is computed using an internal table called gimple_ops_offset_[].
This table is indexed by the gimple code of G.
When the compiler is built, this table is filled-in using the
sizes of the structures used by each statement code defined in
gimple.def. Since the operand vector is at the bottom of the
structure, for a gimple code C the offset is computed as sizeof
(struct-of C) - sizeof (tree).
This mechanism adds one memory indirection to every access when
using gimple_op(), if this becomes a bottleneck, a pass can
choose to memoize the result from gimple_ops() and use that to
access the operands.
When adding a new operand to a gimple statement, the operand will
be validated according to what each tuple accepts in its operand
vector. These predicates are called by the
gimple_name_set_...(). Each tuple will use one of the
following predicates (Note, this list is not exhaustive):
Returns true if t is a "GIMPLE value", which are all the
non-addressable stack variables (variables for which
is_gimple_reg returns true) and constants (expressions for which
is_gimple_min_invariant returns true).
Returns true if t is a symbol or memory reference whose address can be taken.
Similar to is_gimple_val but it also accepts hard registers.
Return true if t is a valid expression to use as the function
called by a GIMPLE_CALL.
Return true if t is a valid expression to use as first operand
of a MEM_REF expression.
Return true if t is a valid gimple constant.
Return true if t is a valid minimal invariant. This is different from constants, in that the specific value of t may not be known at compile time, but it is known that it doesn’t change (e.g., the address of a function local variable).
Return true if t is an interprocedural invariant. This means that t is a valid invariant in all functions (e.g. it can be an address of a global variable but not of a local one).
Return true if t is an ADDR_EXPR that does not change once the
program is running (and which is valid in all functions).
Return true if the code of g is GIMPLE_ASSIGN.
Return true if the code of g is GIMPLE_CALL.
Return true if the code of g is GIMPLE_DEBUG.
Return true if g is a GIMPLE_ASSIGN that performs a type cast
operation.
Return true if g is a GIMPLE_DEBUG that binds the value of an
expression to a variable.
Next: Tuple specific accessors, Previous: Operands, Up: GIMPLE [Contents][Index]
This section documents all the functions available to handle each of the GIMPLE instructions.
The following are common accessors for gimple statements.
Return the code for statement G.
Return the basic block to which statement G belongs to.
Return the lexical scope block holding statement G.
Return the type of the main expression computed by STMT. Return
void_type_node if STMT computes nothing. This will only return
something meaningful for GIMPLE_ASSIGN, GIMPLE_COND and
GIMPLE_CALL. For all other tuple codes, it will return
void_type_node.
Return the tree code for the expression computed by STMT. This
is only meaningful for GIMPLE_CALL, GIMPLE_ASSIGN and
GIMPLE_COND. If STMT is GIMPLE_CALL, it will return CALL_EXPR.
For GIMPLE_COND, it returns the code of the comparison predicate.
For GIMPLE_ASSIGN it returns the code of the operation performed
by the RHS of the assignment.
Set the lexical scope block of G to BLOCK.
Return locus information for statement G.
Set locus information for statement G.
Return true if G does not have locus information.
Return true if no warnings should be emitted for statement STMT.
Set the visited status on statement STMT to VISITED_P.
Return the visited status on statement STMT.
Set pass local flag PLF on statement STMT to VAL_P.
Return the value of pass local flag PLF on statement STMT.
Return true if statement G has register or memory operands.
Return true if statement G has memory operands.
Return the number of operands for statement G.
Return the array of operands for statement G.
Return operand I for statement G.
Return a pointer to operand I for statement G.
Set operand I of statement G to OP.
Return the set of symbols that have had their address taken by
STMT.
Return the set of DEF operands for statement G.
Set DEF to be the set of DEF operands for statement G.
Return the set of USE operands for statement G.
Set USE to be the set of USE operands for statement G.
Return the set of VUSE operands for statement G.
Set OPS to be the set of VUSE operands for statement G.
Return the set of VDEF operands for statement G.
Set OPS to be the set of VDEF operands for statement G.
Return the set of symbols loaded by statement G. Each element of
the set is the DECL_UID of the corresponding symbol.
Return the set of symbols stored by statement G. Each element of
the set is the DECL_UID of the corresponding symbol.
Return true if statement G has operands and the modified field
has been set.
Return true if statement STMT contains volatile operands.
Return true if statement STMT contains volatile operands.
Mark statement S as modified, and update it.
Update statement S if it has been marked modified.
Return a deep copy of statement STMT.
Next: GIMPLE sequences, Previous: Manipulating GIMPLE statements, Up: GIMPLE [Contents][Index]
• GIMPLE_ASM: | ||
• GIMPLE_ASSIGN: | ||
• GIMPLE_BIND: | ||
• GIMPLE_CALL: | ||
• GIMPLE_CATCH: | ||
• GIMPLE_COND: | ||
• GIMPLE_DEBUG: | ||
• GIMPLE_EH_FILTER: | ||
• GIMPLE_LABEL: | ||
• GIMPLE_NOP: | ||
• GIMPLE_OMP_ATOMIC_LOAD: | ||
• GIMPLE_OMP_ATOMIC_STORE: | ||
• GIMPLE_OMP_CONTINUE: | ||
• GIMPLE_OMP_CRITICAL: | ||
• GIMPLE_OMP_FOR: | ||
• GIMPLE_OMP_MASTER: | ||
• GIMPLE_OMP_ORDERED: | ||
• GIMPLE_OMP_PARALLEL: | ||
• GIMPLE_OMP_RETURN: | ||
• GIMPLE_OMP_SECTION: | ||
• GIMPLE_OMP_SECTIONS: | ||
• GIMPLE_OMP_SINGLE: | ||
• GIMPLE_PHI: | ||
• GIMPLE_RESX: | ||
• GIMPLE_RETURN: | ||
• GIMPLE_SWITCH: | ||
• GIMPLE_TRY: | ||
• GIMPLE_WITH_CLEANUP_EXPR: |
Next: GIMPLE_ASSIGN, Up: Tuple specific accessors [Contents][Index]
GIMPLE_ASMBuild a GIMPLE_ASM statement. This statement is used for
building in-line assembly constructs. STRING is the assembly
code. NINPUT is the number of register inputs. NOUTPUT is the
number of register outputs. NCLOBBERS is the number of clobbered
registers. The rest of the arguments trees for each input,
output, and clobbered registers.
Identical to gimple_build_asm, but the arguments are passed in VECs.
Return the number of input operands for GIMPLE_ASM G.
Return the number of output operands for GIMPLE_ASM G.
Return the number of clobber operands for GIMPLE_ASM G.
Return input operand INDEX of GIMPLE_ASM G.
Set IN_OP to be input operand INDEX in GIMPLE_ASM G.
Return output operand INDEX of GIMPLE_ASM G.
Set OUT_OP to be output operand INDEX in GIMPLE_ASM G.
Return clobber operand INDEX of GIMPLE_ASM G.
Set CLOBBER_OP to be clobber operand INDEX in GIMPLE_ASM G.
Return the string representing the assembly instruction in
GIMPLE_ASM G.
Return true if G is an asm statement marked volatile.
Mark asm statement G as volatile.
Remove volatile marker from asm statement G.
Next: GIMPLE_BIND, Previous: GIMPLE_ASM, Up: Tuple specific accessors [Contents][Index]
GIMPLE_ASSIGNBuild a GIMPLE_ASSIGN statement. The left-hand side is an lvalue
passed in lhs. The right-hand side can be either a unary or
binary tree expression. The expression tree rhs will be
flattened and its operands assigned to the corresponding operand
slots in the new statement. This function is useful when you
already have a tree expression that you want to convert into a
tuple. However, try to avoid building expression trees for the
sole purpose of calling this function. If you already have the
operands in separate trees, it is better to use
gimple_build_assign_with_ops.
Build a new GIMPLE_ASSIGN tuple and append it to the end of
*SEQ_P.
DST/SRC are the destination and source respectively. You can
pass ungimplified trees in DST or SRC, in which
case they will be converted to a gimple operand if necessary.
This function returns the newly created GIMPLE_ASSIGN tuple.
This function is similar to gimple_build_assign, but is used to
build a GIMPLE_ASSIGN statement when the operands of the
right-hand side of the assignment are already split into
different operands.
The left-hand side is an lvalue passed in lhs. Subcode is the
tree_code for the right-hand side of the assignment. Op1 and op2
are the operands. If op2 is null, subcode must be a tree_code
for a unary expression.
Return the code of the expression computed on the RHS of
assignment statement G.
Return the gimple rhs class of the code for the expression
computed on the rhs of assignment statement G. This will never
return GIMPLE_INVALID_RHS.
Return the LHS of assignment statement G.
Return a pointer to the LHS of assignment statement G.
Return the first operand on the RHS of assignment statement G.
Return the address of the first operand on the RHS of assignment
statement G.
Return the second operand on the RHS of assignment statement G.
Return the address of the second operand on the RHS of assignment
statement G.
Return the third operand on the RHS of assignment statement G.
Return the address of the third operand on the RHS of assignment
statement G.
Set LHS to be the LHS operand of assignment statement G.
Set RHS to be the first operand on the RHS of assignment
statement G.
Set RHS to be the second operand on the RHS of assignment
statement G.
Set RHS to be the third operand on the RHS of assignment
statement G.
Return true if S is a type-cast assignment.
Next: GIMPLE_CALL, Previous: GIMPLE_ASSIGN, Up: Tuple specific accessors [Contents][Index]
GIMPLE_BINDBuild a GIMPLE_BIND statement with a list of variables in VARS
and a body of statements in sequence BODY.
Return the variables declared in the GIMPLE_BIND statement G.
Set VARS to be the set of variables declared in the GIMPLE_BIND
statement G.
Append VARS to the set of variables declared in the GIMPLE_BIND
statement G.
Return the GIMPLE sequence contained in the GIMPLE_BIND statement
G.
Set SEQ to be sequence contained in the GIMPLE_BIND statement G.
Append a statement to the end of a GIMPLE_BIND’s body.
Append a sequence of statements to the end of a GIMPLE_BIND’s
body.
Return the TREE_BLOCK node associated with GIMPLE_BIND statement
G. This is analogous to the BIND_EXPR_BLOCK field in trees.
Set BLOCK to be the TREE_BLOCK node associated with GIMPLE_BIND
statement G.
Next: GIMPLE_CATCH, Previous: GIMPLE_BIND, Up: Tuple specific accessors [Contents][Index]
GIMPLE_CALLBuild a GIMPLE_CALL statement to function FN. The argument FN
must be either a FUNCTION_DECL or a gimple call address as
determined by is_gimple_call_addr. NARGS are the number of
arguments. The rest of the arguments follow the argument NARGS,
and must be trees that are valid as rvalues in gimple (i.e., each
operand is validated with is_gimple_operand).
Build a GIMPLE_CALL from a CALL_EXPR node. The arguments and the
function are taken from the expression directly. This routine
assumes that call_expr is already in GIMPLE form. That is, its
operands are GIMPLE values and the function call needs no further
simplification. All the call flags in call_expr are copied over
to the new GIMPLE_CALL.
VEC(tree, heap) *args)Identical to gimple_build_call but the arguments are stored in a
VEC().
Return the LHS of call statement G.
Return a pointer to the LHS of call statement G.
Set LHS to be the LHS operand of call statement G.
Return the tree node representing the function called by call
statement G.
Set FN to be the function called by call statement G. This has
to be a gimple value specifying the address of the called
function.
If a given GIMPLE_CALL’s callee is a FUNCTION_DECL, return it.
Otherwise return NULL. This function is analogous to
get_callee_fndecl in GENERIC.
Set the called function to FNDECL.
Return the type returned by call statement G.
Return the static chain for call statement G.
Set CHAIN to be the static chain for call statement G.
Return the number of arguments used by call statement G.
Return the argument at position INDEX for call statement G. The
first argument is 0.
Return a pointer to the argument at position INDEX for call
statement G.
Set ARG to be the argument at position INDEX for call statement
G.
Mark call statement S as being a tail call (i.e., a call just
before the exit of a function). These calls are candidate for
tail call optimization.
Return true if GIMPLE_CALL S is marked as a tail call.
Mark GIMPLE_CALL S as being uninlinable.
Return true if GIMPLE_CALL S cannot be inlined.
Return true if S is a noreturn call.
Build a GIMPLE_CALL identical to STMT but skipping the arguments
in the positions marked by the set ARGS_TO_SKIP.
Next: GIMPLE_COND, Previous: GIMPLE_CALL, Up: Tuple specific accessors [Contents][Index]
GIMPLE_CATCHBuild a GIMPLE_CATCH statement. TYPES are the tree types this
catch handles. HANDLER is a sequence of statements with the code
for the handler.
Return the types handled by GIMPLE_CATCH statement G.
Return a pointer to the types handled by GIMPLE_CATCH statement
G.
Return the GIMPLE sequence representing the body of the handler
of GIMPLE_CATCH statement G.
Set T to be the set of types handled by GIMPLE_CATCH G.
Set HANDLER to be the body of GIMPLE_CATCH G.
Next: GIMPLE_DEBUG, Previous: GIMPLE_CATCH, Up: Tuple specific accessors [Contents][Index]
GIMPLE_CONDBuild a GIMPLE_COND statement. A GIMPLE_COND statement compares
LHS and RHS and if the condition in PRED_CODE is true, jump to
the label in t_label, otherwise jump to the label in f_label.
PRED_CODE are relational operator tree codes like EQ_EXPR,
LT_EXPR, LE_EXPR, NE_EXPR, etc.
Build a GIMPLE_COND statement from the conditional expression
tree COND. T_LABEL and F_LABEL are as in gimple_build_cond.
Return the code of the predicate computed by conditional
statement G.
Set CODE to be the predicate code for the conditional statement
G.
Return the LHS of the predicate computed by conditional statement
G.
Set LHS to be the LHS operand of the predicate computed by
conditional statement G.
Return the RHS operand of the predicate computed by conditional
G.
Set RHS to be the RHS operand of the predicate computed by
conditional statement G.
Return the label used by conditional statement G when its
predicate evaluates to true.
Set LABEL to be the label used by conditional statement G when
its predicate evaluates to true.
Set LABEL to be the label used by conditional statement G when
its predicate evaluates to false.
Return the label used by conditional statement G when its
predicate evaluates to false.
Set the conditional COND_STMT to be of the form ’if (1 == 0)’.
Set the conditional COND_STMT to be of the form ’if (1 == 1)’.
Next: GIMPLE_EH_FILTER, Previous: GIMPLE_COND, Up: Tuple specific accessors [Contents][Index]
GIMPLE_DEBUGBuild a GIMPLE_DEBUG statement with GIMPLE_DEBUG_BIND of
subcode. The effect of this statement is to tell debug
information generation machinery that the value of user variable
var is given by value at that point, and to remain with
that value until var runs out of scope, a
dynamically-subsequent debug bind statement overrides the binding, or
conflicting values reach a control flow merge point. Even if
components of the value expression change afterwards, the
variable is supposed to retain the same value, though not necessarily
the same location.
It is expected that var be most often a tree for automatic user
variables (VAR_DECL or PARM_DECL) that satisfy the
requirements for gimple registers, but it may also be a tree for a
scalarized component of a user variable (ARRAY_REF,
COMPONENT_REF), or a debug temporary (DEBUG_EXPR_DECL).
As for value, it can be an arbitrary tree expression, but it is
recommended that it be in a suitable form for a gimple assignment
RHS. It is not expected that user variables that could appear
as var ever appear in value, because in the latter we’d
have their SSA_NAMEs instead, but even if they were not in SSA
form, user variables appearing in value are to be regarded as
part of the executable code space, whereas those in var are to
be regarded as part of the source code space. There is no way to
refer to the value bound to a user variable within a value
expression.
If value is GIMPLE_DEBUG_BIND_NOVALUE, debug information
generation machinery is informed that the variable var is
unbound, i.e., that its value is indeterminate, which sometimes means
it is really unavailable, and other times that the compiler could not
keep track of it.
Block and location information for the newly-created stmt are
taken from stmt, if given.
Return the user variable var that is bound at stmt.
Return the value expression that is bound to a user variable at
stmt.
Return a pointer to the value expression that is bound to a user
variable at stmt.
Modify the user variable bound at stmt to var.
Modify the value bound to the user variable bound at stmt to
value.
Modify the value bound to the user variable bound at stmt so
that the variable becomes unbound.
Return TRUE if stmt binds a user variable to a value,
and FALSE if it unbinds the variable.
Next: GIMPLE_LABEL, Previous: GIMPLE_DEBUG, Up: Tuple specific accessors [Contents][Index]
GIMPLE_EH_FILTERBuild a GIMPLE_EH_FILTER statement. TYPES are the filter’s
types. FAILURE is a sequence with the filter’s failure action.
Return the types handled by GIMPLE_EH_FILTER statement G.
Return a pointer to the types handled by GIMPLE_EH_FILTER
statement G.
Return the sequence of statement to execute when GIMPLE_EH_FILTER
statement fails.
Set TYPES to be the set of types handled by GIMPLE_EH_FILTER G.
Set FAILURE to be the sequence of statements to execute on
failure for GIMPLE_EH_FILTER G.
Return the EH_FILTER_MUST_NOT_THROW flag.
Set the EH_FILTER_MUST_NOT_THROW flag.
Next: GIMPLE_NOP, Previous: GIMPLE_EH_FILTER, Up: Tuple specific accessors [Contents][Index]
GIMPLE_LABELBuild a GIMPLE_LABEL statement with corresponding to the tree
label, LABEL.
Return the LABEL_DECL node used by GIMPLE_LABEL statement G.
Set LABEL to be the LABEL_DECL node used by GIMPLE_LABEL
statement G.
Build a GIMPLE_GOTO statement to label DEST.
Return the destination of the unconditional jump G.
Set DEST to be the destination of the unconditional jump G.
Next: GIMPLE_OMP_ATOMIC_LOAD, Previous: GIMPLE_LABEL, Up: Tuple specific accessors [Contents][Index]
GIMPLE_NOPBuild a GIMPLE_NOP statement.
Returns TRUE if statement G is a GIMPLE_NOP.
Next: GIMPLE_OMP_ATOMIC_STORE, Previous: GIMPLE_NOP, Up: Tuple specific accessors [Contents][Index]
GIMPLE_OMP_ATOMIC_LOADBuild a GIMPLE_OMP_ATOMIC_LOAD statement. LHS is the left-hand
side of the assignment. RHS is the right-hand side of the
assignment.
Set the LHS of an atomic load.
Get the LHS of an atomic load.
Set the RHS of an atomic set.
Get the RHS of an atomic set.
Next: GIMPLE_OMP_CONTINUE, Previous: GIMPLE_OMP_ATOMIC_LOAD, Up: Tuple specific accessors [Contents][Index]
GIMPLE_OMP_ATOMIC_STOREBuild a GIMPLE_OMP_ATOMIC_STORE statement. VAL is the value to be
stored.
Set the value being stored in an atomic store.
Return the value being stored in an atomic store.
Next: GIMPLE_OMP_CRITICAL, Previous: GIMPLE_OMP_ATOMIC_STORE, Up: Tuple specific accessors [Contents][Index]
GIMPLE_OMP_CONTINUEBuild a GIMPLE_OMP_CONTINUE statement. CONTROL_DEF is the
definition of the control variable. CONTROL_USE is the use of
the control variable.
Return the definition of the control variable on a
GIMPLE_OMP_CONTINUE in S.
Same as above, but return the pointer.
Set the control variable definition for a GIMPLE_OMP_CONTINUE
statement in S.
Return the use of the control variable on a GIMPLE_OMP_CONTINUE
in S.
Same as above, but return the pointer.
Set the control variable use for a GIMPLE_OMP_CONTINUE statement
in S.
Next: GIMPLE_OMP_FOR, Previous: GIMPLE_OMP_CONTINUE, Up: Tuple specific accessors [Contents][Index]
GIMPLE_OMP_CRITICALBuild a GIMPLE_OMP_CRITICAL statement. BODY is the sequence of
statements for which only one thread can execute. NAME is an
optional identifier for this critical block.
Return the name associated with OMP_CRITICAL statement G.
Return a pointer to the name associated with OMP critical
statement G.
Set NAME to be the name associated with OMP critical statement G.
Next: GIMPLE_OMP_MASTER, Previous: GIMPLE_OMP_CRITICAL, Up: Tuple specific accessors [Contents][Index]
GIMPLE_OMP_FORBuild a GIMPLE_OMP_FOR statement. BODY is sequence of statements
inside the for loop. CLAUSES, are any of the OMP loop
construct’s clauses: private, firstprivate, lastprivate,
reductions, ordered, schedule, and nowait. PRE_BODY is the
sequence of statements that are loop invariant. INDEX is the
index variable. INITIAL is the initial value of INDEX. FINAL is
final value of INDEX. OMP_FOR_COND is the predicate used to
compare INDEX and FINAL. INCR is the increment expression.
Return the clauses associated with OMP_FOR G.
Return a pointer to the OMP_FOR G.
Set CLAUSES to be the list of clauses associated with OMP_FOR G.
Return the index variable for OMP_FOR G.
Return a pointer to the index variable for OMP_FOR G.
Set INDEX to be the index variable for OMP_FOR G.
Return the initial value for OMP_FOR G.
Return a pointer to the initial value for OMP_FOR G.
Set INITIAL to be the initial value for OMP_FOR G.
Return the final value for OMP_FOR G.
turn a pointer to the final value for OMP_FOR G.
Set FINAL to be the final value for OMP_FOR G.
Return the increment value for OMP_FOR G.
Return a pointer to the increment value for OMP_FOR G.
Set INCR to be the increment value for OMP_FOR G.
Return the sequence of statements to execute before the OMP_FOR
statement G starts.
Set PRE_BODY to be the sequence of statements to execute before
the OMP_FOR statement G starts.
Set COND to be the condition code for OMP_FOR G.
Return the condition code associated with OMP_FOR G.
Next: GIMPLE_OMP_ORDERED, Previous: GIMPLE_OMP_FOR, Up: Tuple specific accessors [Contents][Index]
GIMPLE_OMP_MASTERBuild a GIMPLE_OMP_MASTER statement. BODY is the sequence of
statements to be executed by just the master.
Next: GIMPLE_OMP_PARALLEL, Previous: GIMPLE_OMP_MASTER, Up: Tuple specific accessors [Contents][Index]
GIMPLE_OMP_ORDEREDBuild a GIMPLE_OMP_ORDERED statement.
BODY is the sequence of statements inside a loop that will
executed in sequence.
Next: GIMPLE_OMP_RETURN, Previous: GIMPLE_OMP_ORDERED, Up: Tuple specific accessors [Contents][Index]
GIMPLE_OMP_PARALLELBuild a GIMPLE_OMP_PARALLEL statement.
BODY is sequence of statements which are executed in parallel.
CLAUSES, are the OMP parallel construct’s clauses. CHILD_FN is
the function created for the parallel threads to execute.
DATA_ARG are the shared data argument(s).
Return true if OMP parallel statement G has the
GF_OMP_PARALLEL_COMBINED flag set.
Set the GF_OMP_PARALLEL_COMBINED field in OMP parallel statement
G.
Return the body for the OMP statement G.
Set BODY to be the body for the OMP statement G.
Return the clauses associated with OMP_PARALLEL G.
Return a pointer to the clauses associated with OMP_PARALLEL G.
Set CLAUSES to be the list of clauses associated with
OMP_PARALLEL G.
Return the child function used to hold the body of OMP_PARALLEL
G.
Return a pointer to the child function used to hold the body of
OMP_PARALLEL G.
Set CHILD_FN to be the child function for OMP_PARALLEL G.
Return the artificial argument used to send variables and values
from the parent to the children threads in OMP_PARALLEL G.
Return a pointer to the data argument for OMP_PARALLEL G.
Set DATA_ARG to be the data argument for OMP_PARALLEL G.
Returns true when the gimple statement STMT is any of the OpenMP
types.
Next: GIMPLE_OMP_SECTION, Previous: GIMPLE_OMP_PARALLEL, Up: Tuple specific accessors [Contents][Index]
GIMPLE_OMP_RETURNBuild a GIMPLE_OMP_RETURN statement. WAIT_P is true if this is a
non-waiting return.
Set the nowait flag on GIMPLE_OMP_RETURN statement S.
Return true if OMP return statement G has the
GF_OMP_RETURN_NOWAIT flag set.
Next: GIMPLE_OMP_SECTIONS, Previous: GIMPLE_OMP_RETURN, Up: Tuple specific accessors [Contents][Index]
GIMPLE_OMP_SECTIONBuild a GIMPLE_OMP_SECTION statement for a sections statement.
BODY is the sequence of statements in the section.
Return true if OMP section statement G has the
GF_OMP_SECTION_LAST flag set.
Set the GF_OMP_SECTION_LAST flag on G.
Next: GIMPLE_OMP_SINGLE, Previous: GIMPLE_OMP_SECTION, Up: Tuple specific accessors [Contents][Index]
GIMPLE_OMP_SECTIONSBuild a GIMPLE_OMP_SECTIONS statement. BODY is a sequence of
section statements. CLAUSES are any of the OMP sections
construct’s clauses: private, firstprivate, lastprivate,
reduction, and nowait.
Build a GIMPLE_OMP_SECTIONS_SWITCH statement.
Return the control variable associated with the
GIMPLE_OMP_SECTIONS in G.
Return a pointer to the clauses associated with the
GIMPLE_OMP_SECTIONS in G.
Set CONTROL to be the set of clauses associated with the
GIMPLE_OMP_SECTIONS in G.
Return the clauses associated with OMP_SECTIONS G.
Return a pointer to the clauses associated with OMP_SECTIONS G.
Set CLAUSES to be the set of clauses associated with OMP_SECTIONS
G.
Next: GIMPLE_PHI, Previous: GIMPLE_OMP_SECTIONS, Up: Tuple specific accessors [Contents][Index]
GIMPLE_OMP_SINGLEBuild a GIMPLE_OMP_SINGLE statement. BODY is the sequence of
statements that will be executed once. CLAUSES are any of the
OMP single construct’s clauses: private, firstprivate,
copyprivate, nowait.
Return the clauses associated with OMP_SINGLE G.
Return a pointer to the clauses associated with OMP_SINGLE G.
Set CLAUSES to be the clauses associated with OMP_SINGLE G.
Next: GIMPLE_RESX, Previous: GIMPLE_OMP_SINGLE, Up: Tuple specific accessors [Contents][Index]
GIMPLE_PHIBuild a PHI node with len argument slots for variable var.
Return the maximum number of arguments supported by GIMPLE_PHI G.
Return the number of arguments in GIMPLE_PHI G. This must always
be exactly the number of incoming edges for the basic block
holding G.
Return the SSA name created by GIMPLE_PHI G.
Return a pointer to the SSA name created by GIMPLE_PHI G.
Set RESULT to be the SSA name created by GIMPLE_PHI G.
Return the PHI argument corresponding to incoming edge INDEX for
GIMPLE_PHI G.
Set PHIARG to be the argument corresponding to incoming edge
INDEX for GIMPLE_PHI G.
Next: GIMPLE_RETURN, Previous: GIMPLE_PHI, Up: Tuple specific accessors [Contents][Index]
GIMPLE_RESXBuild a GIMPLE_RESX statement which is a statement. This
statement is a placeholder for _Unwind_Resume before we know if a
function call or a branch is needed. REGION is the exception
region from which control is flowing.
Return the region number for GIMPLE_RESX G.
Set REGION to be the region number for GIMPLE_RESX G.
Next: GIMPLE_SWITCH, Previous: GIMPLE_RESX, Up: Tuple specific accessors [Contents][Index]
GIMPLE_RETURNBuild a GIMPLE_RETURN statement whose return value is retval.
Return the return value for GIMPLE_RETURN G.
Set RETVAL to be the return value for GIMPLE_RETURN G.
Next: GIMPLE_TRY, Previous: GIMPLE_RETURN, Up: Tuple specific accessors [Contents][Index]
GIMPLE_SWITCHBuild a GIMPLE_SWITCH statement. NLABELS are the number of
labels excluding the default label. The default label is passed
in DEFAULT_LABEL. The rest of the arguments are trees
representing the labels. Each label is a tree of code
CASE_LABEL_EXPR.
VEC(tree,heap) *args)This function is an alternate way of building GIMPLE_SWITCH
statements. INDEX and DEFAULT_LABEL are as in
gimple_build_switch. ARGS is a vector of CASE_LABEL_EXPR trees
that contain the labels.
Return the number of labels associated with the switch statement
G.
Set NLABELS to be the number of labels for the switch statement
G.
Return the index variable used by the switch statement G.
Set INDEX to be the index variable for switch statement G.
Return the label numbered INDEX. The default label is 0, followed
by any labels in a switch statement.
Set the label number INDEX to LABEL. 0 is always the default
label.
Return the default label for a switch statement.
Set the default label for a switch statement.
Next: GIMPLE_WITH_CLEANUP_EXPR, Previous: GIMPLE_SWITCH, Up: Tuple specific accessors [Contents][Index]
GIMPLE_TRYBuild a GIMPLE_TRY statement. EVAL is a sequence with the
expression to evaluate. CLEANUP is a sequence of statements to
run at clean-up time. KIND is the enumeration value
GIMPLE_TRY_CATCH if this statement denotes a try/catch construct
or GIMPLE_TRY_FINALLY if this statement denotes a try/finally
construct.
Return the kind of try block represented by GIMPLE_TRY G. This is
either GIMPLE_TRY_CATCH or GIMPLE_TRY_FINALLY.
Return the GIMPLE_TRY_CATCH_IS_CLEANUP flag.
Return the sequence of statements used as the body for GIMPLE_TRY
G.
Return the sequence of statements used as the cleanup body for
GIMPLE_TRY G.
Set the GIMPLE_TRY_CATCH_IS_CLEANUP flag.
Set EVAL to be the sequence of statements to use as the body for
GIMPLE_TRY G.
Set CLEANUP to be the sequence of statements to use as the
cleanup body for GIMPLE_TRY G.
Previous: GIMPLE_TRY, Up: Tuple specific accessors [Contents][Index]
GIMPLE_WITH_CLEANUP_EXPRBuild a GIMPLE_WITH_CLEANUP_EXPR statement. CLEANUP is the
clean-up expression.
Return the cleanup sequence for cleanup statement G.
Set CLEANUP to be the cleanup sequence for G.
Return the CLEANUP_EH_ONLY flag for a WCE tuple.
Set the CLEANUP_EH_ONLY flag for a WCE tuple.
Next: Sequence iterators, Previous: Tuple specific accessors, Up: GIMPLE [Contents][Index]
GIMPLE sequences are the tuple equivalent of STATEMENT_LIST’s
used in GENERIC. They are used to chain statements together, and
when used in conjunction with sequence iterators, provide a
framework for iterating through statements.
GIMPLE sequences are of type struct gimple_sequence, but are more
commonly passed by reference to functions dealing with sequences.
The type for a sequence pointer is gimple_seq which is the same
as struct gimple_sequence *. When declaring a local sequence,
you can define a local variable of type struct gimple_sequence.
When declaring a sequence allocated on the garbage collected
heap, use the function gimple_seq_alloc documented below.
There are convenience functions for iterating through sequences in the section entitled Sequence Iterators.
Below is a list of functions to manipulate and query sequences.
Link a gimple statement to the end of the sequence *SEQ if G is
not NULL. If *SEQ is NULL, allocate a sequence before linking.
Append sequence SRC to the end of sequence *DEST if SRC is not
NULL. If *DEST is NULL, allocate a new sequence before
appending.
Perform a deep copy of sequence SRC and return the result.
Reverse the order of the statements in the sequence SEQ. Return
SEQ.
Return the first statement in sequence S.
Return the last statement in sequence S.
Set the last statement in sequence S to the statement in LAST.
Set the first statement in sequence S to the statement in FIRST.
Initialize sequence S to an empty sequence.
Allocate a new sequence in the garbage collected store and return it.
Copy the sequence SRC into the sequence DEST.
Return true if the sequence S is empty.
Returns the sequence of statements in BB.
Sets the sequence of statements in BB to SEQ.
Determine whether SEQ contains exactly one statement.
Next: Adding a new GIMPLE statement code, Previous: GIMPLE sequences, Up: GIMPLE [Contents][Index]
Sequence iterators are convenience constructs for iterating
through statements in a sequence. Given a sequence SEQ, here is
a typical use of gimple sequence iterators:
gimple_stmt_iterator gsi;
for (gsi = gsi_start (seq); !gsi_end_p (gsi); gsi_next (&gsi))
{
gimple g = gsi_stmt (gsi);
/* Do something with gimple statement G. */
}
Backward iterations are possible:
for (gsi = gsi_last (seq); !gsi_end_p (gsi); gsi_prev (&gsi))
Forward and backward iterations on basic blocks are possible with
gsi_start_bb and gsi_last_bb.
In the documentation below we sometimes refer to enum
gsi_iterator_update. The valid options for this enumeration are:
GSI_NEW_STMT
Only valid when a single statement is added. Move the iterator to it.
GSI_SAME_STMT
Leave the iterator at the same statement.
GSI_CONTINUE_LINKING
Move iterator to whatever position is suitable for linking other
statements in the same direction.
Below is a list of the functions used to manipulate and use statement iterators.
Return a new iterator pointing to the sequence SEQ’s first
statement. If SEQ is empty, the iterator’s basic block is NULL.
Use gsi_start_bb instead when the iterator needs to always have
the correct basic block set.
Return a new iterator pointing to the first statement in basic
block BB.
Return a new iterator initially pointing to the last statement of
sequence SEQ. If SEQ is empty, the iterator’s basic block is
NULL. Use gsi_last_bb instead when the iterator needs to always
have the correct basic block set.
Return a new iterator pointing to the last statement in basic
block BB.
Return TRUE if at the end of I.
Return TRUE if we’re one statement before the end of I.
Advance the iterator to the next gimple statement.
Advance the iterator to the previous gimple statement.
Return the current stmt.
Return a block statement iterator that points to the first
non-label statement in block BB.
Return a pointer to the current stmt.
Return the basic block associated with this iterator.
Return the sequence associated with this iterator.
Remove the current stmt from the sequence. The iterator is
updated to point to the next statement. When REMOVE_EH_INFO is
true we remove the statement pointed to by iterator I from the EH
tables. Otherwise we do not modify the EH tables. Generally,
REMOVE_EH_INFO should be true when the statement is going to be
removed from the IL and not reinserted elsewhere.
Links the sequence of statements SEQ before the statement pointed
by iterator I. MODE indicates what to do with the iterator
after insertion (see enum gsi_iterator_update above).
Links statement G before the statement pointed-to by iterator I.
Updates iterator I according to MODE.
Links sequence SEQ after the statement pointed-to by iterator I.
MODE is as in gsi_insert_after.
Links statement G after the statement pointed-to by iterator I.
MODE is as in gsi_insert_after.
Move all statements in the sequence after I to a new sequence.
Return this new sequence.
Move all statements in the sequence before I to a new sequence.
Return this new sequence.
Replace the statement pointed-to by I to STMT. If UPDATE_EH_INFO
is true, the exception handling information of the original
statement is moved to the new statement.
Insert statement STMT before the statement pointed-to by iterator
I, update STMT’s basic block and scan it for new operands. MODE
specifies how to update iterator I after insertion (see enum
gsi_iterator_update).
Like gsi_insert_before, but for all the statements in SEQ.
Insert statement STMT after the statement pointed-to by iterator
I, update STMT’s basic block and scan it for new operands. MODE
specifies how to update iterator I after insertion (see enum
gsi_iterator_update).
Like gsi_insert_after, but for all the statements in SEQ.
Finds iterator for STMT.
Move the statement at FROM so it comes right after the statement
at TO.
Move the statement at FROM so it comes right before the statement
at TO.
Move the statement at FROM to the end of basic block BB.
Add STMT to the pending list of edge E. No actual insertion is
made until a call to gsi_commit_edge_inserts() is made.
Add the sequence of statements in SEQ to the pending list of edge
E. No actual insertion is made until a call to
gsi_commit_edge_inserts() is made.
Similar to gsi_insert_on_edge+gsi_commit_edge_inserts. If a new
block has to be created, it is returned.
Commit insertions pending at edge E. If a new block is created,
set NEW_BB to this block, otherwise set it to NULL.
This routine will commit all pending edge insertions, creating any new basic blocks which are necessary.
Next: Statement and operand traversals, Previous: Sequence iterators, Up: GIMPLE [Contents][Index]
The first step in adding a new GIMPLE statement code, is
modifying the file gimple.def, which contains all the GIMPLE
codes. Then you must add a corresponding structure, and an entry
in union gimple_statement_d, both of which are located in
gimple.h. This in turn, will require you to add a corresponding
GTY tag in gsstruct.def, and code to handle this tag in
gss_for_code which is located in gimple.c.
In order for the garbage collector to know the size of the
structure you created in gimple.h, you need to add a case to
handle your new GIMPLE statement in gimple_size which is located
in gimple.c.
You will probably want to create a function to build the new
gimple statement in gimple.c. The function should be called
gimple_build_new-tuple-name, and should return the new tuple
of type gimple.
If your new statement requires accessors for any members or
operands it may have, put simple inline accessors in
gimple.h and any non-trivial accessors in gimple.c with a
corresponding prototype in gimple.h.
Previous: Adding a new GIMPLE statement code, Up: GIMPLE [Contents][Index]
There are two functions available for walking statements and
sequences: walk_gimple_stmt and walk_gimple_seq,
accordingly, and a third function for walking the operands in a
statement: walk_gimple_op.
This function is used to walk the current statement in GSI,
optionally using traversal state stored in WI. If WI is NULL, no
state is kept during the traversal.
The callback CALLBACK_STMT is called. If CALLBACK_STMT returns
true, it means that the callback function has handled all the
operands of the statement and it is not necessary to walk its
operands.
If CALLBACK_STMT is NULL or it returns false, CALLBACK_OP is
called on each operand of the statement via walk_gimple_op. If
walk_gimple_op returns non-NULL for any operand, the remaining
operands are not scanned.
The return value is that returned by the last call to
walk_gimple_op, or NULL_TREE if no CALLBACK_OP is specified.
Use this function to walk the operands of statement STMT. Every
operand is walked via walk_tree with optional state information
in WI.
CALLBACK_OP is called on each operand of STMT via walk_tree.
Additional parameters to walk_tree must be stored in WI. For
each operand OP, walk_tree is called as:
walk_tree (&OP,CALLBACK_OP,WI,PSET)
If CALLBACK_OP returns non-NULL for an operand, the remaining
operands are not scanned. The return value is that returned by
the last call to walk_tree, or NULL_TREE if no CALLBACK_OP is
specified.
This function walks all the statements in the sequence SEQ
calling walk_gimple_stmt on each one. WI is as in
walk_gimple_stmt. If walk_gimple_stmt returns non-NULL, the walk
is stopped and the value returned. Otherwise, all the statements
are walked and NULL_TREE returned.
Next: Loop Analysis and Representation, Previous: GIMPLE, Up: Top [Contents][Index]
GCC uses three main intermediate languages to represent the program during compilation: GENERIC, GIMPLE and RTL. GENERIC is a language-independent representation generated by each front end. It is used to serve as an interface between the parser and optimizer. GENERIC is a common representation that is able to represent programs written in all the languages supported by GCC.
GIMPLE and RTL are used to optimize the program. GIMPLE is used for target and language independent optimizations (e.g., inlining, constant propagation, tail call elimination, redundancy elimination, etc). Much like GENERIC, GIMPLE is a language independent, tree based representation. However, it differs from GENERIC in that the GIMPLE grammar is more restrictive: expressions contain no more than 3 operands (except function calls), it has no control flow structures and expressions with side-effects are only allowed on the right hand side of assignments. See the chapter describing GENERIC and GIMPLE for more details.
This chapter describes the data structures and functions used in the GIMPLE optimizers (also known as “tree optimizers” or “middle end”). In particular, it focuses on all the macros, data structures, functions and programming constructs needed to implement optimization passes for GIMPLE.
| • Annotations: | Attributes for variables. | |
| • SSA Operands: | SSA names referenced by GIMPLE statements. | |
| • SSA: | Static Single Assignment representation. | |
| • Alias analysis: | Representing aliased loads and stores. | |
| • Memory model: | Memory model used by the middle-end. |
Next: SSA Operands, Up: Tree SSA [Contents][Index]
The optimizers need to associate attributes with variables during the
optimization process. For instance, we need to know whether a
variable has aliases. All these attributes are stored in data
structures called annotations which are then linked to the field
ann in struct tree_common.
Presently, we define annotations for variables (var_ann_t).
Annotations are defined and documented in tree-flow.h.
Next: SSA, Previous: Annotations, Up: Tree SSA [Contents][Index]
Almost every GIMPLE statement will contain a reference to a variable
or memory location. Since statements come in different shapes and
sizes, their operands are going to be located at various spots inside
the statement’s tree. To facilitate access to the statement’s
operands, they are organized into lists associated inside each
statement’s annotation. Each element in an operand list is a pointer
to a VAR_DECL, PARM_DECL or SSA_NAME tree node.
This provides a very convenient way of examining and replacing
operands.
Data flow analysis and optimization is done on all tree nodes
representing variables. Any node for which SSA_VAR_P returns
nonzero is considered when scanning statement operands. However, not
all SSA_VAR_P variables are processed in the same way. For the
purposes of optimization, we need to distinguish between references to
local scalar variables and references to globals, statics, structures,
arrays, aliased variables, etc. The reason is simple, the compiler
can gather complete data flow information for a local scalar. On the
other hand, a global variable may be modified by a function call, it
may not be possible to keep track of all the elements of an array or
the fields of a structure, etc.
The operand scanner gathers two kinds of operands: real and
virtual. An operand for which is_gimple_reg returns true
is considered real, otherwise it is a virtual operand. We also
distinguish between uses and definitions. An operand is used if its
value is loaded by the statement (e.g., the operand at the RHS of an
assignment). If the statement assigns a new value to the operand, the
operand is considered a definition (e.g., the operand at the LHS of
an assignment).
Virtual and real operands also have very different data flow properties. Real operands are unambiguous references to the full object that they represent. For instance, given
{
int a, b;
a = b
}
Since a and b are non-aliased locals, the statement
a = b will have one real definition and one real use because
variable a is completely modified with the contents of
variable b. Real definition are also known as killing
definitions. Similarly, the use of b reads all its bits.
In contrast, virtual operands are used with variables that can have a partial or ambiguous reference. This includes structures, arrays, globals, and aliased variables. In these cases, we have two types of definitions. For globals, structures, and arrays, we can determine from a statement whether a variable of these types has a killing definition. If the variable does, then the statement is marked as having a must definition of that variable. However, if a statement is only defining a part of the variable (i.e. a field in a structure), or if we know that a statement might define the variable but we cannot say for sure, then we mark that statement as having a may definition. For instance, given
{
int a, b, *p;
if (…)
p = &a;
else
p = &b;
*p = 5;
return *p;
}
The assignment *p = 5 may be a definition of a or
b. If we cannot determine statically where p is
pointing to at the time of the store operation, we create virtual
definitions to mark that statement as a potential definition site for
a and b. Memory loads are similarly marked with virtual
use operands. Virtual operands are shown in tree dumps right before
the statement that contains them. To request a tree dump with virtual
operands, use the -vops option to -fdump-tree:
{
int a, b, *p;
if (…)
p = &a;
else
p = &b;
# a = VDEF <a>
# b = VDEF <b>
*p = 5;
# VUSE <a>
# VUSE <b>
return *p;
}
Notice that VDEF operands have two copies of the referenced
variable. This indicates that this is not a killing definition of
that variable. In this case we refer to it as a may definition
or aliased store. The presence of the second copy of the
variable in the VDEF operand will become important when the
function is converted into SSA form. This will be used to link all
the non-killing definitions to prevent optimizations from making
incorrect assumptions about them.
Operands are updated as soon as the statement is finished via a call
to update_stmt. If statement elements are changed via
SET_USE or SET_DEF, then no further action is required
(i.e., those macros take care of updating the statement). If changes
are made by manipulating the statement’s tree directly, then a call
must be made to update_stmt when complete. Calling one of the
bsi_insert routines or bsi_replace performs an implicit
call to update_stmt.
Operands are collected by tree-ssa-operands.c. They are stored inside each statement’s annotation and can be accessed through either the operand iterators or an access routine.
The following access routines are available for examining operands:
SINGLE_SSA_{USE,DEF,TREE}_OPERAND: These accessors will return
NULL unless there is exactly one operand matching the specified flags. If
there is exactly one operand, the operand is returned as either a tree,
def_operand_p, or use_operand_p.
tree t = SINGLE_SSA_TREE_OPERAND (stmt, flags); use_operand_p u = SINGLE_SSA_USE_OPERAND (stmt, SSA_ALL_VIRTUAL_USES); def_operand_p d = SINGLE_SSA_DEF_OPERAND (stmt, SSA_OP_ALL_DEFS);
ZERO_SSA_OPERANDS: This macro returns true if there are no
operands matching the specified flags.
if (ZERO_SSA_OPERANDS (stmt, SSA_OP_ALL_VIRTUALS)) return;
NUM_SSA_OPERANDS: This macro Returns the number of operands
matching ’flags’. This actually executes a loop to perform the count, so
only use this if it is really needed.
int count = NUM_SSA_OPERANDS (stmt, flags)
If you wish to iterate over some or all operands, use the
FOR_EACH_SSA_{USE,DEF,TREE}_OPERAND iterator. For example, to print
all the operands for a statement:
void
print_ops (tree stmt)
{
ssa_op_iter;
tree var;
FOR_EACH_SSA_TREE_OPERAND (var, stmt, iter, SSA_OP_ALL_OPERANDS)
print_generic_expr (stderr, var, TDF_SLIM);
}
How to choose the appropriate iterator:
Need Macro: ---- ------- use_operand_p FOR_EACH_SSA_USE_OPERAND def_operand_p FOR_EACH_SSA_DEF_OPERAND tree FOR_EACH_SSA_TREE_OPERAND
#define SSA_OP_USE 0x01 /* Real USE operands. */ #define SSA_OP_DEF 0x02 /* Real DEF operands. */ #define SSA_OP_VUSE 0x04 /* VUSE operands. */ #define SSA_OP_VMAYUSE 0x08 /* USE portion of VDEFS. */ #define SSA_OP_VDEF 0x10 /* DEF portion of VDEFS. */ /* These are commonly grouped operand flags. */ #define SSA_OP_VIRTUAL_USES (SSA_OP_VUSE | SSA_OP_VMAYUSE) #define SSA_OP_VIRTUAL_DEFS (SSA_OP_VDEF) #define SSA_OP_ALL_USES (SSA_OP_VIRTUAL_USES | SSA_OP_USE) #define SSA_OP_ALL_DEFS (SSA_OP_VIRTUAL_DEFS | SSA_OP_DEF) #define SSA_OP_ALL_OPERANDS (SSA_OP_ALL_USES | SSA_OP_ALL_DEFS)
So if you want to look at the use pointers for all the USE and
VUSE operands, you would do something like:
use_operand_p use_p;
ssa_op_iter iter;
FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, (SSA_OP_USE | SSA_OP_VUSE))
{
process_use_ptr (use_p);
}
The TREE macro is basically the same as the USE and
DEF macros, only with the use or def dereferenced via
USE_FROM_PTR (use_p) and DEF_FROM_PTR (def_p). Since we
aren’t using operand pointers, use and defs flags can be mixed.
tree var;
ssa_op_iter iter;
FOR_EACH_SSA_TREE_OPERAND (var, stmt, iter, SSA_OP_VUSE)
{
print_generic_expr (stderr, var, TDF_SLIM);
}
VDEFs are broken into two flags, one for the
DEF portion (SSA_OP_VDEF) and one for the USE portion
(SSA_OP_VMAYUSE). If all you want to look at are the
VDEFs together, there is a fourth iterator macro for this,
which returns both a def_operand_p and a use_operand_p for each
VDEF in the statement. Note that you don’t need any flags for
this one.
use_operand_p use_p;
def_operand_p def_p;
ssa_op_iter iter;
FOR_EACH_SSA_MAYDEF_OPERAND (def_p, use_p, stmt, iter)
{
my_code;
}
There are many examples in the code as well, as well as the documentation in tree-ssa-operands.h.
There are also a couple of variants on the stmt iterators regarding PHI nodes.
FOR_EACH_PHI_ARG Works exactly like
FOR_EACH_SSA_USE_OPERAND, except it works over PHI arguments
instead of statement operands.
/* Look at every virtual PHI use. */
FOR_EACH_PHI_ARG (use_p, phi_stmt, iter, SSA_OP_VIRTUAL_USES)
{
my_code;
}
/* Look at every real PHI use. */
FOR_EACH_PHI_ARG (use_p, phi_stmt, iter, SSA_OP_USES)
my_code;
/* Look at every PHI use. */
FOR_EACH_PHI_ARG (use_p, phi_stmt, iter, SSA_OP_ALL_USES)
my_code;
FOR_EACH_PHI_OR_STMT_{USE,DEF} works exactly like
FOR_EACH_SSA_{USE,DEF}_OPERAND, except it will function on
either a statement or a PHI node. These should be used when it is
appropriate but they are not quite as efficient as the individual
FOR_EACH_PHI and FOR_EACH_SSA routines.
FOR_EACH_PHI_OR_STMT_USE (use_operand_p, stmt, iter, flags)
{
my_code;
}
FOR_EACH_PHI_OR_STMT_DEF (def_operand_p, phi, iter, flags)
{
my_code;
}
Immediate use information is now always available. Using the immediate use
iterators, you may examine every use of any SSA_NAME. For instance,
to change each use of ssa_var to ssa_var2 and call fold_stmt on
each stmt after that is done:
use_operand_p imm_use_p;
imm_use_iterator iterator;
tree ssa_var, stmt;
FOR_EACH_IMM_USE_STMT (stmt, iterator, ssa_var)
{
FOR_EACH_IMM_USE_ON_STMT (imm_use_p, iterator)
SET_USE (imm_use_p, ssa_var_2);
fold_stmt (stmt);
}
There are 2 iterators which can be used. FOR_EACH_IMM_USE_FAST is
used when the immediate uses are not changed, i.e., you are looking at the
uses, but not setting them.
If they do get changed, then care must be taken that things are not changed
under the iterators, so use the FOR_EACH_IMM_USE_STMT and
FOR_EACH_IMM_USE_ON_STMT iterators. They attempt to preserve the
sanity of the use list by moving all the uses for a statement into
a controlled position, and then iterating over those uses. Then the
optimization can manipulate the stmt when all the uses have been
processed. This is a little slower than the FAST version since it adds a
placeholder element and must sort through the list a bit for each statement.
This placeholder element must be also be removed if the loop is
terminated early. The macro BREAK_FROM_IMM_USE_SAFE is provided
to do this :
FOR_EACH_IMM_USE_STMT (stmt, iterator, ssa_var)
{
if (stmt == last_stmt)
BREAK_FROM_SAFE_IMM_USE (iter);
FOR_EACH_IMM_USE_ON_STMT (imm_use_p, iterator)
SET_USE (imm_use_p, ssa_var_2);
fold_stmt (stmt);
}
There are checks in verify_ssa which verify that the immediate use list
is up to date, as well as checking that an optimization didn’t break from the
loop without using this macro. It is safe to simply ’break’; from a
FOR_EACH_IMM_USE_FAST traverse.
Some useful functions and macros:
has_zero_uses (ssa_var) : Returns true if there are no uses of
ssa_var.
has_single_use (ssa_var) : Returns true if there is only a
single use of ssa_var.
single_imm_use (ssa_var, use_operand_p *ptr, tree *stmt) :
Returns true if there is only a single use of ssa_var, and also returns
the use pointer and statement it occurs in, in the second and third parameters.
num_imm_uses (ssa_var) : Returns the number of immediate uses of
ssa_var. It is better not to use this if possible since it simply
utilizes a loop to count the uses.
PHI_ARG_INDEX_FROM_USE (use_p) : Given a use within a PHI
node, return the index number for the use. An assert is triggered if the use
isn’t located in a PHI node.
USE_STMT (use_p) : Return the statement a use occurs in.
Note that uses are not put into an immediate use list until their statement is
actually inserted into the instruction stream via a bsi_* routine.
It is also still possible to utilize lazy updating of statements, but this should be used only when absolutely required. Both alias analysis and the dominator optimizations currently do this.
When lazy updating is being used, the immediate use information is out of date
and cannot be used reliably. Lazy updating is achieved by simply marking
statements modified via calls to mark_stmt_modified instead of
update_stmt. When lazy updating is no longer required, all the
modified statements must have update_stmt called in order to bring them
up to date. This must be done before the optimization is finished, or
verify_ssa will trigger an abort.
This is done with a simple loop over the instruction stream:
block_stmt_iterator bsi;
basic_block bb;
FOR_EACH_BB (bb)
{
for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
update_stmt_if_modified (bsi_stmt (bsi));
}
Next: Alias analysis, Previous: SSA Operands, Up: Tree SSA [Contents][Index]
Most of the tree optimizers rely on the data flow information provided by the Static Single Assignment (SSA) form. We implement the SSA form as described in R. Cytron, J. Ferrante, B. Rosen, M. Wegman, and K. Zadeck. Efficiently Computing Static Single Assignment Form and the Control Dependence Graph. ACM Transactions on Programming Languages and Systems, 13(4):451-490, October 1991.
The SSA form is based on the premise that program variables are assigned in exactly one location in the program. Multiple assignments to the same variable create new versions of that variable. Naturally, actual programs are seldom in SSA form initially because variables tend to be assigned multiple times. The compiler modifies the program representation so that every time a variable is assigned in the code, a new version of the variable is created. Different versions of the same variable are distinguished by subscripting the variable name with its version number. Variables used in the right-hand side of expressions are renamed so that their version number matches that of the most recent assignment.
We represent variable versions using SSA_NAME nodes. The
renaming process in tree-ssa.c wraps every real and
virtual operand with an SSA_NAME node which contains
the version number and the statement that created the
SSA_NAME. Only definitions and virtual definitions may
create new SSA_NAME nodes.
Sometimes, flow of control makes it impossible to determine the most recent version of a variable. In these cases, the compiler inserts an artificial definition for that variable called PHI function or PHI node. This new definition merges all the incoming versions of the variable to create a new name for it. For instance,
if (…) a_1 = 5; else if (…) a_2 = 2; else a_3 = 13; # a_4 = PHI <a_1, a_2, a_3> return a_4;
Since it is not possible to determine which of the three branches
will be taken at runtime, we don’t know which of a_1,
a_2 or a_3 to use at the return statement. So, the
SSA renamer creates a new version a_4 which is assigned
the result of “merging” a_1, a_2 and a_3.
Hence, PHI nodes mean “one of these operands. I don’t know
which”.
The following macros can be used to examine PHI nodes
Returns the SSA_NAME created by PHI node phi (i.e.,
phi’s LHS).
Returns the number of arguments in phi. This number is exactly the number of incoming edges to the basic block holding phi.
Returns a tuple representing the ith argument of phi.
Each element of this tuple contains an SSA_NAME var and
the incoming edge through which var flows.
Returns the incoming edge for the ith argument of phi.
Returns the SSA_NAME for the ith argument of phi.
Some optimization passes make changes to the function that invalidate the SSA property. This can happen when a pass has added new symbols or changed the program so that variables that were previously aliased aren’t anymore. Whenever something like this happens, the affected symbols must be renamed into SSA form again. Transformations that emit new code or replicate existing statements will also need to update the SSA form.
Since GCC implements two different SSA forms for register and virtual variables, keeping the SSA form up to date depends on whether you are updating register or virtual names. In both cases, the general idea behind incremental SSA updates is similar: when new SSA names are created, they typically are meant to replace other existing names in the program.
For instance, given the following code:
1 L0:
2 x_1 = PHI (0, x_5)
3 if (x_1 < 10)
4 if (x_1 > 7)
5 y_2 = 0
6 else
7 y_3 = x_1 + x_7
8 endif
9 x_5 = x_1 + 1
10 goto L0;
11 endif
Suppose that we insert new names x_10 and x_11 (lines
4 and 8).
1 L0:
2 x_1 = PHI (0, x_5)
3 if (x_1 < 10)
4 x_10 = …
5 if (x_1 > 7)
6 y_2 = 0
7 else
8 x_11 = …
9 y_3 = x_1 + x_7
10 endif
11 x_5 = x_1 + 1
12 goto L0;
13 endif
We want to replace all the uses of x_1 with the new definitions
of x_10 and x_11. Note that the only uses that should
be replaced are those at lines 5, 9 and 11.
Also, the use of x_7 at line 9 should not be
replaced (this is why we cannot just mark symbol x for
renaming).
Additionally, we may need to insert a PHI node at line 11
because that is a merge point for x_10 and x_11. So the
use of x_1 at line 11 will be replaced with the new PHI
node. The insertion of PHI nodes is optional. They are not strictly
necessary to preserve the SSA form, and depending on what the caller
inserted, they may not even be useful for the optimizers.
Updating the SSA form is a two step process. First, the pass has to
identify which names need to be updated and/or which symbols need to
be renamed into SSA form for the first time. When new names are
introduced to replace existing names in the program, the mapping
between the old and the new names are registered by calling
register_new_name_mapping (note that if your pass creates new
code by duplicating basic blocks, the call to tree_duplicate_bb
will set up the necessary mappings automatically). On the other hand,
if your pass exposes a new symbol that should be put in SSA form for
the first time, the new symbol should be registered with
mark_sym_for_renaming.
After the replacement mappings have been registered and new symbols
marked for renaming, a call to update_ssa makes the registered
changes. This can be done with an explicit call or by creating
TODO flags in the tree_opt_pass structure for your pass.
There are several TODO flags that control the behavior of
update_ssa:
TODO_update_ssa. Update the SSA form inserting PHI nodes
for newly exposed symbols and virtual names marked for updating.
When updating real names, only insert PHI nodes for a real name
O_j in blocks reached by all the new and old definitions for
O_j. If the iterated dominance frontier for O_j
is not pruned, we may end up inserting PHI nodes in blocks that
have one or more edges with no incoming definition for
O_j. This would lead to uninitialized warnings for
O_j’s symbol.
TODO_update_ssa_no_phi. Update the SSA form without
inserting any new PHI nodes at all. This is used by passes that
have either inserted all the PHI nodes themselves or passes that
need only to patch use-def and def-def chains for virtuals
(e.g., DCE).
TODO_update_ssa_full_phi. Insert PHI nodes everywhere
they are needed. No pruning of the IDF is done. This is used
by passes that need the PHI nodes for O_j even if it
means that some arguments will come from the default definition
of O_j’s symbol (e.g., pass_linear_transform).
WARNING: If you need to use this flag, chances are that your pass may be doing something wrong. Inserting PHI nodes for an old name where not all edges carry a new replacement may lead to silent codegen errors or spurious uninitialized warnings.
TODO_update_ssa_only_virtuals. Passes that update the
SSA form on their own may want to delegate the updating of
virtual names to the generic updater. Since FUD chains are
easier to maintain, this simplifies the work they need to do.
NOTE: If this flag is used, any OLD->NEW mappings for real names
are explicitly destroyed and only the symbols marked for
renaming are processed.
The virtual SSA form is harder to preserve than the non-virtual SSA form
mainly because the set of virtual operands for a statement may change at
what some would consider unexpected times. In general, statement
modifications should be bracketed between calls to
push_stmt_changes and pop_stmt_changes. For example,
munge_stmt (tree stmt)
{
push_stmt_changes (&stmt);
… rewrite STMT …
pop_stmt_changes (&stmt);
}
The call to push_stmt_changes saves the current state of the
statement operands and the call to pop_stmt_changes compares
the saved state with the current one and does the appropriate symbol
marking for the SSA renamer.
It is possible to modify several statements at a time, provided that
push_stmt_changes and pop_stmt_changes are called in
LIFO order, as when processing a stack of statements.
Additionally, if the pass discovers that it did not need to make
changes to the statement after calling push_stmt_changes, it
can simply discard the topmost change buffer by calling
discard_stmt_changes. This will avoid the expensive operand
re-scan operation and the buffer comparison that determines if symbols
need to be marked for renaming.
SSA_NAME nodesThe following macros can be used to examine SSA_NAME nodes
Returns the statement s that creates the SSA_NAME
var. If s is an empty statement (i.e., IS_EMPTY_STMT
(s) returns true), it means that the first reference to
this variable is a USE or a VUSE.
Returns the version number of the SSA_NAME object var.
Walks use-def chains starting at the SSA_NAME node var.
Calls function fn at each reaching definition found. Function
FN takes three arguments: var, its defining statement
(def_stmt) and a generic pointer to whatever state information
that fn may want to maintain (data). Function fn is
able to stop the walk by returning true, otherwise in order to
continue the walk, fn should return false.
Note, that if def_stmt is a PHI node, the semantics are
slightly different. For each argument arg of the PHI node, this
function will:
FN (arg, phi, data).
Note how the first argument to fn is no longer the original
variable var, but the PHI argument currently being examined.
If fn wants to get at var, it should call
PHI_RESULT (phi).
This function walks the dominator tree for the current CFG calling a set of callback functions defined in struct dom_walk_data in domwalk.h. The call back functions you need to define give you hooks to execute custom code at various points during traversal:
Next: Memory model, Previous: SSA, Up: Tree SSA [Contents][Index]
Alias analysis in GIMPLE SSA form consists of two pieces. First the virtual SSA web ties conflicting memory accesses and provides a SSA use-def chain and SSA immediate-use chains for walking possibly dependent memory accesses. Second an alias-oracle can be queried to disambiguate explicit and implicit memory references.
All statements that may use memory have exactly one accompanied use of a virtual SSA name that represents the state of memory at the given point in the IL.
All statements that may define memory have exactly one accompanied definition of a virtual SSA name using the previous state of memory and defining the new state of memory after the given point in the IL.
int i;
int foo (void)
{
# .MEM_3 = VDEF <.MEM_2(D)>
i = 1;
# VUSE <.MEM_3>
return i;
}
The virtual SSA names in this case are .MEM_2(D) and
.MEM_3. The store to the global variable i
defines .MEM_3 invalidating .MEM_2(D). The
load from i uses that new state .MEM_3.
The virtual SSA web serves as constraints to SSA optimizers preventing illegitimate code-motion and optimization. It also provides a way to walk related memory statements.
Points-to analysis builds a set of constraints from the GIMPLE SSA IL representing all pointer operations and facts we do or do not know about pointers. Solving this set of constraints yields a conservatively correct solution for each pointer variable in the program (though we are only interested in SSA name pointers) as to what it may possibly point to.
This points-to solution for a given SSA name pointer is stored
in the pt_solution sub-structure of the
SSA_NAME_PTR_INFO record. The following accessor
functions are available:
pt_solution_includes
pt_solutions_intersect
Points-to analysis also computes the solution for two special
set of pointers, ESCAPED and CALLUSED. Those
represent all memory that has escaped the scope of analysis
or that is used by pure or nested const calls.
Type-based alias analysis is frontend dependent though generic
support is provided by the middle-end in alias.c. TBAA
code is used by both tree optimizers and RTL optimizers.
Every language that wishes to perform language-specific alias analysis
should define a function that computes, given a tree
node, an alias set for the node. Nodes in different alias sets are not
allowed to alias. For an example, see the C front-end function
c_get_alias_set.
The tree alias-oracle provides means to disambiguate two memory references and memory references against statements. The following queries are available:
refs_may_alias_p
ref_maybe_used_by_stmt_p
stmt_may_clobber_ref_p
In addition to those two kind of statement walkers are available
walking statements related to a reference ref.
walk_non_aliased_vuses walks over dominating memory defining
statements and calls back if the statement does not clobber ref
providing the non-aliased VUSE. The walk stops at
the first clobbering statement or if asked to.
walk_aliased_vdefs walks over dominating memory defining
statements and calls back on each statement clobbering ref
providing its aliasing VDEF. The walk stops if asked to.
Previous: Alias analysis, Up: Tree SSA [Contents][Index]
The memory model used by the middle-end models that of the C/C++ languages. The middle-end has the notion of an effective type of a memory region which is used for type-based alias analysis.
The following is a refinement of ISO C99 6.5/6, clarifying the block copy case to follow common sense and extending the concept of a dynamic effective type to objects with a declared type as required for C++.
The effective type of an object for an access to its stored value is the declared type of the object or the effective type determined by a previous store to it. If a value is stored into an object through an lvalue having a type that is not a character type, then the type of the lvalue becomes the effective type of the object for that access and for subsequent accesses that do not modify the stored value. If a value is copied into an object usingmemcpyormemmove, or is copied as an array of character type, then the effective type of the modified object for that access and for subsequent accesses that do not modify the value is undetermined. For all other accesses to an object, the effective type of the object is simply the type of the lvalue used for the access.
Next: Control Flow, Previous: Tree SSA, Up: Top [Contents][Index]
GCC provides extensive infrastructure for work with natural loops, i.e., strongly connected components of CFG with only one entry block. This chapter describes representation of loops in GCC, both on GIMPLE and in RTL, as well as the interfaces to loop-related analyses (induction variable analysis and number of iterations analysis).
| • Loop representation: | Representation and analysis of loops. | |
| • Loop querying: | Getting information about loops. | |
| • Loop manipulation: | Loop manipulation functions. | |
| • LCSSA: | Loop-closed SSA form. | |
| • Scalar evolutions: | Induction variables on GIMPLE. | |
| • loop-iv: | Induction variables on RTL. | |
| • Number of iterations: | Number of iterations analysis. | |
| • Dependency analysis: | Data dependency analysis. | |
| • Lambda: | Linear loop transformations framework. | |
| • Omega: | A solver for linear programming problems. |
Next: Loop querying, Up: Loop Analysis and Representation [Contents][Index]
This chapter describes the representation of loops in GCC, and functions that can be used to build, modify and analyze this representation. Most of the interfaces and data structures are declared in cfgloop.h. At the moment, loop structures are analyzed and this information is updated only by the optimization passes that deal with loops, but some efforts are being made to make it available throughout most of the optimization passes.
In general, a natural loop has one entry block (header) and possibly
several back edges (latches) leading to the header from the inside of
the loop. Loops with several latches may appear if several loops share
a single header, or if there is a branching in the middle of the loop.
The representation of loops in GCC however allows only loops with a
single latch. During loop analysis, headers of such loops are split and
forwarder blocks are created in order to disambiguate their structures.
Heuristic based on profile information and structure of the induction
variables in the loops is used to determine whether the latches
correspond to sub-loops or to control flow in a single loop. This means
that the analysis sometimes changes the CFG, and if you run it in the
middle of an optimization pass, you must be able to deal with the new
blocks. You may avoid CFG changes by passing
LOOPS_MAY_HAVE_MULTIPLE_LATCHES flag to the loop discovery,
note however that most other loop manipulation functions will not work
correctly for loops with multiple latch edges (the functions that only
query membership of blocks to loops and subloop relationships, or
enumerate and test loop exits, can be expected to work).
Body of the loop is the set of blocks that are dominated by its header,
and reachable from its latch against the direction of edges in CFG. The
loops are organized in a containment hierarchy (tree) such that all the
loops immediately contained inside loop L are the children of L in the
tree. This tree is represented by the struct loops structure.
The root of this tree is a fake loop that contains all blocks in the
function. Each of the loops is represented in a struct loop
structure. Each loop is assigned an index (num field of the
struct loop structure), and the pointer to the loop is stored in
the corresponding field of the larray vector in the loops
structure. The indices do not have to be continuous, there may be
empty (NULL) entries in the larray created by deleting
loops. Also, there is no guarantee on the relative order of a loop
and its subloops in the numbering. The index of a loop never changes.
The entries of the larray field should not be accessed directly.
The function get_loop returns the loop description for a loop with
the given index. number_of_loops function returns number of
loops in the function. To traverse all loops, use FOR_EACH_LOOP
macro. The flags argument of the macro is used to determine
the direction of traversal and the set of loops visited. Each loop is
guaranteed to be visited exactly once, regardless of the changes to the
loop tree, and the loops may be removed during the traversal. The newly
created loops are never traversed, if they need to be visited, this
must be done separately after their creation. The FOR_EACH_LOOP
macro allocates temporary variables. If the FOR_EACH_LOOP loop
were ended using break or goto, they would not be released;
FOR_EACH_LOOP_BREAK macro must be used instead.
Each basic block contains the reference to the innermost loop it belongs
to (loop_father). For this reason, it is only possible to have
one struct loops structure initialized at the same time for each
CFG. The global variable current_loops contains the
struct loops structure. Many of the loop manipulation functions
assume that dominance information is up-to-date.
The loops are analyzed through loop_optimizer_init function. The
argument of this function is a set of flags represented in an integer
bitmask. These flags specify what other properties of the loop
structures should be calculated/enforced and preserved later:
LOOPS_MAY_HAVE_MULTIPLE_LATCHES: If this flag is set, no
changes to CFG will be performed in the loop analysis, in particular,
loops with multiple latch edges will not be disambiguated. If a loop
has multiple latches, its latch block is set to NULL. Most of
the loop manipulation functions will not work for loops in this shape.
No other flags that require CFG changes can be passed to
loop_optimizer_init.
LOOPS_HAVE_PREHEADERS: Forwarder blocks are created in such
a way that each loop has only one entry edge, and additionally, the
source block of this entry edge has only one successor. This creates a
natural place where the code can be moved out of the loop, and ensures
that the entry edge of the loop leads from its immediate super-loop.
LOOPS_HAVE_SIMPLE_LATCHES: Forwarder blocks are created to
force the latch block of each loop to have only one successor. This
ensures that the latch of the loop does not belong to any of its
sub-loops, and makes manipulation with the loops significantly easier.
Most of the loop manipulation functions assume that the loops are in
this shape. Note that with this flag, the “normal” loop without any
control flow inside and with one exit consists of two basic blocks.
LOOPS_HAVE_MARKED_IRREDUCIBLE_REGIONS: Basic blocks and
edges in the strongly connected components that are not natural loops
(have more than one entry block) are marked with
BB_IRREDUCIBLE_LOOP and EDGE_IRREDUCIBLE_LOOP flags. The
flag is not set for blocks and edges that belong to natural loops that
are in such an irreducible region (but it is set for the entry and exit
edges of such a loop, if they lead to/from this region).
LOOPS_HAVE_RECORDED_EXITS: The lists of exits are recorded
and updated for each loop. This makes some functions (e.g.,
get_loop_exit_edges) more efficient. Some functions (e.g.,
single_exit) can be used only if the lists of exits are
recorded.
These properties may also be computed/enforced later, using functions
create_preheaders, force_single_succ_latches,
mark_irreducible_loops and record_loop_exits.
The memory occupied by the loops structures should be freed with
loop_optimizer_finalize function.
The CFG manipulation functions in general do not update loop structures.
Specialized versions that additionally do so are provided for the most
common tasks. On GIMPLE, cleanup_tree_cfg_loop function can be
used to cleanup CFG while updating the loops structures if
current_loops is set.
Next: Loop manipulation, Previous: Loop representation, Up: Loop Analysis and Representation [Contents][Index]
The functions to query the information about loops are declared in
cfgloop.h. Some of the information can be taken directly from
the structures. loop_father field of each basic block contains
the innermost loop to that the block belongs. The most useful fields of
loop structure (that are kept up-to-date at all times) are:
header, latch: Header and latch basic blocks of the
loop.
num_nodes: Number of basic blocks in the loop (including
the basic blocks of the sub-loops).
depth: The depth of the loop in the loops tree, i.e., the
number of super-loops of the loop.
outer, inner, next: The super-loop, the first
sub-loop, and the sibling of the loop in the loops tree.
There are other fields in the loop structures, many of them used only by some of the passes, or not updated during CFG changes; in general, they should not be accessed directly.
The most important functions to query loop structures are:
flow_loops_dump: Dumps the information about loops to a
file.
verify_loop_structure: Checks consistency of the loop
structures.
loop_latch_edge: Returns the latch edge of a loop.
loop_preheader_edge: If loops have preheaders, returns
the preheader edge of a loop.
flow_loop_nested_p: Tests whether loop is a sub-loop of
another loop.
flow_bb_inside_loop_p: Tests whether a basic block belongs
to a loop (including its sub-loops).
find_common_loop: Finds the common super-loop of two loops.
superloop_at_depth: Returns the super-loop of a loop with
the given depth.
tree_num_loop_insns, num_loop_insns: Estimates the
number of insns in the loop, on GIMPLE and on RTL.
loop_exit_edge_p: Tests whether edge is an exit from a
loop.
mark_loop_exit_edges: Marks all exit edges of all loops
with EDGE_LOOP_EXIT flag.
get_loop_body, get_loop_body_in_dom_order,
get_loop_body_in_bfs_order: Enumerates the basic blocks in the
loop in depth-first search order in reversed CFG, ordered by dominance
relation, and breath-first search order, respectively.
single_exit: Returns the single exit edge of the loop, or
NULL if the loop has more than one exit. You can only use this
function if LOOPS_HAVE_MARKED_SINGLE_EXITS property is used.
get_loop_exit_edges: Enumerates the exit edges of a loop.
just_once_each_iteration_p: Returns true if the basic block
is executed exactly once during each iteration of a loop (that is, it
does not belong to a sub-loop, and it dominates the latch of the loop).
Next: LCSSA, Previous: Loop querying, Up: Loop Analysis and Representation [Contents][Index]
The loops tree can be manipulated using the following functions:
flow_loop_tree_node_add: Adds a node to the tree.
flow_loop_tree_node_remove: Removes a node from the tree.
add_bb_to_loop: Adds a basic block to a loop.
remove_bb_from_loops: Removes a basic block from loops.
Most low-level CFG functions update loops automatically. The following functions handle some more complicated cases of CFG manipulations:
remove_path: Removes an edge and all blocks it dominates.
split_loop_exit_edge: Splits exit edge of the loop,
ensuring that PHI node arguments remain in the loop (this ensures that
loop-closed SSA form is preserved). Only useful on GIMPLE.
Finally, there are some higher-level loop transformations implemented. While some of them are written so that they should work on non-innermost loops, they are mostly untested in that case, and at the moment, they are only reliable for the innermost loops:
create_iv: Creates a new induction variable. Only works on
GIMPLE. standard_iv_increment_position can be used to find a
suitable place for the iv increment.
duplicate_loop_to_header_edge,
tree_duplicate_loop_to_header_edge: These functions (on RTL and
on GIMPLE) duplicate the body of the loop prescribed number of times on
one of the edges entering loop header, thus performing either loop
unrolling or loop peeling. can_duplicate_loop_p
(can_unroll_loop_p on GIMPLE) must be true for the duplicated
loop.
loop_version, tree_ssa_loop_version: These function
create a copy of a loop, and a branch before them that selects one of
them depending on the prescribed condition. This is useful for
optimizations that need to verify some assumptions in runtime (one of
the copies of the loop is usually left unchanged, while the other one is
transformed in some way).
tree_unroll_loop: Unrolls the loop, including peeling the
extra iterations to make the number of iterations divisible by unroll
factor, updating the exit condition, and removing the exits that now
cannot be taken. Works only on GIMPLE.
Next: Scalar evolutions, Previous: Loop manipulation, Up: Loop Analysis and Representation [Contents][Index]
Throughout the loop optimizations on tree level, one extra condition is enforced on the SSA form: No SSA name is used outside of the loop in that it is defined. The SSA form satisfying this condition is called “loop-closed SSA form” – LCSSA. To enforce LCSSA, PHI nodes must be created at the exits of the loops for the SSA names that are used outside of them. Only the real operands (not virtual SSA names) are held in LCSSA, in order to save memory.
There are various benefits of LCSSA:
However, it also means LCSSA must be updated. This is usually
straightforward, unless you create a new value in loop and use it
outside, or unless you manipulate loop exit edges (functions are
provided to make these manipulations simple).
rewrite_into_loop_closed_ssa is used to rewrite SSA form to
LCSSA, and verify_loop_closed_ssa to check that the invariant of
LCSSA is preserved.
Next: loop-iv, Previous: LCSSA, Up: Loop Analysis and Representation [Contents][Index]
Scalar evolutions (SCEV) are used to represent results of induction
variable analysis on GIMPLE. They enable us to represent variables with
complicated behavior in a simple and consistent way (we only use it to
express values of polynomial induction variables, but it is possible to
extend it). The interfaces to SCEV analysis are declared in
tree-scalar-evolution.h. To use scalar evolutions analysis,
scev_initialize must be used. To stop using SCEV,
scev_finalize should be used. SCEV analysis caches results in
order to save time and memory. This cache however is made invalid by
most of the loop transformations, including removal of code. If such a
transformation is performed, scev_reset must be called to clean
the caches.
Given an SSA name, its behavior in loops can be analyzed using the
analyze_scalar_evolution function. The returned SCEV however
does not have to be fully analyzed and it may contain references to
other SSA names defined in the loop. To resolve these (potentially
recursive) references, instantiate_parameters or
resolve_mixers functions must be used.
instantiate_parameters is useful when you use the results of SCEV
only for some analysis, and when you work with whole nest of loops at
once. It will try replacing all SSA names by their SCEV in all loops,
including the super-loops of the current loop, thus providing a complete
information about the behavior of the variable in the loop nest.
resolve_mixers is useful if you work with only one loop at a
time, and if you possibly need to create code based on the value of the
induction variable. It will only resolve the SSA names defined in the
current loop, leaving the SSA names defined outside unchanged, even if
their evolution in the outer loops is known.
The SCEV is a normal tree expression, except for the fact that it may
contain several special tree nodes. One of them is
SCEV_NOT_KNOWN, used for SSA names whose value cannot be
expressed. The other one is POLYNOMIAL_CHREC. Polynomial chrec
has three arguments – base, step and loop (both base and step may
contain further polynomial chrecs). Type of the expression and of base
and step must be the same. A variable has evolution
POLYNOMIAL_CHREC(base, step, loop) if it is (in the specified
loop) equivalent to x_1 in the following example
while (…)
{
x_1 = phi (base, x_2);
x_2 = x_1 + step;
}
Note that this includes the language restrictions on the operations.
For example, if we compile C code and x has signed type, then the
overflow in addition would cause undefined behavior, and we may assume
that this does not happen. Hence, the value with this SCEV cannot
overflow (which restricts the number of iterations of such a loop).
In many cases, one wants to restrict the attention just to affine
induction variables. In this case, the extra expressive power of SCEV
is not useful, and may complicate the optimizations. In this case,
simple_iv function may be used to analyze a value – the result
is a loop-invariant base and step.
Next: Number of iterations, Previous: Scalar evolutions, Up: Loop Analysis and Representation [Contents][Index]
The induction variable on RTL is simple and only allows analysis of
affine induction variables, and only in one loop at once. The interface
is declared in cfgloop.h. Before analyzing induction variables
in a loop L, iv_analysis_loop_init function must be called on L.
After the analysis (possibly calling iv_analysis_loop_init for
several loops) is finished, iv_analysis_done should be called.
The following functions can be used to access the results of the
analysis:
iv_analyze: Analyzes a single register used in the given
insn. If no use of the register in this insn is found, the following
insns are scanned, so that this function can be called on the insn
returned by get_condition.
iv_analyze_result: Analyzes result of the assignment in the
given insn.
iv_analyze_expr: Analyzes a more complicated expression.
All its operands are analyzed by iv_analyze, and hence they must
be used in the specified insn or one of the following insns.
The description of the induction variable is provided in struct
rtx_iv. In order to handle subregs, the representation is a bit
complicated; if the value of the extend field is not
UNKNOWN, the value of the induction variable in the i-th
iteration is
delta + mult * extend_{extend_mode} (subreg_{mode} (base + i * step)),
with the following exception: if first_special is true, then the
value in the first iteration (when i is zero) is delta +
mult * base. However, if extend is equal to UNKNOWN,
then first_special must be false, delta 0, mult 1
and the value in the i-th iteration is
subreg_{mode} (base + i * step)
The function get_iv_value can be used to perform these
calculations.
Next: Dependency analysis, Previous: loop-iv, Up: Loop Analysis and Representation [Contents][Index]
Both on GIMPLE and on RTL, there are functions available to determine the number of iterations of a loop, with a similar interface. The number of iterations of a loop in GCC is defined as the number of executions of the loop latch. In many cases, it is not possible to determine the number of iterations unconditionally – the determined number is correct only if some assumptions are satisfied. The analysis tries to verify these conditions using the information contained in the program; if it fails, the conditions are returned together with the result. The following information and conditions are provided by the analysis:
assumptions: If this condition is false, the rest of
the information is invalid.
noloop_assumptions on RTL, may_be_zero on GIMPLE: If
this condition is true, the loop exits in the first iteration.
infinite: If this condition is true, the loop is infinite.
This condition is only available on RTL. On GIMPLE, conditions for
finiteness of the loop are included in assumptions.
niter_expr on RTL, niter on GIMPLE: The expression
that gives number of iterations. The number of iterations is defined as
the number of executions of the loop latch.
Both on GIMPLE and on RTL, it necessary for the induction variable
analysis framework to be initialized (SCEV on GIMPLE, loop-iv on RTL).
On GIMPLE, the results are stored to struct tree_niter_desc
structure. Number of iterations before the loop is exited through a
given exit can be determined using number_of_iterations_exit
function. On RTL, the results are returned in struct niter_desc
structure. The corresponding function is named
check_simple_exit. There are also functions that pass through
all the exits of a loop and try to find one with easy to determine
number of iterations – find_loop_niter on GIMPLE and
find_simple_exit on RTL. Finally, there are functions that
provide the same information, but additionally cache it, so that
repeated calls to number of iterations are not so costly –
number_of_latch_executions on GIMPLE and get_simple_loop_desc
on RTL.
Note that some of these functions may behave slightly differently than
others – some of them return only the expression for the number of
iterations, and fail if there are some assumptions. The function
number_of_latch_executions works only for single-exit loops.
The function number_of_cond_exit_executions can be used to
determine number of executions of the exit condition of a single-exit
loop (i.e., the number_of_latch_executions increased by one).
Next: Lambda, Previous: Number of iterations, Up: Loop Analysis and Representation [Contents][Index]
The code for the data dependence analysis can be found in
tree-data-ref.c and its interface and data structures are
described in tree-data-ref.h. The function that computes the
data dependences for all the array and pointer references for a given
loop is compute_data_dependences_for_loop. This function is
currently used by the linear loop transform and the vectorization
passes. Before calling this function, one has to allocate two vectors:
a first vector will contain the set of data references that are
contained in the analyzed loop body, and the second vector will contain
the dependence relations between the data references. Thus if the
vector of data references is of size n, the vector containing the
dependence relations will contain n*n elements. However if the
analyzed loop contains side effects, such as calls that potentially can
interfere with the data references in the current analyzed loop, the
analysis stops while scanning the loop body for data references, and
inserts a single chrec_dont_know in the dependence relation
array.
The data references are discovered in a particular order during the scanning of the loop body: the loop body is analyzed in execution order, and the data references of each statement are pushed at the end of the data reference array. Two data references syntactically occur in the program in the same order as in the array of data references. This syntactic order is important in some classical data dependence tests, and mapping this order to the elements of this array avoids costly queries to the loop body representation.
Three types of data references are currently handled: ARRAY_REF,
INDIRECT_REF and COMPONENT_REF. The data structure for the data reference
is data_reference, where data_reference_p is a name of a
pointer to the data reference structure. The structure contains the
following elements:
base_object_info: Provides information about the base object
of the data reference and its access functions. These access functions
represent the evolution of the data reference in the loop relative to
its base, in keeping with the classical meaning of the data reference
access function for the support of arrays. For example, for a reference
a.b[i][j], the base object is a.b and the access functions,
one for each array subscript, are:
{i_init, + i_step}_1, {j_init, +, j_step}_2.
first_location_in_loop: Provides information about the first
location accessed by the data reference in the loop and about the access
function used to represent evolution relative to this location. This data
is used to support pointers, and is not used for arrays (for which we
have base objects). Pointer accesses are represented as a one-dimensional
access that starts from the first location accessed in the loop. For
example:
for1 i
for2 j
*((int *)p + i + j) = a[i][j];
The access function of the pointer access is {0, + 4B}_for2
relative to p + i. The access functions of the array are
{i_init, + i_step}_for1 and {j_init, +, j_step}_for2
relative to a.
Usually, the object the pointer refers to is either unknown, or we can’t prove that the access is confined to the boundaries of a certain object.
Two data references can be compared only if at least one of these two representations has all its fields filled for both data references.
The current strategy for data dependence tests is as follows:
If both a and b are represented as arrays, compare
a.base_object and b.base_object;
if they are equal, apply dependence tests (use access functions based on
base_objects).
Else if both a and b are represented as pointers, compare
a.first_location and b.first_location;
if they are equal, apply dependence tests (use access functions based on
first location).
However, if a and b are represented differently, only try
to prove that the bases are definitely different.
The structure describing the relation between two data references is
data_dependence_relation and the shorter name for a pointer to
such a structure is ddr_p. This structure contains:
are_dependent that is set to chrec_known
if the analysis has proved that there is no dependence between these two
data references, chrec_dont_know if the analysis was not able to
determine any useful result and potentially there could exist a
dependence between these data references, and are_dependent is
set to NULL_TREE if there exist a dependence relation between the
data references, and the description of this dependence relation is
given in the subscripts, dir_vects, and dist_vects
arrays,
subscripts that contains a description of each
subscript of the data references. Given two array accesses a
subscript is the tuple composed of the access functions for a given
dimension. For example, given A[f1][f2][f3] and
B[g1][g2][g3], there are three subscripts: (f1, g1), (f2,
g2), (f3, g3).
dir_vects and dist_vects that contain
classical representations of the data dependences under the form of
direction and distance dependence vectors,
loop_nest that contains the loops to
which the distance and direction vectors refer to.
Several functions for pretty printing the information extracted by the
data dependence analysis are available: dump_ddrs prints with a
maximum verbosity the details of a data dependence relations array,
dump_dist_dir_vectors prints only the classical distance and
direction vectors for a data dependence relations array, and
dump_data_references prints the details of the data references
contained in a data reference array.
Next: Omega, Previous: Dependency analysis, Up: Loop Analysis and Representation [Contents][Index]
Lambda is a framework that allows transformations of loops using non-singular matrix based transformations of the iteration space and loop bounds. This allows compositions of skewing, scaling, interchange, and reversal transformations. These transformations are often used to improve cache behavior or remove inner loop dependencies to allow parallelization and vectorization to take place.
To perform these transformations, Lambda requires that the loopnest be
converted into an internal form that can be matrix transformed easily.
To do this conversion, the function
gcc_loopnest_to_lambda_loopnest is provided. If the loop cannot
be transformed using lambda, this function will return NULL.
Once a lambda_loopnest is obtained from the conversion function,
it can be transformed by using lambda_loopnest_transform, which
takes a transformation matrix to apply. Note that it is up to the
caller to verify that the transformation matrix is legal to apply to the
loop (dependence respecting, etc). Lambda simply applies whatever
matrix it is told to provide. It can be extended to make legal matrices
out of any non-singular matrix, but this is not currently implemented.
Legality of a matrix for a given loopnest can be verified using
lambda_transform_legal_p.
Given a transformed loopnest, conversion back into gcc IR is done by
lambda_loopnest_to_gcc_loopnest. This function will modify the
loops so that they match the transformed loopnest.
Previous: Lambda, Up: Loop Analysis and Representation [Contents][Index]
The data dependence analysis contains several solvers triggered sequentially from the less complex ones to the more sophisticated. For ensuring the consistency of the results of these solvers, a data dependence check pass has been implemented based on two different solvers. The second method that has been integrated to GCC is based on the Omega dependence solver, written in the 1990’s by William Pugh and David Wonnacott. Data dependence tests can be formulated using a subset of the Presburger arithmetics that can be translated to linear constraint systems. These linear constraint systems can then be solved using the Omega solver.
The Omega solver is using Fourier-Motzkin’s algorithm for variable
elimination: a linear constraint system containing n variables
is reduced to a linear constraint system with n-1 variables.
The Omega solver can also be used for solving other problems that can
be expressed under the form of a system of linear equalities and
inequalities. The Omega solver is known to have an exponential worst
case, also known under the name of “omega nightmare” in the
literature, but in practice, the omega test is known to be efficient
for the common data dependence tests.
The interface used by the Omega solver for describing the linear
programming problems is described in omega.h, and the solver is
omega_solve_problem.
Next: Machine Desc, Previous: Loop Analysis and Representation, Up: Top [Contents][Index]
A control flow graph (CFG) is a data structure built on top of the
intermediate code representation (the RTL or tree instruction
stream) abstracting the control flow behavior of a function that is
being compiled. The CFG is a directed graph where the vertices
represent basic blocks and edges represent possible transfer of
control flow from one basic block to another. The data structures
used to represent the control flow graph are defined in
basic-block.h.
| • Basic Blocks: | The definition and representation of basic blocks. | |
| • Edges: | Types of edges and their representation. | |
| • Profile information: | Representation of frequencies and probabilities. | |
| • Maintaining the CFG: | Keeping the control flow graph and up to date. | |
| • Liveness information: | Using and maintaining liveness information. |
Next: Edges, Up: Control Flow [Contents][Index]
A basic block is a straight-line sequence of code with only one entry
point and only one exit. In GCC, basic blocks are represented using
the basic_block data type.
Two pointer members of the basic_block structure are the
pointers next_bb and prev_bb. These are used to keep
doubly linked chain of basic blocks in the same order as the
underlying instruction stream. The chain of basic blocks is updated
transparently by the provided API for manipulating the CFG. The macro
FOR_EACH_BB can be used to visit all the basic blocks in
lexicographical order. Dominator traversals are also possible using
walk_dominator_tree. Given two basic blocks A and B, block A
dominates block B if A is always executed before B.
The BASIC_BLOCK array contains all basic blocks in an
unspecified order. Each basic_block structure has a field
that holds a unique integer identifier index that is the
index of the block in the BASIC_BLOCK array.
The total number of basic blocks in the function is
n_basic_blocks. Both the basic block indices and
the total number of basic blocks may vary during the compilation
process, as passes reorder, create, duplicate, and destroy basic
blocks. The index for any block should never be greater than
last_basic_block.
Special basic blocks represent possible entry and exit points of a
function. These blocks are called ENTRY_BLOCK_PTR and
EXIT_BLOCK_PTR. These blocks do not contain any code, and are
not elements of the BASIC_BLOCK array. Therefore they have
been assigned unique, negative index numbers.
Each basic_block also contains pointers to the first
instruction (the head) and the last instruction (the tail)
or end of the instruction stream contained in a basic block. In
fact, since the basic_block data type is used to represent
blocks in both major intermediate representations of GCC (tree
and RTL), there are pointers to the head and end of a basic block for
both representations.
For RTL, these pointers are rtx head, end. In the RTL function
representation, the head pointer always points either to a
NOTE_INSN_BASIC_BLOCK or to a CODE_LABEL, if present.
In the RTL representation of a function, the instruction stream
contains not only the “real” instructions, but also notes.
Any function that moves or duplicates the basic blocks needs
to take care of updating of these notes. Many of these notes expect
that the instruction stream consists of linear regions, making such
updates difficult. The NOTE_INSN_BASIC_BLOCK note is the only
kind of note that may appear in the instruction stream contained in a
basic block. The instruction stream of a basic block always follows a
NOTE_INSN_BASIC_BLOCK, but zero or more CODE_LABEL
nodes can precede the block note. A basic block ends by control flow
instruction or last instruction before following CODE_LABEL or
NOTE_INSN_BASIC_BLOCK. A CODE_LABEL cannot appear in
the instruction stream of a basic block.
In addition to notes, the jump table vectors are also represented as
“pseudo-instructions” inside the insn stream. These vectors never
appear in the basic block and should always be placed just after the
table jump instructions referencing them. After removing the
table-jump it is often difficult to eliminate the code computing the
address and referencing the vector, so cleaning up these vectors is
postponed until after liveness analysis. Thus the jump table vectors
may appear in the insn stream unreferenced and without any purpose.
Before any edge is made fall-thru, the existence of such
construct in the way needs to be checked by calling
can_fallthru function.
For the tree representation, the head and end of the basic block
are being pointed to by the stmt_list field, but this special
tree should never be referenced directly. Instead, at the tree
level abstract containers and iterators are used to access statements
and expressions in basic blocks. These iterators are called
block statement iterators (BSIs). Grep for ^bsi
in the various tree-* files.
The following snippet will pretty-print all the statements of the
program in the GIMPLE representation.
FOR_EACH_BB (bb)
{
block_stmt_iterator si;
for (si = bsi_start (bb); !bsi_end_p (si); bsi_next (&si))
{
tree stmt = bsi_stmt (si);
print_generic_stmt (stderr, stmt, 0);
}
}
Next: Profile information, Previous: Basic Blocks, Up: Control Flow [Contents][Index]
Edges represent possible control flow transfers from the end of some
basic block A to the head of another basic block B. We say that A is
a predecessor of B, and B is a successor of A. Edges are represented
in GCC with the edge data type. Each edge acts as a
link between two basic blocks: the src member of an edge
points to the predecessor basic block of the dest basic block.
The members preds and succs of the basic_block data
type point to type-safe vectors of edges to the predecessors and
successors of the block.
When walking the edges in an edge vector, edge iterators should
be used. Edge iterators are constructed using the
edge_iterator data structure and several methods are available
to operate on them:
ei_start
This function initializes an edge_iterator that points to the
first edge in a vector of edges.
ei_last
This function initializes an edge_iterator that points to the
last edge in a vector of edges.
ei_end_p
This predicate is true if an edge_iterator represents
the last edge in an edge vector.
ei_one_before_end_p
This predicate is true if an edge_iterator represents
the second last edge in an edge vector.
ei_next
This function takes a pointer to an edge_iterator and makes it
point to the next edge in the sequence.
ei_prev
This function takes a pointer to an edge_iterator and makes it
point to the previous edge in the sequence.
ei_edge
This function returns the edge currently pointed to by an
edge_iterator.
ei_safe_safe
This function returns the edge currently pointed to by an
edge_iterator, but returns NULL if the iterator is
pointing at the end of the sequence. This function has been provided
for existing code makes the assumption that a NULL edge
indicates the end of the sequence.
The convenience macro FOR_EACH_EDGE can be used to visit all of
the edges in a sequence of predecessor or successor edges. It must
not be used when an element might be removed during the traversal,
otherwise elements will be missed. Here is an example of how to use
the macro:
edge e;
edge_iterator ei;
FOR_EACH_EDGE (e, ei, bb->succs)
{
if (e->flags & EDGE_FALLTHRU)
break;
}
There are various reasons why control flow may transfer from one block
to another. One possibility is that some instruction, for example a
CODE_LABEL, in a linearized instruction stream just always
starts a new basic block. In this case a fall-thru edge links
the basic block to the first following basic block. But there are
several other reasons why edges may be created. The flags
field of the edge data type is used to store information
about the type of edge we are dealing with. Each edge is of one of
the following types:
No type flags are set for edges corresponding to jump instructions. These edges are used for unconditional or conditional jumps and in RTL also for table jumps. They are the easiest to manipulate as they may be freely redirected when the flow graph is not in SSA form.
Fall-thru edges are present in case where the basic block may continue
execution to the following one without branching. These edges have
the EDGE_FALLTHRU flag set. Unlike other types of edges, these
edges must come into the basic block immediately following in the
instruction stream. The function force_nonfallthru is
available to insert an unconditional jump in the case that redirection
is needed. Note that this may require creation of a new basic block.
Exception handling edges represent possible control transfers from a
trapping instruction to an exception handler. The definition of
“trapping” varies. In C++, only function calls can throw, but for
Java, exceptions like division by zero or segmentation fault are
defined and thus each instruction possibly throwing this kind of
exception needs to be handled as control flow instruction. Exception
edges have the EDGE_ABNORMAL and EDGE_EH flags set.
When updating the instruction stream it is easy to change possibly
trapping instruction to non-trapping, by simply removing the exception
edge. The opposite conversion is difficult, but should not happen
anyway. The edges can be eliminated via purge_dead_edges call.
In the RTL representation, the destination of an exception edge is
specified by REG_EH_REGION note attached to the insn.
In case of a trapping call the EDGE_ABNORMAL_CALL flag is set
too. In the tree representation, this extra flag is not set.
In the RTL representation, the predicate may_trap_p may be used
to check whether instruction still may trap or not. For the tree
representation, the tree_could_trap_p predicate is available,
but this predicate only checks for possible memory traps, as in
dereferencing an invalid pointer location.
Sibling calls or tail calls terminate the function in a non-standard
way and thus an edge to the exit must be present.
EDGE_SIBCALL and EDGE_ABNORMAL are set in such case.
These edges only exist in the RTL representation.
Computed jumps contain edges to all labels in the function referenced
from the code. All those edges have EDGE_ABNORMAL flag set.
The edges used to represent computed jumps often cause compile time
performance problems, since functions consisting of many taken labels
and many computed jumps may have very dense flow graphs, so
these edges need to be handled with special care. During the earlier
stages of the compilation process, GCC tries to avoid such dense flow
graphs by factoring computed jumps. For example, given the following
series of jumps,
goto *x; [ … ] goto *x; [ … ] goto *x; [ … ]
factoring the computed jumps results in the following code sequence which has a much simpler flow graph:
goto y; [ … ] goto y; [ … ] goto y; [ … ] y: goto *x;
However, the classic problem with this transformation is that it has a runtime cost in there resulting code: An extra jump. Therefore, the computed jumps are un-factored in the later passes of the compiler. Be aware of that when you work on passes in that area. There have been numerous examples already where the compile time for code with unfactored computed jumps caused some serious headaches.
GCC allows nested functions to return into caller using a goto
to a label passed to as an argument to the callee. The labels passed
to nested functions contain special code to cleanup after function
call. Such sections of code are referred to as “nonlocal goto
receivers”. If a function contains such nonlocal goto receivers, an
edge from the call to the label is created with the
EDGE_ABNORMAL and EDGE_ABNORMAL_CALL flags set.
By definition, execution of function starts at basic block 0, so there
is always an edge from the ENTRY_BLOCK_PTR to basic block 0.
There is no tree representation for alternate entry points at
this moment. In RTL, alternate entry points are specified by
CODE_LABEL with LABEL_ALTERNATE_NAME defined. This
feature is currently used for multiple entry point prologues and is
limited to post-reload passes only. This can be used by back-ends to
emit alternate prologues for functions called from different contexts.
In future full support for multiple entry functions defined by Fortran
90 needs to be implemented.
In the pre-reload representation a function terminates after the last instruction in the insn chain and no explicit return instructions are used. This corresponds to the fall-thru edge into exit block. After reload, optimal RTL epilogues are used that use explicit (conditional) return instructions that are represented by edges with no flags set.
Next: Maintaining the CFG, Previous: Edges, Up: Control Flow [Contents][Index]
In many cases a compiler must make a choice whether to trade speed in one part of code for speed in another, or to trade code size for code speed. In such cases it is useful to know information about how often some given block will be executed. That is the purpose for maintaining profile within the flow graph. GCC can handle profile information obtained through profile feedback, but it can also estimate branch probabilities based on statics and heuristics.
The feedback based profile is produced by compiling the program with instrumentation, executing it on a train run and reading the numbers of executions of basic blocks and edges back to the compiler while re-compiling the program to produce the final executable. This method provides very accurate information about where a program spends most of its time on the train run. Whether it matches the average run of course depends on the choice of train data set, but several studies have shown that the behavior of a program usually changes just marginally over different data sets.
When profile feedback is not available, the compiler may be asked to attempt to predict the behavior of each branch in the program using a set of heuristics (see predict.def for details) and compute estimated frequencies of each basic block by propagating the probabilities over the graph.
Each basic_block contains two integer fields to represent
profile information: frequency and count. The
frequency is an estimation how often is basic block executed
within a function. It is represented as an integer scaled in the
range from 0 to BB_FREQ_BASE. The most frequently executed
basic block in function is initially set to BB_FREQ_BASE and
the rest of frequencies are scaled accordingly. During optimization,
the frequency of the most frequent basic block can both decrease (for
instance by loop unrolling) or grow (for instance by cross-jumping
optimization), so scaling sometimes has to be performed multiple
times.
The count contains hard-counted numbers of execution measured
during training runs and is nonzero only when profile feedback is
available. This value is represented as the host’s widest integer
(typically a 64 bit integer) of the special type gcov_type.
Most optimization passes can use only the frequency information of a basic block, but a few passes may want to know hard execution counts. The frequencies should always match the counts after scaling, however during updating of the profile information numerical error may accumulate into quite large errors.
Each edge also contains a branch probability field: an integer in the
range from 0 to REG_BR_PROB_BASE. It represents probability of
passing control from the end of the src basic block to the
dest basic block, i.e. the probability that control will flow
along this edge. The EDGE_FREQUENCY macro is available to
compute how frequently a given edge is taken. There is a count
field for each edge as well, representing same information as for a
basic block.
The basic block frequencies are not represented in the instruction
stream, but in the RTL representation the edge frequencies are
represented for conditional jumps (via the REG_BR_PROB
macro) since they are used when instructions are output to the
assembly file and the flow graph is no longer maintained.
The probability that control flow arrives via a given edge to its destination basic block is called reverse probability and is not directly represented, but it may be easily computed from frequencies of basic blocks.
Updating profile information is a delicate task that can unfortunately
not be easily integrated with the CFG manipulation API. Many of the
functions and hooks to modify the CFG, such as
redirect_edge_and_branch, do not have enough information to
easily update the profile, so updating it is in the majority of cases
left up to the caller. It is difficult to uncover bugs in the profile
updating code, because they manifest themselves only by producing
worse code, and checking profile consistency is not possible because
of numeric error accumulation. Hence special attention needs to be
given to this issue in each pass that modifies the CFG.
It is important to point out that REG_BR_PROB_BASE and
BB_FREQ_BASE are both set low enough to be possible to compute
second power of any frequency or probability in the flow graph, it is
not possible to even square the count field, as modern CPUs are
fast enough to execute $2^32$ operations quickly.
Next: Liveness information, Previous: Profile information, Up: Control Flow [Contents][Index]
An important task of each compiler pass is to keep both the control flow graph and all profile information up-to-date. Reconstruction of the control flow graph after each pass is not an option, since it may be very expensive and lost profile information cannot be reconstructed at all.
GCC has two major intermediate representations, and both use the
basic_block and edge data types to represent control
flow. Both representations share as much of the CFG maintenance code
as possible. For each representation, a set of hooks is defined
so that each representation can provide its own implementation of CFG
manipulation routines when necessary. These hooks are defined in
cfghooks.h. There are hooks for almost all common CFG
manipulations, including block splitting and merging, edge redirection
and creating and deleting basic blocks. These hooks should provide
everything you need to maintain and manipulate the CFG in both the RTL
and tree representation.
At the moment, the basic block boundaries are maintained transparently
when modifying instructions, so there rarely is a need to move them
manually (such as in case someone wants to output instruction outside
basic block explicitly).
Often the CFG may be better viewed as integral part of instruction
chain, than structure built on the top of it. However, in principle
the control flow graph for the tree representation is
not an integral part of the representation, in that a function
tree may be expanded without first building a flow graph for the
tree representation at all. This happens when compiling
without any tree optimization enabled. When the tree
optimizations are enabled and the instruction stream is rewritten in
SSA form, the CFG is very tightly coupled with the instruction stream.
In particular, statement insertion and removal has to be done with
care. In fact, the whole tree representation can not be easily
used or maintained without proper maintenance of the CFG
simultaneously.
In the RTL representation, each instruction has a
BLOCK_FOR_INSN value that represents pointer to the basic block
that contains the instruction. In the tree representation, the
function bb_for_stmt returns a pointer to the basic block
containing the queried statement.
When changes need to be applied to a function in its tree
representation, block statement iterators should be used. These
iterators provide an integrated abstraction of the flow graph and the
instruction stream. Block statement iterators are constructed using
the block_stmt_iterator data structure and several modifier are
available, including the following:
bsi_start
This function initializes a block_stmt_iterator that points to
the first non-empty statement in a basic block.
bsi_last
This function initializes a block_stmt_iterator that points to
the last statement in a basic block.
bsi_end_p
This predicate is true if a block_stmt_iterator
represents the end of a basic block.
bsi_next
This function takes a block_stmt_iterator and makes it point to
its successor.
bsi_prev
This function takes a block_stmt_iterator and makes it point to
its predecessor.
bsi_insert_after
This function inserts a statement after the block_stmt_iterator
passed in. The final parameter determines whether the statement
iterator is updated to point to the newly inserted statement, or left
pointing to the original statement.
bsi_insert_before
This function inserts a statement before the block_stmt_iterator
passed in. The final parameter determines whether the statement
iterator is updated to point to the newly inserted statement, or left
pointing to the original statement.
bsi_remove
This function removes the block_stmt_iterator passed in and
rechains the remaining statements in a basic block, if any.
In the RTL representation, the macros BB_HEAD and BB_END
may be used to get the head and end rtx of a basic block. No
abstract iterators are defined for traversing the insn chain, but you
can just use NEXT_INSN and PREV_INSN instead. See Insns.
Usually a code manipulating pass simplifies the instruction stream and
the flow of control, possibly eliminating some edges. This may for
example happen when a conditional jump is replaced with an
unconditional jump, but also when simplifying possibly trapping
instruction to non-trapping while compiling Java. Updating of edges
is not transparent and each optimization pass is required to do so
manually. However only few cases occur in practice. The pass may
call purge_dead_edges on a given basic block to remove
superfluous edges, if any.
Another common scenario is redirection of branch instructions, but
this is best modeled as redirection of edges in the control flow graph
and thus use of redirect_edge_and_branch is preferred over more
low level functions, such as redirect_jump that operate on RTL
chain only. The CFG hooks defined in cfghooks.h should provide
the complete API required for manipulating and maintaining the CFG.
It is also possible that a pass has to insert control flow instruction
into the middle of a basic block, thus creating an entry point in the
middle of the basic block, which is impossible by definition: The
block must be split to make sure it only has one entry point, i.e. the
head of the basic block. The CFG hook split_block may be used
when an instruction in the middle of a basic block has to become the
target of a jump or branch instruction.
For a global optimizer, a common operation is to split edges in the
flow graph and insert instructions on them. In the RTL
representation, this can be easily done using the
insert_insn_on_edge function that emits an instruction
“on the edge”, caching it for a later commit_edge_insertions
call that will take care of moving the inserted instructions off the
edge into the instruction stream contained in a basic block. This
includes the creation of new basic blocks where needed. In the
tree representation, the equivalent functions are
bsi_insert_on_edge which inserts a block statement
iterator on an edge, and bsi_commit_edge_inserts which flushes
the instruction to actual instruction stream.
While debugging the optimization pass, a verify_flow_info
function may be useful to find bugs in the control flow graph updating
code.
Note that at present, the representation of control flow in the
tree representation is discarded before expanding to RTL.
Long term the CFG should be maintained and “expanded” to the
RTL representation along with the function tree itself.
Previous: Maintaining the CFG, Up: Control Flow [Contents][Index]
Liveness information is useful to determine whether some register is “live” at given point of program, i.e. that it contains a value that may be used at a later point in the program. This information is used, for instance, during register allocation, as the pseudo registers only need to be assigned to a unique hard register or to a stack slot if they are live. The hard registers and stack slots may be freely reused for other values when a register is dead.
Liveness information is available in the back end starting with
pass_df_initialize and ending with pass_df_finish. Three
flavors of live analysis are available: With LR, it is possible
to determine at any point P in the function if the register may be
used on some path from P to the end of the function. With
UR, it is possible to determine if there is a path from the
beginning of the function to P that defines the variable.
LIVE is the intersection of the LR and UR and a
variable is live at P if there is both an assignment that reaches
it from the beginning of the function and a use that can be reached on
some path from P to the end of the function.
In general LIVE is the most useful of the three. The macros
DF_[LR,UR,LIVE]_[IN,OUT] can be used to access this information.
The macros take a basic block number and return a bitmap that is indexed
by the register number. This information is only guaranteed to be up to
date after calls are made to df_analyze. See the file
df-core.c for details on using the dataflow.
The liveness information is stored partly in the RTL instruction stream
and partly in the flow graph. Local information is stored in the
instruction stream: Each instruction may contain REG_DEAD notes
representing that the value of a given register is no longer needed, or
REG_UNUSED notes representing that the value computed by the
instruction is never used. The second is useful for instructions
computing multiple values at once.
Next: Target Macros, Previous: Control Flow, Up: Top [Contents][Index]
A machine description has two parts: a file of instruction patterns (.md file) and a C header file of macro definitions.
The .md file for a target machine contains a pattern for each instruction that the target machine supports (or at least each instruction that is worth telling the compiler about). It may also contain comments. A semicolon causes the rest of the line to be a comment, unless the semicolon is inside a quoted string.
See the next chapter for information on the C header file.
| • Overview: | How the machine description is used. | |
| • Patterns: | How to write instruction patterns. | |
| • Example: | An explained example of a define_insn pattern.
| |
| • RTL Template: | The RTL template defines what insns match a pattern. | |
| • Output Template: | The output template says how to make assembler code from such an insn. | |
| • Output Statement: | For more generality, write C code to output the assembler code. | |
| • Predicates: | Controlling what kinds of operands can be used for an insn. | |
| • Constraints: | Fine-tuning operand selection. | |
| • Standard Names: | Names mark patterns to use for code generation. | |
| • Pattern Ordering: | When the order of patterns makes a difference. | |
| • Dependent Patterns: | Having one pattern may make you need another. | |
| • Jump Patterns: | Special considerations for patterns for jump insns. | |
| • Looping Patterns: | How to define patterns for special looping insns. | |
| • Insn Canonicalizations: | Canonicalization of Instructions | |
| • Expander Definitions: | Generating a sequence of several RTL insns for a standard operation. | |
| • Insn Splitting: | Splitting Instructions into Multiple Instructions. | |
| • Including Patterns: | Including Patterns in Machine Descriptions. | |
| • Peephole Definitions: | Defining machine-specific peephole optimizations. | |
| • Insn Attributes: | Specifying the value of attributes for generated insns. | |
| • Conditional Execution: | Generating define_insn patterns for
predication.
| |
| • Constant Definitions: | Defining symbolic constants that can be used in the md file. | |
| • Iterators: | Using iterators to generate patterns from a template. |
Next: Patterns, Up: Machine Desc [Contents][Index]
There are three main conversions that happen in the compiler:
For the generate pass, only the names of the insns matter, from either a
named define_insn or a define_expand. The compiler will
choose the pattern with the right name and apply the operands according
to the documentation later in this chapter, without regard for the RTL
template or operand constraints. Note that the names the compiler looks
for are hard-coded in the compiler—it will ignore unnamed patterns and
patterns with names it doesn’t know about, but if you don’t provide a
named pattern it needs, it will abort.
If a define_insn is used, the template given is inserted into the
insn list. If a define_expand is used, one of three things
happens, based on the condition logic. The condition logic may manually
create new insns for the insn list, say via emit_insn(), and
invoke DONE. For certain named patterns, it may invoke FAIL to tell the
compiler to use an alternate way of performing that task. If it invokes
neither DONE nor FAIL, the template given in the pattern
is inserted, as if the define_expand were a define_insn.
Once the insn list is generated, various optimization passes convert,
replace, and rearrange the insns in the insn list. This is where the
define_split and define_peephole patterns get used, for
example.
Finally, the insn list’s RTL is matched up with the RTL templates in the
define_insn patterns, and those patterns are used to emit the
final assembly code. For this purpose, each named define_insn
acts like it’s unnamed, since the names are ignored.
Next: Example, Previous: Overview, Up: Machine Desc [Contents][Index]
Each instruction pattern contains an incomplete RTL expression, with pieces
to be filled in later, operand constraints that restrict how the pieces can
be filled in, and an output pattern or C code to generate the assembler
output, all wrapped up in a define_insn expression.
A define_insn is an RTL expression containing four or five operands:
The absence of a name is indicated by writing an empty string where the name should go. Nameless instruction patterns are never used for generating RTL code, but they may permit several simpler insns to be combined later on.
Names that are not thus known and used in RTL-generation have no effect; they are equivalent to no name at all.
For the purpose of debugging the compiler, you may also specify a name beginning with the ‘*’ character. Such a name is used only for identifying the instruction in RTL dumps; it is entirely equivalent to having a nameless pattern for all other purposes.
match_operand,
match_operator, and match_dup expressions that stand for
operands of the instruction.
If the vector has only one element, that element is the template for the
instruction pattern. If the vector has multiple elements, then the
instruction pattern is a parallel expression containing the
elements described.
For a named pattern, the condition (if present) may not depend on the data in the insn being matched, but only the target-machine-type flags. The compiler needs to test these conditions during initialization in order to learn exactly which named instructions are available in a particular run.
For nameless patterns, the condition is applied only when matching an
individual insn, and only after the insn has matched the pattern’s
recognition template. The insn’s operands may be found in the vector
operands. For an insn where the condition has once matched, it
can’t be used to control register allocation, for example by excluding
certain hard registers or hard register combinations.
When simple substitution isn’t general enough, you can specify a piece of C code to compute the output. See Output Statement.
Next: RTL Template, Previous: Patterns, Up: Machine Desc [Contents][Index]
define_insnHere is an actual example of an instruction pattern, for the 68000/68020.
(define_insn "tstsi"
[(set (cc0)
(match_operand:SI 0 "general_operand" "rm"))]
""
"*
{
if (TARGET_68020 || ! ADDRESS_REG_P (operands[0]))
return \"tstl %0\";
return \"cmpl #0,%0\";
}")
This can also be written using braced strings:
(define_insn "tstsi"
[(set (cc0)
(match_operand:SI 0 "general_operand" "rm"))]
""
{
if (TARGET_68020 || ! ADDRESS_REG_P (operands[0]))
return "tstl %0";
return "cmpl #0,%0";
})
This is an instruction that sets the condition codes based on the value of
a general operand. It has no condition, so any insn whose RTL description
has the form shown may be handled according to this pattern. The name
‘tstsi’ means “test a SImode value” and tells the RTL generation
pass that, when it is necessary to test such a value, an insn to do so
can be constructed using this pattern.
The output control string is a piece of C code which chooses which output template to return based on the kind of operand and the specific type of CPU for which code is being generated.
‘"rm"’ is an operand constraint. Its meaning is explained below.
Next: Output Template, Previous: Example, Up: Machine Desc [Contents][Index]
The RTL template is used to define which insns match the particular pattern and how to find their operands. For named patterns, the RTL template also says how to construct an insn from specified operands.
Construction involves substituting specified operands into a copy of the template. Matching involves determining the values that serve as the operands in the insn being matched. Both of these activities are controlled by special expression types that direct matching and substitution of the operands.
(match_operand:m n predicate constraint)This expression is a placeholder for operand number n of the insn. When constructing an insn, operand number n will be substituted at this point. When matching an insn, whatever appears at this position in the insn will be taken as operand number n; but it must satisfy predicate or this instruction pattern will not match at all.
Operand numbers must be chosen consecutively counting from zero in
each instruction pattern. There may be only one match_operand
expression in the pattern for each operand number. Usually operands
are numbered in the order of appearance in match_operand
expressions. In the case of a define_expand, any operand numbers
used only in match_dup expressions have higher values than all
other operand numbers.
predicate is a string that is the name of a function that
accepts two arguments, an expression and a machine mode.
See Predicates. During matching, the function will be called with
the putative operand as the expression and m as the mode
argument (if m is not specified, VOIDmode will be used,
which normally causes predicate to accept any mode). If it
returns zero, this instruction pattern fails to match.
predicate may be an empty string; then it means no test is to be
done on the operand, so anything which occurs in this position is
valid.
Most of the time, predicate will reject modes other than m—but
not always. For example, the predicate address_operand uses
m as the mode of memory ref that the address should be valid for.
Many predicates accept const_int nodes even though their mode is
VOIDmode.
constraint controls reloading and the choice of the best register class to use for a value, as explained later (see Constraints). If the constraint would be an empty string, it can be omitted.
People are often unclear on the difference between the constraint and the predicate. The predicate helps decide whether a given insn matches the pattern. The constraint plays no role in this decision; instead, it controls various decisions in the case of an insn which does match.
(match_scratch:m n constraint)This expression is also a placeholder for operand number n
and indicates that operand must be a scratch or reg
expression.
When matching patterns, this is equivalent to
(match_operand:m n "scratch_operand" pred)
but, when generating RTL, it produces a (scratch:m)
expression.
If the last few expressions in a parallel are clobber
expressions whose operands are either a hard register or
match_scratch, the combiner can add or delete them when
necessary. See Side Effects.
(match_dup n)This expression is also a placeholder for operand number n. It is used when the operand needs to appear more than once in the insn.
In construction, match_dup acts just like match_operand:
the operand is substituted into the insn being constructed. But in
matching, match_dup behaves differently. It assumes that operand
number n has already been determined by a match_operand
appearing earlier in the recognition template, and it matches only an
identical-looking expression.
Note that match_dup should not be used to tell the compiler that
a particular register is being used for two operands (example:
add that adds one register to another; the second register is
both an input operand and the output operand). Use a matching
constraint (see Simple Constraints) for those. match_dup is for the cases where one
operand is used in two places in the template, such as an instruction
that computes both a quotient and a remainder, where the opcode takes
two input operands but the RTL template has to refer to each of those
twice; once for the quotient pattern and once for the remainder pattern.
(match_operator:m n predicate [operands…])This pattern is a kind of placeholder for a variable RTL expression code.
When constructing an insn, it stands for an RTL expression whose expression code is taken from that of operand n, and whose operands are constructed from the patterns operands.
When matching an expression, it matches an expression if the function predicate returns nonzero on that expression and the patterns operands match the operands of the expression.
Suppose that the function commutative_operator is defined as
follows, to match any expression whose operator is one of the
commutative arithmetic operators of RTL and whose mode is mode:
int
commutative_integer_operator (x, mode)
rtx x;
enum machine_mode mode;
{
enum rtx_code code = GET_CODE (x);
if (GET_MODE (x) != mode)
return 0;
return (GET_RTX_CLASS (code) == RTX_COMM_ARITH
|| code == EQ || code == NE);
}
Then the following pattern will match any RTL expression consisting of a commutative operator applied to two general operands:
(match_operator:SI 3 "commutative_operator" [(match_operand:SI 1 "general_operand" "g") (match_operand:SI 2 "general_operand" "g")])
Here the vector [operands…] contains two patterns
because the expressions to be matched all contain two operands.
When this pattern does match, the two operands of the commutative
operator are recorded as operands 1 and 2 of the insn. (This is done
by the two instances of match_operand.) Operand 3 of the insn
will be the entire commutative expression: use GET_CODE
(operands[3]) to see which commutative operator was used.
The machine mode m of match_operator works like that of
match_operand: it is passed as the second argument to the
predicate function, and that function is solely responsible for
deciding whether the expression to be matched “has” that mode.
When constructing an insn, argument 3 of the gen-function will specify the operation (i.e. the expression code) for the expression to be made. It should be an RTL expression, whose expression code is copied into a new expression whose operands are arguments 1 and 2 of the gen-function. The subexpressions of argument 3 are not used; only its expression code matters.
When match_operator is used in a pattern for matching an insn,
it usually best if the operand number of the match_operator
is higher than that of the actual operands of the insn. This improves
register allocation because the register allocator often looks at
operands 1 and 2 of insns to see if it can do register tying.
There is no way to specify constraints in match_operator. The
operand of the insn which corresponds to the match_operator
never has any constraints because it is never reloaded as a whole.
However, if parts of its operands are matched by
match_operand patterns, those parts may have constraints of
their own.
(match_op_dup:m n[operands…])Like match_dup, except that it applies to operators instead of
operands. When constructing an insn, operand number n will be
substituted at this point. But in matching, match_op_dup behaves
differently. It assumes that operand number n has already been
determined by a match_operator appearing earlier in the
recognition template, and it matches only an identical-looking
expression.
(match_parallel n predicate [subpat…])This pattern is a placeholder for an insn that consists of a
parallel expression with a variable number of elements. This
expression should only appear at the top level of an insn pattern.
When constructing an insn, operand number n will be substituted at
this point. When matching an insn, it matches if the body of the insn
is a parallel expression with at least as many elements as the
vector of subpat expressions in the match_parallel, if each
subpat matches the corresponding element of the parallel,
and the function predicate returns nonzero on the
parallel that is the body of the insn. It is the responsibility
of the predicate to validate elements of the parallel beyond
those listed in the match_parallel.
A typical use of match_parallel is to match load and store
multiple expressions, which can contain a variable number of elements
in a parallel. For example,
(define_insn ""
[(match_parallel 0 "load_multiple_operation"
[(set (match_operand:SI 1 "gpc_reg_operand" "=r")
(match_operand:SI 2 "memory_operand" "m"))
(use (reg:SI 179))
(clobber (reg:SI 179))])]
""
"loadm 0,0,%1,%2")
This example comes from a29k.md. The function
load_multiple_operation is defined in a29k.c and checks
that subsequent elements in the parallel are the same as the
set in the pattern, except that they are referencing subsequent
registers and memory locations.
An insn that matches this pattern might look like:
(parallel
[(set (reg:SI 20) (mem:SI (reg:SI 100)))
(use (reg:SI 179))
(clobber (reg:SI 179))
(set (reg:SI 21)
(mem:SI (plus:SI (reg:SI 100)
(const_int 4))))
(set (reg:SI 22)
(mem:SI (plus:SI (reg:SI 100)
(const_int 8))))])
(match_par_dup n [subpat…])Like match_op_dup, but for match_parallel instead of
match_operator.
Next: Output Statement, Previous: RTL Template, Up: Machine Desc [Contents][Index]
The output template is a string which specifies how to output the assembler code for an instruction pattern. Most of the template is a fixed string which is output literally. The character ‘%’ is used to specify where to substitute an operand; it can also be used to identify places where different variants of the assembler require different syntax.
In the simplest case, a ‘%’ followed by a digit n says to output operand n at that point in the string.
‘%’ followed by a letter and a digit says to output an operand in an
alternate fashion. Four letters have standard, built-in meanings described
below. The machine description macro PRINT_OPERAND can define
additional letters with nonstandard meanings.
‘%cdigit’ can be used to substitute an operand that is a constant value without the syntax that normally indicates an immediate operand.
‘%ndigit’ is like ‘%cdigit’ except that the value of the constant is negated before printing.
‘%adigit’ can be used to substitute an operand as if it were a memory reference, with the actual operand treated as the address. This may be useful when outputting a “load address” instruction, because often the assembler syntax for such an instruction requires you to write the operand as if it were a memory reference.
‘%ldigit’ is used to substitute a label_ref into a jump
instruction.
‘%=’ outputs a number which is unique to each instruction in the entire compilation. This is useful for making local labels to be referred to more than once in a single template that generates multiple assembler instructions.
‘%’ followed by a punctuation character specifies a substitution that
does not use an operand. Only one case is standard: ‘%%’ outputs a
‘%’ into the assembler code. Other nonstandard cases can be
defined in the PRINT_OPERAND macro. You must also define
which punctuation characters are valid with the
PRINT_OPERAND_PUNCT_VALID_P macro.
The template may generate multiple assembler instructions. Write the text for the instructions, with ‘\;’ between them.
When the RTL contains two operands which are required by constraint to match each other, the output template must refer only to the lower-numbered operand. Matching operands are not always identical, and the rest of the compiler arranges to put the proper RTL expression for printing into the lower-numbered operand.
One use of nonstandard letters or punctuation following ‘%’ is to
distinguish between different assembler languages for the same machine; for
example, Motorola syntax versus MIT syntax for the 68000. Motorola syntax
requires periods in most opcode names, while MIT syntax does not. For
example, the opcode ‘movel’ in MIT syntax is ‘move.l’ in Motorola
syntax. The same file of patterns is used for both kinds of output syntax,
but the character sequence ‘%.’ is used in each place where Motorola
syntax wants a period. The PRINT_OPERAND macro for Motorola syntax
defines the sequence to output a period; the macro for MIT syntax defines
it to do nothing.
As a special case, a template consisting of the single character #
instructs the compiler to first split the insn, and then output the
resulting instructions separately. This helps eliminate redundancy in the
output templates. If you have a define_insn that needs to emit
multiple assembler instructions, and there is a matching define_split
already defined, then you can simply use # as the output template
instead of writing an output template that emits the multiple assembler
instructions.
If the macro ASSEMBLER_DIALECT is defined, you can use construct
of the form ‘{option0|option1|option2}’ in the templates. These
describe multiple variants of assembler language syntax.
See Instruction Output.
Next: Predicates, Previous: Output Template, Up: Machine Desc [Contents][Index]
Often a single fixed template string cannot produce correct and efficient assembler code for all the cases that are recognized by a single instruction pattern. For example, the opcodes may depend on the kinds of operands; or some unfortunate combinations of operands may require extra machine instructions.
If the output control string starts with a ‘@’, then it is actually a series of templates, each on a separate line. (Blank lines and leading spaces and tabs are ignored.) The templates correspond to the pattern’s constraint alternatives (see Multi-Alternative). For example, if a target machine has a two-address add instruction ‘addr’ to add into a register and another ‘addm’ to add a register to memory, you might write this pattern:
(define_insn "addsi3"
[(set (match_operand:SI 0 "general_operand" "=r,m")
(plus:SI (match_operand:SI 1 "general_operand" "0,0")
(match_operand:SI 2 "general_operand" "g,r")))]
""
"@
addr %2,%0
addm %2,%0")
If the output control string starts with a ‘*’, then it is not an
output template but rather a piece of C program that should compute a
template. It should execute a return statement to return the
template-string you want. Most such templates use C string literals, which
require doublequote characters to delimit them. To include these
doublequote characters in the string, prefix each one with ‘\’.
If the output control string is written as a brace block instead of a double-quoted string, it is automatically assumed to be C code. In that case, it is not necessary to put in a leading asterisk, or to escape the doublequotes surrounding C string literals.
The operands may be found in the array operands, whose C data type
is rtx [].
It is very common to select different ways of generating assembler code
based on whether an immediate operand is within a certain range. Be
careful when doing this, because the result of INTVAL is an
integer on the host machine. If the host machine has more bits in an
int than the target machine has in the mode in which the constant
will be used, then some of the bits you get from INTVAL will be
superfluous. For proper results, you must carefully disregard the
values of those bits.
It is possible to output an assembler instruction and then go on to output
or compute more of them, using the subroutine output_asm_insn. This
receives two arguments: a template-string and a vector of operands. The
vector may be operands, or it may be another array of rtx
that you declare locally and initialize yourself.
When an insn pattern has multiple alternatives in its constraints, often
the appearance of the assembler code is determined mostly by which alternative
was matched. When this is so, the C code can test the variable
which_alternative, which is the ordinal number of the alternative
that was actually satisfied (0 for the first, 1 for the second alternative,
etc.).
For example, suppose there are two opcodes for storing zero, ‘clrreg’
for registers and ‘clrmem’ for memory locations. Here is how
a pattern could use which_alternative to choose between them:
(define_insn ""
[(set (match_operand:SI 0 "general_operand" "=r,m")
(const_int 0))]
""
{
return (which_alternative == 0
? "clrreg %0" : "clrmem %0");
})
The example above, where the assembler code to generate was solely determined by the alternative, could also have been specified as follows, having the output control string start with a ‘@’:
(define_insn ""
[(set (match_operand:SI 0 "general_operand" "=r,m")
(const_int 0))]
""
"@
clrreg %0
clrmem %0")
Next: Constraints, Previous: Output Statement, Up: Machine Desc [Contents][Index]
A predicate determines whether a match_operand or
match_operator expression matches, and therefore whether the
surrounding instruction pattern will be used for that combination of
operands. GCC has a number of machine-independent predicates, and you
can define machine-specific predicates as needed. By convention,
predicates used with match_operand have names that end in
‘_operand’, and those used with match_operator have names
that end in ‘_operator’.
All predicates are Boolean functions (in the mathematical sense) of
two arguments: the RTL expression that is being considered at that
position in the instruction pattern, and the machine mode that the
match_operand or match_operator specifies. In this
section, the first argument is called op and the second argument
mode. Predicates can be called from C as ordinary two-argument
functions; this can be useful in output templates or other
machine-specific code.
Operand predicates can allow operands that are not actually acceptable to the hardware, as long as the constraints give reload the ability to fix them up (see Constraints). However, GCC will usually generate better code if the predicates specify the requirements of the machine instructions as closely as possible. Reload cannot fix up operands that must be constants (“immediate operands”); you must use a predicate that allows only constants, or else enforce the requirement in the extra condition.
Most predicates handle their mode argument in a uniform manner.
If mode is VOIDmode (unspecified), then op can have
any mode. If mode is anything else, then op must have the
same mode, unless op is a CONST_INT or integer
CONST_DOUBLE. These RTL expressions always have
VOIDmode, so it would be counterproductive to check that their
mode matches. Instead, predicates that accept CONST_INT and/or
integer CONST_DOUBLE check that the value stored in the
constant will fit in the requested mode.
Predicates with this behavior are called normal.
genrecog can optimize the instruction recognizer based on
knowledge of how normal predicates treat modes. It can also diagnose
certain kinds of common errors in the use of normal predicates; for
instance, it is almost always an error to use a normal predicate
without specifying a mode.
Predicates that do something different with their mode argument
are called special. The generic predicates
address_operand and pmode_register_operand are special
predicates. genrecog does not do any optimizations or
diagnosis when special predicates are used.
| • Machine-Independent Predicates: | Predicates available to all back ends. | |
| • Defining Predicates: | How to write machine-specific predicate functions. |
Next: Defining Predicates, Up: Predicates [Contents][Index]
These are the generic predicates available to all back ends. They are defined in recog.c. The first category of predicates allow only constant, or immediate, operands.
This predicate allows any sort of constant that fits in mode. It is an appropriate choice for instructions that take operands that must be constant.
This predicate allows any CONST_INT expression that fits in
mode. It is an appropriate choice for an immediate operand that
does not allow a symbol or label.
This predicate accepts any CONST_DOUBLE expression that has
exactly mode. If mode is VOIDmode, it will also
accept CONST_INT. It is intended for immediate floating point
constants.
The second category of predicates allow only some kind of machine register.
This predicate allows any REG or SUBREG expression that
is valid for mode. It is often suitable for arithmetic
instruction operands on a RISC machine.
This is a slight variant on register_operand which works around
a limitation in the machine-description reader.
(match_operand n "pmode_register_operand" constraint)
means exactly what
(match_operand:P n "register_operand" constraint)
would mean, if the machine-description reader accepted ‘:P’
mode suffixes. Unfortunately, it cannot, because Pmode is an
alias for some other mode, and might vary with machine-specific
options. See Misc.
This predicate allows hard registers and SCRATCH expressions,
but not pseudo-registers. It is used internally by match_scratch;
it should not be used directly.
The third category of predicates allow only some kind of memory reference.
This predicate allows any valid reference to a quantity of mode
mode in memory, as determined by the weak form of
GO_IF_LEGITIMATE_ADDRESS (see Addressing Modes).
This predicate is a little unusual; it allows any operand that is a
valid expression for the address of a quantity of mode
mode, again determined by the weak form of
GO_IF_LEGITIMATE_ADDRESS. To first order, if
‘(mem:mode (exp))’ is acceptable to
memory_operand, then exp is acceptable to
address_operand. Note that exp does not necessarily have
the mode mode.
This is a stricter form of memory_operand which allows only
memory references with a general_operand as the address
expression. New uses of this predicate are discouraged, because
general_operand is very permissive, so it’s hard to tell what
an indirect_operand does or does not allow. If a target has
different requirements for memory operands for different instructions,
it is better to define target-specific predicates which enforce the
hardware’s requirements explicitly.
This predicate allows a memory reference suitable for pushing a value
onto the stack. This will be a MEM which refers to
stack_pointer_rtx, with a side-effect in its address expression
(see Incdec); which one is determined by the
STACK_PUSH_CODE macro (see Frame Layout).
This predicate allows a memory reference suitable for popping a value
off the stack. Again, this will be a MEM referring to
stack_pointer_rtx, with a side-effect in its address
expression. However, this time STACK_POP_CODE is expected.
The fourth category of predicates allow some combination of the above operands.
This predicate allows any immediate or register operand valid for mode.
This predicate allows any register or memory operand valid for mode.
This predicate allows any immediate, register, or memory operand valid for mode.
Finally, there are two generic operator predicates.
This predicate matches any expression which performs an arithmetic
comparison in mode; that is, COMPARISON_P is true for the
expression code.
This predicate matches any expression which performs an arithmetic
comparison in mode and whose expression code is valid for integer
modes; that is, the expression code will be one of eq, ne,
lt, ltu, le, leu, gt, gtu,
ge, geu.
Previous: Machine-Independent Predicates, Up: Predicates [Contents][Index]
Many machines have requirements for their operands that cannot be
expressed precisely using the generic predicates. You can define
additional predicates using define_predicate and
define_special_predicate expressions. These expressions have
three operands:
match_operand or match_operator expressions.
MATCH_OPERANDWhen written inside a predicate expression, a MATCH_OPERAND
expression evaluates to true if the predicate it names would allow
op. The operand number and constraint are ignored. Due to
limitations in genrecog, you can only refer to generic
predicates and predicates that have already been defined.
MATCH_CODEThis expression evaluates to true if op or a specified subexpression of op has one of a given list of RTX codes.
The first operand of this expression is a string constant containing a
comma-separated list of RTX code names (in lower case). These are the
codes for which the MATCH_CODE will be true.
The second operand is a string constant which indicates what
subexpression of op to examine. If it is absent or the empty
string, op itself is examined. Otherwise, the string constant
must be a sequence of digits and/or lowercase letters. Each character
indicates a subexpression to extract from the current expression; for
the first character this is op, for the second and subsequent
characters it is the result of the previous character. A digit
n extracts ‘XEXP (e, n)’; a letter l
extracts ‘XVECEXP (e, 0, n)’ where n is the
alphabetic ordinal of l (0 for ‘a’, 1 for ’b’, and so on). The
MATCH_CODE then examines the RTX code of the subexpression
extracted by the complete string. It is not possible to extract
components of an rtvec that is not at position 0 within its RTX
object.
MATCH_TESTThis expression has one operand, a string constant containing a C
expression. The predicate’s arguments, op and mode, are
available with those names in the C expression. The MATCH_TEST
evaluates to true if the C expression evaluates to a nonzero value.
MATCH_TEST expressions must not have side effects.
ANDIORNOTIF_THEN_ELSEThe basic ‘MATCH_’ expressions can be combined using these
logical operators, which have the semantics of the C operators
‘&&’, ‘||’, ‘!’, and ‘? :’ respectively. As
in Common Lisp, you may give an AND or IOR expression an
arbitrary number of arguments; this has exactly the same effect as
writing a chain of two-argument AND or IOR expressions.
If a code block is present in a predicate definition, then the RTL expression must evaluate to true and the code block must execute ‘return true’ for the predicate to allow the operand. The RTL expression is evaluated first; do not re-check anything in the code block that was checked in the RTL expression.
The program genrecog scans define_predicate and
define_special_predicate expressions to determine which RTX
codes are possibly allowed. You should always make this explicit in
the RTL predicate expression, using MATCH_OPERAND and
MATCH_CODE.
Here is an example of a simple predicate definition, from the IA64 machine description:
;; True if op is a SYMBOL_REF which refers to the sdata section.
(define_predicate "small_addr_symbolic_operand"
(and (match_code "symbol_ref")
(match_test "SYMBOL_REF_SMALL_ADDR_P (op)")))
And here is another, showing the use of the C block.
;; True if op is a register operand that is (or could be) a GR reg.
(define_predicate "gr_register_operand"
(match_operand 0 "register_operand")
{
unsigned int regno;
if (GET_CODE (op) == SUBREG)
op = SUBREG_REG (op);
regno = REGNO (op);
return (regno >= FIRST_PSEUDO_REGISTER || GENERAL_REGNO_P (regno));
})
Predicates written with define_predicate automatically include
a test that mode is VOIDmode, or op has the same
mode as mode, or op is a CONST_INT or
CONST_DOUBLE. They do not check specifically for
integer CONST_DOUBLE, nor do they test that the value of either
kind of constant fits in the requested mode. This is because
target-specific predicates that take constants usually have to do more
stringent value checks anyway. If you need the exact same treatment
of CONST_INT or CONST_DOUBLE that the generic predicates
provide, use a MATCH_OPERAND subexpression to call
const_int_operand, const_double_operand, or
immediate_operand.
Predicates written with define_special_predicate do not get any
automatic mode checks, and are treated as having special mode handling
by genrecog.
The program genpreds is responsible for generating code to
test predicates. It also writes a header file containing function
declarations for all machine-specific predicates. It is not necessary
to declare these predicates in cpu-protos.h.
Next: Standard Names, Previous: Predicates, Up: Machine Desc [Contents][Index]
Each match_operand in an instruction pattern can specify
constraints for the operands allowed. The constraints allow you to
fine-tune matching within the set of operands allowed by the
predicate.
Constraints can say whether
an operand may be in a register, and which kinds of register; whether the
operand can be a memory reference, and which kinds of address; whether the
operand may be an immediate constant, and which possible values it may
have. Constraints can also require two operands to match.
Side-effects aren’t allowed in operands of inline asm, unless
‘<’ or ‘>’ constraints are used, because there is no guarantee
that the side-effects will happen exactly once in an instruction that can update
the addressing register.
| • Simple Constraints: | Basic use of constraints. | |
| • Multi-Alternative: | When an insn has two alternative constraint-patterns. | |
| • Class Preferences: | Constraints guide which hard register to put things in. | |
| • Modifiers: | More precise control over effects of constraints. | |
| • Disable Insn Alternatives: | Disable insn alternatives using the enabled attribute.
| |
| • Machine Constraints: | Existing constraints for some particular machines. | |
| • Define Constraints: | How to define machine-specific constraints. | |
| • C Constraint Interface: | How to test constraints from C code. |
Next: Multi-Alternative, Up: Constraints [Contents][Index]
The simplest kind of constraint is a string full of letters, each of which describes one kind of operand that is permitted. Here are the letters that are allowed:
Whitespace characters are ignored and can be inserted at any position except the first. This enables each alternative for different operands to be visually aligned in the machine description even if they have different number of constraints and modifiers.
A memory operand is allowed, with any kind of address that the machine
supports in general.
Note that the letter used for the general memory constraint can be
re-defined by a back end using the TARGET_MEM_CONSTRAINT macro.
A memory operand is allowed, but only if the address is offsettable. This means that adding a small integer (actually, the width in bytes of the operand, as determined by its machine mode) may be added to the address and the result is also a valid memory address.
For example, an address which is constant is offsettable; so is an address that is the sum of a register and a constant (as long as a slightly larger constant is also within the range of address-offsets supported by the machine); but an autoincrement or autodecrement address is not offsettable. More complicated indirect/indexed addresses may or may not be offsettable depending on the other addressing modes that the machine supports.
Note that in an output operand which can be matched by another operand, the constraint letter ‘o’ is valid only when accompanied by both ‘<’ (if the target machine has predecrement addressing) and ‘>’ (if the target machine has preincrement addressing).
A memory operand that is not offsettable. In other words, anything that would fit the ‘m’ constraint but not the ‘o’ constraint.
A memory operand with autodecrement addressing (either predecrement or
postdecrement) is allowed. In inline asm this constraint is only
allowed if the operand is used exactly once in an instruction that can
handle the side-effects. Not using an operand with ‘<’ in constraint
string in the inline asm pattern at all or using it in multiple
instructions isn’t valid, because the side-effects wouldn’t be performed
or would be performed more than once. Furthermore, on some targets
the operand with ‘<’ in constraint string must be accompanied by
special instruction suffixes like %U0 instruction suffix on PowerPC
or %P0 on IA-64.
A memory operand with autoincrement addressing (either preincrement or
postincrement) is allowed. In inline asm the same restrictions
as for ‘<’ apply.
A register operand is allowed provided that it is in a general register.
An immediate integer operand (one with constant value) is allowed. This includes symbolic constants whose values will be known only at assembly time or later.
An immediate integer operand with a known numeric value is allowed. Many systems cannot support assembly-time constants for operands less than a word wide. Constraints for these operands should use ‘n’ rather than ‘i’.
Other letters in the range ‘I’ through ‘P’ may be defined in a machine-dependent fashion to permit immediate integer operands with explicit integer values in specified ranges. For example, on the 68000, ‘I’ is defined to stand for the range of values 1 to 8. This is the range permitted as a shift count in the shift instructions.
An immediate floating operand (expression code const_double) is
allowed, but only if the target floating point format is the same as
that of the host machine (on which the compiler is running).
An immediate floating operand (expression code const_double or
const_vector) is allowed.
‘G’ and ‘H’ may be defined in a machine-dependent fashion to permit immediate floating operands in particular ranges of values.
An immediate integer operand whose value is not an explicit integer is allowed.
This might appear strange; if an insn allows a constant operand with a value not known at compile time, it certainly must allow any known value. So why use ‘s’ instead of ‘i’? Sometimes it allows better code to be generated.
For example, on the 68000 in a fullword instruction it is possible to use an immediate operand; but if the immediate value is between -128 and 127, better code results from loading the value into a register and using the register. This is because the load into the register can be done with a ‘moveq’ instruction. We arrange for this to happen by defining the letter ‘K’ to mean “any integer outside the range -128 to 127”, and then specifying ‘Ks’ in the operand constraints.
Any register, memory or immediate integer operand is allowed, except for registers that are not general registers.
Any operand whatsoever is allowed, even if it does not satisfy
general_operand. This is normally used in the constraint of
a match_scratch when certain alternatives will not actually
require a scratch register.
An operand that matches the specified operand number is allowed. If a digit is used together with letters within the same alternative, the digit should come last.
This number is allowed to be more than a single digit. If multiple digits are encountered consecutively, they are interpreted as a single decimal integer. There is scant chance for ambiguity, since to-date it has never been desirable that ‘10’ be interpreted as matching either operand 1 or operand 0. Should this be desired, one can use multiple alternatives instead.
This is called a matching constraint and what it really means is that the assembler has only a single operand that fills two roles considered separate in the RTL insn. For example, an add insn has two input operands and one output operand in the RTL, but on most CISC machines an add instruction really has only two operands, one of them an input-output operand:
addl #35,r12
Matching constraints are used in these circumstances. More precisely, the two operands that match must include one input-only operand and one output-only operand. Moreover, the digit must be a smaller number than the number of the operand that uses it in the constraint.
For operands to match in a particular case usually means that they
are identical-looking RTL expressions. But in a few special cases
specific kinds of dissimilarity are allowed. For example, *x
as an input operand will match *x++ as an output operand.
For proper results in such cases, the output template should always
use the output-operand’s number when printing the operand.
An operand that is a valid memory address is allowed. This is for “load address” and “push address” instructions.
‘p’ in the constraint must be accompanied by address_operand
as the predicate in the match_operand. This predicate interprets
the mode specified in the match_operand as the mode of the memory
reference for which the address would be valid.
Other letters can be defined in machine-dependent fashion to stand for particular classes of registers or other arbitrary operand types. ‘d’, ‘a’ and ‘f’ are defined on the 68000/68020 to stand for data, address and floating point registers.
In order to have valid assembler code, each operand must satisfy its constraint. But a failure to do so does not prevent the pattern from applying to an insn. Instead, it directs the compiler to modify the code so that the constraint will be satisfied. Usually this is done by copying an operand into a register.
Contrast, therefore, the two instruction patterns that follow:
(define_insn ""
[(set (match_operand:SI 0 "general_operand" "=r")
(plus:SI (match_dup 0)
(match_operand:SI 1 "general_operand" "r")))]
""
"…")
which has two operands, one of which must appear in two places, and
(define_insn ""
[(set (match_operand:SI 0 "general_operand" "=r")
(plus:SI (match_operand:SI 1 "general_operand" "0")
(match_operand:SI 2 "general_operand" "r")))]
""
"…")
which has three operands, two of which are required by a constraint to be identical. If we are considering an insn of the form
(insn n prev next
(set (reg:SI 3)
(plus:SI (reg:SI 6) (reg:SI 109)))
…)
the first pattern would not apply at all, because this insn does not contain two identical subexpressions in the right place. The pattern would say, “That does not look like an add instruction; try other patterns”. The second pattern would say, “Yes, that’s an add instruction, but there is something wrong with it”. It would direct the reload pass of the compiler to generate additional insns to make the constraint true. The results might look like this:
(insn n2 prev n
(set (reg:SI 3) (reg:SI 6))
…)
(insn n n2 next
(set (reg:SI 3)
(plus:SI (reg:SI 3) (reg:SI 109)))
…)
It is up to you to make sure that each operand, in each pattern, has constraints that can handle any RTL expression that could be present for that operand. (When multiple alternatives are in use, each pattern must, for each possible combination of operand expressions, have at least one alternative which can handle that combination of operands.) The constraints don’t need to allow any possible operand—when this is the case, they do not constrain—but they must at least point the way to reloading any possible operand so that it will fit.
For example, an operand whose constraints permit everything except registers is safe provided its predicate rejects registers.
An operand whose predicate accepts only constant values is safe provided its constraints include the letter ‘i’. If any possible constant value is accepted, then nothing less than ‘i’ will do; if the predicate is more selective, then the constraints may also be more selective.
If the operand’s predicate can recognize registers, but the constraint does not permit them, it can make the compiler crash. When this operand happens to be a register, the reload pass will be stymied, because it does not know how to copy a register temporarily into memory.
If the predicate accepts a unary operator, the constraint applies to the
operand. For example, the MIPS processor at ISA level 3 supports an
instruction which adds two registers in SImode to produce a
DImode result, but only if the registers are correctly sign
extended. This predicate for the input operands accepts a
sign_extend of an SImode register. Write the constraint
to indicate the type of register that is required for the operand of the
sign_extend.
Next: Class Preferences, Previous: Simple Constraints, Up: Constraints [Contents][Index]
Sometimes a single instruction has multiple alternative sets of possible operands. For example, on the 68000, a logical-or instruction can combine register or an immediate value into memory, or it can combine any kind of operand into a register; but it cannot combine one memory location into another.
These constraints are represented as multiple alternatives. An alternative can be described by a series of letters for each operand. The overall constraint for an operand is made from the letters for this operand from the first alternative, a comma, the letters for this operand from the second alternative, a comma, and so on until the last alternative. Here is how it is done for fullword logical-or on the 68000:
(define_insn "iorsi3"
[(set (match_operand:SI 0 "general_operand" "=m,d")
(ior:SI (match_operand:SI 1 "general_operand" "%0,0")
(match_operand:SI 2 "general_operand" "dKs,dmKs")))]
…)
The first alternative has ‘m’ (memory) for operand 0, ‘0’ for operand 1 (meaning it must match operand 0), and ‘dKs’ for operand 2. The second alternative has ‘d’ (data register) for operand 0, ‘0’ for operand 1, and ‘dmKs’ for operand 2. The ‘=’ and ‘%’ in the constraints apply to all the alternatives; their meaning is explained in the next section (see Class Preferences).
If all the operands fit any one alternative, the instruction is valid. Otherwise, for each alternative, the compiler counts how many instructions must be added to copy the operands so that that alternative applies. The alternative requiring the least copying is chosen. If two alternatives need the same amount of copying, the one that comes first is chosen. These choices can be altered with the ‘?’ and ‘!’ characters:
?Disparage slightly the alternative that the ‘?’ appears in, as a choice when no alternative applies exactly. The compiler regards this alternative as one unit more costly for each ‘?’ that appears in it.
!Disparage severely the alternative that the ‘!’ appears in. This alternative can still be used if it fits without reloading, but if reloading is needed, some other alternative will be used.
When an insn pattern has multiple alternatives in its constraints, often
the appearance of the assembler code is determined mostly by which
alternative was matched. When this is so, the C code for writing the
assembler code can use the variable which_alternative, which is
the ordinal number of the alternative that was actually satisfied (0 for
the first, 1 for the second alternative, etc.). See Output Statement.
Next: Modifiers, Previous: Multi-Alternative, Up: Constraints [Contents][Index]
The operand constraints have another function: they enable the compiler to decide which kind of hardware register a pseudo register is best allocated to. The compiler examines the constraints that apply to the insns that use the pseudo register, looking for the machine-dependent letters such as ‘d’ and ‘a’ that specify classes of registers. The pseudo register is put in whichever class gets the most “votes”. The constraint letters ‘g’ and ‘r’ also vote: they vote in favor of a general register. The machine description says which registers are considered general.
Of course, on some machines all registers are equivalent, and no register classes are defined. Then none of this complexity is relevant.
Next: Machine Constraints, Previous: Class Preferences, Up: Constraints [Contents][Index]
Here are constraint modifier characters.
Means that this operand is write-only for this instruction: the previous value is discarded and replaced by output data.
Means that this operand is both read and written by the instruction.
When the compiler fixes up the operands to satisfy the constraints, it needs to know which operands are inputs to the instruction and which are outputs from it. ‘=’ identifies an output; ‘+’ identifies an operand that is both input and output; all other operands are assumed to be input only.
If you specify ‘=’ or ‘+’ in a constraint, you put it in the first character of the constraint string.
Means (in a particular alternative) that this operand is an earlyclobber operand, which is modified before the instruction is finished using the input operands. Therefore, this operand may not lie in a register that is used as an input operand or as part of any memory address.
‘&’ applies only to the alternative in which it is written. In constraints with multiple alternatives, sometimes one alternative requires ‘&’ while others do not. See, for example, the ‘movdf’ insn of the 68000.
An input operand can be tied to an earlyclobber operand if its only use as an input occurs before the early result is written. Adding alternatives of this form often allows GCC to produce better code when only some of the inputs can be affected by the earlyclobber. See, for example, the ‘mulsi3’ insn of the ARM.
‘&’ does not obviate the need to write ‘=’.
Declares the instruction to be commutative for this operand and the following operand. This means that the compiler may interchange the two operands if that is the cheapest way to make all operands fit the constraints. This is often used in patterns for addition instructions that really have only two operands: the result must go in one of the arguments. Here for example, is how the 68000 halfword-add instruction is defined:
(define_insn "addhi3"
[(set (match_operand:HI 0 "general_operand" "=m,r")
(plus:HI (match_operand:HI 1 "general_operand" "%0,0")
(match_operand:HI 2 "general_operand" "di,g")))]
…)
GCC can only handle one commutative pair in an asm; if you use more,
the compiler may fail. Note that you need not use the modifier if
the two alternatives are strictly identical; this would only waste
time in the reload pass. The modifier is not operational after
register allocation, so the result of define_peephole2
and define_splits performed after reload cannot rely on
‘%’ to make the intended insn match.
Says that all following characters, up to the next comma, are to be ignored as a constraint. They are significant only for choosing register preferences.
Says that the following character should be ignored when choosing register preferences. ‘*’ has no effect on the meaning of the constraint as a constraint, and no effect on reloading.
Here is an example: the 68000 has an instruction to sign-extend a halfword in a data register, and can also sign-extend a value by copying it into an address register. While either kind of register is acceptable, the constraints on an address-register destination are less strict, so it is best if register allocation makes an address register its goal. Therefore, ‘*’ is used so that the ‘d’ constraint letter (for data register) is ignored when computing register preferences.
(define_insn "extendhisi2"
[(set (match_operand:SI 0 "general_operand" "=*d,a")
(sign_extend:SI
(match_operand:HI 1 "general_operand" "0,g")))]
…)
Next: Disable Insn Alternatives, Previous: Modifiers, Up: Constraints [Contents][Index]
Whenever possible, you should use the general-purpose constraint letters
in asm arguments, since they will convey meaning more readily to
people reading your code. Failing that, use the constraint letters
that usually have very similar meanings across architectures. The most
commonly used constraints are ‘m’ and ‘r’ (for memory and
general-purpose registers respectively; see Simple Constraints), and
‘I’, usually the letter indicating the most common
immediate-constant format.
Each architecture defines additional constraints. These constraints
are used by the compiler itself for instruction generation, as well as
for asm statements; therefore, some of the constraints are not
particularly useful for asm. Here is a summary of some of the
machine-dependent constraints available on some particular machines;
it includes both constraints that are useful for asm and
constraints that aren’t. The compiler source file mentioned in the
table heading for each architecture is the definitive reference for
the meanings of that architecture’s constraints.
fFloating-point register
wVFP floating-point register
FOne of the floating-point constants 0.0, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0 or 10.0
GFloating-point constant that would satisfy the constraint ‘F’ if it were negated
IInteger that is valid as an immediate operand in a data processing instruction. That is, an integer in the range 0 to 255 rotated by a multiple of 2
JInteger in the range -4095 to 4095
KInteger that satisfies constraint ‘I’ when inverted (ones complement)
LInteger that satisfies constraint ‘I’ when negated (twos complement)
MInteger in the range 0 to 32
QA memory reference where the exact address is in a single register
(‘‘m’’ is preferable for asm statements)
RAn item in the constant pool
SA symbol in the text segment of the current file
UvA memory reference suitable for VFP load/store insns (reg+constant offset)
UyA memory reference suitable for iWMMXt load/store instructions.
UqA memory reference suitable for the ARMv4 ldrsb instruction.
lRegisters from r0 to r15
aRegisters from r16 to r23
dRegisters from r16 to r31
wRegisters from r24 to r31. These registers can be used in ‘adiw’ command
ePointer register (r26–r31)
bBase pointer register (r28–r31)
qStack pointer register (SPH:SPL)
tTemporary register r0
xRegister pair X (r27:r26)
yRegister pair Y (r29:r28)
zRegister pair Z (r31:r30)
IConstant greater than -1, less than 64
JConstant greater than -64, less than 1
KConstant integer 2
LConstant integer 0
MConstant that fits in 8 bits
NConstant integer -1
OConstant integer 8, 16, or 24
PConstant integer 1
GA floating point constant 0.0
RInteger constant in the range -6 … 5.
QA memory address based on Y or Z pointer with displacement.
bRegisters from r0 to r14 (registers without stack pointer)
lRegister r16 (64-bit accumulator lo register)
hRegister r17 (64-bit accumulator hi register)
kRegister pair r16-r17. (64-bit accumulator lo-hi pair)
IConstant that fits in 3 bits
JConstant that fits in 4 bits
KConstant that fits in 5 bits
LConstant that is one of -1, 4, -4, 7, 8, 12, 16, 20, 32, 48
GFloating point constant that is legal for store immediate
aGeneral register 1
fFloating point register
qShift amount register
xFloating point register (deprecated)
yUpper floating point register (32-bit), floating point register (64-bit)
ZAny register
ISigned 11-bit integer constant
JSigned 14-bit integer constant
KInteger constant that can be deposited with a zdepi instruction
LSigned 5-bit integer constant
MInteger constant 0
NInteger constant that can be loaded with a ldil instruction
OInteger constant whose value plus one is a power of 2
PInteger constant that can be used for and operations in depi
and extru instructions
SInteger constant 31
UInteger constant 63
GFloating-point constant 0.0
AA lo_sum data-linkage-table memory operand
QA memory operand that can be used as the destination operand of an integer store instruction
RA scaled or unscaled indexed memory operand
TA memory operand for floating-point loads and stores
WA register indirect memory operand
kStack register.
fPointer register. A register which can be used to access memory without supplying an offset. Any other register can be used to access memory, but will need a constant offset. In the case of the offset being zero, it is more efficient to use a pointer register, since this reduces code size.
tA twin register. A register which may be paired with an adjacent register to create a 32-bit register.
aAny absolute memory address (e.g., symbolic constant, symbolic constant + offset).
I4-bit signed integer.
J4-bit unsigned integer.
K8-bit signed integer.
MAny constant whose absolute value is no greater than 4-bits.
N10-bit signed integer
O16-bit signed integer.
bAddress base register
dFloating point register (containing 64-bit value)
fFloating point register (containing 32-bit value)
vAltivec vector register
wdVSX vector register to hold vector double data
wfVSX vector register to hold vector float data
wsVSX vector register to hold scalar float data
waAny VSX register
h‘MQ’, ‘CTR’, or ‘LINK’ register
q‘MQ’ register
c‘CTR’ register
l‘LINK’ register
x‘CR’ register (condition register) number 0
y‘CR’ register (condition register)
z‘XER[CA]’ carry bit (part of the XER register)
ISigned 16-bit constant
JUnsigned 16-bit constant shifted left 16 bits (use ‘L’ instead for
SImode constants)
KUnsigned 16-bit constant
LSigned 16-bit constant shifted left 16 bits
MConstant larger than 31
NExact power of 2
OZero
PConstant whose negation is a signed 16-bit constant
GFloating point constant that can be loaded into a register with one instruction per word
HInteger/Floating point constant that can be loaded into a register using three instructions
mMemory operand.
Normally, m does not allow addresses that update the base register.
If ‘<’ or ‘>’ constraint is also used, they are allowed and
therefore on PowerPC targets in that case it is only safe
to use ‘m<>’ in an asm statement if that asm statement
accesses the operand exactly once. The asm statement must also
use ‘%U<opno>’ as a placeholder for the “update” flag in the
corresponding load or store instruction. For example:
asm ("st%U0 %1,%0" : "=m<>" (mem) : "r" (val));
is correct but:
asm ("st %1,%0" : "=m<>" (mem) : "r" (val));
is not.
esA “stable” memory operand; that is, one which does not include any automodification of the base register. This used to be useful when ‘m’ allowed automodification of the base register, but as those are now only allowed when ‘<’ or ‘>’ is used, ‘es’ is basically the same as ‘m’ without ‘<’ and ‘>’.
QMemory operand that is an offset from a register (it is usually better
to use ‘m’ or ‘es’ in asm statements)
ZMemory operand that is an indexed or indirect from a register (it is
usually better to use ‘m’ or ‘es’ in asm statements)
RAIX TOC entry
aAddress operand that is an indexed or indirect from a register (‘p’ is
preferable for asm statements)
SConstant suitable as a 64-bit mask operand
TConstant suitable as a 32-bit mask operand
USystem V Release 4 small data area reference
tAND masks that can be performed by two rldic{l, r} instructions
WVector constant that does not require memory
jVector constant that is all zeros.
RLegacy register—the eight integer registers available on all
i386 processors (a, b, c, d,
si, di, bp, sp).
qAny register accessible as rl. In 32-bit mode, a,
b, c, and d; in 64-bit mode, any integer register.
QAny register accessible as rh: a, b,
c, and d.
lAny register that can be used as the index in a base+index memory access: that is, any general register except the stack pointer.
aThe a register.
bThe b register.
cThe c register.
dThe d register.
SThe si register.
DThe di register.
AThe a and d registers. This class is used for instructions
that return double word results in the ax:dx register pair. Single
word values will be allocated either in ax or dx.
For example on i386 the following implements rdtsc:
unsigned long long rdtsc (void)
{
unsigned long long tick;
__asm__ __volatile__("rdtsc":"=A"(tick));
return tick;
}
This is not correct on x86_64 as it would allocate tick in either ax
or dx. You have to use the following variant instead:
unsigned long long rdtsc (void)
{
unsigned int tickl, tickh;
__asm__ __volatile__("rdtsc":"=a"(tickl),"=d"(tickh));
return ((unsigned long long)tickh << 32)|tickl;
}
fAny 80387 floating-point (stack) register.
tTop of 80387 floating-point stack (%st(0)).
uSecond from top of 80387 floating-point stack (%st(1)).
yAny MMX register.
xAny SSE register.
YzFirst SSE register (%xmm0).
Y2Any SSE register, when SSE2 is enabled.
YiAny SSE register, when SSE2 and inter-unit moves are enabled.
YmAny MMX register, when inter-unit moves are enabled.
IInteger constant in the range 0 … 31, for 32-bit shifts.
JInteger constant in the range 0 … 63, for 64-bit shifts.
KSigned 8-bit integer constant.
L0xFF or 0xFFFF, for andsi as a zero-extending move.
M0, 1, 2, or 3 (shifts for the lea instruction).
NUnsigned 8-bit integer constant (for in and out
instructions).
OInteger constant in the range 0 … 127, for 128-bit shifts.
GStandard 80387 floating point constant.
CStandard SSE floating point constant.
e32-bit signed integer constant, or a symbolic reference known to fit that range (for immediate operands in sign-extending x86-64 instructions).
Z32-bit unsigned integer constant, or a symbolic reference known to fit that range (for immediate operands in zero-extending x86-64 instructions).
aGeneral register r0 to r3 for addl instruction
bBranch register
cPredicate register (‘c’ as in “conditional”)
dApplication register residing in M-unit
eApplication register residing in I-unit
fFloating-point register
mMemory operand. If used together with ‘<’ or ‘>’, the operand can have postincrement and postdecrement which require printing with ‘%Pn’ on IA-64.
GFloating-point constant 0.0 or 1.0
I14-bit signed integer constant
J22-bit signed integer constant
K8-bit signed integer constant for logical instructions
L8-bit adjusted signed integer constant for compare pseudo-ops
M6-bit unsigned integer constant for shift counts
N9-bit signed integer constant for load and store postincrements
OThe constant zero
P0 or -1 for dep instruction
QNon-volatile memory for floating-point loads and stores
RInteger constant in the range 1 to 4 for shladd instruction
SMemory operand except postincrement and postdecrement. This is now roughly the same as ‘m’ when not used together with ‘<’ or ‘>’.
aRegister in the class ACC_REGS (acc0 to acc7).
bRegister in the class EVEN_ACC_REGS (acc0 to acc7).
cRegister in the class CC_REGS (fcc0 to fcc3 and
icc0 to icc3).
dRegister in the class GPR_REGS (gr0 to gr63).
eRegister in the class EVEN_REGS (gr0 to gr63).
Odd registers are excluded not in the class but through the use of a machine
mode larger than 4 bytes.
fRegister in the class FPR_REGS (fr0 to fr63).
hRegister in the class FEVEN_REGS (fr0 to fr63).
Odd registers are excluded not in the class but through the use of a machine
mode larger than 4 bytes.
lRegister in the class LR_REG (the lr register).
qRegister in the class QUAD_REGS (gr2 to gr63).
Register numbers not divisible by 4 are excluded not in the class but through
the use of a machine mode larger than 8 bytes.
tRegister in the class ICC_REGS (icc0 to icc3).
uRegister in the class FCC_REGS (fcc0 to fcc3).
vRegister in the class ICR_REGS (cc4 to cc7).
wRegister in the class FCR_REGS (cc0 to cc3).
xRegister in the class QUAD_FPR_REGS (fr0 to fr63).
Register numbers not divisible by 4 are excluded not in the class but through
the use of a machine mode larger than 8 bytes.
zRegister in the class SPR_REGS (lcr and lr).
ARegister in the class QUAD_ACC_REGS (acc0 to acc7).
BRegister in the class ACCG_REGS (accg0 to accg7).
CRegister in the class CR_REGS (cc0 to cc7).
GFloating point constant zero
I6-bit signed integer constant
J10-bit signed integer constant
L16-bit signed integer constant
M16-bit unsigned integer constant
N12-bit signed integer constant that is negative—i.e. in the range of -2048 to -1
OConstant zero
P12-bit signed integer constant that is greater than zero—i.e. in the range of 1 to 2047.
aP register
dD register
zA call clobbered P register.
qnA single register. If n is in the range 0 to 7, the corresponding D
register. If it is A, then the register P0.
DEven-numbered D register
WOdd-numbered D register
eAccumulator register.
AEven-numbered accumulator register.
BOdd-numbered accumulator register.
bI register
vB register
fM register
cRegisters used for circular buffering, i.e. I, B, or L registers.
CThe CC register.
tLT0 or LT1.
kLC0 or LC1.
uLB0 or LB1.
xAny D, P, B, M, I or L register.
yAdditional registers typically used only in prologues and epilogues: RETS, RETN, RETI, RETX, RETE, ASTAT, SEQSTAT and USP.
wAny register except accumulators or CC.
KshSigned 16 bit integer (in the range -32768 to 32767)
KuhUnsigned 16 bit integer (in the range 0 to 65535)
Ks7Signed 7 bit integer (in the range -64 to 63)
Ku7Unsigned 7 bit integer (in the range 0 to 127)
Ku5Unsigned 5 bit integer (in the range 0 to 31)
Ks4Signed 4 bit integer (in the range -8 to 7)
Ks3Signed 3 bit integer (in the range -3 to 4)
Ku3Unsigned 3 bit integer (in the range 0 to 7)
PnConstant n, where n is a single-digit constant in the range 0 to 4.
PAAn integer equal to one of the MACFLAG_XXX constants that is suitable for use with either accumulator.
PBAn integer equal to one of the MACFLAG_XXX constants that is suitable for use only with accumulator A1.
M1Constant 255.
M2Constant 65535.
JAn integer constant with exactly a single bit set.
LAn integer constant with all bits set except exactly one.
HQAny SYMBOL_REF.
RspRfbRsb‘$sp’, ‘$fb’, ‘$sb’.
RcrAny control register, when they’re 16 bits wide (nothing if control registers are 24 bits wide)
RclAny control register, when they’re 24 bits wide.
R0wR1wR2wR3w$r0, $r1, $r2, $r3.
R02$r0 or $r2, or $r2r0 for 32 bit values.
R13$r1 or $r3, or $r3r1 for 32 bit values.
RdiA register that can hold a 64 bit value.
Rhl$r0 or $r1 (registers with addressable high/low bytes)
R23$r2 or $r3
RaaAddress registers
RawAddress registers when they’re 16 bits wide.
RalAddress registers when they’re 24 bits wide.
RqiRegisters that can hold QI values.
RadRegisters that can be used with displacements ($a0, $a1, $sb).
RsiRegisters that can hold 32 bit values.
RhiRegisters that can hold 16 bit values.
RhcRegisters chat can hold 16 bit values, including all control registers.
Rra$r0 through R1, plus $a0 and $a1.
RflThe flags register.
RmmThe memory-based pseudo-registers $mem0 through $mem15.
RpiRegisters that can hold pointers (16 bit registers for r8c, m16c; 24 bit registers for m32cm, m32c).
RpaMatches multiple registers in a PARALLEL to form a larger register. Used to match function return values.
Is3-8 … 7
IS1-128 … 127
IS2-32768 … 32767
IU20 … 65535
In4-8 … -1 or 1 … 8
In5-16 … -1 or 1 … 16
In6-32 … -1 or 1 … 32
IM2-65536 … -1
IlbAn 8 bit value with exactly one bit set.
IlwA 16 bit value with exactly one bit set.
SdThe common src/dest memory addressing modes.
SaMemory addressed using $a0 or $a1.
SiMemory addressed with immediate addresses.
SsMemory addressed using the stack pointer ($sp).
SfMemory addressed using the frame base register ($fb).
SsMemory addressed using the small base register ($sb).
S1$r1h
aThe $sp register.
bThe $tp register.
cAny control register.
dEither the $hi or the $lo register.
emCoprocessor registers that can be directly loaded ($c0-$c15).
exCoprocessor registers that can be moved to each other.
erCoprocessor registers that can be moved to core registers.
hThe $hi register.
jThe $rpc register.
lThe $lo register.
tRegisters which can be used in $tp-relative addressing.
vThe $gp register.
xThe coprocessor registers.
yThe coprocessor control registers.
zThe $0 register.
AUser-defined register set A.
BUser-defined register set B.
CUser-defined register set C.
DUser-defined register set D.
IOffsets for $gp-rel addressing.
JConstants that can be used directly with boolean insns.
KConstants that can be moved directly to registers.
LSmall constants that can be added to registers.
MLong shift counts.
NSmall constants that can be compared to registers.
OConstants that can be loaded into the top half of registers.
SSigned 8-bit immediates.
TSymbols encoded for $tp-rel or $gp-rel addressing.
UNon-constant addresses for loading/saving coprocessor registers.
WThe top half of a symbol’s value.
YA register indirect address without offset.
ZSymbolic references to the control bus.
dA general register (r0 to r31).
zA status register (rmsr, $fcc1 to $fcc7).
dAn address register. This is equivalent to r unless
generating MIPS16 code.
fA floating-point register (if available).
hFormerly the hi register. This constraint is no longer supported.
lThe lo register. Use this register to store values that are
no bigger than a word.
xThe concatenated hi and lo registers. Use this register
to store doubleword values.
cA register suitable for use in an indirect jump. This will always be
$25 for -mabicalls.
vRegister $3. Do not use this constraint in new code;
it is retained only for compatibility with glibc.
yEquivalent to r; retained for backwards compatibility.
zA floating-point condition code register.
IA signed 16-bit constant (for arithmetic instructions).
JInteger zero.
KAn unsigned 16-bit constant (for logic instructions).
LA signed 32-bit constant in which the lower 16 bits are zero.
Such constants can be loaded using lui.
MA constant that cannot be loaded using lui, addiu
or ori.
NA constant in the range -65535 to -1 (inclusive).
OA signed 15-bit constant.
PA constant in the range 1 to 65535 (inclusive).
GFloating-point zero.
RAn address that can be used in a non-macro load or store.
aAddress register
dData register
f68881 floating-point register, if available
IInteger in the range 1 to 8
J16-bit signed number
KSigned number whose magnitude is greater than 0x80
LInteger in the range -8 to -1
MSigned number whose magnitude is greater than 0x100
NRange 24 to 31, rotatert:SI 8 to 1 expressed as rotate
O16 (for rotate using swap)
PRange 8 to 15, rotatert:HI 8 to 1 expressed as rotate
RNumbers that mov3q can handle
GFloating point constant that is not a 68881 constant
SOperands that satisfy ’m’ when -mpcrel is in effect
TOperands that satisfy ’s’ when -mpcrel is not in effect
QAddress register indirect addressing mode
URegister offset addressing
Wconst_call_operand
Cssymbol_ref or const
Ciconst_int
C0const_int 0
CjRange of signed numbers that don’t fit in 16 bits
CmvqIntegers valid for mvq
CapswIntegers valid for a moveq followed by a swap
CmvzIntegers valid for mvz
CmvsIntegers valid for mvs
Appush_operand
AcNon-register operands allowed in clr
aRegister ‘a’
bRegister ‘b’
dRegister ‘d’
qAn 8-bit register
tTemporary soft register _.tmp
uA soft register _.d1 to _.d31
wStack pointer register
xRegister ‘x’
yRegister ‘y’
zPseudo register ‘z’ (replaced by ‘x’ or ‘y’ at the end)
AAn address register: x, y or z
BAn address register: x or y
DRegister pair (x:d) to form a 32-bit value
LConstants in the range -65536 to 65535
MConstants whose 16-bit low part is zero
NConstant integer 1 or -1
OConstant integer 16
PConstants in the range -8 to 2
AAn absolute address
BAn offset address
WA register indirect memory operand
IA constant in the range of 0 to 255.
NA constant in the range of 0 to -255.
aFloating point registers AC0 through AC3. These can be loaded from/to memory with a single instruction.
dOdd numbered general registers (R1, R3, R5). These are used for 16-bit multiply operations.
fAny of the floating point registers (AC0 through AC5).
GFloating point constant 0.
IAn integer constant that fits in 16 bits.
JAn integer constant whose low order 16 bits are zero.
KAn integer constant that does not meet the constraints for codes ‘I’ or ‘J’.
LThe integer constant 1.
MThe integer constant -1.
NThe integer constant 0.
OInteger constants -4 through -1 and 1 through 4; shifts by these amounts are handled as multiple single-bit shifts rather than a single variable-length shift.
QA memory reference which requires an additional word (address or offset) after the opcode.
RA memory reference that is encoded within the opcode.
QAn address which does not involve register indirect addressing or pre/post increment/decrement addressing.
SymbolA symbol reference.
Int08A constant in the range -256 to 255, inclusive.
Sint08A constant in the range -128 to 127, inclusive.
Sint16A constant in the range -32768 to 32767, inclusive.
Sint24A constant in the range -8388608 to 8388607, inclusive.
Uint04A constant in the range 0 to 15, inclusive.
fFloating-point register on the SPARC-V8 architecture and lower floating-point register on the SPARC-V9 architecture.
eFloating-point register. It is equivalent to ‘f’ on the SPARC-V8 architecture and contains both lower and upper floating-point registers on the SPARC-V9 architecture.
cFloating-point condition code register.
dLower floating-point register. It is only valid on the SPARC-V9 architecture when the Visual Instruction Set is available.
bFloating-point register. It is only valid on the SPARC-V9 architecture when the Visual Instruction Set is available.
h64-bit global or out register for the SPARC-V8+ architecture.
DA vector constant
ISigned 13-bit constant
JZero
K32-bit constant with the low 12 bits clear (a constant that can be
loaded with the sethi instruction)
LA constant in the range supported by movcc instructions
MA constant in the range supported by movrcc instructions
NSame as ‘K’, except that it verifies that bits that are not in the
lower 32-bit range are all zero. Must be used instead of ‘K’ for
modes wider than SImode
OThe constant 4096
GFloating-point zero
HSigned 13-bit constant, sign-extended to 32 or 64 bits
QFloating-point constant whose integral representation can be moved into an integer register using a single sethi instruction
RFloating-point constant whose integral representation can be moved into an integer register using a single mov instruction
SFloating-point constant whose integral representation can be moved into an integer register using a high/lo_sum instruction sequence
TMemory address aligned to an 8-byte boundary
UEven register
WMemory address for ‘e’ constraint registers
YVector zero
aAn immediate which can be loaded with the il/ila/ilh/ilhu instructions. const_int is treated as a 64 bit value.
cAn immediate for and/xor/or instructions. const_int is treated as a 64 bit value.
dAn immediate for the iohl instruction. const_int is treated as a 64 bit value.
fAn immediate which can be loaded with fsmbi.
AAn immediate which can be loaded with the il/ila/ilh/ilhu instructions. const_int is treated as a 32 bit value.
BAn immediate for most arithmetic instructions. const_int is treated as a 32 bit value.
CAn immediate for and/xor/or instructions. const_int is treated as a 32 bit value.
DAn immediate for the iohl instruction. const_int is treated as a 32 bit value.
IA constant in the range [-64, 63] for shift/rotate instructions.
JAn unsigned 7-bit constant for conversion/nop/channel instructions.
KA signed 10-bit constant for most arithmetic instructions.
MA signed 16 bit immediate for stop.
NAn unsigned 16-bit constant for iohl and fsmbi.
OAn unsigned 7-bit constant whose 3 least significant bits are 0.
PAn unsigned 3-bit constant for 16-byte rotates and shifts
RCall operand, reg, for indirect calls
SCall operand, symbol, for relative calls.
TCall operand, const_int, for absolute calls.
UAn immediate which can be loaded with the il/ila/ilh/ilhu instructions. const_int is sign extended to 128 bit.
WAn immediate for shift and rotate instructions. const_int is treated as a 32 bit value.
YAn immediate for and/xor/or instructions. const_int is sign extended as a 128 bit.
ZAn immediate for the iohl instruction. const_int is sign extended to 128 bit.
aAddress register (general purpose register except r0)
cCondition code register
dData register (arbitrary general purpose register)
fFloating-point register
IUnsigned 8-bit constant (0–255)
JUnsigned 12-bit constant (0–4095)
KSigned 16-bit constant (-32768–32767)
LValue appropriate as displacement.
(0..4095)for short displacement
(-524288..524287)for long displacement
MConstant integer with a value of 0x7fffffff.
NMultiple letter constraint followed by 4 parameter letters.
0..9:number of the part counting from most to least significant
H,Q:mode of the part
D,S,H:mode of the containing operand
0,F:value of the other parts (F—all bits set)
The constraint matches if the specified part of a constant has a value different from its other parts.
QMemory reference without index register and with short displacement.
RMemory reference with index register and short displacement.
SMemory reference without index register but with long displacement.
TMemory reference with index register and long displacement.
UPointer with short displacement.
WPointer with long displacement.
YShift count operand.
dRegisters from r0 to r32.
eRegisters from r0 to r16.
tr8—r11 or r22—r27 registers.
hhi register.
llo register.
xhi + lo register.
qcnt register.
ylcb register.
zscb register.
acnt + lcb + scb register.
ccr0—cr15 register.
bcp1 registers.
fcp2 registers.
icp3 registers.
jcp1 + cp2 + cp3 registers.
IHigh 16-bit constant (32-bit constant with 16 LSBs zero).
JUnsigned 5 bit integer (in the range 0 to 31).
KUnsigned 16 bit integer (in the range 0 to 65535).
LSigned 16 bit integer (in the range -32768 to 32767).
MUnsigned 14 bit integer (in the range 0 to 16383).
NSigned 14 bit integer (in the range -8192 to 8191).
ZAny SYMBOL_REF.
aRegister r0.
bRegister r1.
cRegister r2.
dRegister r8.
eRegisters r0 through r7.
tRegisters r0 and r1.
yThe carry register.
zRegisters r8 and r9.
IA constant between 0 and 3 inclusive.
JA constant that has exactly one bit set.
KA constant that has exactly one bit clear.
LA constant between 0 and 255 inclusive.
MA constant between -255 and 0 inclusive.
NA constant between -3 and 0 inclusive.
OA constant between 1 and 4 inclusive.
PA constant between -4 and -1 inclusive.
QA memory reference that is a stack push.
RA memory reference that is a stack pop.
SA memory reference that refers to a constant address of known value.
TThe register indicated by Rx (not implemented yet).
UA constant that is not between 2 and 15 inclusive.
ZThe constant 0.
aGeneral-purpose 32-bit register
bOne-bit boolean register
AMAC16 40-bit accumulator register
ISigned 12-bit integer constant, for use in MOVI instructions
JSigned 8-bit integer constant, for use in ADDI instructions
KInteger constant valid for BccI instructions
LUnsigned constant valid for BccUI instructions
Next: Define Constraints, Previous: Machine Constraints, Up: Constraints [Contents][Index]
enabled attributeThe enabled insn attribute may be used to disable certain insn
alternatives for machine-specific reasons. This is useful when adding
new instructions to an existing pattern which are only available for
certain cpu architecture levels as specified with the -march=
option.
If an insn alternative is disabled, then it will never be used. The compiler treats the constraints for the disabled alternative as unsatisfiable.
In order to make use of the enabled attribute a back end has to add
in the machine description files:
enabled insn attribute. The attribute is
defined as usual using the define_attr command. This
definition should be based on other insn attributes and/or target flags.
The enabled attribute is a numeric attribute and should evaluate to
(const_int 1) for an enabled alternative and to
(const_int 0) otherwise.
cpu_facility as in the example below.
E.g. the following two patterns could easily be merged using the enabled
attribute:
(define_insn "*movdi_old"
[(set (match_operand:DI 0 "register_operand" "=d")
(match_operand:DI 1 "register_operand" " d"))]
"!TARGET_NEW"
"lgr %0,%1")
(define_insn "*movdi_new"
[(set (match_operand:DI 0 "register_operand" "=d,f,d")
(match_operand:DI 1 "register_operand" " d,d,f"))]
"TARGET_NEW"
"@
lgr %0,%1
ldgr %0,%1
lgdr %0,%1")
to:
(define_insn "*movdi_combined"
[(set (match_operand:DI 0 "register_operand" "=d,f,d")
(match_operand:DI 1 "register_operand" " d,d,f"))]
""
"@
lgr %0,%1
ldgr %0,%1
lgdr %0,%1"
[(set_attr "cpu_facility" "*,new,new")])
with the enabled attribute defined like this:
(define_attr "cpu_facility" "standard,new" (const_string "standard"))
(define_attr "enabled" ""
(cond [(eq_attr "cpu_facility" "standard") (const_int 1)
(and (eq_attr "cpu_facility" "new")
(ne (symbol_ref "TARGET_NEW") (const_int 0)))
(const_int 1)]
(const_int 0)))
Next: C Constraint Interface, Previous: Disable Insn Alternatives, Up: Constraints [Contents][Index]
Machine-specific constraints fall into two categories: register and
non-register constraints. Within the latter category, constraints
which allow subsets of all possible memory or address operands should
be specially marked, to give reload more information.
Machine-specific constraints can be given names of arbitrary length, but they must be entirely composed of letters, digits, underscores (‘_’), and angle brackets (‘< >’). Like C identifiers, they must begin with a letter or underscore.
In order to avoid ambiguity in operand constraint strings, no
constraint can have a name that begins with any other constraint’s
name. For example, if x is defined as a constraint name,
xy may not be, and vice versa. As a consequence of this rule,
no constraint may begin with one of the generic constraint letters:
‘E F V X g i m n o p r s’.
Register constraints correspond directly to register classes. See Register Classes. There is thus not much flexibility in their definitions.
All three arguments are string constants.
name is the name of the constraint, as it will appear in
match_operand expressions. If name is a multi-letter
constraint its length shall be the same for all constraints starting
with the same letter. regclass can be either the
name of the corresponding register class (see Register Classes),
or a C expression which evaluates to the appropriate register class.
If it is an expression, it must have no side effects, and it cannot
look at the operand. The usual use of expressions is to map some
register constraints to NO_REGS when the register class
is not available on a given subarchitecture.
docstring is a sentence documenting the meaning of the constraint. Docstrings are explained further below.
Non-register constraints are more like predicates: the constraint definition gives a Boolean expression which indicates whether the constraint matches.
The name and docstring arguments are the same as for
define_register_constraint, but note that the docstring comes
immediately after the name for these expressions. exp is an RTL
expression, obeying the same rules as the RTL expressions in predicate
definitions. See Defining Predicates, for details. If it
evaluates true, the constraint matches; if it evaluates false, it
doesn’t. Constraint expressions should indicate which RTL codes they
might match, just like predicate expressions.
match_test C expressions have access to the
following variables:
The RTL object defining the operand.
The machine mode of op.
‘INTVAL (op)’, if op is a const_int.
‘CONST_DOUBLE_HIGH (op)’, if op is an integer
const_double.
‘CONST_DOUBLE_LOW (op)’, if op is an integer
const_double.
‘CONST_DOUBLE_REAL_VALUE (op)’, if op is a floating-point
const_double.
The *val variables should only be used once another piece of the expression has verified that op is the appropriate kind of RTL object.
Most non-register constraints should be defined with
define_constraint. The remaining two definition expressions
are only appropriate for constraints that should be handled specially
by reload if they fail to match.
Use this expression for constraints that match a subset of all memory
operands: that is, reload can make them match by converting the
operand to the form ‘(mem (reg X))’, where X is a
base register (from the register class specified by
BASE_REG_CLASS, see Register Classes).
For example, on the S/390, some instructions do not accept arbitrary
memory references, but only those that do not make use of an index
register. The constraint letter ‘Q’ is defined to represent a
memory address of this type. If ‘Q’ is defined with
define_memory_constraint, a ‘Q’ constraint can handle any
memory operand, because reload knows it can simply copy the
memory address into a base register if required. This is analogous to
the way an ‘o’ constraint can handle any memory operand.
The syntax and semantics are otherwise identical to
define_constraint.
Use this expression for constraints that match a subset of all address
operands: that is, reload can make the constraint match by
converting the operand to the form ‘(reg X)’, again
with X a base register.
Constraints defined with define_address_constraint can only be
used with the address_operand predicate, or machine-specific
predicates that work the same way. They are treated analogously to
the generic ‘p’ constraint.
The syntax and semantics are otherwise identical to
define_constraint.
For historical reasons, names beginning with the letters ‘G H’
are reserved for constraints that match only const_doubles, and
names beginning with the letters ‘I J K L M N O P’ are reserved
for constraints that match only const_ints. This may change in
the future. For the time being, constraints with these names must be
written in a stylized form, so that genpreds can tell you did
it correctly:
(define_constraint "[GHIJKLMNOP]…" "doc…" (and (match_code "const_int") ;const_doublefor G/H condition…)) ; usually amatch_test
It is fine to use names beginning with other letters for constraints
that match const_doubles or const_ints.
Each docstring in a constraint definition should be one or more complete
sentences, marked up in Texinfo format. They are currently unused.
In the future they will be copied into the GCC manual, in Machine Constraints, replacing the hand-maintained tables currently found in
that section. Also, in the future the compiler may use this to give
more helpful diagnostics when poor choice of asm constraints
causes a reload failure.
If you put the pseudo-Texinfo directive ‘@internal’ at the
beginning of a docstring, then (in the future) it will appear only in
the internals manual’s version of the machine-specific constraint tables.
Use this for constraints that should not appear in asm statements.
Previous: Define Constraints, Up: Constraints [Contents][Index]
It is occasionally useful to test a constraint from C code rather than
implicitly via the constraint string in a match_operand. The
generated file tm_p.h declares a few interfaces for working
with machine-specific constraints. None of these interfaces work with
the generic constraints described in Simple Constraints. This
may change in the future.
Warning: tm_p.h may declare other functions that operate on constraints, besides the ones documented here. Do not use those functions from machine-dependent code. They exist to implement the old constraint interface that machine-independent components of the compiler still expect. They will change or disappear in the future.
Some valid constraint names are not valid C identifiers, so there is a mangling scheme for referring to them from C. Constraint names that do not contain angle brackets or underscores are left unchanged. Underscores are doubled, each ‘<’ is replaced with ‘_l’, and each ‘>’ with ‘_g’. Here are some examples:
Original | Mangled |
| |
| |
| |
| |
| |
| |
Throughout this section, the variable c is either a constraint
in the abstract sense, or a constant from enum constraint_num;
the variable m is a mangled constraint name (usually as part of
a larger identifier).
For each machine-specific constraint, there is a corresponding
enumeration constant: ‘CONSTRAINT_’ plus the mangled name of the
constraint. Functions that take an enum constraint_num as an
argument expect one of these constants.
Machine-independent constraints do not have associated constants. This may change in the future.
For each machine-specific, non-register constraint m, there is
one of these functions; it returns true if exp satisfies the
constraint. These functions are only visible if rtl.h was included
before tm_p.h.
Like the satisfies_constraint_m functions, but the
constraint to test is given as an argument, c. If c
specifies a register constraint, this function will always return
false.
Returns the register class associated with c. If c is not
a register constraint, or those registers are not available for the
currently selected subtarget, returns NO_REGS.
Here is an example use of satisfies_constraint_m. In
peephole optimizations (see Peephole Definitions), operand
constraint strings are ignored, so if there are relevant constraints,
they must be tested in the C condition. In the example, the
optimization is applied if operand 2 does not satisfy the
‘K’ constraint. (This is a simplified version of a peephole
definition from the i386 machine description.)
(define_peephole2
[(match_scratch:SI 3 "r")
(set (match_operand:SI 0 "register_operand" "")
(mult:SI (match_operand:SI 1 "memory_operand" "")
(match_operand:SI 2 "immediate_operand" "")))]
"!satisfies_constraint_K (operands[2])"
[(set (match_dup 3) (match_dup 1))
(set (match_dup 0) (mult:SI (match_dup 3) (match_dup 2)))]
"")
Next: Pattern Ordering, Previous: Constraints, Up: Machine Desc [Contents][Index]
Here is a table of the instruction names that are meaningful in the RTL generation pass of the compiler. Giving one of these names to an instruction pattern tells the RTL generation pass that it can use the pattern to accomplish a certain task.
Here m stands for a two-letter machine mode name, in lowercase. This instruction pattern moves data with that machine mode from operand 1 to operand 0. For example, ‘movsi’ moves full-word data.
If operand 0 is a subreg with mode m of a register whose
own mode is wider than m, the effect of this instruction is
to store the specified value in the part of the register that corresponds
to mode m. Bits outside of m, but which are within the
same target word as the subreg are undefined. Bits which are
outside the target word are left unchanged.
This class of patterns is special in several ways. First of all, each of these names up to and including full word size must be defined, because there is no other way to copy a datum from one place to another. If there are patterns accepting operands in larger modes, ‘movm’ must be defined for integer modes of those sizes.
Second, these patterns are not used solely in the RTL generation pass. Even the reload pass can generate move insns to copy values from stack slots into temporary registers. When it does so, one of the operands is a hard register and the other is an operand that can need to be reloaded into a register.
Therefore, when given such a pair of operands, the pattern must generate
RTL which needs no reloading and needs no temporary registers—no
registers other than the operands. For example, if you support the
pattern with a define_expand, then in such a case the
define_expand mustn’t call force_reg or any other such
function which might generate new pseudo registers.
This requirement exists even for subword modes on a RISC machine where fetching those modes from memory normally requires several insns and some temporary registers.
During reload a memory reference with an invalid address may be passed
as an operand. Such an address will be replaced with a valid address
later in the reload pass. In this case, nothing may be done with the
address except to use it as it stands. If it is copied, it will not be
replaced with a valid address. No attempt should be made to make such
an address into a valid address and no routine (such as
change_address) that will do so may be called. Note that
general_operand will fail when applied to such an address.
The global variable reload_in_progress (which must be explicitly
declared if required) can be used to determine whether such special
handling is required.
The variety of operands that have reloads depends on the rest of the machine description, but typically on a RISC machine these can only be pseudo registers that did not get hard registers, while on other machines explicit memory references will get optional reloads.
If a scratch register is required to move an object to or from memory,
it can be allocated using gen_reg_rtx prior to life analysis.
If there are cases which need scratch registers during or after reload, you must provide an appropriate secondary_reload target hook.
The macro can_create_pseudo_p can be used to determine if it
is unsafe to create new pseudo registers. If this variable is nonzero, then
it is unsafe to call gen_reg_rtx to allocate a new pseudo.
The constraints on a ‘movm’ must permit moving any hard
register to any other hard register provided that
HARD_REGNO_MODE_OK permits mode m in both registers and
TARGET_REGISTER_MOVE_COST applied to their classes returns a value
of 2.
It is obligatory to support floating point ‘movm’
instructions into and out of any registers that can hold fixed point
values, because unions and structures (which have modes SImode or
DImode) can be in those registers and they may have floating
point members.
There may also be a need to support fixed point ‘movm’
instructions in and out of floating point registers. Unfortunately, I
have forgotten why this was so, and I don’t know whether it is still
true. If HARD_REGNO_MODE_OK rejects fixed point values in
floating point registers, then the constraints of the fixed point
‘movm’ instructions must be designed to avoid ever trying to
reload into a floating point register.
These named patterns have been obsoleted by the target hook
secondary_reload.
Like ‘movm’, but used when a scratch register is required to
move between operand 0 and operand 1. Operand 2 describes the scratch
register. See the discussion of the SECONDARY_RELOAD_CLASS
macro in see Register Classes.
There are special restrictions on the form of the match_operands
used in these patterns. First, only the predicate for the reload
operand is examined, i.e., reload_in examines operand 1, but not
the predicates for operand 0 or 2. Second, there may be only one
alternative in the constraints. Third, only a single register class
letter may be used for the constraint; subsequent constraint letters
are ignored. As a special exception, an empty constraint string
matches the ALL_REGS register class. This may relieve ports
of the burden of defining an ALL_REGS constraint letter just
for these patterns.
Like ‘movm’ except that if operand 0 is a subreg
with mode m of a register whose natural mode is wider,
the ‘movstrictm’ instruction is guaranteed not to alter
any of the register except the part which belongs to mode m.
This variant of a move pattern is designed to load or store a value from a memory address that is not naturally aligned for its mode. For a store, the memory will be in operand 0; for a load, the memory will be in operand 1. The other operand is guaranteed not to be a memory, so that it’s easy to tell whether this is a load or store.
This pattern is used by the autovectorizer, and when expanding a
MISALIGNED_INDIRECT_REF expression.
Load several consecutive memory locations into consecutive registers. Operand 0 is the first of the consecutive registers, operand 1 is the first memory location, and operand 2 is a constant: the number of consecutive registers.
Define this only if the target machine really has such an instruction; do not define this if the most efficient way of loading consecutive registers from memory is to do them one at a time.
On some machines, there are restrictions as to which consecutive
registers can be stored into memory, such as particular starting or
ending register numbers or only a range of valid counts. For those
machines, use a define_expand (see Expander Definitions)
and make the pattern fail if the restrictions are not met.
Write the generated insn as a parallel with elements being a
set of one register from the appropriate memory location (you may
also need use or clobber elements). Use a
match_parallel (see RTL Template) to recognize the insn. See
rs6000.md for examples of the use of this insn pattern.
Similar to ‘load_multiple’, but store several consecutive registers into consecutive memory locations. Operand 0 is the first of the consecutive memory locations, operand 1 is the first register, and operand 2 is a constant: the number of consecutive registers.
Perform an interleaved load of several vectors from memory operand 1 into register operand 0. Both operands have mode m. The register operand is viewed as holding consecutive vectors of mode n, while the memory operand is a flat array that contains the same number of elements. The operation is equivalent to:
int c = GET_MODE_SIZE (m) / GET_MODE_SIZE (n);
for (j = 0; j < GET_MODE_NUNITS (n); j++)
for (i = 0; i < c; i++)
operand0[i][j] = operand1[j * c + i];
For example, ‘vec_load_lanestiv4hi’ loads 8 16-bit values from memory into a register of mode ‘TI’. The register contains two consecutive vectors of mode ‘V4HI’.
This pattern can only be used if:
TARGET_ARRAY_MODE_SUPPORTED_P (n, c)
is true. GCC assumes that, if a target supports this kind of instruction for some mode n, it also supports unaligned loads for vectors of mode n.
Equivalent to ‘vec_load_lanesmn’, with the memory and register operands reversed. That is, the instruction is equivalent to:
int c = GET_MODE_SIZE (m) / GET_MODE_SIZE (n);
for (j = 0; j < GET_MODE_NUNITS (n); j++)
for (i = 0; i < c; i++)
operand0[j * c + i] = operand1[i][j];
for a memory operand 0 and register operand 1.
Set given field in the vector value. Operand 0 is the vector to modify, operand 1 is new value of field and operand 2 specify the field index.
Extract given field from the vector value. Operand 1 is the vector, operand 2 specify field index and operand 0 place to store value into.
Extract even elements from the input vectors (operand 1 and operand 2). The even elements of operand 2 are concatenated to the even elements of operand 1 in their original order. The result is stored in operand 0. The output and input vectors should have the same modes.
Extract odd elements from the input vectors (operand 1 and operand 2). The odd elements of operand 2 are concatenated to the odd elements of operand 1 in their original order. The result is stored in operand 0. The output and input vectors should have the same modes.
Merge high elements of the two input vectors into the output vector. The output
and input vectors should have the same modes (N elements). The high
N/2 elements of the first input vector are interleaved with the high
N/2 elements of the second input vector.
Merge low elements of the two input vectors into the output vector. The output
and input vectors should have the same modes (N elements). The low
N/2 elements of the first input vector are interleaved with the low
N/2 elements of the second input vector.
Initialize the vector to given values. Operand 0 is the vector to initialize and operand 1 is parallel containing values for individual fields.
Output a push instruction. Operand 0 is value to push. Used only when
PUSH_ROUNDING is defined. For historical reason, this pattern may be
missing and in such case an mov expander is used instead, with a
MEM expression forming the push operation. The mov expander
method is deprecated.
Add operand 2 and operand 1, storing the result in operand 0. All operands must have mode m. This can be used even on two-address machines, by means of constraints requiring operands 1 and 0 to be the same location.
Similar, for other arithmetic operations.
Multiply operand 2 and operand 1, then add operand 3, storing the
result in operand 0. All operands must have mode m. This
pattern is used to implement the fma, fmaf, and
fmal builtin functions from the ISO C99 standard. The
fma operation may produce different results than doing the
multiply followed by the add if the machine does not perform a
rounding step between the operations.
Like fmam4, except operand 3 subtracted from the
product instead of added to the product. This is represented
in the rtl as
(fma:m op1 op2 (neg:m op3))
Like fmam4 except that the intermediate product
is negated before being added to operand 3. This is represented
in the rtl as
(fma:m (neg:m op1) op2 op3)
Like fmsm4 except that the intermediate product
is negated before subtracting operand 3. This is represented
in the rtl as
(fma:m (neg:m op1) op2 (neg:m op3))
Signed minimum and maximum operations. When used with floating point,
if both operands are zeros, or if either operand is NaN, then
it is unspecified which of the two operands is returned as the result.
Find the signed minimum/maximum of the elements of a vector. The vector is operand 1, and the scalar result is stored in the least significant bits of operand 0 (also a vector). The output and input vector should have the same modes.
Find the unsigned minimum/maximum of the elements of a vector. The vector is operand 1, and the scalar result is stored in the least significant bits of operand 0 (also a vector). The output and input vector should have the same modes.
Compute the sum of the signed elements of a vector. The vector is operand 1, and the scalar result is stored in the least significant bits of operand 0 (also a vector). The output and input vector should have the same modes.
Compute the sum of the unsigned elements of a vector. The vector is operand 1, and the scalar result is stored in the least significant bits of operand 0 (also a vector). The output and input vector should have the same modes.
Compute the sum of the products of two signed/unsigned elements. Operand 1 and operand 2 are of the same mode. Their product, which is of a wider mode, is computed and added to operand 3. Operand 3 is of a mode equal or wider than the mode of the product. The result is placed in operand 0, which is of the same mode as operand 3.
Operands 0 and 2 are of the same mode, which is wider than the mode of operand 1. Add operand 1 to operand 2 and place the widened result in operand 0. (This is used express accumulation of elements into an accumulator of a wider mode.)
Whole vector left/right shift in bits. Operand 1 is a vector to be shifted. Operand 2 is an integer shift amount in bits. Operand 0 is where the resulting shifted vector is stored. The output and input vectors should have the same modes.
Narrow (demote) and merge the elements of two vectors. Operands 1 and 2 are vectors of the same mode having N integral or floating point elements of size S. Operand 0 is the resulting vector in which 2*N elements of size N/2 are concatenated after narrowing them down using truncation.
Narrow (demote) and merge the elements of two vectors. Operands 1 and 2 are vectors of the same mode having N integral elements of size S. Operand 0 is the resulting vector in which the elements of the two input vectors are concatenated after narrowing them down using signed/unsigned saturating arithmetic.
Narrow, convert to signed/unsigned integral type and merge the elements of two vectors. Operands 1 and 2 are vectors of the same mode having N floating point elements of size S. Operand 0 is the resulting vector in which 2*N elements of size N/2 are concatenated.
Extract and widen (promote) the high/low part of a vector of signed integral or floating point elements. The input vector (operand 1) has N elements of size S. Widen (promote) the high/low elements of the vector using signed or floating point extension and place the resulting N/2 values of size 2*S in the output vector (operand 0).
Extract and widen (promote) the high/low part of a vector of unsigned integral elements. The input vector (operand 1) has N elements of size S. Widen (promote) the high/low elements of the vector using zero extension and place the resulting N/2 values of size 2*S in the output vector (operand 0).
Extract, convert to floating point type and widen the high/low part of a vector of signed/unsigned integral elements. The input vector (operand 1) has N elements of size S. Convert the high/low elements of the vector using floating point conversion and place the resulting N/2 values of size 2*S in the output vector (operand 0).
Signed/Unsigned widening multiplication. The two inputs (operands 1 and 2) are vectors with N signed/unsigned elements of size S. Multiply the high/low elements of the two vectors, and put the N/2 products of size 2*S in the output vector (operand 0).
Signed/Unsigned widening shift left. The first input (operand 1) is a vector with N signed/unsigned elements of size S. Operand 2 is a constant. Shift the high/low elements of operand 1, and put the N/2 results of size 2*S in the output vector (operand 0).
Multiply operands 1 and 2, which have mode HImode, and store
a SImode product in operand 0.
Similar widening-multiplication instructions of other widths.
Similar widening-multiplication instructions that do unsigned multiplication.
Similar widening-multiplication instructions that interpret the first operand as unsigned and the second operand as signed, then do a signed multiplication.
Perform a signed multiplication of operands 1 and 2, which have mode m, and store the most significant half of the product in operand 0. The least significant half of the product is discarded.
Similar, but the multiplication is unsigned.
Multiply operands 1 and 2, sign-extend them to mode n, add operand 3, and store the result in operand 0. Operands 1 and 2 have mode m and operands 0 and 3 have mode n. Both modes must be integer or fixed-point modes and n must be twice the size of m.
In other words, maddmn4 is like
mulmn3 except that it also adds operand 3.
These instructions are not allowed to FAIL.
Like maddmn4, but zero-extend the multiplication
operands instead of sign-extending them.
Like maddmn4, but all involved operations must be
signed-saturating.
Like umaddmn4, but all involved operations must be
unsigned-saturating.
Multiply operands 1 and 2, sign-extend them to mode n, subtract the result from operand 3, and store the result in operand 0. Operands 1 and 2 have mode m and operands 0 and 3 have mode n. Both modes must be integer or fixed-point modes and n must be twice the size of m.
In other words, msubmn4 is like
mulmn3 except that it also subtracts the result
from operand 3.
These instructions are not allowed to FAIL.
Like msubmn4, but zero-extend the multiplication
operands instead of sign-extending them.
Like msubmn4, but all involved operations must be
signed-saturating.
Like umsubmn4, but all involved operations must be
unsigned-saturating.
Signed division that produces both a quotient and a remainder. Operand 1 is divided by operand 2 to produce a quotient stored in operand 0 and a remainder stored in operand 3.
For machines with an instruction that produces both a quotient and a remainder, provide a pattern for ‘divmodm4’ but do not provide patterns for ‘divm3’ and ‘modm3’. This allows optimization in the relatively common case when both the quotient and remainder are computed.
If an instruction that just produces a quotient or just a remainder
exists and is more efficient than the instruction that produces both,
write the output routine of ‘divmodm4’ to call
find_reg_note and look for a REG_UNUSED note on the
quotient or remainder and generate the appropriate instruction.
Similar, but does unsigned division.
Arithmetic-shift operand 1 left by a number of bits specified by operand
2, and store the result in operand 0. Here m is the mode of
operand 0 and operand 1; operand 2’s mode is specified by the
instruction pattern, and the compiler will convert the operand to that
mode before generating the instruction. The meaning of out-of-range shift
counts can optionally be specified by TARGET_SHIFT_TRUNCATION_MASK.
See TARGET_SHIFT_TRUNCATION_MASK. Operand 2 is always a scalar type.
Other shift and rotate instructions, analogous to the
ashlm3 instructions. Operand 2 is always a scalar type.
Vector shift and rotate instructions that take vectors as operand 2 instead of a scalar type.
Negate operand 1 and store the result in operand 0.
Store the absolute value of operand 1 into operand 0.
Store the square root of operand 1 into operand 0.
The sqrt built-in function of C always uses the mode which
corresponds to the C data type double and the sqrtf
built-in function uses the mode which corresponds to the C data
type float.
Store the remainder of dividing operand 1 by operand 2 into operand 0, rounded towards zero to an integer.
The fmod built-in function of C always uses the mode which
corresponds to the C data type double and the fmodf
built-in function uses the mode which corresponds to the C data
type float.
Store the remainder of dividing operand 1 by operand 2 into operand 0, rounded to the nearest integer.
The remainder built-in function of C always uses the mode
which corresponds to the C data type double and the
remainderf built-in function uses the mode which corresponds
to the C data type float.
Store the cosine of operand 1 into operand 0.
The cos built-in function of C always uses the mode which
corresponds to the C data type double and the cosf
built-in function uses the mode which corresponds to the C data
type float.
Store the sine of operand 1 into operand 0.
The sin built-in function of C always uses the mode which
corresponds to the C data type double and the sinf
built-in function uses the mode which corresponds to the C data
type float.
Store the exponential of operand 1 into operand 0.
The exp built-in function of C always uses the mode which
corresponds to the C data type double and the expf
built-in function uses the mode which corresponds to the C data
type float.
Store the natural logarithm of operand 1 into operand 0.
The log built-in function of C always uses the mode which
corresponds to the C data type double and the logf
built-in function uses the mode which corresponds to the C data
type float.
Store the value of operand 1 raised to the exponent operand 2 into operand 0.
The pow built-in function of C always uses the mode which
corresponds to the C data type double and the powf
built-in function uses the mode which corresponds to the C data
type float.
Store the arc tangent (inverse tangent) of operand 1 divided by operand 2 into operand 0, using the signs of both arguments to determine the quadrant of the result.
The atan2 built-in function of C always uses the mode which
corresponds to the C data type double and the atan2f
built-in function uses the mode which corresponds to the C data
type float.
Store the largest integral value not greater than argument.
The floor built-in function of C always uses the mode which
corresponds to the C data type double and the floorf
built-in function uses the mode which corresponds to the C data
type float.
Store the argument rounded to integer towards zero.
The trunc built-in function of C always uses the mode which
corresponds to the C data type double and the truncf
built-in function uses the mode which corresponds to the C data
type float.
Store the argument rounded to integer away from zero.
The round built-in function of C always uses the mode which
corresponds to the C data type double and the roundf
built-in function uses the mode which corresponds to the C data
type float.
Store the argument rounded to integer away from zero.
The ceil built-in function of C always uses the mode which
corresponds to the C data type double and the ceilf
built-in function uses the mode which corresponds to the C data
type float.
Store the argument rounded according to the default rounding mode
The nearbyint built-in function of C always uses the mode which
corresponds to the C data type double and the nearbyintf
built-in function uses the mode which corresponds to the C data
type float.
Store the argument rounded according to the default rounding mode and raise the inexact exception when the result differs in value from the argument
The rint built-in function of C always uses the mode which
corresponds to the C data type double and the rintf
built-in function uses the mode which corresponds to the C data
type float.
Convert operand 1 (valid for floating point mode m) to fixed point mode n as a signed number according to the current rounding mode and store in operand 0 (which has mode n).
Convert operand 1 (valid for floating point mode m) to fixed point mode n as a signed number rounding to nearest and away from zero and store in operand 0 (which has mode n).
Convert operand 1 (valid for floating point mode m) to fixed point mode n as a signed number rounding down and store in operand 0 (which has mode n).
Convert operand 1 (valid for floating point mode m) to fixed point mode n as a signed number rounding up and store in operand 0 (which has mode n).
Store a value with the magnitude of operand 1 and the sign of operand 2 into operand 0.
The copysign built-in function of C always uses the mode which
corresponds to the C data type double and the copysignf
built-in function uses the mode which corresponds to the C data
type float.
Store into operand 0 one plus the index of the least significant 1-bit of operand 1. If operand 1 is zero, store zero. m is the mode of operand 0; operand 1’s mode is specified by the instruction pattern, and the compiler will convert the operand to that mode before generating the instruction.
The ffs built-in function of C always uses the mode which
corresponds to the C data type int.
Store into operand 0 the number of leading 0-bits in x, starting
at the most significant bit position. If x is 0, the
CLZ_DEFINED_VALUE_AT_ZERO (see Misc) macro defines if
the result is undefined or has a useful value.
m is the mode of operand 0; operand 1’s mode is
specified by the instruction pattern, and the compiler will convert the
operand to that mode before generating the instruction.
Store into operand 0 the number of trailing 0-bits in x, starting
at the least significant bit position. If x is 0, the
CTZ_DEFINED_VALUE_AT_ZERO (see Misc) macro defines if
the result is undefined or has a useful value.
m is the mode of operand 0; operand 1’s mode is
specified by the instruction pattern, and the compiler will convert the
operand to that mode before generating the instruction.
Store into operand 0 the number of 1-bits in x. m is the mode of operand 0; operand 1’s mode is specified by the instruction pattern, and the compiler will convert the operand to that mode before generating the instruction.
Store into operand 0 the parity of x, i.e. the number of 1-bits in x modulo 2. m is the mode of operand 0; operand 1’s mode is specified by the instruction pattern, and the compiler will convert the operand to that mode before generating the instruction.
Store the bitwise-complement of operand 1 into operand 0.
Block move instruction. The destination and source blocks of memory
are the first two operands, and both are mem:BLKs with an
address in mode Pmode.
The number of bytes to move is the third operand, in mode m.
Usually, you specify word_mode for m. However, if you can
generate better code knowing the range of valid lengths is smaller than
those representable in a full word, you should provide a pattern with a
mode corresponding to the range of values you can handle efficiently
(e.g., QImode for values in the range 0–127; note we avoid numbers
that appear negative) and also a pattern with word_mode.
The fourth operand is the known shared alignment of the source and
destination, in the form of a const_int rtx. Thus, if the
compiler knows that both source and destination are word-aligned,
it may provide the value 4 for this operand.
Optional operands 5 and 6 specify expected alignment and size of block
respectively. The expected alignment differs from alignment in operand 4
in a way that the blocks are not required to be aligned according to it in
all cases. This expected alignment is also in bytes, just like operand 4.
Expected size, when unknown, is set to (const_int -1).
Descriptions of multiple movmemm patterns can only be
beneficial if the patterns for smaller modes have fewer restrictions
on their first, second and fourth operands. Note that the mode m
in movmemm does not impose any restriction on the mode of
individually moved data units in the block.
These patterns need not give special consideration to the possibility that the source and destination strings might overlap.
String copy instruction, with stpcpy semantics. Operand 0 is
an output operand in mode Pmode. The addresses of the
destination and source strings are operands 1 and 2, and both are
mem:BLKs with addresses in mode Pmode. The execution of
the expansion of this pattern should store in operand 0 the address in
which the NUL terminator was stored in the destination string.
Block set instruction. The destination string is the first operand,
given as a mem:BLK whose address is in mode Pmode. The
number of bytes to set is the second operand, in mode m. The value to
initialize the memory with is the third operand. Targets that only support the
clearing of memory should reject any value that is not the constant 0. See
‘movmemm’ for a discussion of the choice of mode.
The fourth operand is the known alignment of the destination, in the form
of a const_int rtx. Thus, if the compiler knows that the
destination is word-aligned, it may provide the value 4 for this
operand.
Optional operands 5 and 6 specify expected alignment and size of block
respectively. The expected alignment differs from alignment in operand 4
in a way that the blocks are not required to be aligned according to it in
all cases. This expected alignment is also in bytes, just like operand 4.
Expected size, when unknown, is set to (const_int -1).
The use for multiple setmemm is as for movmemm.
String compare instruction, with five operands. Operand 0 is the output; it has mode m. The remaining four operands are like the operands of ‘movmemm’. The two memory blocks specified are compared byte by byte in lexicographic order starting at the beginning of each string. The instruction is not allowed to prefetch more than one byte at a time since either string may end in the first byte and reading past that may access an invalid page or segment and cause a fault. The comparison terminates early if the fetched bytes are different or if they are equal to zero. The effect of the instruction is to store a value in operand 0 whose sign indicates the result of the comparison.
String compare instruction, without known maximum length. Operand 0 is the
output; it has mode m. The second and third operand are the blocks of
memory to be compared; both are mem:BLK with an address in mode
Pmode.
The fourth operand is the known shared alignment of the source and
destination, in the form of a const_int rtx. Thus, if the
compiler knows that both source and destination are word-aligned,
it may provide the value 4 for this operand.
The two memory blocks specified are compared byte by byte in lexicographic order starting at the beginning of each string. The instruction is not allowed to prefetch more than one byte at a time since either string may end in the first byte and reading past that may access an invalid page or segment and cause a fault. The comparison will terminate when the fetched bytes are different or if they are equal to zero. The effect of the instruction is to store a value in operand 0 whose sign indicates the result of the comparison.
Block compare instruction, with five operands like the operands of ‘cmpstrm’. The two memory blocks specified are compared byte by byte in lexicographic order starting at the beginning of each block. Unlike ‘cmpstrm’ the instruction can prefetch any bytes in the two memory blocks. Also unlike ‘cmpstrm’ the comparison will not stop if both bytes are zero. The effect of the instruction is to store a value in operand 0 whose sign indicates the result of the comparison.
Compute the length of a string, with three operands.
Operand 0 is the result (of mode m), operand 1 is
a mem referring to the first character of the string,
operand 2 is the character to search for (normally zero),
and operand 3 is a constant describing the known alignment
of the beginning of the string.
Convert signed integer operand 1 (valid for fixed point mode m) to floating point mode n and store in operand 0 (which has mode n).
Convert unsigned integer operand 1 (valid for fixed point mode m) to floating point mode n and store in operand 0 (which has mode n).
Convert operand 1 (valid for floating point mode m) to fixed point mode n as a signed number and store in operand 0 (which has mode n). This instruction’s result is defined only when the value of operand 1 is an integer.
If the machine description defines this pattern, it also needs to
define the ftrunc pattern.
Convert operand 1 (valid for floating point mode m) to fixed point mode n as an unsigned number and store in operand 0 (which has mode n). This instruction’s result is defined only when the value of operand 1 is an integer.
Convert operand 1 (valid for floating point mode m) to an integer value, still represented in floating point mode m, and store it in operand 0 (valid for floating point mode m).
Like ‘fixmn2’ but works for any floating point value of mode m by converting the value to an integer.
Like ‘fixunsmn2’ but works for any floating point value of mode m by converting the value to an integer.
Truncate operand 1 (valid for mode m) to mode n and store in operand 0 (which has mode n). Both modes must be fixed point or both floating point.
Sign-extend operand 1 (valid for mode m) to mode n and store in operand 0 (which has mode n). Both modes must be fixed point or both floating point.
Zero-extend operand 1 (valid for mode m) to mode n and store in operand 0 (which has mode n). Both modes must be fixed point.
Convert operand 1 of mode m to mode n and store in operand 0 (which has mode n). Mode m and mode n could be fixed-point to fixed-point, signed integer to fixed-point, fixed-point to signed integer, floating-point to fixed-point, or fixed-point to floating-point. When overflows or underflows happen, the results are undefined.
Convert operand 1 of mode m to mode n and store in operand 0 (which has mode n). Mode m and mode n could be fixed-point to fixed-point, signed integer to fixed-point, or floating-point to fixed-point. When overflows or underflows happen, the instruction saturates the results to the maximum or the minimum.
Convert operand 1 of mode m to mode n and store in operand 0 (which has mode n). Mode m and mode n could be unsigned integer to fixed-point, or fixed-point to unsigned integer. When overflows or underflows happen, the results are undefined.
Convert unsigned integer operand 1 of mode m to fixed-point mode n and store in operand 0 (which has mode n). When overflows or underflows happen, the instruction saturates the results to the maximum or the minimum.
Extract a bit-field from operand 1 (a register or memory operand), where
operand 2 specifies the width in bits and operand 3 the starting bit,
and store it in operand 0. Operand 0 must have mode word_mode.
Operand 1 may have mode byte_mode or word_mode; often
word_mode is allowed only for registers. Operands 2 and 3 must
be valid for word_mode.
The RTL generation pass generates this instruction only with constants for operands 2 and 3 and the constant is never zero for operand 2.
The bit-field value is sign-extended to a full word integer before it is stored in operand 0.
Like ‘extv’ except that the bit-field value is zero-extended.
Store operand 3 (which must be valid for word_mode) into a
bit-field in operand 0, where operand 1 specifies the width in bits and
operand 2 the starting bit. Operand 0 may have mode byte_mode or
word_mode; often word_mode is allowed only for registers.
Operands 1 and 2 must be valid for word_mode.
The RTL generation pass generates this instruction only with constants for operands 1 and 2 and the constant is never zero for operand 1.
Conditionally move operand 2 or operand 3 into operand 0 according to the comparison in operand 1. If the comparison is true, operand 2 is moved into operand 0, otherwise operand 3 is moved.
The mode of the operands being compared need not be the same as the operands being moved. Some machines, sparc64 for example, have instructions that conditionally move an integer value based on the floating point condition codes and vice versa.
If the machine does not have conditional move instructions, do not define these patterns.
Similar to ‘movmodecc’ but for conditional addition. Conditionally move operand 2 or (operands 2 + operand 3) into operand 0 according to the comparison in operand 1. If the comparison is true, operand 2 is moved into operand 0, otherwise (operand 2 + operand 3) is moved.
Store zero or nonzero in operand 0 according to whether a comparison
is true. Operand 1 is a comparison operator. Operand 2 and operand 3
are the first and second operand of the comparison, respectively.
You specify the mode that operand 0 must have when you write the
match_operand expression. The compiler automatically sees which
mode you have used and supplies an operand of that mode.
The value stored for a true condition must have 1 as its low bit, or
else must be negative. Otherwise the instruction is not suitable and
you should omit it from the machine description. You describe to the
compiler exactly which value is stored by defining the macro
STORE_FLAG_VALUE (see Misc). If a description cannot be
found that can be used for all the possible comparison operators, you
should pick one and use a define_expand to map all results
onto the one you chose.
These operations may FAIL, but should do so only in relatively
uncommon cases; if they would FAIL for common cases involving
integer comparisons, it is best to restrict the predicates to not
allow these operands. Likewise if a given comparison operator will
always fail, independent of the operands (for floating-point modes, the
ordered_comparison_operator predicate is often useful in this case).
If this pattern is omitted, the compiler will generate a conditional
branch—for example, it may copy a constant one to the target and branching
around an assignment of zero to the target—or a libcall. If the predicate
for operand 1 only rejects some operators, it will also try reordering the
operands and/or inverting the result value (e.g. by an exclusive OR).
These possibilities could be cheaper or equivalent to the instructions
used for the ‘cstoremode4’ pattern followed by those required
to convert a positive result from STORE_FLAG_VALUE to 1; in this
case, you can and should make operand 1’s predicate reject some operators
in the ‘cstoremode4’ pattern, or remove the pattern altogether
from the machine description.
Conditional branch instruction combined with a compare instruction.
Operand 0 is a comparison operator. Operand 1 and operand 2 are the
first and second operands of the comparison, respectively. Operand 3
is a label_ref that refers to the label to jump to.
A jump inside a function; an unconditional branch. Operand 0 is the
label_ref of the label to jump to. This pattern name is mandatory
on all machines.
Subroutine call instruction returning no value. Operand 0 is the
function to call; operand 1 is the number of bytes of arguments pushed
as a const_int; operand 2 is the number of registers used as
operands.
On most machines, operand 2 is not actually stored into the RTL pattern. It is supplied for the sake of some RISC machines which need to put this information into the assembler code; they can put it in the RTL instead of operand 1.
Operand 0 should be a mem RTX whose address is the address of the
function. Note, however, that this address can be a symbol_ref
expression even if it would not be a legitimate memory address on the
target machine. If it is also not a valid argument for a call
instruction, the pattern for this operation should be a
define_expand (see Expander Definitions) that places the
address into a register and uses that register in the call instruction.
Subroutine call instruction returning a value. Operand 0 is the hard register in which the value is returned. There are three more operands, the same as the three operands of the ‘call’ instruction (but with numbers increased by one).
Subroutines that return BLKmode objects use the ‘call’
insn.
Similar to ‘call’ and ‘call_value’, except used if defined and
if RETURN_POPS_ARGS is nonzero. They should emit a parallel
that contains both the function call and a set to indicate the
adjustment made to the frame pointer.
For machines where RETURN_POPS_ARGS can be nonzero, the use of these
patterns increases the number of functions for which the frame pointer
can be eliminated, if desired.
Subroutine call instruction returning a value of any type. Operand 0 is
the function to call; operand 1 is a memory location where the result of
calling the function is to be stored; operand 2 is a parallel
expression where each element is a set expression that indicates
the saving of a function return value into the result block.
This instruction pattern should be defined to support
__builtin_apply on machines where special instructions are needed
to call a subroutine with arbitrary arguments or to save the value
returned. This instruction pattern is required on machines that have
multiple registers that can hold a return value
(i.e. FUNCTION_VALUE_REGNO_P is true for more than one register).
Subroutine return instruction. This instruction pattern name should be defined only if a single instruction can do all the work of returning from a function.
Like the ‘movm’ patterns, this pattern is also used after the RTL generation phase. In this case it is to support machines where multiple instructions are usually needed to return from a function, but some class of functions only requires one instruction to implement a return. Normally, the applicable functions are those which do not need to save any registers or allocate stack space.
For such machines, the condition specified in this pattern should only
be true when reload_completed is nonzero and the function’s
epilogue would only be a single instruction. For machines with register
windows, the routine leaf_function_p may be used to determine if
a register window push is required.
Machines that have conditional return instructions should define patterns such as
(define_insn ""
[(set (pc)
(if_then_else (match_operator
0 "comparison_operator"
[(cc0) (const_int 0)])
(return)
(pc)))]
"condition"
"…")
where condition would normally be the same condition specified on the named ‘return’ pattern.
Untyped subroutine return instruction. This instruction pattern should
be defined to support __builtin_return on machines where special
instructions are needed to return a value of any type.
Operand 0 is a memory location where the result of calling a function
with __builtin_apply is stored; operand 1 is a parallel
expression where each element is a set expression that indicates
the restoring of a function return value from the result block.
No-op instruction. This instruction pattern name should always be defined
to output a no-op in assembler code. (const_int 0) will do as an
RTL pattern.
An instruction to jump to an address which is operand zero. This pattern name is mandatory on all machines.
Instruction to jump through a dispatch table, including bounds checking. This instruction takes five operands:
SImode.
The table is an addr_vec or addr_diff_vec inside of a
jump_insn. The number of elements in the table is one plus the
difference between the upper bound and the lower bound.
Instruction to jump to a variable address. This is a low-level capability which can be used to implement a dispatch table when there is no ‘casesi’ pattern.
This pattern requires two operands: the address or offset, and a label
which should immediately precede the jump table. If the macro
CASE_VECTOR_PC_RELATIVE evaluates to a nonzero value then the first
operand is an offset which counts from the address of the table; otherwise,
it is an absolute address to jump to. In either case, the first operand has
mode Pmode.
The ‘tablejump’ insn is always the last insn before the jump table it uses. Its assembler code normally has no need to use the second operand, but you should incorporate it in the RTL pattern so that the jump optimizer will not delete the table as unreachable code.
Conditional branch instruction that decrements a register and jumps if the register is nonzero. Operand 0 is the register to decrement and test; operand 1 is the label to jump to if the register is nonzero. See Looping Patterns.
This optional instruction pattern is only used by the combiner, typically for loops reversed by the loop optimizer when strength reduction is enabled.
Conditional branch instruction that decrements a register and jumps if
the register is nonzero. This instruction takes five operands: Operand
0 is the register to decrement and test; operand 1 is the number of loop
iterations as a const_int or const0_rtx if this cannot be
determined until run-time; operand 2 is the actual or estimated maximum
number of iterations as a const_int; operand 3 is the number of
enclosed loops as a const_int (an innermost loop has a value of
1); operand 4 is the label to jump to if the register is nonzero.
See Looping Patterns.
This optional instruction pattern should be defined for machines with
low-overhead looping instructions as the loop optimizer will try to
modify suitable loops to utilize it. If nested low-overhead looping is
not supported, use a define_expand (see Expander Definitions)
and make the pattern fail if operand 3 is not const1_rtx.
Similarly, if the actual or estimated maximum number of iterations is
too large for this instruction, make it fail.
Companion instruction to doloop_end required for machines that
need to perform some initialization, such as loading special registers
used by a low-overhead looping instruction. If initialization insns do
not always need to be emitted, use a define_expand
(see Expander Definitions) and make it fail.
Canonicalize the function pointer in operand 1 and store the result into operand 0.
Operand 0 is always a reg and has mode Pmode; operand 1
may be a reg, mem, symbol_ref, const_int, etc
and also has mode Pmode.
Canonicalization of a function pointer usually involves computing the address of the function which would be called if the function pointer were used in an indirect call.
Only define this pattern if function pointers on the target machine can have different values but still call the same function when used in an indirect call.
Most machines save and restore the stack pointer by copying it to or
from an object of mode Pmode. Do not define these patterns on
such machines.
Some machines require special handling for stack pointer saves and
restores. On those machines, define the patterns corresponding to the
non-standard cases by using a define_expand (see Expander Definitions) that produces the required insns. The three types of
saves and restores are:
alloca. Only
the epilogue uses the restored stack pointer, allowing a simpler save or
restore sequence on some machines.
When saving the stack pointer, operand 0 is the save area and operand 1
is the stack pointer. The mode used to allocate the save area defaults
to Pmode but you can override that choice by defining the
STACK_SAVEAREA_MODE macro (see Storage Layout). You must
specify an integral mode, or VOIDmode if no save area is needed
for a particular type of save (either because no save is needed or
because a machine-specific save area can be used). Operand 0 is the
stack pointer and operand 1 is the save area for restore operations. If
‘save_stack_block’ is defined, operand 0 must not be
VOIDmode since these saves can be arbitrarily nested.
A save area is a mem that is at a constant offset from
virtual_stack_vars_rtx when the stack pointer is saved for use by
nonlocal gotos and a reg in the other two cases.
Subtract (or add if STACK_GROWS_DOWNWARD is undefined) operand 1 from
the stack pointer to create space for dynamically allocated data.
Store the resultant pointer to this space into operand 0. If you
are allocating space from the main stack, do this by emitting a
move insn to copy virtual_stack_dynamic_rtx to operand 0.
If you are allocating the space elsewhere, generate code to copy the
location of the space to operand 0. In the latter case, you must
ensure this space gets freed when the corresponding space on the main
stack is free.
Do not define this pattern if all that must be done is the subtraction. Some machines require other operations such as stack probes or maintaining the back chain. Define this pattern to emit those operations in addition to updating the stack pointer.
If stack checking (see Stack Checking) cannot be done on your system by probing the stack, define this pattern to perform the needed check and signal an error if the stack has overflowed. The single operand is the address in the stack farthest from the current stack pointer that you need to validate. Normally, on platforms where this pattern is needed, you would obtain the stack limit from a global or thread-specific variable or register.
If stack checking (see Stack Checking) can be done on your system by probing the stack but doing it with a “store zero” instruction is not valid or optimal, define this pattern to do the probing differently and signal an error if the stack has overflowed. The single operand is the memory reference in the stack that needs to be probed.
Emit code to generate a non-local goto, e.g., a jump from one function to a label in an outer function. This pattern has four arguments, each representing a value to be used in the jump. The first argument is to be loaded into the frame pointer, the second is the address to branch to (code to dispatch to the actual label), the third is the address of a location where the stack is saved, and the last is the address of the label, to be placed in the location for the incoming static chain.
On most machines you need not define this pattern, since GCC will already generate the correct code, which is to load the frame pointer and static chain, restore the stack (using the ‘restore_stack_nonlocal’ pattern, if defined), and jump indirectly to the dispatcher. You need only define this pattern if this code will not work on your machine.
This pattern, if defined, contains code needed at the target of a nonlocal goto after the code already generated by GCC. You will not normally need to define this pattern. A typical reason why you might need this pattern is if some value, such as a pointer to a global table, must be restored when the frame pointer is restored. Note that a nonlocal goto only occurs within a unit-of-translation, so a global table pointer that is shared by all functions of a given module need not be restored. There are no arguments.
This pattern, if defined, contains code needed at the site of an exception handler that isn’t needed at the site of a nonlocal goto. You will not normally need to define this pattern. A typical reason why you might need this pattern is if some value, such as a pointer to a global table, must be restored after control flow is branched to the handler of an exception. There are no arguments.
This pattern, if defined, contains additional code needed to initialize
the jmp_buf. You will not normally need to define this pattern.
A typical reason why you might need this pattern is if some value, such
as a pointer to a global table, must be restored. Though it is
preferred that the pointer value be recalculated if possible (given the
address of a label for instance). The single argument is a pointer to
the jmp_buf. Note that the buffer is five words long and that
the first three are normally used by the generic mechanism.
This pattern, if defined, contains code needed at the site of a built-in setjmp that isn’t needed at the site of a nonlocal goto. You will not normally need to define this pattern. A typical reason why you might need this pattern is if some value, such as a pointer to a global table, must be restored. It takes one argument, which is the label to which builtin_longjmp transfered control; this pattern may be emitted at a small offset from that label.
This pattern, if defined, performs the entire action of the longjmp.
You will not normally need to define this pattern unless you also define
builtin_setjmp_setup. The single argument is a pointer to the
jmp_buf.
This pattern, if defined, affects the way __builtin_eh_return,
and thence the call frame exception handling library routines, are
built. It is intended to handle non-trivial actions needed along
the abnormal return path.
The address of the exception handler to which the function should return
is passed as operand to this pattern. It will normally need to copied by
the pattern to some special register or memory location.
If the pattern needs to determine the location of the target call
frame in order to do so, it may use EH_RETURN_STACKADJ_RTX,
if defined; it will have already been assigned.
If this pattern is not defined, the default action will be to simply
copy the return address to EH_RETURN_HANDLER_RTX. Either
that macro or this pattern needs to be defined if call frame exception
handling is to be used.
This pattern, if defined, emits RTL for entry to a function. The function entry is responsible for setting up the stack frame, initializing the frame pointer register, saving callee saved registers, etc.
Using a prologue pattern is generally preferred over defining
TARGET_ASM_FUNCTION_PROLOGUE to emit assembly code for the prologue.
The prologue pattern is particularly useful for targets which perform
instruction scheduling.
This pattern emits RTL for exit from a function. The function exit is responsible for deallocating the stack frame, restoring callee saved registers and emitting the return instruction.
Using an epilogue pattern is generally preferred over defining
TARGET_ASM_FUNCTION_EPILOGUE to emit assembly code for the epilogue.
The epilogue pattern is particularly useful for targets which perform
instruction scheduling or which have delay slots for their return instruction.
This pattern, if defined, emits RTL for exit from a function without the final branch back to the calling function. This pattern will be emitted before any sibling call (aka tail call) sites.
The sibcall_epilogue pattern must not clobber any arguments used for
parameter passing or any stack slots for arguments passed to the current
function.
This pattern, if defined, signals an error, typically by causing some kind of signal to be raised. Among other places, it is used by the Java front end to signal ‘invalid array index’ exceptions.
Conditional trap instruction. Operand 0 is a piece of RTL which performs a comparison, and operands 1 and 2 are the arms of the comparison. Operand 3 is the trap code, an integer.
A typical ctrap pattern looks like
(define_insn "ctrapsi4"
[(trap_if (match_operator 0 "trap_operator"
[(match_operand 1 "register_operand")
(match_operand 2 "immediate_operand")])
(match_operand 3 "const_int_operand" "i"))]
""
"…")
This pattern, if defined, emits code for a non-faulting data prefetch instruction. Operand 0 is the address of the memory to prefetch. Operand 1 is a constant 1 if the prefetch is preparing for a write to the memory address, or a constant 0 otherwise. Operand 2 is the expected degree of temporal locality of the data and is a value between 0 and 3, inclusive; 0 means that the data has no temporal locality, so it need not be left in the cache after the access; 3 means that the data has a high degree of temporal locality and should be left in all levels of cache possible; 1 and 2 mean, respectively, a low or moderate degree of temporal locality.
Targets that do not support write prefetches or locality hints can ignore the values of operands 1 and 2.
This pattern defines a pseudo insn that prevents the instruction scheduler from moving instructions across the boundary defined by the blockage insn. Normally an UNSPEC_VOLATILE pattern.
If the target memory model is not fully synchronous, then this pattern should be defined to an instruction that orders both loads and stores before the instruction with respect to loads and stores after the instruction. This pattern has no operands.
This pattern, if defined, emits code for an atomic compare-and-swap operation. Operand 1 is the memory on which the atomic operation is performed. Operand 2 is the “old” value to be compared against the current contents of the memory location. Operand 3 is the “new” value to store in the memory if the compare succeeds. Operand 0 is the result of the operation; it should contain the contents of the memory before the operation. If the compare succeeds, this should obviously be a copy of operand 2.
This pattern must show that both operand 0 and operand 1 are modified.
This pattern must issue any memory barrier instructions such that all memory operations before the atomic operation occur before the atomic operation and all memory operations after the atomic operation occur after the atomic operation.
For targets where the success or failure of the compare-and-swap
operation is available via the status flags, it is possible to
avoid a separate compare operation and issue the subsequent
branch or store-flag operation immediately after the compare-and-swap.
To this end, GCC will look for a MODE_CC set in the
output of sync_compare_and_swapmode; if the machine
description includes such a set, the target should also define special
cbranchcc4 and/or cstorecc4 instructions. GCC will then
be able to take the destination of the MODE_CC set and pass it
to the cbranchcc4 or cstorecc4 pattern as the first
operand of the comparison (the second will be (const_int 0)).
These patterns emit code for an atomic operation on memory. Operand 0 is the memory on which the atomic operation is performed. Operand 1 is the second operand to the binary operator.
This pattern must issue any memory barrier instructions such that all memory operations before the atomic operation occur before the atomic operation and all memory operations after the atomic operation occur after the atomic operation.
If these patterns are not defined, the operation will be constructed from a compare-and-swap operation, if defined.
These patterns are emit code for an atomic operation on memory, and return the value that the memory contained before the operation. Operand 0 is the result value, operand 1 is the memory on which the atomic operation is performed, and operand 2 is the second operand to the binary operator.
This pattern must issue any memory barrier instructions such that all memory operations before the atomic operation occur before the atomic operation and all memory operations after the atomic operation occur after the atomic operation.
If these patterns are not defined, the operation will be constructed from a compare-and-swap operation, if defined.
These patterns are like their sync_old_op counterparts,
except that they return the value that exists in the memory location
after the operation, rather than before the operation.
This pattern takes two forms, based on the capabilities of the target. In either case, operand 0 is the result of the operand, operand 1 is the memory on which the atomic operation is performed, and operand 2 is the value to set in the lock.
In the ideal case, this operation is an atomic exchange operation, in which the previous value in memory operand is copied into the result operand, and the value operand is stored in the memory operand.
For less capable targets, any value operand that is not the constant 1
should be rejected with FAIL. In this case the target may use
an atomic test-and-set bit operation. The result operand should contain
1 if the bit was previously set and 0 if the bit was previously clear.
The true contents of the memory operand are implementation defined.
This pattern must issue any memory barrier instructions such that the pattern as a whole acts as an acquire barrier, that is all memory operations after the pattern do not occur until the lock is acquired.
If this pattern is not defined, the operation will be constructed from a compare-and-swap operation, if defined.
This pattern, if defined, releases a lock set by
sync_lock_test_and_setmode. Operand 0 is the memory
that contains the lock; operand 1 is the value to store in the lock.
If the target doesn’t implement full semantics for
sync_lock_test_and_setmode, any value operand which is not
the constant 0 should be rejected with FAIL, and the true contents
of the memory operand are implementation defined.
This pattern must issue any memory barrier instructions such that the pattern as a whole acts as a release barrier, that is the lock is released only after all previous memory operations have completed.
If this pattern is not defined, then a memory_barrier pattern
will be emitted, followed by a store of the value to the memory operand.
This pattern, if defined, moves a ptr_mode value from the memory
in operand 1 to the memory in operand 0 without leaving the value in
a register afterward. This is to avoid leaking the value some place
that an attacker might use to rewrite the stack guard slot after
having clobbered it.
If this pattern is not defined, then a plain move pattern is generated.
This pattern, if defined, compares a ptr_mode value from the
memory in operand 1 with the memory in operand 0 without leaving the
value in a register afterward and branches to operand 2 if the values
weren’t equal.
If this pattern is not defined, then a plain compare pattern and conditional branch pattern is used.
This pattern, if defined, flushes the instruction cache for a region of memory. The region is bounded to by the Pmode pointers in operand 0 inclusive and operand 1 exclusive.
If this pattern is not defined, a call to the library function
__clear_cache is used.
Next: Dependent Patterns, Previous: Standard Names, Up: Machine Desc [Contents][Index]
Sometimes an insn can match more than one instruction pattern. Then the pattern that appears first in the machine description is the one used. Therefore, more specific patterns (patterns that will match fewer things) and faster instructions (those that will produce better code when they do match) should usually go first in the description.
In some cases the effect of ordering the patterns can be used to hide a pattern when it is not valid. For example, the 68000 has an instruction for converting a fullword to floating point and another for converting a byte to floating point. An instruction converting an integer to floating point could match either one. We put the pattern to convert the fullword first to make sure that one will be used rather than the other. (Otherwise a large integer might be generated as a single-byte immediate quantity, which would not work.) Instead of using this pattern ordering it would be possible to make the pattern for convert-a-byte smart enough to deal properly with any constant value.
Next: Jump Patterns, Previous: Pattern Ordering, Up: Machine Desc [Contents][Index]
In some cases machines support instructions identical except for the machine mode of one or more operands. For example, there may be “sign-extend halfword” and “sign-extend byte” instructions whose patterns are
(set (match_operand:SI 0 …)
(extend:SI (match_operand:HI 1 …)))
(set (match_operand:SI 0 …)
(extend:SI (match_operand:QI 1 …)))
Constant integers do not specify a machine mode, so an instruction to
extend a constant value could match either pattern. The pattern it
actually will match is the one that appears first in the file. For correct
results, this must be the one for the widest possible mode (HImode,
here). If the pattern matches the QImode instruction, the results
will be incorrect if the constant value does not actually fit that mode.
Such instructions to extend constants are rarely generated because they are optimized away, but they do occasionally happen in nonoptimized compilations.
If a constraint in a pattern allows a constant, the reload pass may replace a register with a constant permitted by the constraint in some cases. Similarly for memory references. Because of this substitution, you should not provide separate patterns for increment and decrement instructions. Instead, they should be generated from the same pattern that supports register-register add insns by examining the operands and generating the appropriate machine instruction.
Next: Looping Patterns, Previous: Dependent Patterns, Up: Machine Desc [Contents][Index]
GCC does not assume anything about how the machine realizes jumps.
The machine description should define a single pattern, usually
a define_expand, which expands to all the required insns.
Usually, this would be a comparison insn to set the condition code
and a separate branch insn testing the condition code and branching
or not according to its value. For many machines, however,
separating compares and branches is limiting, which is why the
more flexible approach with one define_expand is used in GCC.
The machine description becomes clearer for architectures that
have compare-and-branch instructions but no condition code. It also
works better when different sets of comparison operators are supported
by different kinds of conditional branches (e.g. integer vs. floating-point),
or by conditional branches with respect to conditional stores.
Two separate insns are always used if the machine description represents
a condition code register using the legacy RTL expression (cc0),
and on most machines that use a separate condition code register
(see Condition Code). For machines that use (cc0), in
fact, the set and use of the condition code must be separate and
adjacent4, thus
allowing flags in cc_status to be used (see Condition Code) and
so that the comparison and branch insns could be located from each other
by using the functions prev_cc0_setter and next_cc0_user.
Even in this case having a single entry point for conditional branches is advantageous, because it handles equally well the case where a single comparison instruction records the results of both signed and unsigned comparison of the given operands (with the branch insns coming in distinct signed and unsigned flavors) as in the x86 or SPARC, and the case where there are distinct signed and unsigned compare instructions and only one set of conditional branch instructions as in the PowerPC.
Next: Insn Canonicalizations, Previous: Jump Patterns, Up: Machine Desc [Contents][Index]
Some machines have special jump instructions that can be utilized to make loops more efficient. A common example is the 68000 ‘dbra’ instruction which performs a decrement of a register and a branch if the result was greater than zero. Other machines, in particular digital signal processors (DSPs), have special block repeat instructions to provide low-overhead loop support. For example, the TI TMS320C3x/C4x DSPs have a block repeat instruction that loads special registers to mark the top and end of a loop and to count the number of loop iterations. This avoids the need for fetching and executing a ‘dbra’-like instruction and avoids pipeline stalls associated with the jump.
GCC has three special named patterns to support low overhead looping.
They are ‘decrement_and_branch_until_zero’, ‘doloop_begin’,
and ‘doloop_end’. The first pattern,
‘decrement_and_branch_until_zero’, is not emitted during RTL
generation but may be emitted during the instruction combination phase.
This requires the assistance of the loop optimizer, using information
collected during strength reduction, to reverse a loop to count down to
zero. Some targets also require the loop optimizer to add a
REG_NONNEG note to indicate that the iteration count is always
positive. This is needed if the target performs a signed loop
termination test. For example, the 68000 uses a pattern similar to the
following for its dbra instruction:
(define_insn "decrement_and_branch_until_zero"
[(set (pc)
(if_then_else
(ge (plus:SI (match_operand:SI 0 "general_operand" "+d*am")
(const_int -1))
(const_int 0))
(label_ref (match_operand 1 "" ""))
(pc)))
(set (match_dup 0)
(plus:SI (match_dup 0)
(const_int -1)))]
"find_reg_note (insn, REG_NONNEG, 0)"
"…")
Note that since the insn is both a jump insn and has an output, it must deal with its own reloads, hence the ‘m’ constraints. Also note that since this insn is generated by the instruction combination phase combining two sequential insns together into an implicit parallel insn, the iteration counter needs to be biased by the same amount as the decrement operation, in this case -1. Note that the following similar pattern will not be matched by the combiner.
(define_insn "decrement_and_branch_until_zero"
[(set (pc)
(if_then_else
(ge (match_operand:SI 0 "general_operand" "+d*am")
(const_int 1))
(label_ref (match_operand 1 "" ""))
(pc)))
(set (match_dup 0)
(plus:SI (match_dup 0)
(const_int -1)))]
"find_reg_note (insn, REG_NONNEG, 0)"
"…")
The other two special looping patterns, ‘doloop_begin’ and ‘doloop_end’, are emitted by the loop optimizer for certain well-behaved loops with a finite number of loop iterations using information collected during strength reduction.
The ‘doloop_end’ pattern describes the actual looping instruction (or the implicit looping operation) and the ‘doloop_begin’ pattern is an optional companion pattern that can be used for initialization needed for some low-overhead looping instructions.
Note that some machines require the actual looping instruction to be
emitted at the top of the loop (e.g., the TMS320C3x/C4x DSPs). Emitting
the true RTL for a looping instruction at the top of the loop can cause
problems with flow analysis. So instead, a dummy doloop insn is
emitted at the end of the loop. The machine dependent reorg pass checks
for the presence of this doloop insn and then searches back to
the top of the loop, where it inserts the true looping insn (provided
there are no instructions in the loop which would cause problems). Any
additional labels can be emitted at this point. In addition, if the
desired special iteration counter register was not allocated, this
machine dependent reorg pass could emit a traditional compare and jump
instruction pair.
The essential difference between the ‘decrement_and_branch_until_zero’ and the ‘doloop_end’ patterns is that the loop optimizer allocates an additional pseudo register for the latter as an iteration counter. This pseudo register cannot be used within the loop (i.e., general induction variables cannot be derived from it), however, in many cases the loop induction variable may become redundant and removed by the flow pass.
Next: Expander Definitions, Previous: Looping Patterns, Up: Machine Desc [Contents][Index]
There are often cases where multiple RTL expressions could represent an operation performed by a single machine instruction. This situation is most commonly encountered with logical, branch, and multiply-accumulate instructions. In such cases, the compiler attempts to convert these multiple RTL expressions into a single canonical form to reduce the number of insn patterns required.
In addition to algebraic simplifications, following canonicalizations are performed:
plus
can itself be a plus. and, ior, xor,
plus, mult, smin, smax, umin, and
umax are associative when applied to integers, and sometimes to
floating-point.
neg, not,
mult, plus, or minus expression, it will be the
first operand.
neg, mult, plus, and
minus, the neg operations (if any) will be moved inside
the operations as far as possible. For instance,
(neg (mult A B)) is canonicalized as (mult (neg A) B), but
(plus (mult (neg B) C) A) is canonicalized as
(minus A (mult B C)).
compare operator, a constant is always the second operand
if the first argument is a condition code register or (cc0).
neg, not, mult, plus, or
minus is made the first operand under the same conditions as
above.
(ltu (plus a b) b) is converted to
(ltu (plus a b) a). Likewise with geu instead
of ltu.
(minus x (const_int n)) is converted to
(plus x (const_int -n)).
mem), a left shift is
converted into the appropriate multiplication by a power of two.
not expression, it will be the first one.
A machine that has an instruction that performs a bitwise logical-and of one operand with the bitwise negation of the other should specify the pattern for that instruction as
(define_insn ""
[(set (match_operand:m 0 …)
(and:m (not:m (match_operand:m 1 …))
(match_operand:m 2 …)))]
"…"
"…")
Similarly, a pattern for a “NAND” instruction should be written
(define_insn ""
[(set (match_operand:m 0 …)
(ior:m (not:m (match_operand:m 1 …))
(not:m (match_operand:m 2 …))))]
"…"
"…")
In both cases, it is not necessary to include patterns for the many logically equivalent RTL expressions.
(xor:m x y)
and (not:m (xor:m x y)).
(plus:m (plus:m x y) constant)
zero_extract rather than the equivalent
and or sign_extract operations.
(sign_extend:m1 (mult:m2 (sign_extend:m2 x)
(sign_extend:m2 y))) is converted to (mult:m1
(sign_extend:m1 x) (sign_extend:m1 y)), and likewise
for zero_extend.
(sign_extend:m1 (mult:m2 (ashiftrt:m2
x s) (sign_extend:m2 y))) is converted
to (mult:m1 (sign_extend:m1 (ashiftrt:m2
x s)) (sign_extend:m1 y)), and likewise for
patterns using zero_extend and lshiftrt. If the second
operand of mult is also a shift, then that is extended also.
This transformation is only applied when it can be proven that the
original operation had sufficient precision to prevent overflow.
Further canonicalization rules are defined in the function
commutative_operand_precedence in gcc/rtlanal.c.
Next: Insn Splitting, Previous: Insn Canonicalizations, Up: Machine Desc [Contents][Index]
On some target machines, some standard pattern names for RTL generation
cannot be handled with single insn, but a sequence of RTL insns can
represent them. For these target machines, you can write a
define_expand to specify how to generate the sequence of RTL.
A define_expand is an RTL expression that looks almost like a
define_insn; but, unlike the latter, a define_expand is used
only for RTL generation and it can produce more than one RTL insn.
A define_expand RTX has four operands:
define_expand must have a name, since the only
use for it is to refer to it by name.
define_insn, there
is no implicit surrounding PARALLEL.
define_insn that
has a standard name. Therefore, the condition (if present) may not
depend on the data in the insn being matched, but only the
target-machine-type flags. The compiler needs to test these conditions
during initialization in order to learn exactly which named instructions
are available in a particular run.
Usually these statements prepare temporary registers for use as
internal operands in the RTL template, but they can also generate RTL
insns directly by calling routines such as emit_insn, etc.
Any such insns precede the ones that come from the RTL template.
Every RTL insn emitted by a define_expand must match some
define_insn in the machine description. Otherwise, the compiler
will crash when trying to generate code for the insn or trying to optimize
it.
The RTL template, in addition to controlling generation of RTL insns, also describes the operands that need to be specified when this pattern is used. In particular, it gives a predicate for each operand.
A true operand, which needs to be specified in order to generate RTL from
the pattern, should be described with a match_operand in its first
occurrence in the RTL template. This enters information on the operand’s
predicate into the tables that record such things. GCC uses the
information to preload the operand into a register if that is required for
valid RTL code. If the operand is referred to more than once, subsequent
references should use match_dup.
The RTL template may also refer to internal “operands” which are
temporary registers or labels used only within the sequence made by the
define_expand. Internal operands are substituted into the RTL
template with match_dup, never with match_operand. The
values of the internal operands are not passed in as arguments by the
compiler when it requests use of this pattern. Instead, they are computed
within the pattern, in the preparation statements. These statements
compute the values and store them into the appropriate elements of
operands so that match_dup can find them.
There are two special macros defined for use in the preparation statements:
DONE and FAIL. Use them with a following semicolon,
as a statement.
DONEUse the DONE macro to end RTL generation for the pattern. The
only RTL insns resulting from the pattern on this occasion will be
those already emitted by explicit calls to emit_insn within the
preparation statements; the RTL template will not be generated.
FAILMake the pattern fail on this occasion. When a pattern fails, it means that the pattern was not truly available. The calling routines in the compiler will try other strategies for code generation using other patterns.
Failure is currently supported only for binary (addition, multiplication,
shifting, etc.) and bit-field (extv, extzv, and insv)
operations.
If the preparation falls through (invokes neither DONE nor
FAIL), then the define_expand acts like a
define_insn in that the RTL template is used to generate the
insn.
The RTL template is not used for matching, only for generating the
initial insn list. If the preparation statement always invokes
DONE or FAIL, the RTL template may be reduced to a simple
list of operands, such as this example:
(define_expand "addsi3" [(match_operand:SI 0 "register_operand" "") (match_operand:SI 1 "register_operand" "") (match_operand:SI 2 "register_operand" "")]
""
"
{
handle_add (operands[0], operands[1], operands[2]);
DONE;
}")
Here is an example, the definition of left-shift for the SPUR chip:
(define_expand "ashlsi3"
[(set (match_operand:SI 0 "register_operand" "")
(ashift:SI
(match_operand:SI 1 "register_operand" "")
(match_operand:SI 2 "nonmemory_operand" "")))]
""
"
{
if (GET_CODE (operands[2]) != CONST_INT
|| (unsigned) INTVAL (operands[2]) > 3)
FAIL;
}")
This example uses define_expand so that it can generate an RTL insn
for shifting when the shift-count is in the supported range of 0 to 3 but
fail in other cases where machine insns aren’t available. When it fails,
the compiler tries another strategy using different patterns (such as, a
library call).
If the compiler were able to handle nontrivial condition-strings in
patterns with names, then it would be possible to use a
define_insn in that case. Here is another case (zero-extension
on the 68000) which makes more use of the power of define_expand:
(define_expand "zero_extendhisi2"
[(set (match_operand:SI 0 "general_operand" "")
(const_int 0))
(set (strict_low_part
(subreg:HI
(match_dup 0)
0))
(match_operand:HI 1 "general_operand" ""))]
""
"operands[1] = make_safe_from (operands[1], operands[0]);")
Here two RTL insns are generated, one to clear the entire output operand
and the other to copy the input operand into its low half. This sequence
is incorrect if the input operand refers to [the old value of] the output
operand, so the preparation statement makes sure this isn’t so. The
function make_safe_from copies the operands[1] into a
temporary register if it refers to operands[0]. It does this
by emitting another RTL insn.
Finally, a third example shows the use of an internal operand.
Zero-extension on the SPUR chip is done by and-ing the result
against a halfword mask. But this mask cannot be represented by a
const_int because the constant value is too large to be legitimate
on this machine. So it must be copied into a register with
force_reg and then the register used in the and.
(define_expand "zero_extendhisi2"
[(set (match_operand:SI 0 "register_operand" "")
(and:SI (subreg:SI
(match_operand:HI 1 "register_operand" "")
0)
(match_dup 2)))]
""
"operands[2]
= force_reg (SImode, GEN_INT (65535)); ")
Note: If the define_expand is used to serve a
standard binary or unary arithmetic operation or a bit-field operation,
then the last insn it generates must not be a code_label,
barrier or note. It must be an insn,
jump_insn or call_insn. If you don’t need a real insn
at the end, emit an insn to copy the result of the operation into
itself. Such an insn will generate no code, but it can avoid problems
in the compiler.
Next: Including Patterns, Previous: Expander Definitions, Up: Machine Desc [Contents][Index]
There are two cases where you should specify how to split a pattern into multiple insns. On machines that have instructions requiring delay slots (see Delay Slots) or that have instructions whose output is not available for multiple cycles (see Processor pipeline description), the compiler phases that optimize these cases need to be able to move insns into one-instruction delay slots. However, some insns may generate more than one machine instruction. These insns cannot be placed into a delay slot.
Often you can rewrite the single insn as a list of individual insns, each corresponding to one machine instruction. The disadvantage of doing so is that it will cause the compilation to be slower and require more space. If the resulting insns are too complex, it may also suppress some optimizations. The compiler splits the insn if there is a reason to believe that it might improve instruction or delay slot scheduling.
The insn combiner phase also splits putative insns. If three insns are
merged into one insn with a complex expression that cannot be matched by
some define_insn pattern, the combiner phase attempts to split
the complex pattern into two insns that are recognized. Usually it can
break the complex pattern into two patterns by splitting out some
subexpression. However, in some other cases, such as performing an
addition of a large constant in two insns on a RISC machine, the way to
split the addition into two insns is machine-dependent.
The define_split definition tells the compiler how to split a
complex insn into several simpler insns. It looks like this:
(define_split [insn-pattern] "condition" [new-insn-pattern-1 new-insn-pattern-2 …] "preparation-statements")
insn-pattern is a pattern that needs to be split and
condition is the final condition to be tested, as in a
define_insn. When an insn matching insn-pattern and
satisfying condition is found, it is replaced in the insn list
with the insns given by new-insn-pattern-1,
new-insn-pattern-2, etc.
The preparation-statements are similar to those statements that
are specified for define_expand (see Expander Definitions)
and are executed before the new RTL is generated to prepare for the
generated code or emit some insns whose pattern is not fixed. Unlike
those in define_expand, however, these statements must not
generate any new pseudo-registers. Once reload has completed, they also
must not allocate any space in the stack frame.
Patterns are matched against insn-pattern in two different
circumstances. If an insn needs to be split for delay slot scheduling
or insn scheduling, the insn is already known to be valid, which means
that it must have been matched by some define_insn and, if
reload_completed is nonzero, is known to satisfy the constraints
of that define_insn. In that case, the new insn patterns must
also be insns that are matched by some define_insn and, if
reload_completed is nonzero, must also satisfy the constraints
of those definitions.
As an example of this usage of define_split, consider the following
example from a29k.md, which splits a sign_extend from
HImode to SImode into a pair of shift insns:
(define_split
[(set (match_operand:SI 0 "gen_reg_operand" "")
(sign_extend:SI (match_operand:HI 1 "gen_reg_operand" "")))]
""
[(set (match_dup 0)
(ashift:SI (match_dup 1)
(const_int 16)))
(set (match_dup 0)
(ashiftrt:SI (match_dup 0)
(const_int 16)))]
"
{ operands[1] = gen_lowpart (SImode, operands[1]); }")
When the combiner phase tries to split an insn pattern, it is always the
case that the pattern is not matched by any define_insn.
The combiner pass first tries to split a single set expression
and then the same set expression inside a parallel, but
followed by a clobber of a pseudo-reg to use as a scratch
register. In these cases, the combiner expects exactly two new insn
patterns to be generated. It will verify that these patterns match some
define_insn definitions, so you need not do this test in the
define_split (of course, there is no point in writing a
define_split that will never produce insns that match).
Here is an example of this use of define_split, taken from
rs6000.md:
(define_split
[(set (match_operand:SI 0 "gen_reg_operand" "")
(plus:SI (match_operand:SI 1 "gen_reg_operand" "")
(match_operand:SI 2 "non_add_cint_operand" "")))]
""
[(set (match_dup 0) (plus:SI (match_dup 1) (match_dup 3)))
(set (match_dup 0) (plus:SI (match_dup 0) (match_dup 4)))]
"
{
int low = INTVAL (operands[2]) & 0xffff;
int high = (unsigned) INTVAL (operands[2]) >> 16;
if (low & 0x8000)
high++, low |= 0xffff0000;
operands[3] = GEN_INT (high << 16);
operands[4] = GEN_INT (low);
}")
Here the predicate non_add_cint_operand matches any
const_int that is not a valid operand of a single add
insn. The add with the smaller displacement is written so that it
can be substituted into the address of a subsequent operation.
An example that uses a scratch register, from the same file, generates an equality comparison of a register and a large constant:
(define_split
[(set (match_operand:CC 0 "cc_reg_operand" "")
(compare:CC (match_operand:SI 1 "gen_reg_operand" "")
(match_operand:SI 2 "non_short_cint_operand" "")))
(clobber (match_operand:SI 3 "gen_reg_operand" ""))]
"find_single_use (operands[0], insn, 0)
&& (GET_CODE (*find_single_use (operands[0], insn, 0)) == EQ
|| GET_CODE (*find_single_use (operands[0], insn, 0)) == NE)"
[(set (match_dup 3) (xor:SI (match_dup 1) (match_dup 4)))
(set (match_dup 0) (compare:CC (match_dup 3) (match_dup 5)))]
"
{
/* Get the constant we are comparing against, C, and see what it
looks like sign-extended to 16 bits. Then see what constant
could be XOR’ed with C to get the sign-extended value. */
int c = INTVAL (operands[2]);
int sextc = (c << 16) >> 16;
int xorv = c ^ sextc;
operands[4] = GEN_INT (xorv);
operands[5] = GEN_INT (sextc);
}")
To avoid confusion, don’t write a single define_split that
accepts some insns that match some define_insn as well as some
insns that don’t. Instead, write two separate define_split
definitions, one for the insns that are valid and one for the insns that
are not valid.
The splitter is allowed to split jump instructions into sequence of jumps or create new jumps in while splitting non-jump instructions. As the central flowgraph and branch prediction information needs to be updated, several restriction apply.
Splitting of jump instruction into sequence that over by another jump
instruction is always valid, as compiler expect identical behavior of new
jump. When new sequence contains multiple jump instructions or new labels,
more assistance is needed. Splitter is required to create only unconditional
jumps, or simple conditional jump instructions. Additionally it must attach a
REG_BR_PROB note to each conditional jump. A global variable
split_branch_probability holds the probability of the original branch in case
it was a simple conditional jump, -1 otherwise. To simplify
recomputing of edge frequencies, the new sequence is required to have only
forward jumps to the newly created labels.
For the common case where the pattern of a define_split exactly matches the
pattern of a define_insn, use define_insn_and_split. It looks like
this:
(define_insn_and_split [insn-pattern] "condition" "output-template" "split-condition" [new-insn-pattern-1 new-insn-pattern-2 …] "preparation-statements" [insn-attributes])
insn-pattern, condition, output-template, and
insn-attributes are used as in define_insn. The
new-insn-pattern vector and the preparation-statements are used as
in a define_split. The split-condition is also used as in
define_split, with the additional behavior that if the condition starts
with ‘&&’, the condition used for the split will be the constructed as a
logical “and” of the split condition with the insn condition. For example,
from i386.md:
(define_insn_and_split "zero_extendhisi2_and"
[(set (match_operand:SI 0 "register_operand" "=r")
(zero_extend:SI (match_operand:HI 1 "register_operand" "0")))
(clobber (reg:CC 17))]
"TARGET_ZERO_EXTEND_WITH_AND && !optimize_size"
"#"
"&& reload_completed"
[(parallel [(set (match_dup 0)
(and:SI (match_dup 0) (const_int 65535)))
(clobber (reg:CC 17))])]
""
[(set_attr "type" "alu1")])
In this case, the actual split condition will be ‘TARGET_ZERO_EXTEND_WITH_AND && !optimize_size && reload_completed’.
The define_insn_and_split construction provides exactly the same
functionality as two separate define_insn and define_split
patterns. It exists for compactness, and as a maintenance tool to prevent
having to ensure the two patterns’ templates match.
Next: Peephole Definitions, Previous: Insn Splitting, Up: Machine Desc [Contents][Index]
The include pattern tells the compiler tools where to
look for patterns that are in files other than in the file
.md. This is used only at build time and there is no preprocessing allowed.
It looks like:
(include pathname)
For example:
(include "filestuff")
Where pathname is a string that specifies the location of the file, specifies the include file to be in gcc/config/target/filestuff. The directory gcc/config/target is regarded as the default directory.
Machine descriptions may be split up into smaller more manageable subsections and placed into subdirectories.
By specifying:
(include "BOGUS/filestuff")
the include file is specified to be in gcc/config/target/BOGUS/filestuff.
Specifying an absolute path for the include file such as;
(include "/u2/BOGUS/filestuff")
is permitted but is not encouraged.
The -Idir option specifies directories to search for machine descriptions. For example:
genrecog -I/p1/abc/proc1 -I/p2/abcd/pro2 target.md
Add the directory dir to the head of the list of directories to be searched for header files. This can be used to override a system machine definition file, substituting your own version, since these directories are searched before the default machine description file directories. If you use more than one -I option, the directories are scanned in left-to-right order; the standard default directory come after.
Next: Insn Attributes, Previous: Including Patterns, Up: Machine Desc [Contents][Index]
In addition to instruction patterns the md file may contain definitions of machine-specific peephole optimizations.
The combiner does not notice certain peephole optimizations when the data flow in the program does not suggest that it should try them. For example, sometimes two consecutive insns related in purpose can be combined even though the second one does not appear to use a register computed in the first one. A machine-specific peephole optimizer can detect such opportunities.
There are two forms of peephole definitions that may be used. The
original define_peephole is run at assembly output time to
match insns and substitute assembly text. Use of define_peephole
is deprecated.
A newer define_peephole2 matches insns and substitutes new
insns. The peephole2 pass is run after register allocation
but before scheduling, which may result in much better code for
targets that do scheduling.
| • define_peephole: | RTL to Text Peephole Optimizers | |
| • define_peephole2: | RTL to RTL Peephole Optimizers |
Next: define_peephole2, Up: Peephole Definitions [Contents][Index]
A definition looks like this:
(define_peephole [insn-pattern-1 insn-pattern-2 …] "condition" "template" "optional-insn-attributes")
The last string operand may be omitted if you are not using any
machine-specific information in this machine description. If present,
it must obey the same rules as in a define_insn.
In this skeleton, insn-pattern-1 and so on are patterns to match consecutive insns. The optimization applies to a sequence of insns when insn-pattern-1 matches the first one, insn-pattern-2 matches the next, and so on.
Each of the insns matched by a peephole must also match a
define_insn. Peepholes are checked only at the last stage just
before code generation, and only optionally. Therefore, any insn which
would match a peephole but no define_insn will cause a crash in code
generation in an unoptimized compilation, or at various optimization
stages.
The operands of the insns are matched with match_operands,
match_operator, and match_dup, as usual. What is not
usual is that the operand numbers apply to all the insn patterns in the
definition. So, you can check for identical operands in two insns by
using match_operand in one insn and match_dup in the
other.
The operand constraints used in match_operand patterns do not have
any direct effect on the applicability of the peephole, but they will
be validated afterward, so make sure your constraints are general enough
to apply whenever the peephole matches. If the peephole matches
but the constraints are not satisfied, the compiler will crash.
It is safe to omit constraints in all the operands of the peephole; or you can write constraints which serve as a double-check on the criteria previously tested.
Once a sequence of insns matches the patterns, the condition is checked. This is a C expression which makes the final decision whether to perform the optimization (we do so if the expression is nonzero). If condition is omitted (in other words, the string is empty) then the optimization is applied to every sequence of insns that matches the patterns.
The defined peephole optimizations are applied after register allocation is complete. Therefore, the peephole definition can check which operands have ended up in which kinds of registers, just by looking at the operands.
The way to refer to the operands in condition is to write
operands[i] for operand number i (as matched by
(match_operand i …)). Use the variable insn
to refer to the last of the insns being matched; use
prev_active_insn to find the preceding insns.
When optimizing computations with intermediate results, you can use
condition to match only when the intermediate results are not used
elsewhere. Use the C expression dead_or_set_p (insn,
op), where insn is the insn in which you expect the value
to be used for the last time (from the value of insn, together
with use of prev_nonnote_insn), and op is the intermediate
value (from operands[i]).
Applying the optimization means replacing the sequence of insns with one
new insn. The template controls ultimate output of assembler code
for this combined insn. It works exactly like the template of a
define_insn. Operand numbers in this template are the same ones
used in matching the original sequence of insns.
The result of a defined peephole optimizer does not need to match any of the insn patterns in the machine description; it does not even have an opportunity to match them. The peephole optimizer definition itself serves as the insn pattern to control how the insn is output.
Defined peephole optimizers are run as assembler code is being output, so the insns they produce are never combined or rearranged in any way.
Here is an example, taken from the 68000 machine description:
(define_peephole
[(set (reg:SI 15) (plus:SI (reg:SI 15) (const_int 4)))
(set (match_operand:DF 0 "register_operand" "=f")
(match_operand:DF 1 "register_operand" "ad"))]
"FP_REG_P (operands[0]) && ! FP_REG_P (operands[1])"
{
rtx xoperands[2];
xoperands[1] = gen_rtx_REG (SImode, REGNO (operands[1]) + 1);
#ifdef MOTOROLA
output_asm_insn ("move.l %1,(sp)", xoperands);
output_asm_insn ("move.l %1,-(sp)", operands);
return "fmove.d (sp)+,%0";
#else
output_asm_insn ("movel %1,sp@", xoperands);
output_asm_insn ("movel %1,sp@-", operands);
return "fmoved sp@+,%0";
#endif
})
The effect of this optimization is to change
jbsr _foobar addql #4,sp movel d1,sp@- movel d0,sp@- fmoved sp@+,fp0
into
jbsr _foobar movel d1,sp@ movel d0,sp@- fmoved sp@+,fp0
insn-pattern-1 and so on look almost like the second
operand of define_insn. There is one important difference: the
second operand of define_insn consists of one or more RTX’s
enclosed in square brackets. Usually, there is only one: then the same
action can be written as an element of a define_peephole. But
when there are multiple actions in a define_insn, they are
implicitly enclosed in a parallel. Then you must explicitly
write the parallel, and the square brackets within it, in the
define_peephole. Thus, if an insn pattern looks like this,
(define_insn "divmodsi4"
[(set (match_operand:SI 0 "general_operand" "=d")
(div:SI (match_operand:SI 1 "general_operand" "0")
(match_operand:SI 2 "general_operand" "dmsK")))
(set (match_operand:SI 3 "general_operand" "=d")
(mod:SI (match_dup 1) (match_dup 2)))]
"TARGET_68020"
"divsl%.l %2,%3:%0")
then the way to mention this insn in a peephole is as follows:
(define_peephole
[…
(parallel
[(set (match_operand:SI 0 "general_operand" "=d")
(div:SI (match_operand:SI 1 "general_operand" "0")
(match_operand:SI 2 "general_operand" "dmsK")))
(set (match_operand:SI 3 "general_operand" "=d")
(mod:SI (match_dup 1) (match_dup 2)))])
…]
…)
Previous: define_peephole, Up: Peephole Definitions [Contents][Index]
The define_peephole2 definition tells the compiler how to
substitute one sequence of instructions for another sequence,
what additional scratch registers may be needed and what their
lifetimes must be.
(define_peephole2 [insn-pattern-1 insn-pattern-2 …] "condition" [new-insn-pattern-1 new-insn-pattern-2 …] "preparation-statements")
The definition is almost identical to define_split
(see Insn Splitting) except that the pattern to match is not a
single instruction, but a sequence of instructions.
It is possible to request additional scratch registers for use in the output template. If appropriate registers are not free, the pattern will simply not match.
Scratch registers are requested with a match_scratch pattern at
the top level of the input pattern. The allocated register (initially) will
be dead at the point requested within the original sequence. If the scratch
is used at more than a single point, a match_dup pattern at the
top level of the input pattern marks the last position in the input sequence
at which the register must be available.
Here is an example from the IA-32 machine description:
(define_peephole2
[(match_scratch:SI 2 "r")
(parallel [(set (match_operand:SI 0 "register_operand" "")
(match_operator:SI 3 "arith_or_logical_operator"
[(match_dup 0)
(match_operand:SI 1 "memory_operand" "")]))
(clobber (reg:CC 17))])]
"! optimize_size && ! TARGET_READ_MODIFY"
[(set (match_dup 2) (match_dup 1))
(parallel [(set (match_dup 0)
(match_op_dup 3 [(match_dup 0) (match_dup 2)]))
(clobber (reg:CC 17))])]
"")
This pattern tries to split a load from its use in the hopes that we’ll be
able to schedule around the memory load latency. It allocates a single
SImode register of class GENERAL_REGS ("r") that needs
to be live only at the point just before the arithmetic.
A real example requiring extended scratch lifetimes is harder to come by, so here’s a silly made-up example:
(define_peephole2
[(match_scratch:SI 4 "r")
(set (match_operand:SI 0 "" "") (match_operand:SI 1 "" ""))
(set (match_operand:SI 2 "" "") (match_dup 1))
(match_dup 4)
(set (match_operand:SI 3 "" "") (match_dup 1))]
"/* determine 1 does not overlap 0 and 2 */"
[(set (match_dup 4) (match_dup 1))
(set (match_dup 0) (match_dup 4))
(set (match_dup 2) (match_dup 4))]
(set (match_dup 3) (match_dup 4))]
"")
If we had not added the (match_dup 4) in the middle of the input
sequence, it might have been the case that the register we chose at the
beginning of the sequence is killed by the first or second set.
Next: Conditional Execution, Previous: Peephole Definitions, Up: Machine Desc [Contents][Index]
In addition to describing the instruction supported by the target machine,
the md file also defines a group of attributes and a set of
values for each. Every generated insn is assigned a value for each attribute.
One possible attribute would be the effect that the insn has on the machine’s
condition code. This attribute can then be used by NOTICE_UPDATE_CC
to track the condition codes.
| • Defining Attributes: | Specifying attributes and their values. | |
| • Expressions: | Valid expressions for attribute values. | |
| • Tagging Insns: | Assigning attribute values to insns. | |
| • Attr Example: | An example of assigning attributes. | |
| • Insn Lengths: | Computing the length of insns. | |
| • Constant Attributes: | Defining attributes that are constant. | |
| • Delay Slots: | Defining delay slots required for a machine. | |
| • Processor pipeline description: | Specifying information for insn scheduling. |
Next: Expressions, Up: Insn Attributes [Contents][Index]
The define_attr expression is used to define each attribute required
by the target machine. It looks like:
(define_attr name list-of-values default)
name is a string specifying the name of the attribute being defined.
list-of-values is either a string that specifies a comma-separated list of values that can be assigned to the attribute, or a null string to indicate that the attribute takes numeric values.
default is an attribute expression that gives the value of this attribute for insns that match patterns whose definition does not include an explicit value for this attribute. See Attr Example, for more information on the handling of defaults. See Constant Attributes, for information on attributes that do not depend on any particular insn.
For each defined attribute, a number of definitions are written to the insn-attr.h file. For cases where an explicit set of values is specified for an attribute, the following are defined:
For example, if the following is present in the md file:
(define_attr "type" "branch,fp,load,store,arith" …)
the following lines will be written to the file insn-attr.h.
#define HAVE_ATTR_type
enum attr_type {TYPE_BRANCH, TYPE_FP, TYPE_LOAD,
TYPE_STORE, TYPE_ARITH};
extern enum attr_type get_attr_type ();
If the attribute takes numeric values, no enum type will be
defined and the function to obtain the attribute’s value will return
int.
There are attributes which are tied to a specific meaning. These attributes are not free to use for other purposes:
lengthThe length attribute is used to calculate the length of emitted
code chunks. This is especially important when verifying branch
distances. See Insn Lengths.
enabledThe enabled attribute can be defined to prevent certain
alternatives of an insn definition from being used during code
generation. See Disable Insn Alternatives.
Another way of defining an attribute is to use:
(define_enum_attr "attr" "enum" default)
This works in just the same way as define_attr, except that
the list of values is taken from a separate enumeration called
enum (see define_enum). This form allows you to use
the same list of values for several attributes without having to
repeat the list each time. For example:
(define_enum "processor" [ model_a model_b … ]) (define_enum_attr "arch" "processor" (const (symbol_ref "target_arch"))) (define_enum_attr "tune" "processor" (const (symbol_ref "target_tune")))
defines the same attributes as:
(define_attr "arch" "model_a,model_b,…" (const (symbol_ref "target_arch"))) (define_attr "tune" "model_a,model_b,…" (const (symbol_ref "target_tune")))
but without duplicating the processor list. The second example defines two
separate C enums (attr_arch and attr_tune) whereas the first
defines a single C enum (processor).
Next: Tagging Insns, Previous: Defining Attributes, Up: Insn Attributes [Contents][Index]
RTL expressions used to define attributes use the codes described above plus a few specific to attribute definitions, to be discussed below. Attribute value expressions must have one of the following forms:
(const_int i)The integer i specifies the value of a numeric attribute. i must be non-negative.
The value of a numeric attribute can be specified either with a
const_int, or as an integer represented as a string in
const_string, eq_attr (see below), attr,
symbol_ref, simple arithmetic expressions, and set_attr
overrides on specific instructions (see Tagging Insns).
(const_string value)The string value specifies a constant attribute value.
If value is specified as ‘"*"’, it means that the default value of
the attribute is to be used for the insn containing this expression.
‘"*"’ obviously cannot be used in the default expression
of a define_attr.
If the attribute whose value is being specified is numeric, value
must be a string containing a non-negative integer (normally
const_int would be used in this case). Otherwise, it must
contain one of the valid values for the attribute.
(if_then_else test true-value false-value)test specifies an attribute test, whose format is defined below. The value of this expression is true-value if test is true, otherwise it is false-value.
(cond [test1 value1 …] default)The first operand of this expression is a vector containing an even
number of expressions and consisting of pairs of test and value
expressions. The value of the cond expression is that of the
value corresponding to the first true test expression. If
none of the test expressions are true, the value of the cond
expression is that of the default expression.
test expressions can have one of the following forms:
(const_int i)This test is true if i is nonzero and false otherwise.
(not test)(ior test1 test2)(and test1 test2)These tests are true if the indicated logical function is true.
(match_operand:m n pred constraints)This test is true if operand n of the insn whose attribute value
is being determined has mode m (this part of the test is ignored
if m is VOIDmode) and the function specified by the string
pred returns a nonzero value when passed operand n and mode
m (this part of the test is ignored if pred is the null
string).
The constraints operand is ignored and should be the null string.
(le arith1 arith2)(leu arith1 arith2)(lt arith1 arith2)(ltu arith1 arith2)(gt arith1 arith2)(gtu arith1 arith2)(ge arith1 arith2)(geu arith1 arith2)(ne arith1 arith2)(eq arith1 arith2)These tests are true if the indicated comparison of the two arithmetic
expressions is true. Arithmetic expressions are formed with
plus, minus, mult, div, mod,
abs, neg, and, ior, xor, not,
ashift, lshiftrt, and ashiftrt expressions.
const_int and symbol_ref are always valid terms (see Insn Lengths,for additional forms). symbol_ref is a string
denoting a C expression that yields an int when evaluated by the
‘get_attr_…’ routine. It should normally be a global
variable.
(eq_attr name value)name is a string specifying the name of an attribute.
value is a string that is either a valid value for attribute name, a comma-separated list of values, or ‘!’ followed by a value or list. If value does not begin with a ‘!’, this test is true if the value of the name attribute of the current insn is in the list specified by value. If value begins with a ‘!’, this test is true if the attribute’s value is not in the specified list.
For example,
(eq_attr "type" "load,store")
is equivalent to
(ior (eq_attr "type" "load") (eq_attr "type" "store"))
If name specifies an attribute of ‘alternative’, it refers to the
value of the compiler variable which_alternative
(see Output Statement) and the values must be small integers. For
example,
(eq_attr "alternative" "2,3")
is equivalent to
(ior (eq (symbol_ref "which_alternative") (const_int 2))
(eq (symbol_ref "which_alternative") (const_int 3)))
Note that, for most attributes, an eq_attr test is simplified in cases
where the value of the attribute being tested is known for all insns matching
a particular pattern. This is by far the most common case.
(attr_flag name)The value of an attr_flag expression is true if the flag
specified by name is true for the insn currently being
scheduled.
name is a string specifying one of a fixed set of flags to test.
Test the flags forward and backward to determine the
direction of a conditional branch. Test the flags very_likely,
likely, very_unlikely, and unlikely to determine
if a conditional branch is expected to be taken.
If the very_likely flag is true, then the likely flag is also
true. Likewise for the very_unlikely and unlikely flags.
This example describes a conditional branch delay slot which can be nullified for forward branches that are taken (annul-true) or for backward branches which are not taken (annul-false).
(define_delay (eq_attr "type" "cbranch")
[(eq_attr "in_branch_delay" "true")
(and (eq_attr "in_branch_delay" "true")
(attr_flag "forward"))
(and (eq_attr "in_branch_delay" "true")
(attr_flag "backward"))])
The forward and backward flags are false if the current
insn being scheduled is not a conditional branch.
The very_likely and likely flags are true if the
insn being scheduled is not a conditional branch.
The very_unlikely and unlikely flags are false if the
insn being scheduled is not a conditional branch.
attr_flag is only used during delay slot scheduling and has no
meaning to other passes of the compiler.
(attr name)The value of another attribute is returned. This is most useful
for numeric attributes, as eq_attr and attr_flag
produce more efficient code for non-numeric attributes.
Next: Attr Example, Previous: Expressions, Up: Insn Attributes [Contents][Index]
The value assigned to an attribute of an insn is primarily determined by
which pattern is matched by that insn (or which define_peephole
generated it). Every define_insn and define_peephole can
have an optional last argument to specify the values of attributes for
matching insns. The value of any attribute not specified in a particular
insn is set to the default value for that attribute, as specified in its
define_attr. Extensive use of default values for attributes
permits the specification of the values for only one or two attributes
in the definition of most insn patterns, as seen in the example in the
next section.
The optional last argument of define_insn and
define_peephole is a vector of expressions, each of which defines
the value for a single attribute. The most general way of assigning an
attribute’s value is to use a set expression whose first operand is an
attr expression giving the name of the attribute being set. The
second operand of the set is an attribute expression
(see Expressions) giving the value of the attribute.
When the attribute value depends on the ‘alternative’ attribute
(i.e., which is the applicable alternative in the constraint of the
insn), the set_attr_alternative expression can be used. It
allows the specification of a vector of attribute expressions, one for
each alternative.
When the generality of arbitrary attribute expressions is not required,
the simpler set_attr expression can be used, which allows
specifying a string giving either a single attribute value or a list
of attribute values, one for each alternative.
The form of each of the above specifications is shown below. In each case, name is a string specifying the attribute to be set.
(set_attr name value-string)value-string is either a string giving the desired attribute value, or a string containing a comma-separated list giving the values for succeeding alternatives. The number of elements must match the number of alternatives in the constraint of the insn pattern.
Note that it may be useful to specify ‘*’ for some alternative, in which case the attribute will assume its default value for insns matching that alternative.
(set_attr_alternative name [value1 value2 …])Depending on the alternative of the insn, the value will be one of the
specified values. This is a shorthand for using a cond with
tests on the ‘alternative’ attribute.
(set (attr name) value)The first operand of this set must be the special RTL expression
attr, whose sole operand is a string giving the name of the
attribute being set. value is the value of the attribute.
The following shows three different ways of representing the same attribute value specification:
(set_attr "type" "load,store,arith")
(set_attr_alternative "type"
[(const_string "load") (const_string "store")
(const_string "arith")])
(set (attr "type")
(cond [(eq_attr "alternative" "1") (const_string "load")
(eq_attr "alternative" "2") (const_string "store")]
(const_string "arith")))
The define_asm_attributes expression provides a mechanism to
specify the attributes assigned to insns produced from an asm
statement. It has the form:
(define_asm_attributes [attr-sets])
where attr-sets is specified the same as for both the
define_insn and the define_peephole expressions.
These values will typically be the “worst case” attribute values. For example, they might indicate that the condition code will be clobbered.
A specification for a length attribute is handled specially. The
way to compute the length of an asm insn is to multiply the
length specified in the expression define_asm_attributes by the
number of machine instructions specified in the asm statement,
determined by counting the number of semicolons and newlines in the
string. Therefore, the value of the length attribute specified
in a define_asm_attributes should be the maximum possible length
of a single machine instruction.
Next: Insn Lengths, Previous: Tagging Insns, Up: Insn Attributes [Contents][Index]
The judicious use of defaulting is important in the efficient use of
insn attributes. Typically, insns are divided into types and an
attribute, customarily called type, is used to represent this
value. This attribute is normally used only to define the default value
for other attributes. An example will clarify this usage.
Assume we have a RISC machine with a condition code and in which only full-word operations are performed in registers. Let us assume that we can divide all insns into loads, stores, (integer) arithmetic operations, floating point operations, and branches.
Here we will concern ourselves with determining the effect of an insn on the condition code and will limit ourselves to the following possible effects: The condition code can be set unpredictably (clobbered), not be changed, be set to agree with the results of the operation, or only changed if the item previously set into the condition code has been modified.
Here is part of a sample md file for such a machine:
(define_attr "type" "load,store,arith,fp,branch" (const_string "arith"))
(define_attr "cc" "clobber,unchanged,set,change0"
(cond [(eq_attr "type" "load")
(const_string "change0")
(eq_attr "type" "store,branch")
(const_string "unchanged")
(eq_attr "type" "arith")
(if_then_else (match_operand:SI 0 "" "")
(const_string "set")
(const_string "clobber"))]
(const_string "clobber")))
(define_insn ""
[(set (match_operand:SI 0 "general_operand" "=r,r,m")
(match_operand:SI 1 "general_operand" "r,m,r"))]
""
"@
move %0,%1
load %0,%1
store %0,%1"
[(set_attr "type" "arith,load,store")])
Note that we assume in the above example that arithmetic operations performed on quantities smaller than a machine word clobber the condition code since they will set the condition code to a value corresponding to the full-word result.
Next: Constant Attributes, Previous: Attr Example, Up: Insn Attributes [Contents][Index]
For many machines, multiple types of branch instructions are provided, each
for different length branch displacements. In most cases, the assembler
will choose the correct instruction to use. However, when the assembler
cannot do so, GCC can when a special attribute, the length
attribute, is defined. This attribute must be defined to have numeric
values by specifying a null string in its define_attr.
In the case of the length attribute, two additional forms of
arithmetic terms are allowed in test expressions:
(match_dup n)This refers to the address of operand n of the current insn, which
must be a label_ref.
(pc)This refers to the address of the current insn. It might have been more consistent with other usage to make this the address of the next insn but this would be confusing because the length of the current insn is to be computed.
For normal insns, the length will be determined by value of the
length attribute. In the case of addr_vec and
addr_diff_vec insn patterns, the length is computed as
the number of vectors multiplied by the size of each vector.
Lengths are measured in addressable storage units (bytes).
The following macros can be used to refine the length computation:
ADJUST_INSN_LENGTH (insn, length)If defined, modifies the length assigned to instruction insn as a function of the context in which it is used. length is an lvalue that contains the initially computed length of the insn and should be updated with the correct length of the insn.
This macro will normally not be required. A case in which it is
required is the ROMP. On this machine, the size of an addr_vec
insn must be increased by two to compensate for the fact that alignment
may be required.
The routine that returns get_attr_length (the value of the
length attribute) can be used by the output routine to
determine the form of the branch instruction to be written, as the
example below illustrates.
As an example of the specification of variable-length branches, consider the IBM 360. If we adopt the convention that a register will be set to the starting address of a function, we can jump to labels within 4k of the start using a four-byte instruction. Otherwise, we need a six-byte sequence to load the address from memory and then branch to it.
On such a machine, a pattern for a branch instruction might be specified as follows:
(define_insn "jump"
[(set (pc)
(label_ref (match_operand 0 "" "")))]
""
{
return (get_attr_length (insn) == 4
? "b %l0" : "l r15,=a(%l0); br r15");
}
[(set (attr "length")
(if_then_else (lt (match_dup 0) (const_int 4096))
(const_int 4)
(const_int 6)))])
Next: Delay Slots, Previous: Insn Lengths, Up: Insn Attributes [Contents][Index]
A special form of define_attr, where the expression for the
default value is a const expression, indicates an attribute that
is constant for a given run of the compiler. Constant attributes may be
used to specify which variety of processor is used. For example,
(define_attr "cpu" "m88100,m88110,m88000"
(const
(cond [(symbol_ref "TARGET_88100") (const_string "m88100")
(symbol_ref "TARGET_88110") (const_string "m88110")]
(const_string "m88000"))))
(define_attr "memory" "fast,slow"
(const
(if_then_else (symbol_ref "TARGET_FAST_MEM")
(const_string "fast")
(const_string "slow"))))
The routine generated for constant attributes has no parameters as it
does not depend on any particular insn. RTL expressions used to define
the value of a constant attribute may use the symbol_ref form,
but may not use either the match_operand form or eq_attr
forms involving insn attributes.
Next: Processor pipeline description, Previous: Constant Attributes, Up: Insn Attributes [Contents][Index]
The insn attribute mechanism can be used to specify the requirements for delay slots, if any, on a target machine. An instruction is said to require a delay slot if some instructions that are physically after the instruction are executed as if they were located before it. Classic examples are branch and call instructions, which often execute the following instruction before the branch or call is performed.
On some machines, conditional branch instructions can optionally annul instructions in the delay slot. This means that the instruction will not be executed for certain branch outcomes. Both instructions that annul if the branch is true and instructions that annul if the branch is false are supported.
Delay slot scheduling differs from instruction scheduling in that determining whether an instruction needs a delay slot is dependent only on the type of instruction being generated, not on data flow between the instructions. See the next section for a discussion of data-dependent instruction scheduling.
The requirement of an insn needing one or more delay slots is indicated
via the define_delay expression. It has the following form:
(define_delay test
[delay-1 annul-true-1 annul-false-1
delay-2 annul-true-2 annul-false-2
…])
test is an attribute test that indicates whether this
define_delay applies to a particular insn. If so, the number of
required delay slots is determined by the length of the vector specified
as the second argument. An insn placed in delay slot n must
satisfy attribute test delay-n. annul-true-n is an
attribute test that specifies which insns may be annulled if the branch
is true. Similarly, annul-false-n specifies which insns in the
delay slot may be annulled if the branch is false. If annulling is not
supported for that delay slot, (nil) should be coded.
For example, in the common case where branch and call insns require a single delay slot, which may contain any insn other than a branch or call, the following would be placed in the md file:
(define_delay (eq_attr "type" "branch,call")
[(eq_attr "type" "!branch,call") (nil) (nil)])
Multiple define_delay expressions may be specified. In this
case, each such expression specifies different delay slot requirements
and there must be no insn for which tests in two define_delay
expressions are both true.
For example, if we have a machine that requires one delay slot for branches but two for calls, no delay slot can contain a branch or call insn, and any valid insn in the delay slot for the branch can be annulled if the branch is true, we might represent this as follows:
(define_delay (eq_attr "type" "branch")
[(eq_attr "type" "!branch,call")
(eq_attr "type" "!branch,call")
(nil)])
(define_delay (eq_attr "type" "call")
[(eq_attr "type" "!branch,call") (nil) (nil)
(eq_attr "type" "!branch,call") (nil) (nil)])
Previous: Delay Slots, Up: Insn Attributes [Contents][Index]
To achieve better performance, most modern processors (super-pipelined, superscalar RISC, and VLIW processors) have many functional units on which several instructions can be executed simultaneously. An instruction starts execution if its issue conditions are satisfied. If not, the instruction is stalled until its conditions are satisfied. Such interlock (pipeline) delay causes interruption of the fetching of successor instructions (or demands nop instructions, e.g. for some MIPS processors).
There are two major kinds of interlock delays in modern processors. The first one is a data dependence delay determining instruction latency time. The instruction execution is not started until all source data have been evaluated by prior instructions (there are more complex cases when the instruction execution starts even when the data are not available but will be ready in given time after the instruction execution start). Taking the data dependence delays into account is simple. The data dependence (true, output, and anti-dependence) delay between two instructions is given by a constant. In most cases this approach is adequate. The second kind of interlock delays is a reservation delay. The reservation delay means that two instructions under execution will be in need of shared processors resources, i.e. buses, internal registers, and/or functional units, which are reserved for some time. Taking this kind of delay into account is complex especially for modern RISC processors.
The task of exploiting more processor parallelism is solved by an instruction scheduler. For a better solution to this problem, the instruction scheduler has to have an adequate description of the processor parallelism (or pipeline description). GCC machine descriptions describe processor parallelism and functional unit reservations for groups of instructions with the aid of regular expressions.
The GCC instruction scheduler uses a pipeline hazard recognizer to figure out the possibility of the instruction issue by the processor on a given simulated processor cycle. The pipeline hazard recognizer is automatically generated from the processor pipeline description. The pipeline hazard recognizer generated from the machine description is based on a deterministic finite state automaton (DFA): the instruction issue is possible if there is a transition from one automaton state to another one. This algorithm is very fast, and furthermore, its speed is not dependent on processor complexity5.
The rest of this section describes the directives that constitute an automaton-based processor pipeline description. The order of these constructions within the machine description file is not important.
The following optional construction describes names of automata generated and used for the pipeline hazards recognition. Sometimes the generated finite state automaton used by the pipeline hazard recognizer is large. If we use more than one automaton and bind functional units to the automata, the total size of the automata is usually less than the size of the single automaton. If there is no one such construction, only one finite state automaton is generated.
(define_automaton automata-names)
automata-names is a string giving names of the automata. The
names are separated by commas. All the automata should have unique names.
The automaton name is used in the constructions define_cpu_unit and
define_query_cpu_unit.
Each processor functional unit used in the description of instruction reservations should be described by the following construction.
(define_cpu_unit unit-names [automaton-name])
unit-names is a string giving the names of the functional units separated by commas. Don’t use name ‘nothing’, it is reserved for other goals.
automaton-name is a string giving the name of the automaton with
which the unit is bound. The automaton should be described in
construction define_automaton. You should give
automaton-name, if there is a defined automaton.
The assignment of units to automata are constrained by the uses of the units in insn reservations. The most important constraint is: if a unit reservation is present on a particular cycle of an alternative for an insn reservation, then some unit from the same automaton must be present on the same cycle for the other alternatives of the insn reservation. The rest of the constraints are mentioned in the description of the subsequent constructions.
The following construction describes CPU functional units analogously
to define_cpu_unit. The reservation of such units can be
queried for an automaton state. The instruction scheduler never
queries reservation of functional units for given automaton state. So
as a rule, you don’t need this construction. This construction could
be used for future code generation goals (e.g. to generate
VLIW insn templates).
(define_query_cpu_unit unit-names [automaton-name])
unit-names is a string giving names of the functional units separated by commas.
automaton-name is a string giving the name of the automaton with which the unit is bound.
The following construction is the major one to describe pipeline characteristics of an instruction.
(define_insn_reservation insn-name default_latency
condition regexp)
default_latency is a number giving latency time of the
instruction. There is an important difference between the old
description and the automaton based pipeline description. The latency
time is used for all dependencies when we use the old description. In
the automaton based pipeline description, the given latency time is only
used for true dependencies. The cost of anti-dependencies is always
zero and the cost of output dependencies is the difference between
latency times of the producing and consuming insns (if the difference
is negative, the cost is considered to be zero). You can always
change the default costs for any description by using the target hook
TARGET_SCHED_ADJUST_COST (see Scheduling).
insn-name is a string giving the internal name of the insn. The
internal names are used in constructions define_bypass and in
the automaton description file generated for debugging. The internal
name has nothing in common with the names in define_insn. It is a
good practice to use insn classes described in the processor manual.
condition defines what RTL insns are described by this
construction. You should remember that you will be in trouble if
condition for two or more different
define_insn_reservation constructions is TRUE for an insn. In
this case what reservation will be used for the insn is not defined.
Such cases are not checked during generation of the pipeline hazards
recognizer because in general recognizing that two conditions may have
the same value is quite difficult (especially if the conditions
contain symbol_ref). It is also not checked during the
pipeline hazard recognizer work because it would slow down the
recognizer considerably.
regexp is a string describing the reservation of the cpu’s functional units by the instruction. The reservations are described by a regular expression according to the following syntax:
regexp = regexp "," oneof
| oneof
oneof = oneof "|" allof
| allof
allof = allof "+" repeat
| repeat
repeat = element "*" number
| element
element = cpu_function_unit_name
| reservation_name
| result_name
| "nothing"
| "(" regexp ")"
Sometimes unit reservations for different insns contain common parts. In such case, you can simplify the pipeline description by describing the common part by the following construction
(define_reservation reservation-name regexp)
reservation-name is a string giving name of regexp. Functional unit names and reservation names are in the same name space. So the reservation names should be different from the functional unit names and can not be the reserved name ‘nothing’.
The following construction is used to describe exceptions in the latency time for given instruction pair. This is so called bypasses.
(define_bypass number out_insn_names in_insn_names
[guard])
number defines when the result generated by the instructions given in string out_insn_names will be ready for the instructions given in string in_insn_names. The instructions in the string are separated by commas.
guard is an optional string giving the name of a C function which defines an additional guard for the bypass. The function will get the two insns as parameters. If the function returns zero the bypass will be ignored for this case. The additional guard is necessary to recognize complicated bypasses, e.g. when the consumer is only an address of insn ‘store’ (not a stored value).
If there are more one bypass with the same output and input insns, the chosen bypass is the first bypass with a guard in description whose guard function returns nonzero. If there is no such bypass, then bypass without the guard function is chosen.
The following five constructions are usually used to describe VLIW processors, or more precisely, to describe a placement of small instructions into VLIW instruction slots. They can be used for RISC processors, too.
(exclusion_set unit-names unit-names) (presence_set unit-names patterns) (final_presence_set unit-names patterns) (absence_set unit-names patterns) (final_absence_set unit-names patterns)
unit-names is a string giving names of functional units separated by commas.
patterns is a string giving patterns of functional units separated by comma. Currently pattern is one unit or units separated by white-spaces.
The first construction (‘exclusion_set’) means that each functional unit in the first string can not be reserved simultaneously with a unit whose name is in the second string and vice versa. For example, the construction is useful for describing processors (e.g. some SPARC processors) with a fully pipelined floating point functional unit which can execute simultaneously only single floating point insns or only double floating point insns.
The second construction (‘presence_set’) means that each functional unit in the first string can not be reserved unless at least one of pattern of units whose names are in the second string is reserved. This is an asymmetric relation. For example, it is useful for description that VLIW ‘slot1’ is reserved after ‘slot0’ reservation. We could describe it by the following construction
(presence_set "slot1" "slot0")
Or ‘slot1’ is reserved only after ‘slot0’ and unit ‘b0’ reservation. In this case we could write
(presence_set "slot1" "slot0 b0")
The third construction (‘final_presence_set’) is analogous to ‘presence_set’. The difference between them is when checking is done. When an instruction is issued in given automaton state reflecting all current and planned unit reservations, the automaton state is changed. The first state is a source state, the second one is a result state. Checking for ‘presence_set’ is done on the source state reservation, checking for ‘final_presence_set’ is done on the result reservation. This construction is useful to describe a reservation which is actually two subsequent reservations. For example, if we use
(presence_set "slot1" "slot0")
the following insn will be never issued (because ‘slot1’ requires ‘slot0’ which is absent in the source state).
(define_reservation "insn_and_nop" "slot0 + slot1")
but it can be issued if we use analogous ‘final_presence_set’.
The forth construction (‘absence_set’) means that each functional unit in the first string can be reserved only if each pattern of units whose names are in the second string is not reserved. This is an asymmetric relation (actually ‘exclusion_set’ is analogous to this one but it is symmetric). For example it might be useful in a VLIW description to say that ‘slot0’ cannot be reserved after either ‘slot1’ or ‘slot2’ have been reserved. This can be described as:
(absence_set "slot0" "slot1, slot2")
Or ‘slot2’ can not be reserved if ‘slot0’ and unit ‘b0’ are reserved or ‘slot1’ and unit ‘b1’ are reserved. In this case we could write
(absence_set "slot2" "slot0 b0, slot1 b1")
All functional units mentioned in a set should belong to the same automaton.
The last construction (‘final_absence_set’) is analogous to ‘absence_set’ but checking is done on the result (state) reservation. See comments for ‘final_presence_set’.
You can control the generator of the pipeline hazard recognizer with the following construction.
(automata_option options)
options is a string giving options which affect the generated code. Currently there are the following options:
As an example, consider a superscalar RISC machine which can issue three insns (two integer insns and one floating point insn) on the cycle but can finish only two insns. To describe this, we define the following functional units.
(define_cpu_unit "i0_pipeline, i1_pipeline, f_pipeline") (define_cpu_unit "port0, port1")
All simple integer insns can be executed in any integer pipeline and their result is ready in two cycles. The simple integer insns are issued into the first pipeline unless it is reserved, otherwise they are issued into the second pipeline. Integer division and multiplication insns can be executed only in the second integer pipeline and their results are ready correspondingly in 8 and 4 cycles. The integer division is not pipelined, i.e. the subsequent integer division insn can not be issued until the current division insn finished. Floating point insns are fully pipelined and their results are ready in 3 cycles. Where the result of a floating point insn is used by an integer insn, an additional delay of one cycle is incurred. To describe all of this we could specify
(define_cpu_unit "div")
(define_insn_reservation "simple" 2 (eq_attr "type" "int")
"(i0_pipeline | i1_pipeline), (port0 | port1)")
(define_insn_reservation "mult" 4 (eq_attr "type" "mult")
"i1_pipeline, nothing*2, (port0 | port1)")
(define_insn_reservation "div" 8 (eq_attr "type" "div")
"i1_pipeline, div*7, div + (port0 | port1)")
(define_insn_reservation "float" 3 (eq_attr "type" "float")
"f_pipeline, nothing, (port0 | port1))
(define_bypass 4 "float" "simple,mult,div")
To simplify the description we could describe the following reservation
(define_reservation "finish" "port0|port1")
and use it in all define_insn_reservation as in the following
construction
(define_insn_reservation "simple" 2 (eq_attr "type" "int")
"(i0_pipeline | i1_pipeline), finish")
Next: Constant Definitions, Previous: Insn Attributes, Up: Machine Desc [Contents][Index]
A number of architectures provide for some form of conditional
execution, or predication. The hallmark of this feature is the
ability to nullify most of the instructions in the instruction set.
When the instruction set is large and not entirely symmetric, it
can be quite tedious to describe these forms directly in the
.md file. An alternative is the define_cond_exec template.
(define_cond_exec [predicate-pattern] "condition" "output-template")
predicate-pattern is the condition that must be true for the
insn to be executed at runtime and should match a relational operator.
One can use match_operator to match several relational operators
at once. Any match_operand operands must have no more than one
alternative.
condition is a C expression that must be true for the generated pattern to match.
output-template is a string similar to the define_insn
output template (see Output Template), except that the ‘*’
and ‘@’ special cases do not apply. This is only useful if the
assembly text for the predicate is a simple prefix to the main insn.
In order to handle the general case, there is a global variable
current_insn_predicate that will contain the entire predicate
if the current insn is predicated, and will otherwise be NULL.
When define_cond_exec is used, an implicit reference to
the predicable instruction attribute is made.
See Insn Attributes. This attribute must be boolean (i.e. have
exactly two elements in its list-of-values). Further, it must
not be used with complex expressions. That is, the default and all
uses in the insns must be a simple constant, not dependent on the
alternative or anything else.
For each define_insn for which the predicable
attribute is true, a new define_insn pattern will be
generated that matches a predicated version of the instruction.
For example,
(define_insn "addsi"
[(set (match_operand:SI 0 "register_operand" "r")
(plus:SI (match_operand:SI 1 "register_operand" "r")
(match_operand:SI 2 "register_operand" "r")))]
"test1"
"add %2,%1,%0")
(define_cond_exec
[(ne (match_operand:CC 0 "register_operand" "c")
(const_int 0))]
"test2"
"(%0)")
generates a new pattern
(define_insn ""
[(cond_exec
(ne (match_operand:CC 3 "register_operand" "c") (const_int 0))
(set (match_operand:SI 0 "register_operand" "r")
(plus:SI (match_operand:SI 1 "register_operand" "r")
(match_operand:SI 2 "register_operand" "r"))))]
"(test2) && (test1)"
"(%3) add %2,%1,%0")
Next: Iterators, Previous: Conditional Execution, Up: Machine Desc [Contents][Index]
Using literal constants inside instruction patterns reduces legibility and can be a maintenance problem.
To overcome this problem, you may use the define_constants
expression. It contains a vector of name-value pairs. From that
point on, wherever any of the names appears in the MD file, it is as
if the corresponding value had been written instead. You may use
define_constants multiple times; each appearance adds more
constants to the table. It is an error to redefine a constant with
a different value.
To come back to the a29k load multiple example, instead of
(define_insn ""
[(match_parallel 0 "load_multiple_operation"
[(set (match_operand:SI 1 "gpc_reg_operand" "=r")
(match_operand:SI 2 "memory_operand" "m"))
(use (reg:SI 179))
(clobber (reg:SI 179))])]
""
"loadm 0,0,%1,%2")
You could write:
(define_constants [
(R_BP 177)
(R_FC 178)
(R_CR 179)
(R_Q 180)
])
(define_insn ""
[(match_parallel 0 "load_multiple_operation"
[(set (match_operand:SI 1 "gpc_reg_operand" "=r")
(match_operand:SI 2 "memory_operand" "m"))
(use (reg:SI R_CR))
(clobber (reg:SI R_CR))])]
""
"loadm 0,0,%1,%2")
The constants that are defined with a define_constant are also output in the insn-codes.h header file as #defines.
You can also use the machine description file to define enumerations.
Like the constants defined by define_constant, these enumerations
are visible to both the machine description file and the main C code.
The syntax is as follows:
(define_c_enum "name" [ value0 value1 … valuen ])
This definition causes the equivalent of the following C code to appear in insn-constants.h:
enum name {
value0 = 0,
value1 = 1,
…
valuen = n
};
#define NUM_cname_VALUES (n + 1)
where cname is the capitalized form of name. It also makes each valuei available in the machine description file, just as if it had been declared with:
(define_constants [(valuei i)])
Each valuei is usually an upper-case identifier and usually begins with cname.
You can split the enumeration definition into as many statements as you like. The above example is directly equivalent to:
(define_c_enum "name" [value0]) (define_c_enum "name" [value1]) … (define_c_enum "name" [valuen])
Splitting the enumeration helps to improve the modularity of each
individual .md file. For example, if a port defines its
synchronization instructions in a separate sync.md file,
it is convenient to define all synchronization-specific enumeration
values in sync.md rather than in the main .md file.
Some enumeration names have special significance to GCC:
unspecvIf an enumeration called unspecv is defined, GCC will use it
when printing out unspec_volatile expressions. For example:
(define_c_enum "unspecv" [ UNSPECV_BLOCKAGE ])
causes GCC to print ‘(unspec_volatile … 0)’ as:
(unspec_volatile ... UNSPECV_BLOCKAGE)
unspecIf an enumeration called unspec is defined, GCC will use
it when printing out unspec expressions. GCC will also use
it when printing out unspec_volatile expressions unless an
unspecv enumeration is also defined. You can therefore
decide whether to keep separate enumerations for volatile and
non-volatile expressions or whether to use the same enumeration
for both.
Another way of defining an enumeration is to use define_enum:
(define_enum "name" [ value0 value1 … valuen ])
This directive implies:
(define_c_enum "name" [ cname_cvalue0 cname_cvalue1 … cname_cvaluen ])
where cvaluei is the capitalized form of valuei.
However, unlike define_c_enum, the enumerations defined
by define_enum can be used in attribute specifications
(see define_enum_attr).
Previous: Constant Definitions, Up: Machine Desc [Contents][Index]
Ports often need to define similar patterns for more than one machine mode or for more than one rtx code. GCC provides some simple iterator facilities to make this process easier.
| • Mode Iterators: | Generating variations of patterns for different modes. | |
| • Code Iterators: | Doing the same for codes. |
Next: Code Iterators, Up: Iterators [Contents][Index]
Ports often need to define similar patterns for two or more different modes. For example:
SFmode patterns tend to be
very similar to the DFmode ones.
SImode pointers in one configuration and
DImode pointers in another, it will usually have very similar
SImode and DImode patterns for manipulating pointers.
Mode iterators allow several patterns to be instantiated from one
.md file template. They can be used with any type of
rtx-based construct, such as a define_insn,
define_split, or define_peephole2.
| • Defining Mode Iterators: | Defining a new mode iterator. | |
| • Substitutions: | Combining mode iterators with substitutions | |
| • Examples: | Examples |
Next: Substitutions, Up: Mode Iterators [Contents][Index]
The syntax for defining a mode iterator is:
(define_mode_iterator name [(mode1 "cond1") … (moden "condn")])
This allows subsequent .md file constructs to use the mode suffix
:name. Every construct that does so will be expanded
n times, once with every use of :name replaced by
:mode1, once with every use replaced by :mode2,
and so on. In the expansion for a particular modei, every
C condition will also require that condi be true.
For example:
(define_mode_iterator P [(SI "Pmode == SImode") (DI "Pmode == DImode")])
defines a new mode suffix :P. Every construct that uses
:P will be expanded twice, once with every :P replaced
by :SI and once with every :P replaced by :DI.
The :SI version will only apply if Pmode == SImode and
the :DI version will only apply if Pmode == DImode.
As with other .md conditions, an empty string is treated
as “always true”. (mode "") can also be abbreviated
to mode. For example:
(define_mode_iterator GPR [SI (DI "TARGET_64BIT")])
means that the :DI expansion only applies if TARGET_64BIT
but that the :SI expansion has no such constraint.
Iterators are applied in the order they are defined. This can be significant if two iterators are used in a construct that requires substitutions. See Substitutions.
Next: Examples, Previous: Defining Mode Iterators, Up: Mode Iterators [Contents][Index]
If an .md file construct uses mode iterators, each version of the construct will often need slightly different strings or modes. For example:
define_expand defines several addm3 patterns
(see Standard Names), each expander will need to use the
appropriate mode name for m.
define_insn defines several instruction patterns,
each instruction will often use a different assembler mnemonic.
define_insn requires operands with different modes,
using an iterator for one of the operand modes usually requires a specific
mode for the other operand(s).
GCC supports such variations through a system of “mode attributes”.
There are two standard attributes: mode, which is the name of
the mode in lower case, and MODE, which is the same thing in
upper case. You can define other attributes using:
(define_mode_attr name [(mode1 "value1") … (moden "valuen")])
where name is the name of the attribute and valuei is the value associated with modei.
When GCC replaces some :iterator with :mode, it will scan
each string and mode in the pattern for sequences of the form
<iterator:attr>, where attr is the name of a
mode attribute. If the attribute is defined for mode, the whole
<…> sequence will be replaced by the appropriate attribute
value.
For example, suppose an .md file has:
(define_mode_iterator P [(SI "Pmode == SImode") (DI "Pmode == DImode")]) (define_mode_attr load [(SI "lw") (DI "ld")])
If one of the patterns that uses :P contains the string
"<P:load>\t%0,%1", the SI version of that pattern
will use "lw\t%0,%1" and the DI version will use
"ld\t%0,%1".
Here is an example of using an attribute for a mode:
(define_mode_iterator LONG [SI DI]) (define_mode_attr SHORT [(SI "HI") (DI "SI")]) (define_insn … (sign_extend:LONG (match_operand:<LONG:SHORT> …)) …)
The iterator: prefix may be omitted, in which case the
substitution will be attempted for every iterator expansion.
Previous: Substitutions, Up: Mode Iterators [Contents][Index]
Here is an example from the MIPS port. It defines the following modes and attributes (among others):
(define_mode_iterator GPR [SI (DI "TARGET_64BIT")]) (define_mode_attr d [(SI "") (DI "d")])
and uses the following template to define both subsi3
and subdi3:
(define_insn "sub<mode>3"
[(set (match_operand:GPR 0 "register_operand" "=d")
(minus:GPR (match_operand:GPR 1 "register_operand" "d")
(match_operand:GPR 2 "register_operand" "d")))]
""
"<d>subu\t%0,%1,%2"
[(set_attr "type" "arith")
(set_attr "mode" "<MODE>")])
This is exactly equivalent to:
(define_insn "subsi3"
[(set (match_operand:SI 0 "register_operand" "=d")
(minus:SI (match_operand:SI 1 "register_operand" "d")
(match_operand:SI 2 "register_operand" "d")))]
""
"subu\t%0,%1,%2"
[(set_attr "type" "arith")
(set_attr "mode" "SI")])
(define_insn "subdi3"
[(set (match_operand:DI 0 "register_operand" "=d")
(minus:DI (match_operand:DI 1 "register_operand" "d")
(match_operand:DI 2 "register_operand" "d")))]
""
"dsubu\t%0,%1,%2"
[(set_attr "type" "arith")
(set_attr "mode" "DI")])
Previous: Mode Iterators, Up: Iterators [Contents][Index]
Code iterators operate in a similar way to mode iterators. See Mode Iterators.
The construct:
(define_code_iterator name [(code1 "cond1") … (coden "condn")])
defines a pseudo rtx code name that can be instantiated as codei if condition condi is true. Each codei must have the same rtx format. See RTL Classes.
As with mode iterators, each pattern that uses name will be expanded n times, once with all uses of name replaced by code1, once with all uses replaced by code2, and so on. See Defining Mode Iterators.
It is possible to define attributes for codes as well as for modes.
There are two standard code attributes: code, the name of the
code in lower case, and CODE, the name of the code in upper case.
Other attributes are defined using:
(define_code_attr name [(code1 "value1") … (coden "valuen")])
Here’s an example of code iterators in action, taken from the MIPS port:
(define_code_iterator any_cond [unordered ordered unlt unge uneq ltgt unle ungt
eq ne gt ge lt le gtu geu ltu leu])
(define_expand "b<code>"
[(set (pc)
(if_then_else (any_cond:CC (cc0)
(const_int 0))
(label_ref (match_operand 0 ""))
(pc)))]
""
{
gen_conditional_branch (operands, <CODE>);
DONE;
})
This is equivalent to:
(define_expand "bunordered"
[(set (pc)
(if_then_else (unordered:CC (cc0)
(const_int 0))
(label_ref (match_operand 0 ""))
(pc)))]
""
{
gen_conditional_branch (operands, UNORDERED);
DONE;
})
(define_expand "bordered"
[(set (pc)
(if_then_else (ordered:CC (cc0)
(const_int 0))
(label_ref (match_operand 0 ""))
(pc)))]
""
{
gen_conditional_branch (operands, ORDERED);
DONE;
})
…
Next: Host Config, Previous: Machine Desc, Up: Top [Contents][Index]
In addition to the file machine.md, a machine description
includes a C header file conventionally given the name
machine.h and a C source file named machine.c.
The header file defines numerous macros that convey the information
about the target machine that does not fit into the scheme of the
.md file. The file tm.h should be a link to
machine.h. The header file config.h includes
tm.h and most compiler source files include config.h. The
source file defines a variable targetm, which is a structure
containing pointers to functions and data relating to the target
machine. machine.c should also contain their definitions,
if they are not defined elsewhere in GCC, and other functions called
through the macros defined in the .h file.
| • Target Structure: | The targetm variable.
| |
| • Driver: | Controlling how the driver runs the compilation passes. | |
| • Run-time Target: | Defining ‘-m’ options like -m68000 and -m68020. | |
| • Per-Function Data: | Defining data structures for per-function information. | |
| • Storage Layout: | Defining sizes and alignments of data. | |
| • Type Layout: | Defining sizes and properties of basic user data types. | |
| • Registers: | Naming and describing the hardware registers. | |
| • Register Classes: | Defining the classes of hardware registers. | |
| • Old Constraints: | The old way to define machine-specific constraints. | |
| • Stack and Calling: | Defining which way the stack grows and by how much. | |
| • Varargs: | Defining the varargs macros. | |
| • Trampolines: | Code set up at run time to enter a nested function. | |
| • Library Calls: | Controlling how library routines are implicitly called. | |
| • Addressing Modes: | Defining addressing modes valid for memory operands. | |
| • Anchored Addresses: | Defining how -fsection-anchors should work. | |
| • Condition Code: | Defining how insns update the condition code. | |
| • Costs: | Defining relative costs of different operations. | |
| • Scheduling: | Adjusting the behavior of the instruction scheduler. | |
| • Sections: | Dividing storage into text, data, and other sections. | |
| • PIC: | Macros for position independent code. | |
| • Assembler Format: | Defining how to write insns and pseudo-ops to output. | |
| • Debugging Info: | Defining the format of debugging output. | |
| • Floating Point: | Handling floating point for cross-compilers. | |
| • Mode Switching: | Insertion of mode-switching instructions. | |
| • Target Attributes: | Defining target-specific uses of __attribute__.
| |
| • Emulated TLS: | Emulated TLS support. | |
| • MIPS Coprocessors: | MIPS coprocessor support and how to customize it. | |
| • PCH Target: | Validity checking for precompiled headers. | |
| • C++ ABI: | Controlling C++ ABI changes. | |
| • Named Address Spaces: | Adding support for named address spaces | |
| • Misc: | Everything else. |
Next: Driver, Up: Target Macros [Contents][Index]
targetm VariableThe target .c file must define the global targetm variable
which contains pointers to functions and data relating to the target
machine. The variable is declared in target.h;
target-def.h defines the macro TARGET_INITIALIZER which is
used to initialize the variable, and macros for the default initializers
for elements of the structure. The .c file should override those
macros for which the default definition is inappropriate. For example:
#include "target.h"
#include "target-def.h"
/* Initialize the GCC target structure. */
#undef TARGET_COMP_TYPE_ATTRIBUTES
#define TARGET_COMP_TYPE_ATTRIBUTES machine_comp_type_attributes
struct gcc_target targetm = TARGET_INITIALIZER;
Where a macro should be defined in the .c file in this manner to
form part of the targetm structure, it is documented below as a
“Target Hook” with a prototype. Many macros will change in future
from being defined in the .h file to being part of the
targetm structure.
Next: Run-time Target, Previous: Target Structure, Up: Target Macros [Contents][Index]
You can control the compilation driver.
A list of specs for the driver itself. It should be a suitable initializer for an array of strings, with no surrounding braces.
The driver applies these specs to its own command line between loading default specs files (but not command-line specified ones) and choosing the multilib directory or running any subcommands. It applies them in the order given, so each spec can depend on the options added by earlier ones. It is also possible to remove options using ‘%<option’ in the usual way.
This macro can be useful when a port has several interdependent target options. It provides a way of standardizing the command line so that the other specs are easier to write.
Do not define this macro if it does not need to do anything.
A list of specs used to support configure-time default options (i.e. --with options) in the driver. It should be a suitable initializer for an array of structures, each containing two strings, without the outermost pair of surrounding braces.
The first item in the pair is the name of the default. This must match the code in config.gcc for the target. The second item is a spec to apply if a default with this name was specified. The string ‘%(VALUE)’ in the spec will be replaced by the value of the default everywhere it occurs.
The driver will apply these specs to its own command line between loading
default specs files and processing DRIVER_SELF_SPECS, using
the same mechanism as DRIVER_SELF_SPECS.
Do not define this macro if it does not need to do anything.
A C string constant that tells the GCC driver program options to pass to CPP. It can also specify how to translate options you give to GCC into options for GCC to pass to the CPP.
Do not define this macro if it does not need to do anything.
This macro is just like CPP_SPEC, but is used for C++, rather
than C. If you do not define this macro, then the value of
CPP_SPEC (if any) will be used instead.
A C string constant that tells the GCC driver program options to
pass to cc1, cc1plus, f771, and the other language
front ends.
It can also specify how to translate options you give to GCC into options
for GCC to pass to front ends.
Do not define this macro if it does not need to do anything.
A C string constant that tells the GCC driver program options to
pass to cc1plus. It can also specify how to translate options you
give to GCC into options for GCC to pass to the cc1plus.
Do not define this macro if it does not need to do anything.
Note that everything defined in CC1_SPEC is already passed to
cc1plus so there is no need to duplicate the contents of
CC1_SPEC in CC1PLUS_SPEC.
A C string constant that tells the GCC driver program options to pass to the assembler. It can also specify how to translate options you give to GCC into options for GCC to pass to the assembler. See the file sun3.h for an example of this.
Do not define this macro if it does not need to do anything.
A C string constant that tells the GCC driver program how to run any programs which cleanup after the normal assembler. Normally, this is not needed. See the file mips.h for an example of this.
Do not define this macro if it does not need to do anything.
Define this macro, with no value, if the driver should give the assembler an argument consisting of a single dash, -, to instruct it to read from its standard input (which will be a pipe connected to the output of the compiler proper). This argument is given after any -o option specifying the name of the output file.
If you do not define this macro, the assembler is assumed to read its standard input if given no non-option arguments. If your assembler cannot read standard input at all, use a ‘%{pipe:%e}’ construct; see mips.h for instance.
A C string constant that tells the GCC driver program options to pass to the linker. It can also specify how to translate options you give to GCC into options for GCC to pass to the linker.
Do not define this macro if it does not need to do anything.
Another C string constant used much like LINK_SPEC. The difference
between the two is that LIB_SPEC is used at the end of the
command given to the linker.
If this macro is not defined, a default is provided that loads the standard C library from the usual place. See gcc.c.
Another C string constant that tells the GCC driver program
how and when to place a reference to libgcc.a into the
linker command line. This constant is placed both before and after
the value of LIB_SPEC.
If this macro is not defined, the GCC driver provides a default that passes the string -lgcc to the linker.
By default, if ENABLE_SHARED_LIBGCC is defined, the
LIBGCC_SPEC is not directly used by the driver program but is
instead modified to refer to different versions of libgcc.a
depending on the values of the command line flags -static,
-shared, -static-libgcc, and -shared-libgcc. On
targets where these modifications are inappropriate, define
REAL_LIBGCC_SPEC instead. REAL_LIBGCC_SPEC tells the
driver how to place a reference to libgcc on the link command
line, but, unlike LIBGCC_SPEC, it is used unmodified.
A macro that controls the modifications to LIBGCC_SPEC
mentioned in REAL_LIBGCC_SPEC. If nonzero, a spec will be
generated that uses –as-needed and the shared libgcc in place of the
static exception handler library, when linking without any of
-static, -static-libgcc, or -shared-libgcc.
If defined, this C string constant is added to LINK_SPEC.
When USE_LD_AS_NEEDED is zero or undefined, it also affects
the modifications to LIBGCC_SPEC mentioned in
REAL_LIBGCC_SPEC.
Another C string constant used much like LINK_SPEC. The
difference between the two is that STARTFILE_SPEC is used at
the very beginning of the command given to the linker.
If this macro is not defined, a default is provided that loads the standard C startup file from the usual place. See gcc.c.
Another C string constant used much like LINK_SPEC. The
difference between the two is that ENDFILE_SPEC is used at
the very end of the command given to the linker.
Do not define this macro if it does not need to do anything.
GCC -v will print the thread model GCC was configured to use.
However, this doesn’t work on platforms that are multilibbed on thread
models, such as AIX 4.3. On such platforms, define
THREAD_MODEL_SPEC such that it evaluates to a string without
blanks that names one of the recognized thread models. %*, the
default value of this macro, will expand to the value of
thread_file set in config.gcc.
Define this macro to add a suffix to the target sysroot when GCC is configured with a sysroot. This will cause GCC to search for usr/lib, et al, within sysroot+suffix.
Define this macro to add a headers_suffix to the target sysroot when GCC is configured with a sysroot. This will cause GCC to pass the updated sysroot+headers_suffix to CPP, causing it to search for usr/include, et al, within sysroot+headers_suffix.
Define this macro to provide additional specifications to put in the
specs file that can be used in various specifications like
CC1_SPEC.
The definition should be an initializer for an array of structures, containing a string constant, that defines the specification name, and a string constant that provides the specification.
Do not define this macro if it does not need to do anything.
EXTRA_SPECS is useful when an architecture contains several
related targets, which have various …_SPECS which are similar
to each other, and the maintainer would like one central place to keep
these definitions.
For example, the PowerPC System V.4 targets use EXTRA_SPECS to
define either _CALL_SYSV when the System V calling sequence is
used or _CALL_AIX when the older AIX-based calling sequence is
used.
The config/rs6000/rs6000.h target file defines:
#define EXTRA_SPECS \
{ "cpp_sysv_default", CPP_SYSV_DEFAULT },
#define CPP_SYS_DEFAULT ""
The config/rs6000/sysv.h target file defines:
#undef CPP_SPEC
#define CPP_SPEC \
"%{posix: -D_POSIX_SOURCE } \
%{mcall-sysv: -D_CALL_SYSV } \
%{!mcall-sysv: %(cpp_sysv_default) } \
%{msoft-float: -D_SOFT_FLOAT} %{mcpu=403: -D_SOFT_FLOAT}"
#undef CPP_SYSV_DEFAULT
#define CPP_SYSV_DEFAULT "-D_CALL_SYSV"
while the config/rs6000/eabiaix.h target file defines
CPP_SYSV_DEFAULT as:
#undef CPP_SYSV_DEFAULT #define CPP_SYSV_DEFAULT "-D_CALL_AIX"
Define this macro if the driver program should find the library libgcc.a. If you do not define this macro, the driver program will pass the argument -lgcc to tell the linker to do the search.
The sequence in which libgcc and libc are specified to the linker.
By default this is %G %L %G.
A C string constant giving the complete command line need to execute the
linker. When you do this, you will need to update your port each time a
change is made to the link command line within gcc.c. Therefore,
define this macro only if you need to completely redefine the command
line for invoking the linker and there is no other way to accomplish
the effect you need. Overriding this macro may be avoidable by overriding
LINK_GCC_C_SEQUENCE_SPEC instead.
A nonzero value causes collect2 to remove duplicate -Ldirectory search
directories from linking commands. Do not give it a nonzero value if
removing duplicate search directories changes the linker’s semantics.
Define this macro as a C expression for the initializer of an array of
string to tell the driver program which options are defaults for this
target and thus do not need to be handled specially when using
MULTILIB_OPTIONS.
Do not define this macro if MULTILIB_OPTIONS is not defined in
the target makefile fragment or if none of the options listed in
MULTILIB_OPTIONS are set by default.
See Target Fragment.
Define this macro to tell gcc that it should only translate
a -B prefix into a -L linker option if the prefix
indicates an absolute file name.
If defined, this macro is an additional prefix to try after
STANDARD_EXEC_PREFIX. MD_EXEC_PREFIX is not searched
when the compiler is built as a cross
compiler. If you define MD_EXEC_PREFIX, then be sure to add it
to the list of directories used to find the assembler in configure.in.
Define this macro as a C string constant if you wish to override the
standard choice of libdir as the default prefix to
try when searching for startup files such as crt0.o.
STANDARD_STARTFILE_PREFIX is not searched when the compiler
is built as a cross compiler.
Define this macro as a C string constant if you wish to override the
standard choice of /lib as a prefix to try after the default prefix
when searching for startup files such as crt0.o.
STANDARD_STARTFILE_PREFIX_1 is not searched when the compiler
is built as a cross compiler.
Define this macro as a C string constant if you wish to override the
standard choice of /lib as yet another prefix to try after the
default prefix when searching for startup files such as crt0.o.
STANDARD_STARTFILE_PREFIX_2 is not searched when the compiler
is built as a cross compiler.
If defined, this macro supplies an additional prefix to try after the
standard prefixes. MD_EXEC_PREFIX is not searched when the
compiler is built as a cross compiler.
If defined, this macro supplies yet another prefix to try after the standard prefixes. It is not searched when the compiler is built as a cross compiler.
Define this macro as a C string constant if you wish to set environment
variables for programs called by the driver, such as the assembler and
loader. The driver passes the value of this macro to putenv to
initialize the necessary environment variables.
Define this macro as a C string constant if you wish to override the
standard choice of /usr/local/include as the default prefix to
try when searching for local header files. LOCAL_INCLUDE_DIR
comes before SYSTEM_INCLUDE_DIR in the search order.
Cross compilers do not search either /usr/local/include or its replacement.
Define this macro as a C string constant if you wish to specify a
system-specific directory to search for header files before the standard
directory. SYSTEM_INCLUDE_DIR comes before
STANDARD_INCLUDE_DIR in the search order.
Cross compilers do not use this macro and do not search the directory specified.
Define this macro as a C string constant if you wish to override the standard choice of /usr/include as the default prefix to try when searching for header files.
Cross compilers ignore this macro and do not search either /usr/include or its replacement.
The “component” corresponding to STANDARD_INCLUDE_DIR.
See INCLUDE_DEFAULTS, below, for the description of components.
If you do not define this macro, no component is used.
Define this macro if you wish to override the entire default search path
for include files. For a native compiler, the default search path
usually consists of GCC_INCLUDE_DIR, LOCAL_INCLUDE_DIR,
SYSTEM_INCLUDE_DIR, GPLUSPLUS_INCLUDE_DIR, and
STANDARD_INCLUDE_DIR. In addition, GPLUSPLUS_INCLUDE_DIR
and GCC_INCLUDE_DIR are defined automatically by Makefile,
and specify private search areas for GCC. The directory
GPLUSPLUS_INCLUDE_DIR is used only for C++ programs.
The definition should be an initializer for an array of structures.
Each array element should have four elements: the directory name (a
string constant), the component name (also a string constant), a flag
for C++-only directories,
and a flag showing that the includes in the directory don’t need to be
wrapped in extern ‘C’ when compiling C++. Mark the end of
the array with a null element.
The component name denotes what GNU package the include file is part of, if any, in all uppercase letters. For example, it might be ‘GCC’ or ‘BINUTILS’. If the package is part of a vendor-supplied operating system, code the component name as ‘0’.
For example, here is the definition used for VAX/VMS:
#define INCLUDE_DEFAULTS \
{ \
{ "GNU_GXX_INCLUDE:", "G++", 1, 1}, \
{ "GNU_CC_INCLUDE:", "GCC", 0, 0}, \
{ "SYS$SYSROOT:[SYSLIB.]", 0, 0, 0}, \
{ ".", 0, 0, 0}, \
{ 0, 0, 0, 0} \
}
Here is the order of prefixes tried for exec files:
GCC_EXEC_PREFIX or, if GCC_EXEC_PREFIX
is not set and the compiler has not been installed in the configure-time
prefix, the location in which the compiler has actually been installed.
COMPILER_PATH.
STANDARD_EXEC_PREFIX, if the compiler has been installed
in the configured-time prefix.
MD_EXEC_PREFIX, if defined, but only if this is a native
compiler.
Here is the order of prefixes tried for startfiles:
GCC_EXEC_PREFIX or its automatically determined
value based on the installed toolchain location.
LIBRARY_PATH
(or port-specific name; native only, cross compilers do not use this).
STANDARD_EXEC_PREFIX, but only if the toolchain is installed
in the configured prefix or this is a native compiler.
MD_EXEC_PREFIX, if defined, but only if this is a native
compiler.
MD_STARTFILE_PREFIX, if defined, but only if this is a
native compiler, or we have a target system root.
MD_STARTFILE_PREFIX_1, if defined, but only if this is a
native compiler, or we have a target system root.
STANDARD_STARTFILE_PREFIX, with any sysroot modifications.
If this path is relative it will be prefixed by GCC_EXEC_PREFIX and
the machine suffix or STANDARD_EXEC_PREFIX and the machine suffix.
STANDARD_STARTFILE_PREFIX_1, but only if this is a native
compiler, or we have a target system root. The default for this macro is
/lib/.
STANDARD_STARTFILE_PREFIX_2, but only if this is a native
compiler, or we have a target system root. The default for this macro is
/usr/lib/.
Next: Per-Function Data, Previous: Driver, Up: Target Macros [Contents][Index]
Here are run-time target specifications.
This function-like macro expands to a block of code that defines
built-in preprocessor macros and assertions for the target CPU, using
the functions builtin_define, builtin_define_std and
builtin_assert. When the front end
calls this macro it provides a trailing semicolon, and since it has
finished command line option processing your code can use those
results freely.
builtin_assert takes a string in the form you pass to the
command-line option -A, such as cpu=mips, and creates
the assertion. builtin_define takes a string in the form
accepted by option -D and unconditionally defines the macro.
builtin_define_std takes a string representing the name of an
object-like macro. If it doesn’t lie in the user’s namespace,
builtin_define_std defines it unconditionally. Otherwise, it
defines a version with two leading underscores, and another version
with two leading and trailing underscores, and defines the original
only if an ISO standard was not requested on the command line. For
example, passing unix defines __unix, __unix__
and possibly unix; passing _mips defines __mips,
__mips__ and possibly _mips, and passing _ABI64
defines only _ABI64.
You can also test for the C dialect being compiled. The variable
c_language is set to one of clk_c, clk_cplusplus
or clk_objective_c. Note that if we are preprocessing
assembler, this variable will be clk_c but the function-like
macro preprocessing_asm_p() will return true, so you might want
to check for that first. If you need to check for strict ANSI, the
variable flag_iso can be used. The function-like macro
preprocessing_trad_p() can be used to check for traditional
preprocessing.
Similarly to TARGET_CPU_CPP_BUILTINS but this macro is optional
and is used for the target operating system instead.
Similarly to TARGET_CPU_CPP_BUILTINS but this macro is optional
and is used for the target object format. elfos.h uses this
macro to define __ELF__, so you probably do not need to define
it yourself.
This variable is declared in options.h, which is included before any target-specific headers.
This variable specifies the initial value of target_flags.
Its default setting is 0.
This hook is called whenever the user specifies one of the target-specific options described by the .opt definition files (see Options). It has the opportunity to do some option-specific processing and should return true if the option is valid. The default definition does nothing but return true.
code specifies the OPT_name enumeration value
associated with the selected option; name is just a rendering of
the option name in which non-alphanumeric characters are replaced by
underscores. arg specifies the string argument and is null if
no argument was given. If the option is flagged as a UInteger
(see Option properties), value is the numeric value of the
argument. Otherwise value is 1 if the positive form of the
option was used and 0 if the “no-” form was.
This target hook is called whenever the user specifies one of the
target-specific C language family options described by the .opt
definition files(see Options). It has the opportunity to do some
option-specific processing and should return true if the option is
valid. The arguments are like for TARGET_HANDLE_OPTION. The
default definition does nothing but return false.
In general, you should use TARGET_HANDLE_OPTION to handle
options. However, if processing an option requires routines that are
only available in the C (and related language) front ends, then you
should use TARGET_HANDLE_C_OPTION instead.
Targets may provide a string object type that can be used within and between C, C++ and their respective Objective-C dialects. A string object might, for example, embed encoding and length information. These objects are considered opaque to the compiler and handled as references. An ideal implementation makes the composition of the string object match that of the Objective-C NSString (NXString for GNUStep), allowing efficient interworking between C-only and Objective-C code. If a target implements string objects then this hook should return a reference to such an object constructed from the normal ‘C’ string representation provided in string. At present, the hook is used by Objective-C only, to obtain a common-format string object when the target provides one.
If a target implements string objects then this hook should return true if stringref is a valid reference to such an object.
If a target implements string objects then this hook should should provide a facility to check the function arguments in args_list against the format specifiers in format_arg where the type of format_arg is one recognized as a valid string reference type.
This macro is a C statement to print on stderr a string
describing the particular machine description choice. Every machine
description should define TARGET_VERSION. For example:
#ifdef MOTOROLA #define TARGET_VERSION \ fprintf (stderr, " (68k, Motorola syntax)"); #else #define TARGET_VERSION \ fprintf (stderr, " (68k, MIT syntax)"); #endif
This target function is similar to the hook TARGET_OPTION_OVERRIDE
but is called when the optimize level is changed via an attribute or
pragma or when it is reset at the end of the code affected by the
attribute or pragma. It is not called at the beginning of compilation
when TARGET_OPTION_OVERRIDE is called so if you want to perform these
actions then, you should have TARGET_OPTION_OVERRIDE call
TARGET_OVERRIDE_OPTIONS_AFTER_CHANGE.
This is similar to the TARGET_OPTION_OVERRIDE hook
but is only used in the C
language frontends (C, Objective-C, C++, Objective-C++) and so can be
used to alter option flag variables which only exist in those
frontends.
Some machines may desire to change what optimizations are performed for various optimization levels. This variable, if defined, describes options to enable at particular sets of optimization levels. These options are processed once just after the optimization level is determined and before the remainder of the command options have been parsed, so may be overridden by other options passed explicitly.
This processing is run once at program startup and when the optimization
options are changed via #pragma GCC optimize or by using the
optimize attribute.
Set target-dependent initial values of fields in opts.
Set target-dependent default values for --param settings, using calls to set_default_param_value.
This hook is called in response to the user invoking --target-help on the command line. It gives the target a chance to display extra information on the target specific command line options found in its .opt file.
Some targets need to switch between substantially different subtargets
during compilation. For example, the MIPS target has one subtarget for
the traditional MIPS architecture and another for MIPS16. Source code
can switch between these two subarchitectures using the mips16
and nomips16 attributes.
Such subtargets can differ in things like the set of available registers, the set of available instructions, the costs of various operations, and so on. GCC caches a lot of this type of information in global variables, and recomputing them for each subtarget takes a significant amount of time. The compiler therefore provides a facility for maintaining several versions of the global variables and quickly switching between them; see target-globals.h for details.
Define this macro to 1 if your target needs this facility. The default is 0.
Next: Storage Layout, Previous: Run-time Target, Up: Target Macros [Contents][Index]
If the target needs to store information on a per-function basis, GCC provides a macro and a couple of variables to allow this. Note, just using statics to store the information is a bad idea, since GCC supports nested functions, so you can be halfway through encoding one function when another one comes along.
GCC defines a data structure called struct function which
contains all of the data specific to an individual function. This
structure contains a field called machine whose type is
struct machine_function *, which can be used by targets to point
to their own specific data.
If a target needs per-function specific data it should define the type
struct machine_function and also the macro INIT_EXPANDERS.
This macro should be used to initialize the function pointer
init_machine_status. This pointer is explained below.
One typical use of per-function, target specific data is to create an
RTX to hold the register containing the function’s return address. This
RTX can then be used to implement the __builtin_return_address
function, for level 0.
Note—earlier implementations of GCC used a single data area to hold
all of the per-function information. Thus when processing of a nested
function began the old per-function data had to be pushed onto a
stack, and when the processing was finished, it had to be popped off the
stack. GCC used to provide function pointers called
save_machine_status and restore_machine_status to handle
the saving and restoring of the target specific information. Since the
single data area approach is no longer used, these pointers are no
longer supported.
Macro called to initialize any target specific information. This macro
is called once per function, before generation of any RTL has begun.
The intention of this macro is to allow the initialization of the
function pointer init_machine_status.
If this function pointer is non-NULL it will be called once per
function, before function compilation starts, in order to allow the
target to perform any target specific initialization of the
struct function structure. It is intended that this would be
used to initialize the machine of that structure.
struct machine_function structures are expected to be freed by GC.
Generally, any memory that they reference must be allocated by using
GC allocation, including the structure itself.
Next: Type Layout, Previous: Per-Function Data, Up: Target Macros [Contents][Index]
Note that the definitions of the macros in this table which are sizes or
alignments measured in bits do not need to be constant. They can be C
expressions that refer to static variables, such as the target_flags.
See Run-time Target.
Define this macro to have the value 1 if the most significant bit in a byte has the lowest number; otherwise define it to have the value zero. This means that bit-field instructions count from the most significant bit. If the machine has no bit-field instructions, then this must still be defined, but it doesn’t matter which value it is defined to. This macro need not be a constant.
This macro does not affect the way structure fields are packed into
bytes or words; that is controlled by BYTES_BIG_ENDIAN.
Define this macro to have the value 1 if the most significant byte in a word has the lowest number. This macro need not be a constant.
Define this macro to have the value 1 if, in a multiword object, the most significant word has the lowest number. This applies to both memory locations and registers; GCC fundamentally assumes that the order of words in memory is the same as the order in registers. This macro need not be a constant.
Define this macro to have the value 1 if DFmode, XFmode or
TFmode floating point numbers are stored in memory with the word
containing the sign bit at the lowest address; otherwise define it to
have the value 0. This macro need not be a constant.
You need not define this macro if the ordering is the same as for multi-word integers.
Define this macro to be the number of bits in an addressable storage unit (byte). If you do not define this macro the default is 8.
Number of bits in a word. If you do not define this macro, the default
is BITS_PER_UNIT * UNITS_PER_WORD.
Maximum number of bits in a word. If this is undefined, the default is
BITS_PER_WORD. Otherwise, it is the constant value that is the
largest value that BITS_PER_WORD can have at run-time.
Number of storage units in a word; normally the size of a general-purpose register, a power of two from 1 or 8.
Minimum number of units in a word. If this is undefined, the default is
UNITS_PER_WORD. Otherwise, it is the constant value that is the
smallest value that UNITS_PER_WORD can have at run-time.
Width of a pointer, in bits. You must specify a value no wider than the
width of Pmode. If it is not equal to the width of Pmode,
you must define POINTERS_EXTEND_UNSIGNED. If you do not specify
a value the default is BITS_PER_WORD.
A C expression that determines how pointers should be extended from
ptr_mode to either Pmode or word_mode. It is
greater than zero if pointers should be zero-extended, zero if they
should be sign-extended, and negative if some other sort of conversion
is needed. In the last case, the extension is done by the target’s
ptr_extend instruction.
You need not define this macro if the ptr_mode, Pmode
and word_mode are all the same width.
A macro to update m and unsignedp when an object whose type is type and which has the specified mode and signedness is to be stored in a register. This macro is only called when type is a scalar type.
On most RISC machines, which only have operations that operate on a full
register, define this macro to set m to word_mode if
m is an integer mode narrower than BITS_PER_WORD. In most
cases, only integer modes should be widened because wider-precision
floating-point operations are usually more expensive than their narrower
counterparts.
For most machines, the macro definition does not change unsignedp. However, some machines, have instructions that preferentially handle either signed or unsigned quantities of certain modes. For example, on the DEC Alpha, 32-bit loads from memory and 32-bit add instructions sign-extend the result to 64 bits. On such machines, set unsignedp according to which kind of extension is more efficient.
Do not define this macro if it would never modify m.
Like PROMOTE_MODE, but it is applied to outgoing function arguments or
function return values. The target hook should return the new mode
and possibly change *punsignedp if the promotion should
change signedness. This function is called only for scalar or
pointer types.
for_return allows to distinguish the promotion of arguments and
return values. If it is 1, a return value is being promoted and
TARGET_FUNCTION_VALUE must perform the same promotions done here.
If it is 2, the returned mode should be that of the register in
which an incoming parameter is copied, or the outgoing result is computed;
then the hook should return the same mode as promote_mode, though
the signedness may be different.
The default is to not promote arguments and return values. You can
also define the hook to default_promote_function_mode_always_promote
if you would like to apply the same rules given by PROMOTE_MODE.
Normal alignment required for function parameters on the stack, in bits. All stack parameters receive at least this much alignment regardless of data type. On most machines, this is the same as the size of an integer.
Define this macro to the minimum alignment enforced by hardware for the
stack pointer on this machine. The definition is a C expression for the
desired alignment (measured in bits). This value is used as a default
if PREFERRED_STACK_BOUNDARY is not defined. On most machines,
this should be the same as PARM_BOUNDARY.
Define this macro if you wish to preserve a certain alignment for the
stack pointer, greater than what the hardware enforces. The definition
is a C expression for the desired alignment (measured in bits). This
macro must evaluate to a value equal to or larger than
STACK_BOUNDARY.
Define this macro if the incoming stack boundary may be different
from PREFERRED_STACK_BOUNDARY. This macro must evaluate
to a value equal to or larger than STACK_BOUNDARY.
Alignment required for a function entry point, in bits.
Biggest alignment that any data type can require on this machine, in bits. Note that this is not the biggest alignment that is supported, just the biggest alignment that, when violated, may cause a fault.
Alignment, in bits, a C conformant malloc implementation has to
provide. If not defined, the default value is BITS_PER_WORD.
Alignment used by the __attribute__ ((aligned)) construct. If
not defined, the default value is BIGGEST_ALIGNMENT.
If defined, the smallest alignment, in bits, that can be given to an
object that can be referenced in one operation, without disturbing any
nearby object. Normally, this is BITS_PER_UNIT, but may be larger
on machines that don’t have byte or half-word store operations.
Biggest alignment that any structure or union field can require on this
machine, in bits. If defined, this overrides BIGGEST_ALIGNMENT for
structure and union fields only, unless the field alignment has been set
by the __attribute__ ((aligned (n))) construct.
An expression for the alignment of a structure field field if the
alignment computed in the usual way (including applying of
BIGGEST_ALIGNMENT and BIGGEST_FIELD_ALIGNMENT to the
alignment) is computed. It overrides alignment only if the
field alignment has not been set by the
__attribute__ ((aligned (n))) construct.
Biggest stack alignment guaranteed by the backend. Use this macro to specify the maximum alignment of a variable on stack.
If not defined, the default value is STACK_BOUNDARY.
Biggest alignment supported by the object file format of this machine.
Use this macro to limit the alignment which can be specified using the
__attribute__ ((aligned (n))) construct. If not defined,
the default value is BIGGEST_ALIGNMENT.
On systems that use ELF, the default (in config/elfos.h) is the largest supported 32-bit ELF section alignment representable on a 32-bit host e.g. ‘(((unsigned HOST_WIDEST_INT) 1 << 28) * 8)’. On 32-bit ELF the largest supported section alignment in bits is ‘(0x80000000 * 8)’, but this is not representable on 32-bit hosts.
If defined, a C expression to compute the alignment for a variable in the static store. type is the data type, and basic-align is the alignment that the object would ordinarily have. The value of this macro is used instead of that alignment to align the object.
If this macro is not defined, then basic-align is used.
One use of this macro is to increase alignment of medium-size data to
make it all fit in fewer cache lines. Another is to cause character
arrays to be word-aligned so that strcpy calls that copy
constants to character arrays can be done inline.
If defined, a C expression to compute the alignment given to a constant that is being placed in memory. constant is the constant and basic-align is the alignment that the object would ordinarily have. The value of this macro is used instead of that alignment to align the object.
If this macro is not defined, then basic-align is used.
The typical use of this macro is to increase alignment for string
constants to be word aligned so that strcpy calls that copy
constants can be done inline.
If defined, a C expression to compute the alignment for a variable in the local store. type is the data type, and basic-align is the alignment that the object would ordinarily have. The value of this macro is used instead of that alignment to align the object.
If this macro is not defined, then basic-align is used.
One use of this macro is to increase alignment of medium-size data to make it all fit in fewer cache lines.
If the value of this macro has a type, it should be an unsigned type.
If defined, a C expression to compute the alignment for stack slot. type is the data type, mode is the widest mode available, and basic-align is the alignment that the slot would ordinarily have. The value of this macro is used instead of that alignment to align the slot.
If this macro is not defined, then basic-align is used when
type is NULL. Otherwise, LOCAL_ALIGNMENT will
be used.
This macro is to set alignment of stack slot to the maximum alignment of all possible modes which the slot may have.
If the value of this macro has a type, it should be an unsigned type.
If defined, a C expression to compute the alignment for a local variable decl.
If this macro is not defined, then
LOCAL_ALIGNMENT (TREE_TYPE (decl), DECL_ALIGN (decl))
is used.
One use of this macro is to increase alignment of medium-size data to make it all fit in fewer cache lines.
If the value of this macro has a type, it should be an unsigned type.
If defined, a C expression to compute the minimum required alignment for dynamic stack realignment purposes for exp (a type or decl), mode, assuming normal alignment align.
If this macro is not defined, then align will be used.
Alignment in bits to be given to a structure bit-field that follows an
empty field such as int : 0;.
If PCC_BITFIELD_TYPE_MATTERS is true, it overrides this macro.
Number of bits which any structure or union’s size must be a multiple of. Each structure or union’s size is rounded up to a multiple of this.
If you do not define this macro, the default is the same as
BITS_PER_UNIT.
Define this macro to be the value 1 if instructions will fail to work if given data not on the nominal alignment. If instructions will merely go slower in that case, define this macro as 0.
Define this if you wish to imitate the way many other C compilers handle alignment of bit-fields and the structures that contain them.
The behavior is that the type written for a named bit-field (int,
short, or other integer type) imposes an alignment for the entire
structure, as if the structure really did contain an ordinary field of
that type. In addition, the bit-field is placed within the structure so
that it would fit within such a field, not crossing a boundary for it.
Thus, on most machines, a named bit-field whose type is written as
int would not cross a four-byte boundary, and would force
four-byte alignment for the whole structure. (The alignment used may
not be four bytes; it is controlled by the other alignment parameters.)
An unnamed bit-field will not affect the alignment of the containing structure.
If the macro is defined, its definition should be a C expression; a nonzero value for the expression enables this behavior.
Note that if this macro is not defined, or its value is zero, some bit-fields may cross more than one alignment boundary. The compiler can support such references if there are ‘insv’, ‘extv’, and ‘extzv’ insns that can directly reference memory.
The other known way of making bit-fields work is to define
STRUCTURE_SIZE_BOUNDARY as large as BIGGEST_ALIGNMENT.
Then every structure can be accessed with fullwords.
Unless the machine has bit-field instructions or you define
STRUCTURE_SIZE_BOUNDARY that way, you must define
PCC_BITFIELD_TYPE_MATTERS to have a nonzero value.
If your aim is to make GCC use the same conventions for laying out bit-fields as are used by another compiler, here is how to investigate what the other compiler does. Compile and run this program:
struct foo1
{
char x;
char :0;
char y;
};
struct foo2
{
char x;
int :0;
char y;
};
main ()
{
printf ("Size of foo1 is %d\n",
sizeof (struct foo1));
printf ("Size of foo2 is %d\n",
sizeof (struct foo2));
exit (0);
}
If this prints 2 and 5, then the compiler’s behavior is what you would
get from PCC_BITFIELD_TYPE_MATTERS.
Like PCC_BITFIELD_TYPE_MATTERS except that its effect is limited
to aligning a bit-field within the structure.
When PCC_BITFIELD_TYPE_MATTERS is true this hook will determine
whether unnamed bitfields affect the alignment of the containing
structure. The hook should return true if the structure should inherit
the alignment requirements of an unnamed bitfield’s type.
This target hook should return true if accesses to volatile bitfields
should use the narrowest mode possible. It should return false if
these accesses should use the bitfield container type.
The default is !TARGET_STRICT_ALIGN.
Return 1 if a structure or array containing field should be accessed using
BLKMODE.
If field is the only field in the structure, mode is its mode, otherwise mode is VOIDmode. mode is provided in the case where structures of one field would require the structure’s mode to retain the field’s mode.
Normally, this is not needed.
Define this macro as an expression for the alignment of a type (given by type as a tree node) if the alignment computed in the usual way is computed and the alignment explicitly specified was specified.
The default is to use specified if it is larger; otherwise, use
the smaller of computed and BIGGEST_ALIGNMENT
An integer expression for the size in bits of the largest integer
machine mode that should actually be used. All integer machine modes of
this size or smaller can be used for structures and unions with the
appropriate sizes. If this macro is undefined, GET_MODE_BITSIZE
(DImode) is assumed.
If defined, an expression of type enum machine_mode that
specifies the mode of the save area operand of a
save_stack_level named pattern (see Standard Names).
save_level is one of SAVE_BLOCK, SAVE_FUNCTION, or
SAVE_NONLOCAL and selects which of the three named patterns is
having its mode specified.
You need not define this macro if it always returns Pmode. You
would most commonly define this macro if the
save_stack_level patterns need to support both a 32- and a
64-bit mode.
If defined, an expression of type enum machine_mode that
specifies the mode of the size increment operand of an
allocate_stack named pattern (see Standard Names).
You need not define this macro if it always returns word_mode.
You would most commonly define this macro if the allocate_stack
pattern needs to support both a 32- and a 64-bit mode.
This target hook should return the mode to be used for the return value
of compare instructions expanded to libgcc calls. If not defined
word_mode is returned which is the right choice for a majority of
targets.
This target hook should return the mode to be used for the shift count operand
of shift instructions expanded to libgcc calls. If not defined
word_mode is returned which is the right choice for a majority of
targets.
Return machine mode to be used for _Unwind_Word type.
The default is to use word_mode.
If defined, this macro should be true if the prevailing rounding mode is towards zero.
Defining this macro only affects the way libgcc.a emulates floating-point arithmetic.
Not defining this macro is equivalent to returning zero.
This macro should return true if floats with size bits do not have a NaN or infinity representation, but use the largest exponent for normal numbers instead.
Defining this macro only affects the way libgcc.a emulates floating-point arithmetic.
The default definition of this macro returns false for all sizes.
This target hook returns true if bit-fields in the given
record_type are to be laid out following the rules of Microsoft
Visual C/C++, namely: (i) a bit-field won’t share the same storage
unit with the previous bit-field if their underlying types have
different sizes, and the bit-field will be aligned to the highest
alignment of the underlying types of itself and of the previous
bit-field; (ii) a zero-sized bit-field will affect the alignment of
the whole enclosing structure, even if it is unnamed; except that
(iii) a zero-sized bit-field will be disregarded unless it follows
another bit-field of nonzero size. If this hook returns true,
other macros that control bit-field layout are ignored.
When a bit-field is inserted into a packed record, the whole size of the underlying type is used by one or more same-size adjacent bit-fields (that is, if its long:3, 32 bits is used in the record, and any additional adjacent long bit-fields are packed into the same chunk of 32 bits. However, if the size changes, a new field of that size is allocated). In an unpacked record, this is the same as using alignment, but not equivalent when packing.
If both MS bit-fields and ‘__attribute__((packed))’ are used, the latter will take precedence. If ‘__attribute__((packed))’ is used on a single field when MS bit-fields are in use, it will take precedence for that field, but the alignment of the rest of the structure may affect its placement.
Returns true if the target supports decimal floating point.
Returns true if the target supports fixed-point arithmetic.
This hook is called just before expansion into rtl, allowing the target to perform additional initializations or analysis before the expansion. For example, the rs6000 port uses it to allocate a scratch stack slot for use in copying SDmode values between memory and floating point registers whenever the function being expanded has any SDmode usage.
This hook allows the backend to perform additional instantiations on rtl that are not actually in any insns yet, but will be later.
If your target defines any fundamental types, or any types your target
uses should be mangled differently from the default, define this hook
to return the appropriate encoding for these types as part of a C++
mangled name. The type argument is the tree structure representing
the type to be mangled. The hook may be applied to trees which are
not target-specific fundamental types; it should return NULL
for all such types, as well as arguments it does not recognize. If the
return value is not NULL, it must point to a statically-allocated
string constant.
Target-specific fundamental types might be new fundamental types or
qualified versions of ordinary fundamental types. Encode new
fundamental types as ‘u n name’, where name
is the name used for the type in source code, and n is the
length of name in decimal. Encode qualified versions of
ordinary types as ‘U n name code’, where
name is the name used for the type qualifier in source code,
n is the length of name as above, and code is the
code used to represent the unqualified version of this type. (See
write_builtin_type in cp/mangle.c for the list of
codes.) In both cases the spaces are for clarity; do not include any
spaces in your string.
This hook is applied to types prior to typedef resolution. If the mangled
name for a particular type depends only on that type’s main variant, you
can perform typedef resolution yourself using TYPE_MAIN_VARIANT
before mangling.
The default version of this hook always returns NULL, which is
appropriate for a target that does not define any new fundamental
types.
Next: Registers, Previous: Storage Layout, Up: Target Macros [Contents][Index]
These macros define the sizes and other characteristics of the standard basic data types used in programs being compiled. Unlike the macros in the previous section, these apply to specific features of C and related languages, rather than to fundamental aspects of storage layout.
A C expression for the size in bits of the type int on the
target machine. If you don’t define this, the default is one word.
A C expression for the size in bits of the type short on the
target machine. If you don’t define this, the default is half a word.
(If this would be less than one storage unit, it is rounded up to one
unit.)
A C expression for the size in bits of the type long on the
target machine. If you don’t define this, the default is one word.
On some machines, the size used for the Ada equivalent of the type
long by a native Ada compiler differs from that used by C. In
that situation, define this macro to be a C expression to be used for
the size of that type. If you don’t define this, the default is the
value of LONG_TYPE_SIZE.
A C expression for the size in bits of the type long long on the
target machine. If you don’t define this, the default is two
words. If you want to support GNU Ada on your machine, the value of this
macro must be at least 64.
A C expression for the size in bits of the type char on the
target machine. If you don’t define this, the default is
BITS_PER_UNIT.
A C expression for the size in bits of the C++ type bool and
C99 type _Bool on the target machine. If you don’t define
this, and you probably shouldn’t, the default is CHAR_TYPE_SIZE.
A C expression for the size in bits of the type float on the
target machine. If you don’t define this, the default is one word.
A C expression for the size in bits of the type double on the
target machine. If you don’t define this, the default is two
words.
A C expression for the size in bits of the type long double on
the target machine. If you don’t define this, the default is two
words.
A C expression for the size in bits of the type short _Fract on
the target machine. If you don’t define this, the default is
BITS_PER_UNIT.
A C expression for the size in bits of the type _Fract on
the target machine. If you don’t define this, the default is
BITS_PER_UNIT * 2.
A C expression for the size in bits of the type long _Fract on
the target machine. If you don’t define this, the default is
BITS_PER_UNIT * 4.
A C expression for the size in bits of the type long long _Fract on
the target machine. If you don’t define this, the default is
BITS_PER_UNIT * 8.
A C expression for the size in bits of the type short _Accum on
the target machine. If you don’t define this, the default is
BITS_PER_UNIT * 2.
A C expression for the size in bits of the type _Accum on
the target machine. If you don’t define this, the default is
BITS_PER_UNIT * 4.
A C expression for the size in bits of the type long _Accum on
the target machine. If you don’t define this, the default is
BITS_PER_UNIT * 8.
A C expression for the size in bits of the type long long _Accum on
the target machine. If you don’t define this, the default is
BITS_PER_UNIT * 16.
Define this macro if LONG_DOUBLE_TYPE_SIZE is not constant or
if you want routines in libgcc2.a for a size other than
LONG_DOUBLE_TYPE_SIZE. If you don’t define this, the
default is LONG_DOUBLE_TYPE_SIZE.
Define this macro if neither DOUBLE_TYPE_SIZE nor
LIBGCC2_LONG_DOUBLE_TYPE_SIZE is
DFmode but you want DFmode routines in libgcc2.a
anyway. If you don’t define this and either DOUBLE_TYPE_SIZE
or LIBGCC2_LONG_DOUBLE_TYPE_SIZE is 64 then the default is 1,
otherwise it is 0.
Define this macro if LIBGCC2_LONG_DOUBLE_TYPE_SIZE is not
XFmode but you want XFmode routines in libgcc2.a
anyway. If you don’t define this and LIBGCC2_LONG_DOUBLE_TYPE_SIZE
is 80 then the default is 1, otherwise it is 0.
Define this macro if LIBGCC2_LONG_DOUBLE_TYPE_SIZE is not
TFmode but you want TFmode routines in libgcc2.a
anyway. If you don’t define this and LIBGCC2_LONG_DOUBLE_TYPE_SIZE
is 128 then the default is 1, otherwise it is 0.
Define these macros to be the size in bits of the mantissa of
SFmode, DFmode, XFmode and TFmode values,
if the defaults in libgcc2.h are inappropriate. By default,
FLT_MANT_DIG is used for SF_SIZE, LDBL_MANT_DIG
for XF_SIZE and TF_SIZE, and DBL_MANT_DIG or
LDBL_MANT_DIG for DF_SIZE according to whether
DOUBLE_TYPE_SIZE or
LIBGCC2_LONG_DOUBLE_TYPE_SIZE is 64.
A C expression for the value for FLT_EVAL_METHOD in float.h,
assuming, if applicable, that the floating-point control word is in its
default state. If you do not define this macro the value of
FLT_EVAL_METHOD will be zero.
A C expression for the size in bits of the widest floating-point format
supported by the hardware. If you define this macro, you must specify a
value less than or equal to the value of LONG_DOUBLE_TYPE_SIZE.
If you do not define this macro, the value of LONG_DOUBLE_TYPE_SIZE
is the default.
An expression whose value is 1 or 0, according to whether the type
char should be signed or unsigned by default. The user can
always override this default with the options -fsigned-char
and -funsigned-char.
This target hook should return true if the compiler should give an
enum type only as many bytes as it takes to represent the range
of possible values of that type. It should return false if all
enum types should be allocated like int.
The default is to return false.
A C expression for a string describing the name of the data type to use
for size values. The typedef name size_t is defined using the
contents of the string.
The string can contain more than one keyword. If so, separate them with
spaces, and write first any length keyword, then unsigned if
appropriate, and finally int. The string must exactly match one
of the data type names defined in the function
init_decl_processing in the file c-decl.c. You may not
omit int or change the order—that would cause the compiler to
crash on startup.
If you don’t define this macro, the default is "long unsigned
int".
A C expression for a string describing the name of the data type to use
for the result of subtracting two pointers. The typedef name
ptrdiff_t is defined using the contents of the string. See
SIZE_TYPE above for more information.
If you don’t define this macro, the default is "long int".
A C expression for a string describing the name of the data type to use
for wide characters. The typedef name wchar_t is defined using
the contents of the string. See SIZE_TYPE above for more
information.
If you don’t define this macro, the default is "int".
A C expression for the size in bits of the data type for wide
characters. This is used in cpp, which cannot make use of
WCHAR_TYPE.
A C expression for a string describing the name of the data type to
use for wide characters passed to printf and returned from
getwc. The typedef name wint_t is defined using the
contents of the string. See SIZE_TYPE above for more
information.
If you don’t define this macro, the default is "unsigned int".
A C expression for a string describing the name of the data type that
can represent any value of any standard or extended signed integer type.
The typedef name intmax_t is defined using the contents of the
string. See SIZE_TYPE above for more information.
If you don’t define this macro, the default is the first of
"int", "long int", or "long long int" that has as
much precision as long long int.
A C expression for a string describing the name of the data type that
can represent any value of any standard or extended unsigned integer
type. The typedef name uintmax_t is defined using the contents
of the string. See SIZE_TYPE above for more information.
If you don’t define this macro, the default is the first of
"unsigned int", "long unsigned int", or "long long
unsigned int" that has as much precision as long long unsigned
int.
C expressions for the standard types sig_atomic_t,
int8_t, int16_t, int32_t, int64_t,
uint8_t, uint16_t, uint32_t, uint64_t,
int_least8_t, int_least16_t, int_least32_t,
int_least64_t, uint_least8_t, uint_least16_t,
uint_least32_t, uint_least64_t, int_fast8_t,
int_fast16_t, int_fast32_t, int_fast64_t,
uint_fast8_t, uint_fast16_t, uint_fast32_t,
uint_fast64_t, intptr_t, and uintptr_t. See
SIZE_TYPE above for more information.
If any of these macros evaluates to a null pointer, the corresponding
type is not supported; if GCC is configured to provide
<stdint.h> in such a case, the header provided may not conform
to C99, depending on the type in question. The defaults for all of
these macros are null pointers.
The C++ compiler represents a pointer-to-member-function with a struct that looks like:
struct {
union {
void (*fn)();
ptrdiff_t vtable_index;
};
ptrdiff_t delta;
};
The C++ compiler must use one bit to indicate whether the function that
will be called through a pointer-to-member-function is virtual.
Normally, we assume that the low-order bit of a function pointer must
always be zero. Then, by ensuring that the vtable_index is odd, we can
distinguish which variant of the union is in use. But, on some
platforms function pointers can be odd, and so this doesn’t work. In
that case, we use the low-order bit of the delta field, and shift
the remainder of the delta field to the left.
GCC will automatically make the right selection about where to store
this bit using the FUNCTION_BOUNDARY setting for your platform.
However, some platforms such as ARM/Thumb have FUNCTION_BOUNDARY
set such that functions always start at even addresses, but the lowest
bit of pointers to functions indicate whether the function at that
address is in ARM or Thumb mode. If this is the case of your
architecture, you should define this macro to
ptrmemfunc_vbit_in_delta.
In general, you should not have to define this macro. On architectures
in which function addresses are always even, according to
FUNCTION_BOUNDARY, GCC will automatically define this macro to
ptrmemfunc_vbit_in_pfn.
Normally, the C++ compiler uses function pointers in vtables. This macro allows the target to change to use “function descriptors” instead. Function descriptors are found on targets for whom a function pointer is actually a small data structure. Normally the data structure consists of the actual code address plus a data pointer to which the function’s data is relative.
If vtables are used, the value of this macro should be the number of words that the function descriptor occupies.
By default, the vtable entries are void pointers, the so the alignment is the same as pointer alignment. The value of this macro specifies the alignment of the vtable entry in bits. It should be defined only when special alignment is necessary. */
There are a few non-descriptor entries in the vtable at offsets below
zero. If these entries must be padded (say, to preserve the alignment
specified by TARGET_VTABLE_ENTRY_ALIGN), set this to the number
of words in each data entry.
Next: Register Classes, Previous: Type Layout, Up: Target Macros [Contents][Index]
This section explains how to describe what registers the target machine has, and how (in general) they can be used.
The description of which registers a specific instruction can use is done with register classes; see Register Classes. For information on using registers to access a stack frame, see Frame Registers. For passing values in registers, see Register Arguments. For returning values in registers, see Scalar Return.
| • Register Basics: | Number and kinds of registers. | |
| • Allocation Order: | Order in which registers are allocated. | |
| • Values in Registers: | What kinds of values each reg can hold. | |
| • Leaf Functions: | Renumbering registers for leaf functions. | |
| • Stack Registers: | Handling a register stack such as 80387. |
Next: Allocation Order, Up: Registers [Contents][Index]
Registers have various characteristics.
Number of hardware registers known to the compiler. They receive
numbers 0 through FIRST_PSEUDO_REGISTER-1; thus, the first
pseudo register’s number really is assigned the number
FIRST_PSEUDO_REGISTER.
An initializer that says which registers are used for fixed purposes all throughout the compiled code and are therefore not available for general allocation. These would include the stack pointer, the frame pointer (except on machines where that can be used as a general register when no frame pointer is needed), the program counter on machines where that is considered one of the addressable registers, and any other numbered register with a standard use.
This information is expressed as a sequence of numbers, separated by commas and surrounded by braces. The nth number is 1 if register n is fixed, 0 otherwise.
The table initialized from this macro, and the table initialized by
the following one, may be overridden at run time either automatically,
by the actions of the macro CONDITIONAL_REGISTER_USAGE, or by
the user with the command options -ffixed-reg,
-fcall-used-reg and -fcall-saved-reg.
Like FIXED_REGISTERS but has 1 for each register that is
clobbered (in general) by function calls as well as for fixed
registers. This macro therefore identifies the registers that are not
available for general allocation of values that must live across
function calls.
If a register has 0 in CALL_USED_REGISTERS, the compiler
automatically saves it on function entry and restores it on function
exit, if the register is used within the function.
Like CALL_USED_REGISTERS except this macro doesn’t require
that the entire set of FIXED_REGISTERS be included.
(CALL_USED_REGISTERS must be a superset of FIXED_REGISTERS).
This macro is optional. If not specified, it defaults to the value
of CALL_USED_REGISTERS.
A C expression that is nonzero if it is not permissible to store a value of mode mode in hard register number regno across a call without some part of it being clobbered. For most machines this macro need not be defined. It is only required for machines that do not preserve the entire contents of a register across a call.
This hook may conditionally modify five variables
fixed_regs, call_used_regs, global_regs,
reg_names, and reg_class_contents, to take into account
any dependence of these register sets on target flags. The first three
of these are of type char [] (interpreted as Boolean vectors).
global_regs is a const char *[], and
reg_class_contents is a HARD_REG_SET. Before the macro is
called, fixed_regs, call_used_regs,
reg_class_contents, and reg_names have been initialized
from FIXED_REGISTERS, CALL_USED_REGISTERS,
REG_CLASS_CONTENTS, and REGISTER_NAMES, respectively.
global_regs has been cleared, and any -ffixed-reg,
-fcall-used-reg and -fcall-saved-reg
command options have been applied.
If the usage of an entire class of registers depends on the target
flags, you may indicate this to GCC by using this macro to modify
fixed_regs and call_used_regs to 1 for each of the
registers in the classes which should not be used by GCC. Also define
the macro REG_CLASS_FROM_LETTER / REG_CLASS_FROM_CONSTRAINT
to return NO_REGS if it
is called with a letter for a class that shouldn’t be used.
(However, if this class is not included in GENERAL_REGS and all
of the insn patterns whose constraints permit this class are
controlled by target switches, then GCC will automatically avoid using
these registers when the target switches are opposed to them.)
Define this macro if the target machine has register windows. This C expression returns the register number as seen by the called function corresponding to the register number out as seen by the calling function. Return out if register number out is not an outbound register.
Define this macro if the target machine has register windows. This C expression returns the register number as seen by the calling function corresponding to the register number in as seen by the called function. Return in if register number in is not an inbound register.
Define this macro if the target machine has register windows. This C expression returns true if the register is call-saved but is in the register window. Unlike most call-saved registers, such registers need not be explicitly restored on function exit or during non-local gotos.
If the program counter has a register number, define this as that register number. Otherwise, do not define it.
Next: Values in Registers, Previous: Register Basics, Up: Registers [Contents][Index]
Registers are allocated in order.
If defined, an initializer for a vector of integers, containing the numbers of hard registers in the order in which GCC should prefer to use them (from most preferred to least).
If this macro is not defined, registers are used lowest numbered first (all else being equal).
One use of this macro is on machines where the highest numbered
registers must always be saved and the save-multiple-registers
instruction supports only sequences of consecutive registers. On such
machines, define REG_ALLOC_ORDER to be an initializer that lists
the highest numbered allocable register first.
A C statement (sans semicolon) to choose the order in which to allocate hard registers for pseudo-registers local to a basic block.
Store the desired register order in the array reg_alloc_order.
Element 0 should be the register to allocate first; element 1, the next
register; and so on.
The macro body should not assume anything about the contents of
reg_alloc_order before execution of the macro.
On most machines, it is not necessary to define this macro.
Normally, IRA tries to estimate the costs for saving a register in the prologue and restoring it in the epilogue. This discourages it from using call-saved registers. If a machine wants to ensure that IRA allocates registers in the order given by REG_ALLOC_ORDER even if some call-saved registers appear earlier than call-used ones, this macro should be defined.
In some case register allocation order is not enough for the
Integrated Register Allocator (IRA) to generate a good code.
If this macro is defined, it should return a floating point value
based on regno. The cost of using regno for a pseudo will
be increased by approximately the pseudo’s usage frequency times the
value returned by this macro. Not defining this macro is equivalent
to having it always return 0.0.
On most machines, it is not necessary to define this macro.
Next: Leaf Functions, Previous: Allocation Order, Up: Registers [Contents][Index]
This section discusses the macros that describe which kinds of values (specifically, which machine modes) each register can hold, and how many consecutive registers are needed for a given mode.
A C expression for the number of consecutive hard registers, starting at register number regno, required to hold a value of mode mode. This macro must never return zero, even if a register cannot hold the requested mode - indicate that with HARD_REGNO_MODE_OK and/or CANNOT_CHANGE_MODE_CLASS instead.
On a machine where all registers are exactly one word, a suitable definition of this macro is
#define HARD_REGNO_NREGS(REGNO, MODE) \
((GET_MODE_SIZE (MODE) + UNITS_PER_WORD - 1) \
/ UNITS_PER_WORD)
A C expression that is nonzero if a value of mode mode, stored
in memory, ends with padding that causes it to take up more space than
in registers starting at register number regno (as determined by
multiplying GCC’s notion of the size of the register when containing
this mode by the number of registers returned by
HARD_REGNO_NREGS). By default this is zero.
For example, if a floating-point value is stored in three 32-bit registers but takes up 128 bits in memory, then this would be nonzero.
This macros only needs to be defined if there are cases where
subreg_get_info
would otherwise wrongly determine that a subreg can be
represented by an offset to the register number, when in fact such a
subreg would contain some of the padding not stored in
registers and so not be representable.
For values of regno and mode for which
HARD_REGNO_NREGS_HAS_PADDING returns nonzero, a C expression
returning the greater number of registers required to hold the value
including any padding. In the example above, the value would be four.
Define this macro if the natural size of registers that hold values of mode mode is not the word size. It is a C expression that should give the natural size in bytes for the specified mode. It is used by the register allocator to try to optimize its results. This happens for example on SPARC 64-bit where the natural size of floating-point registers is still 32-bit.
A C expression that is nonzero if it is permissible to store a value of mode mode in hard register number regno (or in several registers starting with that one). For a machine where all registers are equivalent, a suitable definition is
#define HARD_REGNO_MODE_OK(REGNO, MODE) 1
You need not include code to check for the numbers of fixed registers, because the allocation mechanism considers them to be always occupied.
On some machines, double-precision values must be kept in even/odd register pairs. You can implement that by defining this macro to reject odd register numbers for such modes.
The minimum requirement for a mode to be OK in a register is that the ‘movmode’ instruction pattern support moves between the register and other hard register in the same class and that moving a value into the register and back out not alter it.
Since the same instruction used to move word_mode will work for
all narrower integer modes, it is not necessary on any machine for
HARD_REGNO_MODE_OK to distinguish between these modes, provided
you define patterns ‘movhi’, etc., to take advantage of this. This
is useful because of the interaction between HARD_REGNO_MODE_OK
and MODES_TIEABLE_P; it is very desirable for all integer modes
to be tieable.
Many machines have special registers for floating point arithmetic. Often people assume that floating point machine modes are allowed only in floating point registers. This is not true. Any registers that can hold integers can safely hold a floating point machine mode, whether or not floating arithmetic can be done on it in those registers. Integer move instructions can be used to move the values.
On some machines, though, the converse is true: fixed-point machine
modes may not go in floating registers. This is true if the floating
registers normalize any value stored in them, because storing a
non-floating value there would garble it. In this case,
HARD_REGNO_MODE_OK should reject fixed-point machine modes in
floating registers. But if the floating registers do not automatically
normalize, if you can store any bit pattern in one and retrieve it
unchanged without a trap, then any machine mode may go in a floating
register, so you can define this macro to say so.
The primary significance of special floating registers is rather that
they are the registers acceptable in floating point arithmetic
instructions. However, this is of no concern to
HARD_REGNO_MODE_OK. You handle it by writing the proper
constraints for those instructions.
On some machines, the floating registers are especially slow to access,
so that it is better to store a value in a stack frame than in such a
register if floating point arithmetic is not being done. As long as the
floating registers are not in class GENERAL_REGS, they will not
be used unless some pattern’s constraint asks for one.
A C expression that is nonzero if it is OK to rename a hard register from to another hard register to.
One common use of this macro is to prevent renaming of a register to another register that is not saved by a prologue in an interrupt handler.
The default is always nonzero.
A C expression that is nonzero if a value of mode mode1 is accessible in mode mode2 without copying.
If HARD_REGNO_MODE_OK (r, mode1) and
HARD_REGNO_MODE_OK (r, mode2) are always the same for
any r, then MODES_TIEABLE_P (mode1, mode2)
should be nonzero. If they differ for any r, you should define
this macro to return zero unless some other mechanism ensures the
accessibility of the value in a narrower mode.
You should define this macro to return nonzero in as many cases as possible since doing so will allow GCC to perform better register allocation.
This target hook should return true if it is OK to use a hard register
regno as scratch reg in peephole2.
One common use of this macro is to prevent using of a register that is not saved by a prologue in an interrupt handler.
The default version of this hook always returns true.
Define this macro if the compiler should avoid copies to/from CCmode
registers. You should only define this macro if support for copying to/from
CCmode is incomplete.
Next: Stack Registers, Previous: Values in Registers, Up: Registers [Contents][Index]
On some machines, a leaf function (i.e., one which makes no calls) can run more efficiently if it does not make its own register window. Often this means it is required to receive its arguments in the registers where they are passed by the caller, instead of the registers where they would normally arrive.
The special treatment for leaf functions generally applies only when other conditions are met; for example, often they may use only those registers for its own variables and temporaries. We use the term “leaf function” to mean a function that is suitable for this special handling, so that functions with no calls are not necessarily “leaf functions”.
GCC assigns register numbers before it knows whether the function is suitable for leaf function treatment. So it needs to renumber the registers in order to output a leaf function. The following macros accomplish this.
Name of a char vector, indexed by hard register number, which contains 1 for a register that is allowable in a candidate for leaf function treatment.
If leaf function treatment involves renumbering the registers, then the registers marked here should be the ones before renumbering—those that GCC would ordinarily allocate. The registers which will actually be used in the assembler code, after renumbering, should not be marked with 1 in this vector.
Define this macro only if the target machine offers a way to optimize the treatment of leaf functions.
A C expression whose value is the register number to which regno should be renumbered, when a function is treated as a leaf function.
If regno is a register number which should not appear in a leaf function before renumbering, then the expression should yield -1, which will cause the compiler to abort.
Define this macro only if the target machine offers a way to optimize the treatment of leaf functions, and registers need to be renumbered to do this.
TARGET_ASM_FUNCTION_PROLOGUE and
TARGET_ASM_FUNCTION_EPILOGUE must usually treat leaf functions
specially. They can test the C variable current_function_is_leaf
which is nonzero for leaf functions. current_function_is_leaf is
set prior to local register allocation and is valid for the remaining
compiler passes. They can also test the C variable
current_function_uses_only_leaf_regs which is nonzero for leaf
functions which only use leaf registers.
current_function_uses_only_leaf_regs is valid after all passes
that modify the instructions have been run and is only useful if
LEAF_REGISTERS is defined.
Previous: Leaf Functions, Up: Registers [Contents][Index]
There are special features to handle computers where some of the “registers” form a stack. Stack registers are normally written by pushing onto the stack, and are numbered relative to the top of the stack.
Currently, GCC can only handle one group of stack-like registers, and they must be consecutively numbered. Furthermore, the existing support for stack-like registers is specific to the 80387 floating point coprocessor. If you have a new architecture that uses stack-like registers, you will need to do substantial work on reg-stack.c and write your machine description to cooperate with it, as well as defining these macros.
Define this if the machine has any stack-like registers.
This is a cover class containing the stack registers. Define this if the machine has any stack-like registers.
The number of the first stack-like register. This one is the top of the stack.
The number of the last stack-like register. This one is the bottom of the stack.
Next: Old Constraints, Previous: Registers, Up: Target Macros [Contents][Index]
On many machines, the numbered registers are not all equivalent. For example, certain registers may not be allowed for indexed addressing; certain registers may not be allowed in some instructions. These machine restrictions are described to the compiler using register classes.
You define a number of register classes, giving each one a name and saying which of the registers belong to it. Then you can specify register classes that are allowed as operands to particular instruction patterns.
In general, each register will belong to several classes. In fact, one
class must be named ALL_REGS and contain all the registers. Another
class must be named NO_REGS and contain no registers. Often the
union of two classes will be another class; however, this is not required.
One of the classes must be named GENERAL_REGS. There is nothing
terribly special about the name, but the operand constraint letters
‘r’ and ‘g’ specify this class. If GENERAL_REGS is
the same as ALL_REGS, just define it as a macro which expands
to ALL_REGS.
Order the classes so that if class x is contained in class y then x has a lower class number than y.
The way classes other than GENERAL_REGS are specified in operand
constraints is through machine-dependent operand constraint letters.
You can define such letters to correspond to various classes, then use
them in operand constraints.
You should define a class for the union of two classes whenever some
instruction allows both classes. For example, if an instruction allows
either a floating point (coprocessor) register or a general register for a
certain operand, you should define a class FLOAT_OR_GENERAL_REGS
which includes both of them. Otherwise you will get suboptimal code,
or even internal compiler errors when reload cannot find a register in the
the class computed via reg_class_subunion.
You must also specify certain redundant information about the register classes: for each class, which classes contain it and which ones are contained in it; for each pair of classes, the largest class contained in their union.
When a value occupying several consecutive registers is expected in a
certain class, all the registers used must belong to that class.
Therefore, register classes cannot be used to enforce a requirement for
a register pair to start with an even-numbered register. The way to
specify this requirement is with HARD_REGNO_MODE_OK.
Register classes used for input-operands of bitwise-and or shift
instructions have a special requirement: each such class must have, for
each fixed-point machine mode, a subclass whose registers can transfer that
mode to or from memory. For example, on some machines, the operations for
single-byte values (QImode) are limited to certain registers. When
this is so, each register class that is used in a bitwise-and or shift
instruction must have a subclass consisting of registers from which
single-byte values can be loaded or stored. This is so that
PREFERRED_RELOAD_CLASS can always have a possible value to return.
An enumerated type that must be defined with all the register class names
as enumerated values. NO_REGS must be first. ALL_REGS
must be the last register class, followed by one more enumerated value,
LIM_REG_CLASSES, which is not a register class but rather
tells how many classes there are.
Each register class has a number, which is the value of casting
the class name to type int. The number serves as an index
in many of the tables described below.
The number of distinct register classes, defined as follows:
#define N_REG_CLASSES (int) LIM_REG_CLASSES
An initializer containing the names of the register classes as C string constants. These names are used in writing some of the debugging dumps.
An initializer containing the contents of the register classes, as integers
which are bit masks. The nth integer specifies the contents of class
n. The way the integer mask is interpreted is that
register r is in the class if mask & (1 << r) is 1.
When the machine has more than 32 registers, an integer does not suffice.
Then the integers are replaced by sub-initializers, braced groupings containing
several integers. Each sub-initializer must be suitable as an initializer
for the type HARD_REG_SET which is defined in hard-reg-set.h.
In this situation, the first integer in each sub-initializer corresponds to
registers 0 through 31, the second integer to registers 32 through 63, and
so on.
A C expression whose value is a register class containing hard register regno. In general there is more than one such class; choose a class which is minimal, meaning that no smaller class also contains the register.
A macro whose definition is the name of the class to which a valid base register must belong. A base register is one used in an address which is the register value plus a displacement.
This is a variation of the BASE_REG_CLASS macro which allows
the selection of a base register in a mode dependent manner. If
mode is VOIDmode then it should return the same value as
BASE_REG_CLASS.
A C expression whose value is the register class to which a valid base register must belong in order to be used in a base plus index register address. You should define this macro if base plus index addresses have different requirements than other base register uses.
A C expression whose value is the register class to which a valid
base register must belong. outer_code and index_code define the
context in which the base register occurs. outer_code is the code of
the immediately enclosing expression (MEM for the top level of an
address, ADDRESS for something that occurs in an
address_operand). index_code is the code of the corresponding
index expression if outer_code is PLUS; SCRATCH otherwise.
A macro whose definition is the name of the class to which a valid index register must belong. An index register is one used in an address where its value is either multiplied by a scale factor or added to another register (as well as added to a displacement).
A C expression which is nonzero if register number num is suitable for use as a base register in operand addresses.
A C expression that is just like REGNO_OK_FOR_BASE_P, except that
that expression may examine the mode of the memory reference in
mode. You should define this macro if the mode of the memory
reference affects whether a register may be used as a base register. If
you define this macro, the compiler will use it instead of
REGNO_OK_FOR_BASE_P. The mode may be VOIDmode for
addresses that appear outside a MEM, i.e., as an
address_operand.
A C expression which is nonzero if register number num is suitable for use as a base register in base plus index operand addresses, accessing memory in mode mode. It may be either a suitable hard register or a pseudo register that has been allocated such a hard register. You should define this macro if base plus index addresses have different requirements than other base register uses.
Use of this macro is deprecated; please use the more general
REGNO_MODE_CODE_OK_FOR_BASE_P.
A C expression that is just like REGNO_MODE_OK_FOR_BASE_P, except
that that expression may examine the context in which the register
appears in the memory reference. outer_code is the code of the
immediately enclosing expression (MEM if at the top level of the
address, ADDRESS for something that occurs in an
address_operand). index_code is the code of the
corresponding index expression if outer_code is PLUS;
SCRATCH otherwise. The mode may be VOIDmode for addresses
that appear outside a MEM, i.e., as an address_operand.
A C expression which is nonzero if register number num is suitable for use as an index register in operand addresses. It may be either a suitable hard register or a pseudo register that has been allocated such a hard register.
The difference between an index register and a base register is that the index register may be scaled. If an address involves the sum of two registers, neither one of them scaled, then either one may be labeled the “base” and the other the “index”; but whichever labeling is used must fit the machine’s constraints of which registers may serve in each capacity. The compiler will try both labelings, looking for one that is valid, and will reload one or both registers only if neither labeling works.
A target hook that places additional preference on the register class to use when it is necessary to rename a register in class rclass to another class, or perhaps NO_REGS, if no preferred register class is found or hook preferred_rename_class is not implemented. Sometimes returning a more restrictive class makes better code. For example, on ARM, thumb-2 instructions using LO_REGS may be smaller than instructions using GENERIC_REGS. By returning LO_REGS from preferred_rename_class, code size can be reduced.
A target hook that places additional restrictions on the register class to use when it is necessary to copy value x into a register in class rclass. The value is a register class; perhaps rclass, or perhaps another, smaller class.
The default version of this hook always returns value of rclass argument.
Sometimes returning a more restrictive class makes better code. For
example, on the 68000, when x is an integer constant that is in range
for a ‘moveq’ instruction, the value of this macro is always
DATA_REGS as long as rclass includes the data registers.
Requiring a data register guarantees that a ‘moveq’ will be used.
One case where TARGET_PREFERRED_RELOAD_CLASS must not return
rclass is if x is a legitimate constant which cannot be
loaded into some register class. By returning NO_REGS you can
force x into a memory location. For example, rs6000 can load
immediate values into general-purpose registers, but does not have an
instruction for loading an immediate value into a floating-point
register, so TARGET_PREFERRED_RELOAD_CLASS returns NO_REGS when
x is a floating-point constant. If the constant can’t be loaded
into any kind of register, code generation will be better if
TARGET_LEGITIMATE_CONSTANT_P makes the constant illegitimate instead
of using TARGET_PREFERRED_RELOAD_CLASS.
If an insn has pseudos in it after register allocation, reload will go
through the alternatives and call repeatedly TARGET_PREFERRED_RELOAD_CLASS
to find the best one. Returning NO_REGS, in this case, makes
reload add a ! in front of the constraint: the x86 back-end uses
this feature to discourage usage of 387 registers when math is done in
the SSE registers (and vice versa).
A C expression that places additional restrictions on the register class to use when it is necessary to copy value x into a register in class class. The value is a register class; perhaps class, or perhaps another, smaller class. On many machines, the following definition is safe:
#define PREFERRED_RELOAD_CLASS(X,CLASS) CLASS
Sometimes returning a more restrictive class makes better code. For
example, on the 68000, when x is an integer constant that is in range
for a ‘moveq’ instruction, the value of this macro is always
DATA_REGS as long as class includes the data registers.
Requiring a data register guarantees that a ‘moveq’ will be used.
One case where PREFERRED_RELOAD_CLASS must not return
class is if x is a legitimate constant which cannot be
loaded into some register class. By returning NO_REGS you can
force x into a memory location. For example, rs6000 can load
immediate values into general-purpose registers, but does not have an
instruction for loading an immediate value into a floating-point
register, so PREFERRED_RELOAD_CLASS returns NO_REGS when
x is a floating-point constant. If the constant can’t be loaded
into any kind of register, code generation will be better if
TARGET_LEGITIMATE_CONSTANT_P makes the constant illegitimate instead
of using TARGET_PREFERRED_RELOAD_CLASS.
If an insn has pseudos in it after register allocation, reload will go
through the alternatives and call repeatedly PREFERRED_RELOAD_CLASS
to find the best one. Returning NO_REGS, in this case, makes
reload add a ! in front of the constraint: the x86 back-end uses
this feature to discourage usage of 387 registers when math is done in
the SSE registers (and vice versa).
Like PREFERRED_RELOAD_CLASS, but for output reloads instead of
input reloads. If you don’t define this macro, the default is to use
class, unchanged.
You can also use PREFERRED_OUTPUT_RELOAD_CLASS to discourage
reload from using some alternatives, like PREFERRED_RELOAD_CLASS.
Like TARGET_PREFERRED_RELOAD_CLASS, but for output reloads instead of
input reloads.
The default version of this hook always returns value of rclass
argument.
You can also use TARGET_PREFERRED_OUTPUT_RELOAD_CLASS to discourage
reload from using some alternatives, like TARGET_PREFERRED_RELOAD_CLASS.
A C expression that places additional restrictions on the register class to use when it is necessary to be able to hold a value of mode mode in a reload register for which class class would ordinarily be used.
Unlike PREFERRED_RELOAD_CLASS, this macro should be used when
there are certain modes that simply can’t go in certain reload classes.
The value is a register class; perhaps class, or perhaps another, smaller class.
Don’t define this macro unless the target machine has limitations which require the macro to do something nontrivial.
Many machines have some registers that cannot be copied directly to or from memory or even from other types of registers. An example is the ‘MQ’ register, which on most machines, can only be copied to or from general registers, but not memory. Below, we shall be using the term ’intermediate register’ when a move operation cannot be performed directly, but has to be done by copying the source into the intermediate register first, and then copying the intermediate register to the destination. An intermediate register always has the same mode as source and destination. Since it holds the actual value being copied, reload might apply optimizations to re-use an intermediate register and eliding the copy from the source when it can determine that the intermediate register still holds the required value.
Another kind of secondary reload is required on some machines which allow copying all registers to and from memory, but require a scratch register for stores to some memory locations (e.g., those with symbolic address on the RT, and those with certain symbolic address on the SPARC when compiling PIC). Scratch registers need not have the same mode as the value being copied, and usually hold a different value than that being copied. Special patterns in the md file are needed to describe how the copy is performed with the help of the scratch register; these patterns also describe the number, register class(es) and mode(s) of the scratch register(s).
In some cases, both an intermediate and a scratch register are required.
For input reloads, this target hook is called with nonzero in_p, and x is an rtx that needs to be copied to a register of class reload_class in reload_mode. For output reloads, this target hook is called with zero in_p, and a register of class reload_class needs to be copied to rtx x in reload_mode.
If copying a register of reload_class from/to x requires
an intermediate register, the hook secondary_reload should
return the register class required for this intermediate register.
If no intermediate register is required, it should return NO_REGS.
If more than one intermediate register is required, describe the one
that is closest in the copy chain to the reload register.
If scratch registers are needed, you also have to describe how to perform the copy from/to the reload register to/from this closest intermediate register. Or if no intermediate register is required, but still a scratch register is needed, describe the copy from/to the reload register to/from the reload operand x.
You do this by setting sri->icode to the instruction code of a pattern
in the md file which performs the move. Operands 0 and 1 are the output
and input of this copy, respectively. Operands from operand 2 onward are
for scratch operands. These scratch operands must have a mode, and a
single-register-class
output constraint.
When an intermediate register is used, the secondary_reload
hook will be called again to determine how to copy the intermediate
register to/from the reload operand x, so your hook must also
have code to handle the register class of the intermediate operand.
x might be a pseudo-register or a subreg of a
pseudo-register, which could either be in a hard register or in memory.
Use true_regnum to find out; it will return -1 if the pseudo is
in memory and the hard register number if it is in a register.
Scratch operands in memory (constraint "=m" / "=&m") are
currently not supported. For the time being, you will have to continue
to use SECONDARY_MEMORY_NEEDED for that purpose.
copy_cost also uses this target hook to find out how values are
copied. If you want it to include some extra cost for the need to allocate
(a) scratch register(s), set sri->extra_cost to the additional cost.
Or if two dependent moves are supposed to have a lower cost than the sum
of the individual moves due to expected fortuitous scheduling and/or special
forwarding logic, you can set sri->extra_cost to a negative amount.
These macros are obsolete, new ports should use the target hook
TARGET_SECONDARY_RELOAD instead.
These are obsolete macros, replaced by the TARGET_SECONDARY_RELOAD
target hook. Older ports still define these macros to indicate to the
reload phase that it may
need to allocate at least one register for a reload in addition to the
register to contain the data. Specifically, if copying x to a
register class in mode requires an intermediate register,
you were supposed to define SECONDARY_INPUT_RELOAD_CLASS to return the
largest register class all of whose registers can be used as
intermediate registers or scratch registers.
If copying a register class in mode to x requires an
intermediate or scratch register, SECONDARY_OUTPUT_RELOAD_CLASS
was supposed to be defined be defined to return the largest register
class required. If the
requirements for input and output reloads were the same, the macro
SECONDARY_RELOAD_CLASS should have been used instead of defining both
macros identically.
The values returned by these macros are often GENERAL_REGS.
Return NO_REGS if no spare register is needed; i.e., if x
can be directly copied to or from a register of class in
mode without requiring a scratch register. Do not define this
macro if it would always return NO_REGS.
If a scratch register is required (either with or without an
intermediate register), you were supposed to define patterns for
‘reload_inm’ or ‘reload_outm’, as required
(see Standard Names. These patterns, which were normally
implemented with a define_expand, should be similar to the
‘movm’ patterns, except that operand 2 is the scratch
register.
These patterns need constraints for the reload register and scratch register that contain a single register class. If the original reload register (whose class is class) can meet the constraint given in the pattern, the value returned by these macros is used for the class of the scratch register. Otherwise, two additional reload registers are required. Their classes are obtained from the constraints in the insn pattern.
x might be a pseudo-register or a subreg of a
pseudo-register, which could either be in a hard register or in memory.
Use true_regnum to find out; it will return -1 if the pseudo is
in memory and the hard register number if it is in a register.
These macros should not be used in the case where a particular class of
registers can only be copied to memory and not to another class of
registers. In that case, secondary reload registers are not needed and
would not be helpful. Instead, a stack location must be used to perform
the copy and the movm pattern should use memory as an
intermediate storage. This case often occurs between floating-point and
general registers.
Certain machines have the property that some registers cannot be copied to some other registers without using memory. Define this macro on those machines to be a C expression that is nonzero if objects of mode m in registers of class1 can only be copied to registers of class class2 by storing a register of class1 into memory and loading that memory location into a register of class2.
Do not define this macro if its value would always be zero.
Normally when SECONDARY_MEMORY_NEEDED is defined, the compiler
allocates a stack slot for a memory location needed for register copies.
If this macro is defined, the compiler instead uses the memory location
defined by this macro.
Do not define this macro if you do not define
SECONDARY_MEMORY_NEEDED.
When the compiler needs a secondary memory location to copy between two
registers of mode mode, it normally allocates sufficient memory to
hold a quantity of BITS_PER_WORD bits and performs the store and
load operations in a mode that many bits wide and whose class is the
same as that of mode.
This is right thing to do on most machines because it ensures that all bits of the register are copied and prevents accesses to the registers in a narrower mode, which some machines prohibit for floating-point registers.
However, this default behavior is not correct on some machines, such as the DEC Alpha, that store short integers in floating-point registers differently than in integer registers. On those machines, the default widening will not work correctly and you must define this macro to suppress that widening in some cases. See the file alpha.h for details.
Do not define this macro if you do not define
SECONDARY_MEMORY_NEEDED or if widening mode to a mode that
is BITS_PER_WORD bits wide is correct for your machine.
A target hook which returns true if pseudos that have been assigned
to registers of class rclass would likely be spilled because
registers of rclass are needed for spill registers.
The default version of this target hook returns true if rclass
has exactly one register and false otherwise. On most machines, this
default should be used. Only use this target hook to some other expression
if pseudos allocated by local-alloc.c end up in memory because their
hard registers were needed for spill registers. If this target hook returns
false for those classes, those pseudos will only be allocated by
global.c, which knows how to reallocate the pseudo to another
register. If there would not be another register available for reallocation,
you should not change the implementation of this target hook since
the only effect of such implementation would be to slow down register
allocation.
A C expression for the maximum number of consecutive registers of class class needed to hold a value of mode mode.
This is closely related to the macro HARD_REGNO_NREGS. In fact,
the value of the macro CLASS_MAX_NREGS (class, mode)
should be the maximum value of HARD_REGNO_NREGS (regno,
mode) for all regno values in the class class.
This macro helps control the handling of multiple-word values in the reload pass.
If defined, a C expression that returns nonzero for a class for which a change from mode from to mode to is invalid.
For the example, loading 32-bit integer or floating-point objects into
floating-point registers on the Alpha extends them to 64 bits.
Therefore loading a 64-bit object and then storing it as a 32-bit object
does not store the low-order 32 bits, as would be the case for a normal
register. Therefore, alpha.h defines CANNOT_CHANGE_MODE_CLASS
as below:
#define CANNOT_CHANGE_MODE_CLASS(FROM, TO, CLASS) \ (GET_MODE_SIZE (FROM) != GET_MODE_SIZE (TO) \ ? reg_classes_intersect_p (FLOAT_REGS, (CLASS)) : 0)
Return an array of cover classes for the Integrated Register Allocator
(IRA). Cover classes are a set of non-intersecting register
classes covering all hard registers used for register allocation
purposes. If a move between two registers in the same cover class is
possible, it should be cheaper than a load or store of the registers.
The array is terminated by a LIM_REG_CLASSES element.
The order of cover classes in the array is important. If two classes have the same cost of usage for a pseudo, the class occurred first in the array is chosen for the pseudo.
This hook is called once at compiler startup, after the command-line
options have been processed. It is then re-examined by every call to
target_reinit.
The default implementation returns IRA_COVER_CLASSES, if defined,
otherwise there is no default implementation. You must define either this
macro or IRA_COVER_CLASSES in order to use the integrated register
allocator with Chaitin-Briggs coloring. If the macro is not defined,
the only available coloring algorithm is Chow’s priority coloring.
This hook must not be modified from NULL to non-NULL or
vice versa by command-line option processing.
See the documentation for TARGET_IRA_COVER_CLASSES.
Next: Stack and Calling, Previous: Register Classes, Up: Target Macros [Contents][Index]
Machine-specific constraints can be defined with these macros instead of the machine description constructs described in Define Constraints. This mechanism is obsolete. New ports should not use it; old ports should convert to the new mechanism.
For the constraint at the start of str, which starts with the letter c, return the length. This allows you to have register class / constant / extra constraints that are longer than a single letter; you don’t need to define this macro if you can do with single-letter constraints only. The definition of this macro should use DEFAULT_CONSTRAINT_LEN for all the characters that you don’t want to handle specially. There are some sanity checks in genoutput.c that check the constraint lengths for the md file, so you can also use this macro to help you while you are transitioning from a byzantine single-letter-constraint scheme: when you return a negative length for a constraint you want to re-use, genoutput will complain about every instance where it is used in the md file.
A C expression which defines the machine-dependent operand constraint
letters for register classes. If char is such a letter, the
value should be the register class corresponding to it. Otherwise,
the value should be NO_REGS. The register letter ‘r’,
corresponding to class GENERAL_REGS, will not be passed
to this macro; you do not need to handle it.
Like REG_CLASS_FROM_LETTER, but you also get the constraint string
passed in str, so that you can use suffixes to distinguish between
different variants.
A C expression that defines the machine-dependent operand constraint letters (‘I’, ‘J’, ‘K’, … ‘P’) that specify particular ranges of integer values. If c is one of those letters, the expression should check that value, an integer, is in the appropriate range and return 1 if so, 0 otherwise. If c is not one of those letters, the value should be 0 regardless of value.
Like CONST_OK_FOR_LETTER_P, but you also get the constraint
string passed in str, so that you can use suffixes to distinguish
between different variants.
A C expression that defines the machine-dependent operand constraint
letters that specify particular ranges of const_double values
(‘G’ or ‘H’).
If c is one of those letters, the expression should check that
value, an RTX of code const_double, is in the appropriate
range and return 1 if so, 0 otherwise. If c is not one of those
letters, the value should be 0 regardless of value.
const_double is used for all floating-point constants and for
DImode fixed-point constants. A given letter can accept either
or both kinds of values. It can use GET_MODE to distinguish
between these kinds.
Like CONST_DOUBLE_OK_FOR_LETTER_P, but you also get the constraint
string passed in str, so that you can use suffixes to distinguish
between different variants.
A C expression that defines the optional machine-dependent constraint
letters that can be used to segregate specific types of operands, usually
memory references, for the target machine. Any letter that is not
elsewhere defined and not matched by REG_CLASS_FROM_LETTER /
REG_CLASS_FROM_CONSTRAINT
may be used. Normally this macro will not be defined.
If it is required for a particular target machine, it should return 1 if value corresponds to the operand type represented by the constraint letter c. If c is not defined as an extra constraint, the value returned should be 0 regardless of value.
For example, on the ROMP, load instructions cannot have their output in r0 if the memory reference contains a symbolic address. Constraint letter ‘Q’ is defined as representing a memory address that does not contain a symbolic address. An alternative is specified with a ‘Q’ constraint on the input and ‘r’ on the output. The next alternative specifies ‘m’ on the input and a register class that does not include r0 on the output.
Like EXTRA_CONSTRAINT, but you also get the constraint string passed
in str, so that you can use suffixes to distinguish between different
variants.
A C expression that defines the optional machine-dependent constraint
letters, amongst those accepted by EXTRA_CONSTRAINT, that should
be treated like memory constraints by the reload pass.
It should return 1 if the operand type represented by the constraint at the start of str, the first letter of which is the letter c, comprises a subset of all memory references including all those whose address is simply a base register. This allows the reload pass to reload an operand, if it does not directly correspond to the operand type of c, by copying its address into a base register.
For example, on the S/390, some instructions do not accept arbitrary
memory references, but only those that do not make use of an index
register. The constraint letter ‘Q’ is defined via
EXTRA_CONSTRAINT as representing a memory address of this type.
If the letter ‘Q’ is marked as EXTRA_MEMORY_CONSTRAINT,
a ‘Q’ constraint can handle any memory operand, because the
reload pass knows it can be reloaded by copying the memory address
into a base register if required. This is analogous to the way
an ‘o’ constraint can handle any memory operand.
A C expression that defines the optional machine-dependent constraint
letters, amongst those accepted by EXTRA_CONSTRAINT /
EXTRA_CONSTRAINT_STR, that should
be treated like address constraints by the reload pass.
It should return 1 if the operand type represented by the constraint at the start of str, which starts with the letter c, comprises a subset of all memory addresses including all those that consist of just a base register. This allows the reload pass to reload an operand, if it does not directly correspond to the operand type of str, by copying it into a base register.
Any constraint marked as EXTRA_ADDRESS_CONSTRAINT can only
be used with the address_operand predicate. It is treated
analogously to the ‘p’ constraint.
Next: Varargs, Previous: Old Constraints, Up: Target Macros [Contents][Index]
This describes the stack layout and calling conventions.
| • Frame Layout: | ||
| • Exception Handling: | ||
| • Stack Checking: | ||
| • Frame Registers: | ||
| • Elimination: | ||
| • Stack Arguments: | ||
| • Register Arguments: | ||
| • Scalar Return: | ||
| • Aggregate Return: | ||
| • Caller Saves: | ||
| • Function Entry: | ||
| • Profiling: | ||
| • Tail Calls: | ||
| • Stack Smashing Protection: |
Next: Exception Handling, Up: Stack and Calling [Contents][Index]
Here is the basic stack layout.
Define this macro if pushing a word onto the stack moves the stack pointer to a smaller address.
When we say, “define this macro if …”, it means that the
compiler checks this macro only with #ifdef so the precise
definition used does not matter.
This macro defines the operation used when something is pushed
on the stack. In RTL, a push operation will be
(set (mem (STACK_PUSH_CODE (reg sp))) …)
The choices are PRE_DEC, POST_DEC, PRE_INC,
and POST_INC. Which of these is correct depends on
the stack direction and on whether the stack pointer points
to the last item on the stack or whether it points to the
space for the next item on the stack.
The default is PRE_DEC when STACK_GROWS_DOWNWARD is
defined, which is almost always right, and PRE_INC otherwise,
which is often wrong.
Define this macro to nonzero value if the addresses of local variable slots are at negative offsets from the frame pointer.
Define this macro if successive arguments to a function occupy decreasing addresses on the stack.
Offset from the frame pointer to the first local variable slot to be allocated.
If FRAME_GROWS_DOWNWARD, find the next slot’s offset by
subtracting the first slot’s length from STARTING_FRAME_OFFSET.
Otherwise, it is found by adding the length of the first slot to the
value STARTING_FRAME_OFFSET.
Define to zero to disable final alignment of the stack during reload. The nonzero default for this macro is suitable for most ports.
On ports where STARTING_FRAME_OFFSET is nonzero or where there
is a register save block following the local block that doesn’t require
alignment to STACK_BOUNDARY, it may be beneficial to disable
stack alignment and do it in the backend.
Offset from the stack pointer register to the first location at which outgoing arguments are placed. If not specified, the default value of zero is used. This is the proper value for most machines.
If ARGS_GROW_DOWNWARD, this is the offset to the location above
the first location at which outgoing arguments are placed.
Offset from the argument pointer register to the first argument’s address. On some machines it may depend on the data type of the function.
If ARGS_GROW_DOWNWARD, this is the offset to the location above
the first argument’s address.
Offset from the stack pointer register to an item dynamically allocated
on the stack, e.g., by alloca.
The default value for this macro is STACK_POINTER_OFFSET plus the
length of the outgoing arguments. The default is correct for most
machines. See function.c for details.
A C expression whose value is RTL representing the address of the initial
stack frame. This address is passed to RETURN_ADDR_RTX and
DYNAMIC_CHAIN_ADDRESS. If you don’t define this macro, a reasonable
default value will be used. Define this macro in order to make frame pointer
elimination work in the presence of __builtin_frame_address (count) and
__builtin_return_address (count) for count not equal to zero.
A C expression whose value is RTL representing the address in a stack frame where the pointer to the caller’s frame is stored. Assume that frameaddr is an RTL expression for the address of the stack frame itself.
If you don’t define this macro, the default is to return the value of frameaddr—that is, the stack frame address is also the address of the stack word that points to the previous frame.
If defined, a C expression that produces the machine-specific code to setup the stack so that arbitrary frames can be accessed. For example, on the SPARC, we must flush all of the register windows to the stack before we can access arbitrary stack frames. You will seldom need to define this macro.
This target hook should return an rtx that is used to store
the address of the current frame into the built in setjmp buffer.
The default value, virtual_stack_vars_rtx, is correct for most
machines. One reason you may need to define this target hook is if
hard_frame_pointer_rtx is the appropriate value on your machine.
A C expression whose value is RTL representing the value of the frame address for the current frame. frameaddr is the frame pointer of the current frame. This is used for __builtin_frame_address. You need only define this macro if the frame address is not the same as the frame pointer. Most machines do not need to define it.
A C expression whose value is RTL representing the value of the return
address for the frame count steps up from the current frame, after
the prologue. frameaddr is the frame pointer of the count
frame, or the frame pointer of the count - 1 frame if
RETURN_ADDR_IN_PREVIOUS_FRAME is defined.
The value of the expression must always be the correct address when
count is zero, but may be NULL_RTX if there is no way to
determine the return address of other frames.
Define this if the return address of a particular stack frame is accessed from the frame pointer of the previous stack frame.
A C expression whose value is RTL representing the location of the
incoming return address at the beginning of any function, before the
prologue. This RTL is either a REG, indicating that the return
value is saved in ‘REG’, or a MEM representing a location in
the stack.
You only need to define this macro if you want to support call frame debugging information like that provided by DWARF 2.
If this RTL is a REG, you should also define
DWARF_FRAME_RETURN_COLUMN to DWARF_FRAME_REGNUM (REGNO).
A C expression whose value is an integer giving a DWARF 2 column
number that may be used as an alternative return column. The column
must not correspond to any gcc hard register (that is, it must not
be in the range of DWARF_FRAME_REGNUM).
This macro can be useful if DWARF_FRAME_RETURN_COLUMN is set to a
general register, but an alternative column needs to be used for signal
frames. Some targets have also used different frame return columns
over time.
A C expression whose value is an integer giving a DWARF 2 register number that is considered to always have the value zero. This should only be defined if the target has an architected zero register, and someone decided it was a good idea to use that register number to terminate the stack backtrace. New ports should avoid this.
This target hook allows the backend to emit frame-related insns that contain UNSPECs or UNSPEC_VOLATILEs. The DWARF 2 call frame debugging info engine will invoke it on insns of the form
(set (reg) (unspec […] UNSPEC_INDEX))
and
(set (reg) (unspec_volatile […] UNSPECV_INDEX)).
to let the backend emit the call frame instructions. label is
the CFI label attached to the insn, pattern is the pattern of
the insn and index is UNSPEC_INDEX or UNSPECV_INDEX.
A C expression whose value is an integer giving the offset, in bytes, from the value of the stack pointer register to the top of the stack frame at the beginning of any function, before the prologue. The top of the frame is defined to be the value of the stack pointer in the previous frame, just before the call instruction.
You only need to define this macro if you want to support call frame debugging information like that provided by DWARF 2.
A C expression whose value is an integer giving the offset, in bytes,
from the argument pointer to the canonical frame address (cfa). The
final value should coincide with that calculated by
INCOMING_FRAME_SP_OFFSET. Which is unfortunately not usable
during virtual register instantiation.
The default value for this macro is
FIRST_PARM_OFFSET (fundecl) + crtl->args.pretend_args_size,
which is correct for most machines; in general, the arguments are found
immediately before the stack frame. Note that this is not the case on
some targets that save registers into the caller’s frame, such as SPARC
and rs6000, and so such targets need to define this macro.
You only need to define this macro if the default is incorrect, and you want to support call frame debugging information like that provided by DWARF 2.
If defined, a C expression whose value is an integer giving the offset
in bytes from the frame pointer to the canonical frame address (cfa).
The final value should coincide with that calculated by
INCOMING_FRAME_SP_OFFSET.
Normally the CFA is calculated as an offset from the argument pointer,
via ARG_POINTER_CFA_OFFSET, but if the argument pointer is
variable due to the ABI, this may not be possible. If this macro is
defined, it implies that the virtual register instantiation should be
based on the frame pointer instead of the argument pointer. Only one
of FRAME_POINTER_CFA_OFFSET and ARG_POINTER_CFA_OFFSET
should be defined.
If defined, a C expression whose value is an integer giving the offset in bytes from the canonical frame address (cfa) to the frame base used in DWARF 2 debug information. The default is zero. A different value may reduce the size of debug information on some ports.
Next: Stack Checking, Previous: Frame Layout, Up: Stack and Calling [Contents][Index]
A C expression whose value is the Nth register number used for
data by exception handlers, or INVALID_REGNUM if fewer than
N registers are usable.
The exception handling library routines communicate with the exception handlers via a set of agreed upon registers. Ideally these registers should be call-clobbered; it is possible to use call-saved registers, but may negatively impact code size. The target must support at least 2 data registers, but should define 4 if there are enough free registers.
You must define this macro if you want to support call frame exception handling like that provided by DWARF 2.
A C expression whose value is RTL representing a location in which to store a stack adjustment to be applied before function return. This is used to unwind the stack to an exception handler’s call frame. It will be assigned zero on code paths that return normally.
Typically this is a call-clobbered hard register that is otherwise untouched by the epilogue, but could also be a stack slot.
Do not define this macro if the stack pointer is saved and restored by the regular prolog and epilog code in the call frame itself; in this case, the exception handling library routines will update the stack location to be restored in place. Otherwise, you must define this macro if you want to support call frame exception handling like that provided by DWARF 2.
A C expression whose value is RTL representing a location in which to store the address of an exception handler to which we should return. It will not be assigned on code paths that return normally.
Typically this is the location in the call frame at which the normal
return address is stored. For targets that return by popping an
address off the stack, this might be a memory address just below
the target call frame rather than inside the current call
frame. If defined, EH_RETURN_STACKADJ_RTX will have already
been assigned, so it may be used to calculate the location of the
target call frame.
Some targets have more complex requirements than storing to an
address calculable during initial code generation. In that case
the eh_return instruction pattern should be used instead.
If you want to support call frame exception handling, you must
define either this macro or the eh_return instruction pattern.
If defined, an integer-valued C expression for which rtl will be generated to add it to the exception handler address before it is searched in the exception handling tables, and to subtract it again from the address before using it to return to the exception handler.
This macro chooses the encoding of pointers embedded in the exception handling sections. If at all possible, this should be defined such that the exception handling section will not require dynamic relocations, and so may be read-only.
code is 0 for data, 1 for code labels, 2 for function pointers.
global is true if the symbol may be affected by dynamic relocations.
The macro should return a combination of the DW_EH_PE_* defines
as found in dwarf2.h.
If this macro is not defined, pointers will not be encoded but represented directly.
This macro allows the target to emit whatever special magic is required
to represent the encoding chosen by ASM_PREFERRED_EH_DATA_FORMAT.
Generic code takes care of pc-relative and indirect encodings; this must
be defined if the target uses text-relative or data-relative encodings.
This is a C statement that branches to done if the format was
handled. encoding is the format chosen, size is the number
of bytes that the format occupies, addr is the SYMBOL_REF
to be emitted.
A string specifying a file to be #include’d in unwind-dw2.c. The file
so included typically defines MD_FALLBACK_FRAME_STATE_FOR.
This macro allows the target to add CPU and operating system specific code to the call-frame unwinder for use when there is no unwind data available. The most common reason to implement this macro is to unwind through signal frames.
This macro is called from uw_frame_state_for in
unwind-dw2.c, unwind-dw2-xtensa.c and
unwind-ia64.c. context is an _Unwind_Context;
fs is an _Unwind_FrameState. Examine context->ra
for the address of the code being executed and context->cfa for
the stack pointer value. If the frame can be decoded, the register
save addresses should be updated in fs and the macro should
evaluate to _URC_NO_REASON. If the frame cannot be decoded,
the macro should evaluate to _URC_END_OF_STACK.
For proper signal handling in Java this macro is accompanied by
MAKE_THROW_FRAME, defined in libjava/include/*-signal.h headers.
This macro allows the target to add operating system specific code to the
call-frame unwinder to handle the IA-64 .unwabi unwinding directive,
usually used for signal or interrupt frames.
This macro is called from uw_update_context in unwind-ia64.c.
context is an _Unwind_Context;
fs is an _Unwind_FrameState. Examine fs->unwabi
for the abi and context in the .unwabi directive. If the
.unwabi directive can be handled, the register save addresses should
be updated in fs.
A C expression that evaluates to true if the target requires unwind
info to be given comdat linkage. Define it to be 1 if comdat
linkage is necessary. The default is 0.
Next: Frame Registers, Previous: Exception Handling, Up: Stack and Calling [Contents][Index]
GCC will check that stack references are within the boundaries of the stack, if the option -fstack-check is specified, in one of three ways:
STACK_CHECK_BUILTIN macro is nonzero, GCC
will assume that you have arranged for full stack checking to be done
at appropriate places in the configuration files. GCC will not do
other special processing.
STACK_CHECK_BUILTIN is zero and the value of the
STACK_CHECK_STATIC_BUILTIN macro is nonzero, GCC will assume
that you have arranged for static stack checking (checking of the
static stack frame of functions) to be done at appropriate places
in the configuration files. GCC will only emit code to do dynamic
stack checking (checking on dynamic stack allocations) using the third
approach below.
If neither STACK_CHECK_BUILTIN nor STACK_CHECK_STATIC_BUILTIN is defined,
GCC will change its allocation strategy for large objects if the option
-fstack-check is specified: they will always be allocated
dynamically if their size exceeds STACK_CHECK_MAX_VAR_SIZE bytes.
A nonzero value if stack checking is done by the configuration files in a machine-dependent manner. You should define this macro if stack checking is required by the ABI of your machine or if you would like to do stack checking in some more efficient way than the generic approach. The default value of this macro is zero.
A nonzero value if static stack checking is done by the configuration files in a machine-dependent manner. You should define this macro if you would like to do static stack checking in some more efficient way than the generic approach. The default value of this macro is zero.
An integer specifying the interval at which GCC must generate stack probe instructions, defined as 2 raised to this integer. You will normally define this macro so that the interval be no larger than the size of the “guard pages” at the end of a stack area. The default value of 12 (4096-byte interval) is suitable for most systems.
An integer which is nonzero if GCC should move the stack pointer page by page when doing probes. This can be necessary on systems where the stack pointer contains the bottom address of the memory area accessible to the executing thread at any point in time. In this situation an alternate signal stack is required in order to be able to recover from a stack overflow. The default value of this macro is zero.
The number of bytes of stack needed to recover from a stack overflow, for
languages where such a recovery is supported. The default value of 75 words
with the setjmp/longjmp-based exception handling mechanism and
8192 bytes with other exception handling mechanisms should be adequate for
most machines.
The following macros are relevant only if neither STACK_CHECK_BUILTIN nor STACK_CHECK_STATIC_BUILTIN is defined; you can omit them altogether in the opposite case.
The maximum size of a stack frame, in bytes. GCC will generate probe instructions in non-leaf functions to ensure at least this many bytes of stack are available. If a stack frame is larger than this size, stack checking will not be reliable and GCC will issue a warning. The default is chosen so that GCC only generates one instruction on most systems. You should normally not change the default value of this macro.
GCC uses this value to generate the above warning message. It represents the amount of fixed frame used by a function, not including space for any callee-saved registers, temporaries and user variables. You need only specify an upper bound for this amount and will normally use the default of four words.
The maximum size, in bytes, of an object that GCC will place in the fixed area of the stack frame when the user specifies -fstack-check. GCC computed the default from the values of the above macros and you will normally not need to override that default.
Next: Elimination, Previous: Stack Checking, Up: Stack and Calling [Contents][Index]
This discusses registers that address the stack frame.
The register number of the stack pointer register, which must also be a
fixed register according to FIXED_REGISTERS. On most machines,
the hardware determines which register this is.
The register number of the frame pointer register, which is used to access automatic variables in the stack frame. On some machines, the hardware determines which register this is. On other machines, you can choose any register you wish for this purpose.
On some machines the offset between the frame pointer and starting
offset of the automatic variables is not known until after register
allocation has been done (for example, because the saved registers are
between these two locations). On those machines, define
FRAME_POINTER_REGNUM the number of a special, fixed register to
be used internally until the offset is known, and define
HARD_FRAME_POINTER_REGNUM to be the actual hard register number
used for the frame pointer.
You should define this macro only in the very rare circumstances when it
is not possible to calculate the offset between the frame pointer and
the automatic variables until after register allocation has been
completed. When this macro is defined, you must also indicate in your
definition of ELIMINABLE_REGS how to eliminate
FRAME_POINTER_REGNUM into either HARD_FRAME_POINTER_REGNUM
or STACK_POINTER_REGNUM.
Do not define this macro if it would be the same as
FRAME_POINTER_REGNUM.
The register number of the arg pointer register, which is used to access
the function’s argument list. On some machines, this is the same as the
frame pointer register. On some machines, the hardware determines which
register this is. On other machines, you can choose any register you
wish for this purpose. If this is not the same register as the frame
pointer register, then you must mark it as a fixed register according to
FIXED_REGISTERS, or arrange to be able to eliminate it
(see Elimination).
Define this to a preprocessor constant that is nonzero if
hard_frame_pointer_rtx and frame_pointer_rtx should be
the same. The default definition is ‘(HARD_FRAME_POINTER_REGNUM
== FRAME_POINTER_REGNUM)’; you only need to define this macro if that
definition is not suitable for use in preprocessor conditionals.
Define this to a preprocessor constant that is nonzero if
hard_frame_pointer_rtx and arg_pointer_rtx should be the
same. The default definition is ‘(HARD_FRAME_POINTER_REGNUM ==
ARG_POINTER_REGNUM)’; you only need to define this macro if that
definition is not suitable for use in preprocessor conditionals.
The register number of the return address pointer register, which is used to
access the current function’s return address from the stack. On some
machines, the return address is not at a fixed offset from the frame
pointer or stack pointer or argument pointer. This register can be defined
to point to the return address on the stack, and then be converted by
ELIMINABLE_REGS into either the frame pointer or stack pointer.
Do not define this macro unless there is no other way to get the return address from the stack.
Register numbers used for passing a function’s static chain pointer. If
register windows are used, the register number as seen by the called
function is STATIC_CHAIN_INCOMING_REGNUM, while the register
number as seen by the calling function is STATIC_CHAIN_REGNUM. If
these registers are the same, STATIC_CHAIN_INCOMING_REGNUM need
not be defined.
The static chain register need not be a fixed register.
If the static chain is passed in memory, these macros should not be
defined; instead, the TARGET_STATIC_CHAIN hook should be used.
This hook replaces the use of STATIC_CHAIN_REGNUM et al for
targets that may use different static chain locations for different
nested functions. This may be required if the target has function
attributes that affect the calling conventions of the function and
those calling conventions use different static chain locations.
The default version of this hook uses STATIC_CHAIN_REGNUM et al.
If the static chain is passed in memory, this hook should be used to
provide rtx giving mem expressions that denote where they are stored.
Often the mem expression as seen by the caller will be at an offset
from the stack pointer and the mem expression as seen by the callee
will be at an offset from the frame pointer.
The variables stack_pointer_rtx, frame_pointer_rtx, and
arg_pointer_rtx will have been initialized and should be used
to refer to those items.
This macro specifies the maximum number of hard registers that can be saved in a call frame. This is used to size data structures used in DWARF2 exception handling.
Prior to GCC 3.0, this macro was needed in order to establish a stable exception handling ABI in the face of adding new hard registers for ISA extensions. In GCC 3.0 and later, the EH ABI is insulated from changes in the number of hard registers. Nevertheless, this macro can still be used to reduce the runtime memory requirements of the exception handling routines, which can be substantial if the ISA contains a lot of registers that are not call-saved.
If this macro is not defined, it defaults to
FIRST_PSEUDO_REGISTER.
This macro is similar to DWARF_FRAME_REGISTERS, but is provided
for backward compatibility in pre GCC 3.0 compiled code.
If this macro is not defined, it defaults to
DWARF_FRAME_REGISTERS.
Define this macro if the target’s representation for dwarf registers is different than the internal representation for unwind column. Given a dwarf register, this macro should return the internal unwind column number to use instead.
See the PowerPC’s SPE target for an example.
Define this macro if the target’s representation for dwarf registers
used in .eh_frame or .debug_frame is different from that used in other
debug info sections. Given a GCC hard register number, this macro
should return the .eh_frame register number. The default is
DBX_REGISTER_NUMBER (regno).
Define this macro to map register numbers held in the call frame info
that GCC has collected using DWARF_FRAME_REGNUM to those that
should be output in .debug_frame (for_eh is zero) and
.eh_frame (for_eh is nonzero). The default is to
return regno.
Next: Stack Arguments, Previous: Frame Registers, Up: Stack and Calling [Contents][Index]
This is about eliminating the frame pointer and arg pointer.
This target hook should return true if a function must have and use
a frame pointer. This target hook is called in the reload pass. If its return
value is true the function will have a frame pointer.
This target hook can in principle examine the current function and decide
according to the facts, but on most machines the constant false or the
constant true suffices. Use false when the machine allows code
to be generated with no frame pointer, and doing so saves some time or space.
Use true when there is no possible advantage to avoiding a frame
pointer.
In certain cases, the compiler does not know how to produce valid code
without a frame pointer. The compiler recognizes those cases and
automatically gives the function a frame pointer regardless of what
TARGET_FRAME_POINTER_REQUIRED returns. You don’t need to worry about
them.
In a function that does not require a frame pointer, the frame pointer
register can be allocated for ordinary usage, unless you mark it as a
fixed register. See FIXED_REGISTERS for more information.
Default return value is false.
A C statement to store in the variable depth-var the difference
between the frame pointer and the stack pointer values immediately after
the function prologue. The value would be computed from information
such as the result of get_frame_size () and the tables of
registers regs_ever_live and call_used_regs.
If ELIMINABLE_REGS is defined, this macro will be not be used and
need not be defined. Otherwise, it must be defined even if
TARGET_FRAME_POINTER_REQUIRED always returns true; in that
case, you may set depth-var to anything.
If defined, this macro specifies a table of register pairs used to eliminate unneeded registers that point into the stack frame. If it is not defined, the only elimination attempted by the compiler is to replace references to the frame pointer with references to the stack pointer.
The definition of this macro is a list of structure initializations, each of which specifies an original and replacement register.
On some machines, the position of the argument pointer is not known until the compilation is completed. In such a case, a separate hard register must be used for the argument pointer. This register can be eliminated by replacing it with either the frame pointer or the argument pointer, depending on whether or not the frame pointer has been eliminated.
In this case, you might specify:
#define ELIMINABLE_REGS \
{{ARG_POINTER_REGNUM, STACK_POINTER_REGNUM}, \
{ARG_POINTER_REGNUM, FRAME_POINTER_REGNUM}, \
{FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM}}
Note that the elimination of the argument pointer with the stack pointer is specified first since that is the preferred elimination.
This target hook should returns true if the compiler is allowed to
try to replace register number from_reg with register number
to_reg. This target hook need only be defined if ELIMINABLE_REGS
is defined, and will usually be true, since most of the cases
preventing register elimination are things that the compiler already
knows about.
Default return value is true.
This macro is similar to INITIAL_FRAME_POINTER_OFFSET. It
specifies the initial difference between the specified pair of
registers. This macro must be defined if ELIMINABLE_REGS is
defined.
Next: Register Arguments, Previous: Elimination, Up: Stack and Calling [Contents][Index]
The macros in this section control how arguments are passed on the stack. See the following section for other macros that control passing certain arguments in registers.
This target hook returns true if an argument declared in a
prototype as an integral type smaller than int should actually be
passed as an int. In addition to avoiding errors in certain
cases of mismatch, it also makes for better code on certain machines.
The default is to not promote prototypes.
A C expression. If nonzero, push insns will be used to pass
outgoing arguments.
If the target machine does not have a push instruction, set it to zero.
That directs GCC to use an alternate strategy: to
allocate the entire argument block and then store the arguments into
it. When PUSH_ARGS is nonzero, PUSH_ROUNDING must be defined too.
A C expression. If nonzero, function arguments will be evaluated from
last to first, rather than from first to last. If this macro is not
defined, it defaults to PUSH_ARGS on targets where the stack
and args grow in opposite directions, and 0 otherwise.
A C expression that is the number of bytes actually pushed onto the stack when an instruction attempts to push npushed bytes.
On some machines, the definition
#define PUSH_ROUNDING(BYTES) (BYTES)
will suffice. But on other machines, instructions that appear to push one byte actually push two bytes in an attempt to maintain alignment. Then the definition should be
#define PUSH_ROUNDING(BYTES) (((BYTES) + 1) & ~1)
If the value of this macro has a type, it should be an unsigned type.
A C expression. If nonzero, the maximum amount of space required for outgoing arguments
will be computed and placed into the variable
current_function_outgoing_args_size. No space will be pushed
onto the stack for each call; instead, the function prologue should
increase the stack frame size by this amount.
Setting both PUSH_ARGS and ACCUMULATE_OUTGOING_ARGS
is not proper.
Define this macro if functions should assume that stack space has been allocated for arguments even when their values are passed in registers.
The value of this macro is the size, in bytes, of the area reserved for arguments passed in registers for the function represented by fndecl, which can be zero if GCC is calling a library function. The argument fndecl can be the FUNCTION_DECL, or the type itself of the function.
This space can be allocated by the caller, or be a part of the
machine-dependent stack frame: OUTGOING_REG_PARM_STACK_SPACE says
which.
Define this to a nonzero value if it is the responsibility of the caller to allocate the area reserved for arguments passed in registers when calling a function of fntype. fntype may be NULL if the function called is a library function.
If ACCUMULATE_OUTGOING_ARGS is defined, this macro controls
whether the space for these arguments counts in the value of
current_function_outgoing_args_size.
Define this macro if REG_PARM_STACK_SPACE is defined, but the
stack parameters don’t skip the area specified by it.
Normally, when a parameter is not passed in registers, it is placed on the
stack beyond the REG_PARM_STACK_SPACE area. Defining this macro
suppresses this behavior and causes the parameter to be passed on the
stack in its natural location.
This target hook returns the number of bytes of its own arguments that a function pops on returning, or 0 if the function pops no arguments and the caller must therefore pop them all after the function returns.
fundecl is a C variable whose value is a tree node that describes
the function in question. Normally it is a node of type
FUNCTION_DECL that describes the declaration of the function.
From this you can obtain the DECL_ATTRIBUTES of the function.
funtype is a C variable whose value is a tree node that
describes the function in question. Normally it is a node of type
FUNCTION_TYPE that describes the data type of the function.
From this it is possible to obtain the data types of the value and
arguments (if known).
When a call to a library function is being considered, fundecl will contain an identifier node for the library function. Thus, if you need to distinguish among various library functions, you can do so by their names. Note that “library function” in this context means a function used to perform arithmetic, whose name is known specially in the compiler and was not mentioned in the C code being compiled.
size is the number of bytes of arguments passed on the stack. If a variable number of bytes is passed, it is zero, and argument popping will always be the responsibility of the calling function.
On the VAX, all functions always pop their arguments, so the definition
of this macro is size. On the 68000, using the standard
calling convention, no functions pop their arguments, so the value of
the macro is always 0 in this case. But an alternative calling
convention is available in which functions that take a fixed number of
arguments pop them but other functions (such as printf) pop
nothing (the caller pops all). When this convention is in use,
funtype is examined to determine whether a function takes a fixed
number of arguments.
A C expression that should indicate the number of bytes a call sequence
pops off the stack. It is added to the value of RETURN_POPS_ARGS
when compiling a function call.
cum is the variable in which all arguments to the called function have been accumulated.
On certain architectures, such as the SH5, a call trampoline is used
that pops certain registers off the stack, depending on the arguments
that have been passed to the function. Since this is a property of the
call site, not of the called function, RETURN_POPS_ARGS is not
appropriate.
Next: Scalar Return, Previous: Stack Arguments, Up: Stack and Calling [Contents][Index]
This section describes the macros which let you control how various types of arguments are passed in registers or how they are arranged in the stack.
A C expression that controls whether a function argument is passed in a register, and which register.
The arguments are cum, which summarizes all the previous arguments; mode, the machine mode of the argument; type, the data type of the argument as a tree node or 0 if that is not known (which happens for C support library functions); and named, which is 1 for an ordinary argument and 0 for nameless arguments that correspond to ‘…’ in the called function’s prototype. type can be an incomplete type if a syntax error has previously occurred.
The value of the expression is usually either a reg RTX for the
hard register in which to pass the argument, or zero to pass the
argument on the stack.
For machines like the VAX and 68000, where normally all arguments are pushed, zero suffices as a definition.
The value of the expression can also be a parallel RTX. This is
used when an argument is passed in multiple locations. The mode of the
parallel should be the mode of the entire argument. The
parallel holds any number of expr_list pairs; each one
describes where part of the argument is passed. In each
expr_list the first operand must be a reg RTX for the hard
register in which to pass this part of the argument, and the mode of the
register RTX indicates how large this part of the argument is. The
second operand of the expr_list is a const_int which gives
the offset in bytes into the entire argument of where this part starts.
As a special exception the first expr_list in the parallel
RTX may have a first operand of zero. This indicates that the entire
argument is also stored on the stack.
The last time this macro is called, it is called with MODE ==
VOIDmode, and its result is passed to the call or call_value
pattern as operands 2 and 3 respectively.
The usual way to make the ISO library stdarg.h work on a machine
where some arguments are usually passed in registers, is to cause
nameless arguments to be passed on the stack instead. This is done
by making FUNCTION_ARG return 0 whenever named is 0.
You may use the hook targetm.calls.must_pass_in_stack
in the definition of this macro to determine if this argument is of a
type that must be passed in the stack. If REG_PARM_STACK_SPACE
is not defined and FUNCTION_ARG returns nonzero for such an
argument, the compiler will abort. If REG_PARM_STACK_SPACE is
defined, the argument will be computed in the stack and then loaded into
a register.
This target hook should return true if we should not pass type
solely in registers. The file expr.h defines a
definition that is usually appropriate, refer to expr.h for additional
documentation.
Define this macro if the target machine has “register windows”, so that the register in which a function sees an arguments is not necessarily the same as the one in which the caller passed the argument.
For such machines, FUNCTION_ARG computes the register in which
the caller passes the value, and FUNCTION_INCOMING_ARG should
be defined in a similar fashion to tell the function being called
where the arguments will arrive.
If FUNCTION_INCOMING_ARG is not defined, FUNCTION_ARG
serves both purposes.
This target hook returns the number of bytes at the beginning of an argument that must be put in registers. The value must be zero for arguments that are passed entirely in registers or that are entirely pushed on the stack.
On some machines, certain arguments must be passed partially in
registers and partially in memory. On these machines, typically the
first few words of arguments are passed in registers, and the rest
on the stack. If a multi-word argument (a double or a
structure) crosses that boundary, its first few words must be passed
in registers and the rest must be pushed. This macro tells the
compiler when this occurs, and how many bytes should go in registers.
FUNCTION_ARG for these arguments should return the first
register to be used by the caller for this argument; likewise
FUNCTION_INCOMING_ARG, for the called function.
This target hook should return true if an argument at the
position indicated by cum should be passed by reference. This
predicate is queried after target independent reasons for being
passed by reference, such as TREE_ADDRESSABLE (type).
If the hook returns true, a copy of that argument is made in memory and a pointer to the argument is passed instead of the argument itself. The pointer is passed in whatever way is appropriate for passing a pointer to that type.
The function argument described by the parameters to this hook is known to be passed by reference. The hook should return true if the function argument should be copied by the callee instead of copied by the caller.
For any argument for which the hook returns true, if it can be determined that the argument is not modified, then a copy need not be generated.
The default version of this hook always returns false.
A C type for declaring a variable that is used as the first argument of
FUNCTION_ARG and other related values. For some target machines,
the type int suffices and can hold the number of bytes of
argument so far.
There is no need to record in CUMULATIVE_ARGS anything about the
arguments that have been passed on the stack. The compiler has other
variables to keep track of that. For target machines on which all
arguments are passed on the stack, there is no need to store anything in
CUMULATIVE_ARGS; however, the data structure must exist and
should not be empty, so use int.
If defined, this macro is called before generating any code for a
function, but after the cfun descriptor for the function has been
created. The back end may use this macro to update cfun to
reflect an ABI other than that which would normally be used by default.
If the compiler is generating code for a compiler-generated function,
fndecl may be NULL.
A C statement (sans semicolon) for initializing the variable
cum for the state at the beginning of the argument list. The
variable has type CUMULATIVE_ARGS. The value of fntype
is the tree node for the data type of the function which will receive
the args, or 0 if the args are to a compiler support library function.
For direct calls that are not libcalls, fndecl contain the
declaration node of the function. fndecl is also set when
INIT_CUMULATIVE_ARGS is used to find arguments for the function
being compiled. n_named_args is set to the number of named
arguments, including a structure return address if it is passed as a
parameter, when making a call. When processing incoming arguments,
n_named_args is set to -1.
When processing a call to a compiler support library function,
libname identifies which one. It is a symbol_ref rtx which
contains the name of the function, as a string. libname is 0 when
an ordinary C function call is being processed. Thus, each time this
macro is called, either libname or fntype is nonzero, but
never both of them at once.
Like INIT_CUMULATIVE_ARGS but only used for outgoing libcalls,
it gets a MODE argument instead of fntype, that would be
NULL. indirect would always be zero, too. If this macro
is not defined, INIT_CUMULATIVE_ARGS (cum, NULL_RTX, libname,
0) is used instead.
Like INIT_CUMULATIVE_ARGS but overrides it for the purposes of
finding the arguments for the function being compiled. If this macro is
undefined, INIT_CUMULATIVE_ARGS is used instead.
The value passed for libname is always 0, since library routines
with special calling conventions are never compiled with GCC. The
argument libname exists for symmetry with
INIT_CUMULATIVE_ARGS.
A C statement (sans semicolon) to update the summarizer variable
cum to advance past an argument in the argument list. The
values mode, type and named describe that argument.
Once this is done, the variable cum is suitable for analyzing
the following argument with FUNCTION_ARG, etc.
This macro need not do anything if the argument in question was passed on the stack. The compiler knows how to track the amount of stack space used for arguments without any special help.
If defined, a C expression that is the number of bytes to add to the
offset of the argument passed in memory. This is needed for the SPU,
which passes char and short arguments in the preferred
slot that is in the middle of the quad word instead of starting at the
top.
If defined, a C expression which determines whether, and in which direction,
to pad out an argument with extra space. The value should be of type
enum direction: either upward to pad above the argument,
downward to pad below, or none to inhibit padding.
The amount of padding is always just enough to reach the next
multiple of TARGET_FUNCTION_ARG_BOUNDARY; this macro does not
control it.
This macro has a default definition which is right for most systems.
For little-endian machines, the default is to pad upward. For
big-endian machines, the default is to pad downward for an argument of
constant size shorter than an int, and upward otherwise.
If defined, a C expression which determines whether the default
implementation of va_arg will attempt to pad down before reading the
next argument, if that argument is smaller than its aligned space as
controlled by PARM_BOUNDARY. If this macro is not defined, all such
arguments are padded down if BYTES_BIG_ENDIAN is true.
Specify padding for the last element of a block move between registers and
memory. first is nonzero if this is the only element. Defining this
macro allows better control of register function parameters on big-endian
machines, without using PARALLEL rtl. In particular,
MUST_PASS_IN_STACK need not test padding and mode of types in
registers, as there is no longer a "wrong" part of a register; For example,
a three byte aggregate may be passed in the high part of a register if so
required.
This hook returns the alignment boundary, in bits, of an argument
with the specified mode and type. The default hook returns
PARM_BOUNDARY for all arguments.
A C expression that is nonzero if regno is the number of a hard register in which function arguments are sometimes passed. This does not include implicit arguments such as the static chain and the structure-value address. On many machines, no registers can be used for this purpose since all function arguments are pushed on the stack.
This hook should return true if parameter of type type are passed as two scalar parameters. By default, GCC will attempt to pack complex arguments into the target’s word size. Some ABIs require complex arguments to be split and treated as their individual components. For example, on AIX64, complex floats should be passed in a pair of floating point registers, even though a complex float would fit in one 64-bit floating point register.
The default value of this hook is NULL, which is treated as always
false.
This hook returns a type node for va_list for the target.
The default version of the hook returns void*.
This target hook is used in function c_common_nodes_and_builtins
to iterate through the target specific builtin types for va_list. The
variable idx is used as iterator. pname has to be a pointer
to a const char * and ptree a pointer to a tree typed
variable.
The arguments pname and ptree are used to store the result of
this macro and are set to the name of the va_list builtin type and its
internal type.
If the return value of this macro is zero, then there is no more element.
Otherwise the IDX should be increased for the next call of this
macro to iterate through all types.
This hook returns the va_list type of the calling convention specified by
fndecl.
The default version of this hook returns va_list_type_node.
This hook returns the va_list type of the calling convention specified by the
type of type. If type is not a valid va_list type, it returns
NULL_TREE.
This hook performs target-specific gimplification of
VA_ARG_EXPR. The first two parameters correspond to the
arguments to va_arg; the latter two are as in
gimplify.c:gimplify_expr.
Define this to return nonzero if the port can handle pointers
with machine mode mode. The default version of this
hook returns true for both ptr_mode and Pmode.
Define this to return nonzero if the memory reference ref may alias with the system C library errno location. The default version of this hook assumes the system C library errno location is either a declaration of type int or accessed by dereferencing a pointer to int.
Define this to return nonzero if the port is prepared to handle insns involving scalar mode mode. For a scalar mode to be considered supported, all the basic arithmetic and comparisons must work.
The default version of this hook returns true for any mode required to handle the basic C types (as defined by the port). Included here are the double-word arithmetic supported by the code in optabs.c.
Define this to return nonzero if the port is prepared to handle insns involving vector mode mode. At the very least, it must have move patterns for this mode.
Return true if GCC should try to use a scalar mode to store an array
of nelems elements, given that each element has mode mode.
Returning true here overrides the usual MAX_FIXED_MODE limit
and allows GCC to use any defined integer mode.
One use of this hook is to support vector load and store operations that operate on several homogeneous vectors. For example, ARM NEON has operations like:
int8x8x3_t vld3_s8 (const int8_t *)
where the return type is defined as:
typedef struct int8x8x3_t
{
int8x8_t val[3];
} int8x8x3_t;
If this hook allows val to have a scalar mode, then
int8x8x3_t can have the same mode. GCC can then store
int8x8x3_ts in registers rather than forcing them onto the stack.
Define this to return nonzero for machine modes for which the port has
small register classes. If this target hook returns nonzero for a given
mode, the compiler will try to minimize the lifetime of registers
in mode. The hook may be called with VOIDmode as argument.
In this case, the hook is expected to return nonzero if it returns nonzero
for any mode.
On some machines, it is risky to let hard registers live across arbitrary insns. Typically, these machines have instructions that require values to be in specific registers (like an accumulator), and reload will fail if the required hard register is used for another purpose across such an insn.
Passes before reload do not know which hard registers will be used
in an instruction, but the machine modes of the registers set or used in
the instruction are already known. And for some machines, register
classes are small for, say, integer registers but not for floating point
registers. For example, the AMD x86-64 architecture requires specific
registers for the legacy x86 integer instructions, but there are many
SSE registers for floating point operations. On such targets, a good
strategy may be to return nonzero from this hook for INTEGRAL_MODE_P
machine modes but zero for the SSE register classes.
The default version of this hook returns false for any mode. It is always safe to redefine this hook to return with a nonzero value. But if you unnecessarily define it, you will reduce the amount of optimizations that can be performed in some cases. If you do not define this hook to return a nonzero value when it is required, the compiler will run out of spill registers and print a fatal error message.
If the target has a dedicated flags register, and it needs to use the post-reload comparison elimination pass, then this value should be set appropriately.
Next: Aggregate Return, Previous: Register Arguments, Up: Stack and Calling [Contents][Index]
This section discusses the macros that control returning scalars as values—values that can fit in registers.
Define this to return an RTX representing the place where a function
returns or receives a value of data type ret_type, a tree node
representing a data type. fn_decl_or_type is a tree node
representing FUNCTION_DECL or FUNCTION_TYPE of a
function being called. If outgoing is false, the hook should
compute the register in which the caller will see the return value.
Otherwise, the hook should return an RTX representing the place where
a function returns a value.
On many machines, only TYPE_MODE (ret_type) is relevant.
(Actually, on most machines, scalar values are returned in the same
place regardless of mode.) The value of the expression is usually a
reg RTX for the hard register where the return value is stored.
The value can also be a parallel RTX, if the return value is in
multiple places. See FUNCTION_ARG for an explanation of the
parallel form. Note that the callee will populate every
location specified in the parallel, but if the first element of
the parallel contains the whole return value, callers will use
that element as the canonical location and ignore the others. The m68k
port uses this type of parallel to return pointers in both
‘%a0’ (the canonical location) and ‘%d0’.
If TARGET_PROMOTE_FUNCTION_RETURN returns true, you must apply
the same promotion rules specified in PROMOTE_MODE if
valtype is a scalar type.
If the precise function being called is known, func is a tree
node (FUNCTION_DECL) for it; otherwise, func is a null
pointer. This makes it possible to use a different value-returning
convention for specific functions when all their calls are
known.
Some target machines have “register windows” so that the register in which a function returns its value is not the same as the one in which the caller sees the value. For such machines, you should return different RTX depending on outgoing.
TARGET_FUNCTION_VALUE is not used for return values with
aggregate data types, because these are returned in another way. See
TARGET_STRUCT_VALUE_RTX and related macros, below.
This macro has been deprecated. Use TARGET_FUNCTION_VALUE for
a new target instead.
A C expression to create an RTX representing the place where a library function returns a value of mode mode.
Note that “library function” in this context means a compiler support routine, used to perform arithmetic, whose name is known specially by the compiler and was not mentioned in the C code being compiled.
Define this hook if the back-end needs to know the name of the libcall function in order to determine where the result should be returned.
The mode of the result is given by mode and the name of the called library function is given by fun. The hook should return an RTX representing the place where the library function result will be returned.
If this hook is not defined, then LIBCALL_VALUE will be used.
A C expression that is nonzero if regno is the number of a hard register in which the values of called function may come back.
A register whose use for returning values is limited to serving as the
second of a pair (for a value of type double, say) need not be
recognized by this macro. So for most machines, this definition
suffices:
#define FUNCTION_VALUE_REGNO_P(N) ((N) == 0)
If the machine has register windows, so that the caller and the called function use different registers for the return value, this macro should recognize only the caller’s register numbers.
This macro has been deprecated. Use TARGET_FUNCTION_VALUE_REGNO_P
for a new target instead.
A target hook that return true if regno is the number of a hard
register in which the values of called function may come back.
A register whose use for returning values is limited to serving as the
second of a pair (for a value of type double, say) need not be
recognized by this target hook.
If the machine has register windows, so that the caller and the called function use different registers for the return value, this target hook should recognize only the caller’s register numbers.
If this hook is not defined, then FUNCTION_VALUE_REGNO_P will be used.
Define this macro if ‘untyped_call’ and ‘untyped_return’
need more space than is implied by FUNCTION_VALUE_REGNO_P for
saving and restoring an arbitrary return value.
This hook should return true if values of type type are returned at the most significant end of a register (in other words, if they are padded at the least significant end). You can assume that type is returned in a register; the caller is required to check this.
Note that the register provided by TARGET_FUNCTION_VALUE must
be able to hold the complete return value. For example, if a 1-, 2-
or 3-byte structure is returned at the most significant end of a
4-byte register, TARGET_FUNCTION_VALUE should provide an
SImode rtx.
Next: Caller Saves, Previous: Scalar Return, Up: Stack and Calling [Contents][Index]
When a function value’s mode is BLKmode (and in some other
cases), the value is not returned according to
TARGET_FUNCTION_VALUE (see Scalar Return). Instead, the
caller passes the address of a block of memory in which the value
should be stored. This address is called the structure value
address.
This section describes how to control returning structure values in memory.
This target hook should return a nonzero value to say to return the
function value in memory, just as large structures are always returned.
Here type will be the data type of the value, and fntype
will be the type of the function doing the returning, or NULL for
libcalls.
Note that values of mode BLKmode must be explicitly handled
by this function. Also, the option -fpcc-struct-return
takes effect regardless of this macro. On most systems, it is
possible to leave the hook undefined; this causes a default
definition to be used, whose value is the constant 1 for BLKmode
values, and 0 otherwise.
Do not use this hook to indicate that structures and unions should always
be returned in memory. You should instead use DEFAULT_PCC_STRUCT_RETURN
to indicate this.
Define this macro to be 1 if all structure and union return values must be
in memory. Since this results in slower code, this should be defined
only if needed for compatibility with other compilers or with an ABI.
If you define this macro to be 0, then the conventions used for structure
and union return values are decided by the TARGET_RETURN_IN_MEMORY
target hook.
If not defined, this defaults to the value 1.
This target hook should return the location of the structure value
address (normally a mem or reg), or 0 if the address is
passed as an “invisible” first argument. Note that fndecl may
be NULL, for libcalls. You do not need to define this target
hook if the address is always passed as an “invisible” first
argument.
On some architectures the place where the structure value address
is found by the called function is not the same place that the
caller put it. This can be due to register windows, or it could
be because the function prologue moves it to a different place.
incoming is 1 or 2 when the location is needed in
the context of the called function, and 0 in the context of
the caller.
If incoming is nonzero and the address is to be found on the
stack, return a mem which refers to the frame pointer. If
incoming is 2, the result is being used to fetch the
structure value address at the beginning of a function. If you need
to emit adjusting code, you should do it at this point.
Define this macro if the usual system convention on the target machine for returning structures and unions is for the called function to return the address of a static variable containing the value.
Do not define this if the usual system convention is for the caller to pass an address to the subroutine.
This macro has effect in -fpcc-struct-return mode, but it does nothing when you use -freg-struct-return mode.
This target hook returns the mode to be used when accessing raw return registers in __builtin_return. Define this macro if the value in reg_raw_mode is not correct.
This target hook returns the mode to be used when accessing raw argument registers in __builtin_apply_args. Define this macro if the value in reg_raw_mode is not correct.
Next: Function Entry, Previous: Aggregate Return, Up: Stack and Calling [Contents][Index]
If you enable it, GCC can save registers around function calls. This makes it possible to use call-clobbered registers to hold variables that must live across calls.
A C expression to determine whether it is worthwhile to consider placing a pseudo-register in a call-clobbered hard register and saving and restoring it around each function call. The expression should be 1 when this is worth doing, and 0 otherwise.
If you don’t define this macro, a default is used which is good on most
machines: 4 * calls < refs.
A C expression specifying which mode is required for saving nregs
of a pseudo-register in call-clobbered hard register regno. If
regno is unsuitable for caller save, VOIDmode should be
returned. For most machines this macro need not be defined since GCC
will select the smallest suitable mode.
Next: Profiling, Previous: Caller Saves, Up: Stack and Calling [Contents][Index]
This section describes the macros that output function entry (prologue) and exit (epilogue) code.
If defined, a function that outputs the assembler code for entry to a function. The prologue is responsible for setting up the stack frame, initializing the frame pointer register, saving registers that must be saved, and allocating size additional bytes of storage for the local variables. size is an integer. file is a stdio stream to which the assembler code should be output.
The label for the beginning of the function need not be output by this macro. That has already been done when the macro is run.
To determine which registers to save, the macro can refer to the array
regs_ever_live: element r is nonzero if hard register
r is used anywhere within the function. This implies the function
prologue should save register r, provided it is not one of the
call-used registers. (TARGET_ASM_FUNCTION_EPILOGUE must likewise use
regs_ever_live.)
On machines that have “register windows”, the function entry code does not save on the stack the registers that are in the windows, even if they are supposed to be preserved by function calls; instead it takes appropriate steps to “push” the register stack, if any non-call-used registers are used in the function.
On machines where functions may or may not have frame-pointers, the
function entry code must vary accordingly; it must set up the frame
pointer if one is wanted, and not otherwise. To determine whether a
frame pointer is in wanted, the macro can refer to the variable
frame_pointer_needed. The variable’s value will be 1 at run
time in a function that needs a frame pointer. See Elimination.
The function entry code is responsible for allocating any stack space
required for the function. This stack space consists of the regions
listed below. In most cases, these regions are allocated in the
order listed, with the last listed region closest to the top of the
stack (the lowest address if STACK_GROWS_DOWNWARD is defined, and
the highest address if it is not defined). You can use a different order
for a machine if doing so is more convenient or required for
compatibility reasons. Except in cases where required by standard
or by a debugger, there is no reason why the stack layout used by GCC
need agree with that used by other compilers for a machine.
If defined, a function that outputs assembler code at the end of a prologue. This should be used when the function prologue is being emitted as RTL, and you have some extra assembler that needs to be emitted. See prologue instruction pattern.
If defined, a function that outputs assembler code at the start of an epilogue. This should be used when the function epilogue is being emitted as RTL, and you have some extra assembler that needs to be emitted. See epilogue instruction pattern.
If defined, a function that outputs the assembler code for exit from a
function. The epilogue is responsible for restoring the saved
registers and stack pointer to their values when the function was
called, and returning control to the caller. This macro takes the
same arguments as the macro TARGET_ASM_FUNCTION_PROLOGUE, and the
registers to restore are determined from regs_ever_live and
CALL_USED_REGISTERS in the same way.
On some machines, there is a single instruction that does all the work
of returning from the function. On these machines, give that
instruction the name ‘return’ and do not define the macro
TARGET_ASM_FUNCTION_EPILOGUE at all.
Do not define a pattern named ‘return’ if you want the
TARGET_ASM_FUNCTION_EPILOGUE to be used. If you want the target
switches to control whether return instructions or epilogues are used,
define a ‘return’ pattern with a validity condition that tests the
target switches appropriately. If the ‘return’ pattern’s validity
condition is false, epilogues will be used.
On machines where functions may or may not have frame-pointers, the
function exit code must vary accordingly. Sometimes the code for these
two cases is completely different. To determine whether a frame pointer
is wanted, the macro can refer to the variable
frame_pointer_needed. The variable’s value will be 1 when compiling
a function that needs a frame pointer.
Normally, TARGET_ASM_FUNCTION_PROLOGUE and
TARGET_ASM_FUNCTION_EPILOGUE must treat leaf functions specially.
The C variable current_function_is_leaf is nonzero for such a
function. See Leaf Functions.
On some machines, some functions pop their arguments on exit while others leave that for the caller to do. For example, the 68020 when given -mrtd pops arguments in functions that take a fixed number of arguments.
Your definition of the macro RETURN_POPS_ARGS decides which
functions pop their own arguments. TARGET_ASM_FUNCTION_EPILOGUE
needs to know what was decided. The number of bytes of the current
function’s arguments that this function should pop is available in
crtl->args.pops_args. See Scalar Return.
current_function_pretend_args_size bytes of
uninitialized space just underneath the first argument arriving on the
stack. (This may not be at the very start of the allocated stack region
if the calling sequence has pushed anything else since pushing the stack
arguments. But usually, on such machines, nothing else has been pushed
yet, because the function prologue itself does all the pushing.) This
region is used on machines where an argument may be passed partly in
registers and partly in memory, and, in some cases to support the
features in <stdarg.h>.
ACCUMULATE_OUTGOING_ARGS is defined, a region of
current_function_outgoing_args_size bytes to be used for outgoing
argument lists of the function. See Stack Arguments.
Define this macro as a C expression that is nonzero if the return instruction or the function epilogue ignores the value of the stack pointer; in other words, if it is safe to delete an instruction to adjust the stack pointer before a return from the function. The default is 0.
Note that this macro’s value is relevant only for functions for which
frame pointers are maintained. It is never safe to delete a final
stack adjustment in a function that has no frame pointer, and the
compiler knows this regardless of EXIT_IGNORE_STACK.
Define this macro as a C expression that is nonzero for registers that are used by the epilogue or the ‘return’ pattern. The stack and frame pointer registers are already assumed to be used as needed.
Define this macro as a C expression that is nonzero for registers that are used by the exception handling mechanism, and so should be considered live on entry to an exception edge.
Define this macro if the function epilogue contains delay slots to which instructions from the rest of the function can be “moved”. The definition should be a C expression whose value is an integer representing the number of delay slots there.
A C expression that returns 1 if insn can be placed in delay slot number n of the epilogue.
The argument n is an integer which identifies the delay slot now
being considered (since different slots may have different rules of
eligibility). It is never negative and is always less than the number
of epilogue delay slots (what DELAY_SLOTS_FOR_EPILOGUE returns).
If you reject a particular insn for a given delay slot, in principle, it
may be reconsidered for a subsequent delay slot. Also, other insns may
(at least in principle) be considered for the so far unfilled delay
slot.
The insns accepted to fill the epilogue delay slots are put in an RTL
list made with insn_list objects, stored in the variable
current_function_epilogue_delay_list. The insn for the first
delay slot comes first in the list. Your definition of the macro
TARGET_ASM_FUNCTION_EPILOGUE should fill the delay slots by
outputting the insns in this list, usually by calling
final_scan_insn.
You need not define this macro if you did not define
DELAY_SLOTS_FOR_EPILOGUE.
A function that outputs the assembler code for a thunk function, used to implement C++ virtual function calls with multiple inheritance. The thunk acts as a wrapper around a virtual function, adjusting the implicit object parameter before handing control off to the real function.
First, emit code to add the integer delta to the location that
contains the incoming first argument. Assume that this argument
contains a pointer, and is the one used to pass the this pointer
in C++. This is the incoming argument before the function prologue,
e.g. ‘%o0’ on a sparc. The addition must preserve the values of
all other incoming arguments.
Then, if vcall_offset is nonzero, an additional adjustment should be
made after adding delta. In particular, if p is the
adjusted pointer, the following adjustment should be made:
p += (*((ptrdiff_t **)p))[vcall_offset/sizeof(ptrdiff_t)]
After the additions, emit code to jump to function, which is a
FUNCTION_DECL. This is a direct pure jump, not a call, and does
not touch the return address. Hence returning from FUNCTION will
return to whoever called the current ‘thunk’.
The effect must be as if function had been called directly with
the adjusted first argument. This macro is responsible for emitting all
of the code for a thunk function; TARGET_ASM_FUNCTION_PROLOGUE
and TARGET_ASM_FUNCTION_EPILOGUE are not invoked.
The thunk_fndecl is redundant. (delta and function have already been extracted from it.) It might possibly be useful on some targets, but probably not.
If you do not define this macro, the target-independent code in the C++ front end will generate a less efficient heavyweight thunk that calls function instead of jumping to it. The generic approach does not support varargs.
A function that returns true if TARGET_ASM_OUTPUT_MI_THUNK would be able to output the assembler code for the thunk function specified by the arguments it is passed, and false otherwise. In the latter case, the generic approach will be used by the C++ front end, with the limitations previously exposed.
Next: Tail Calls, Previous: Function Entry, Up: Stack and Calling [Contents][Index]
These macros will help you generate code for profiling.
A C statement or compound statement to output to file some
assembler code to call the profiling subroutine mcount.
The details of how mcount expects to be called are determined by
your operating system environment, not by GCC. To figure them out,
compile a small program for profiling using the system’s installed C
compiler and look at the assembler code that results.
Older implementations of mcount expect the address of a counter
variable to be loaded into some register. The name of this variable is
‘LP’ followed by the number labelno, so you would generate
the name using ‘LP%d’ in a fprintf.
A C statement or compound statement to output to file some assembly
code to call the profiling subroutine mcount even the target does
not support profiling.
Define this macro to be an expression with a nonzero value if the
mcount subroutine on your system does not need a counter variable
allocated for each function. This is true for almost all modern
implementations. If you define this macro, you must not use the
labelno argument to FUNCTION_PROFILER.
Define this macro if the code for function profiling should come before the function prologue. Normally, the profiling code comes after.
Next: Stack Smashing Protection, Previous: Profiling, Up: Stack and Calling [Contents][Index]
True if it is ok to do sibling call optimization for the specified
call expression exp. decl will be the called function,
or NULL if this is an indirect call.
It is not uncommon for limitations of calling conventions to prevent
tail calls to functions outside the current unit of translation, or
during PIC compilation. The hook is used to enforce these restrictions,
as the sibcall md pattern can not fail, or fall over to a
“normal” call. The criteria for successful sibling call optimization
may vary greatly between different architectures.
Add any hard registers to regs that are live on entry to the function. This hook only needs to be defined to provide registers that cannot be found by examination of FUNCTION_ARG_REGNO_P, the callee saved registers, STATIC_CHAIN_INCOMING_REGNUM, STATIC_CHAIN_REGNUM, TARGET_STRUCT_VALUE_RTX, FRAME_POINTER_REGNUM, EH_USES, FRAME_POINTER_REGNUM, ARG_POINTER_REGNUM, and the PIC_OFFSET_TABLE_REGNUM.
Previous: Tail Calls, Up: Stack and Calling [Contents][Index]
This hook returns a DECL node for the external variable to use
for the stack protection guard. This variable is initialized by the
runtime to some random value and is used to initialize the guard value
that is placed at the top of the local stack frame. The type of this
variable must be ptr_type_node.
The default version of this hook creates a variable called ‘__stack_chk_guard’, which is normally defined in libgcc2.c.
This hook returns a tree expression that alerts the runtime that the
stack protect guard variable has been modified. This expression should
involve a call to a noreturn function.
The default version of this hook invokes a function called ‘__stack_chk_fail’, taking no arguments. This function is normally defined in libgcc2.c.
Whether this target supports splitting the stack when the options described in opts have been passed. This is called after options have been parsed, so the target may reject splitting the stack in some configurations. The default version of this hook returns false. If report is true, this function may issue a warning or error; if report is false, it must simply return a value
Next: Trampolines, Previous: Stack and Calling, Up: Target Macros [Contents][Index]
GCC comes with an implementation of <varargs.h> and
<stdarg.h> that work without change on machines that pass arguments
on the stack. Other machines require their own implementations of
varargs, and the two machine independent header files must have
conditionals to include it.
ISO <stdarg.h> differs from traditional <varargs.h> mainly in
the calling convention for va_start. The traditional
implementation takes just one argument, which is the variable in which
to store the argument pointer. The ISO implementation of
va_start takes an additional second argument. The user is
supposed to write the last named argument of the function here.
However, va_start should not use this argument. The way to find
the end of the named arguments is with the built-in functions described
below.
Use this built-in function to save the argument registers in memory so
that the varargs mechanism can access them. Both ISO and traditional
versions of va_start must use __builtin_saveregs, unless
you use TARGET_SETUP_INCOMING_VARARGS (see below) instead.
On some machines, __builtin_saveregs is open-coded under the
control of the target hook TARGET_EXPAND_BUILTIN_SAVEREGS. On
other machines, it calls a routine written in assembler language,
found in libgcc2.c.
Code generated for the call to __builtin_saveregs appears at the
beginning of the function, as opposed to where the call to
__builtin_saveregs is written, regardless of what the code is.
This is because the registers must be saved before the function starts
to use them for its own purposes.
This builtin returns the address of the first anonymous stack
argument, as type void *. If ARGS_GROW_DOWNWARD, it
returns the address of the location above the first anonymous stack
argument. Use it in va_start to initialize the pointer for
fetching arguments from the stack. Also use it in va_start to
verify that the second parameter lastarg is the last named argument
of the current function.
Since each machine has its own conventions for which data types are
passed in which kind of register, your implementation of va_arg
has to embody these conventions. The easiest way to categorize the
specified data type is to use __builtin_classify_type together
with sizeof and __alignof__.
__builtin_classify_type ignores the value of object,
considering only its data type. It returns an integer describing what
kind of type that is—integer, floating, pointer, structure, and so on.
The file typeclass.h defines an enumeration that you can use to
interpret the values of __builtin_classify_type.
These machine description macros help implement varargs:
If defined, this hook produces the machine-specific code for a call to
__builtin_saveregs. This code will be moved to the very
beginning of the function, before any parameter access are made. The
return value of this function should be an RTX that contains the value
to use as the return of __builtin_saveregs.
This target hook offers an alternative to using
__builtin_saveregs and defining the hook
TARGET_EXPAND_BUILTIN_SAVEREGS. Use it to store the anonymous
register arguments into the stack so that all the arguments appear to
have been passed consecutively on the stack. Once this is done, you can
use the standard implementation of varargs that works for machines that
pass all their arguments on the stack.
The argument args_so_far points to the CUMULATIVE_ARGS data
structure, containing the values that are obtained after processing the
named arguments. The arguments mode and type describe the
last named argument—its machine mode and its data type as a tree node.
The target hook should do two things: first, push onto the stack all the
argument registers not used for the named arguments, and second,
store the size of the data thus pushed into the int-valued
variable pointed to by pretend_args_size. The value that you
store here will serve as additional offset for setting up the stack
frame.
Because you must generate code to push the anonymous arguments at
compile time without knowing their data types,
TARGET_SETUP_INCOMING_VARARGS is only useful on machines that
have just a single category of argument register and use it uniformly
for all data types.
If the argument second_time is nonzero, it means that the
arguments of the function are being analyzed for the second time. This
happens for an inline function, which is not actually compiled until the
end of the source file. The hook TARGET_SETUP_INCOMING_VARARGS should
not generate any instructions in this case.
Define this hook to return true if the location where a function
argument is passed depends on whether or not it is a named argument.
This hook controls how the named argument to FUNCTION_ARG
is set for varargs and stdarg functions. If this hook returns
true, the named argument is always true for named
arguments, and false for unnamed arguments. If it returns false,
but TARGET_PRETEND_OUTGOING_VARARGS_NAMED returns true,
then all arguments are treated as named. Otherwise, all named arguments
except the last are treated as named.
You need not define this hook if it always returns false.
If you need to conditionally change ABIs so that one works with
TARGET_SETUP_INCOMING_VARARGS, but the other works like neither
TARGET_SETUP_INCOMING_VARARGS nor TARGET_STRICT_ARGUMENT_NAMING was
defined, then define this hook to return true if
TARGET_SETUP_INCOMING_VARARGS is used, false otherwise.
Otherwise, you should not define this hook.
Next: Library Calls, Previous: Varargs, Up: Target Macros [Contents][Index]
A trampoline is a small piece of code that is created at run time when the address of a nested function is taken. It normally resides on the stack, in the stack frame of the containing function. These macros tell GCC how to generate code to allocate and initialize a trampoline.
The instructions in the trampoline must do two things: load a constant address into the static chain register, and jump to the real address of the nested function. On CISC machines such as the m68k, this requires two instructions, a move immediate and a jump. Then the two addresses exist in the trampoline as word-long immediate operands. On RISC machines, it is often necessary to load each address into a register in two parts. Then pieces of each address form separate immediate operands.
The code generated to initialize the trampoline must store the variable parts—the static chain value and the function address—into the immediate operands of the instructions. On a CISC machine, this is simply a matter of copying each address to a memory reference at the proper offset from the start of the trampoline. On a RISC machine, it may be necessary to take out pieces of the address and store them separately.
This hook is called by assemble_trampoline_template to output,
on the stream f, assembler code for a block of data that contains
the constant parts of a trampoline. This code should not include a
label—the label is taken care of automatically.
If you do not define this hook, it means no template is needed for the target. Do not define this hook on systems where the block move code to copy the trampoline into place would be larger than the code to generate it on the spot.
Return the section into which the trampoline template is to be placed
(see Sections). The default value is readonly_data_section.
A C expression for the size in bytes of the trampoline, as an integer.
Alignment required for trampolines, in bits.
If you don’t define this macro, the value of FUNCTION_ALIGNMENT
is used for aligning trampolines.
This hook is called to initialize a trampoline.
m_tramp is an RTX for the memory block for the trampoline; fndecl
is the FUNCTION_DECL for the nested function; static_chain is an
RTX for the static chain value that should be passed to the function
when it is called.
If the target defines TARGET_ASM_TRAMPOLINE_TEMPLATE, then the
first thing this hook should do is emit a block move into m_tramp
from the memory block returned by assemble_trampoline_template.
Note that the block move need only cover the constant parts of the
trampoline. If the target isolates the variable parts of the trampoline
to the end, not all TRAMPOLINE_SIZE bytes need be copied.
If the target requires any other actions, such as flushing caches or enabling stack execution, these actions should be performed after initializing the trampoline proper.
This hook should perform any machine-specific adjustment in
the address of the trampoline. Its argument contains the address of the
memory block that was passed to TARGET_TRAMPOLINE_INIT. In case
the address to be used for a function call should be different from the
address at which the template was stored, the different address should
be returned; otherwise addr should be returned unchanged.
If this hook is not defined, addr will be used for function calls.
Implementing trampolines is difficult on many machines because they have separate instruction and data caches. Writing into a stack location fails to clear the memory in the instruction cache, so when the program jumps to that location, it executes the old contents.
Here are two possible solutions. One is to clear the relevant parts of the instruction cache whenever a trampoline is set up. The other is to make all trampolines identical, by having them jump to a standard subroutine. The former technique makes trampoline execution faster; the latter makes initialization faster.
To clear the instruction cache when a trampoline is initialized, define the following macro.
If defined, expands to a C expression clearing the instruction
cache in the specified interval. The definition of this macro would
typically be a series of asm statements. Both beg and
end are both pointer expressions.
The operating system may also require the stack to be made executable before calling the trampoline. To implement this requirement, define the following macro.
Define this macro if certain operations must be performed before executing
code located on the stack. The macro should expand to a series of C
file-scope constructs (e.g. functions) and provide a unique entry point
named __enable_execute_stack. The target is responsible for
emitting calls to the entry point in the code, for example from the
TARGET_TRAMPOLINE_INIT hook.
To use a standard subroutine, define the following macro. In addition, you must make sure that the instructions in a trampoline fill an entire cache line with identical instructions, or else ensure that the beginning of the trampoline code is always aligned at the same point in its cache line. Look in m68k.h as a guide.
Define this macro if trampolines need a special subroutine to do their
work. The macro should expand to a series of asm statements
which will be compiled with GCC. They go in a library function named
__transfer_from_trampoline.
If you need to avoid executing the ordinary prologue code of a compiled
C function when you jump to the subroutine, you can do so by placing a
special label of your own in the assembler code. Use one asm
statement to generate an assembler label, and another to make the label
global. Then trampolines can use that label to jump directly to your
special assembler code.
Next: Addressing Modes, Previous: Trampolines, Up: Target Macros [Contents][Index]
Here is an explanation of implicit calls to library routines.
This macro, if defined, should expand to a piece of C code that will get expanded when compiling functions for libgcc.a. It can be used to provide alternate names for GCC’s internal library functions if there are ABI-mandated names that the compiler should provide.
This hook should declare additional library routines or rename
existing ones, using the functions set_optab_libfunc and
init_one_libfunc defined in optabs.c.
init_optabs calls this macro after initializing all the normal
library routines.
The default is to do nothing. Most ports don’t need to define this hook.
This macro should return true if the library routine that
implements the floating point comparison operator comparison in
mode mode will return a boolean, and false if it will
return a tristate.
GCC’s own floating point libraries return tristates from the comparison operators, so the default returns false always. Most ports don’t need to define this macro.
This macro should evaluate to true if the integer comparison
functions (like __cmpdi2) return 0 to indicate that the first
operand is smaller than the second, 1 to indicate that they are equal,
and 2 to indicate that the first operand is greater than the second.
If this macro evaluates to false the comparison functions return
-1, 0, and 1 instead of 0, 1, and 2. If the target uses the routines
in libgcc.a, you do not need to define this macro.
The value of EDOM on the target machine, as a C integer constant
expression. If you don’t define this macro, GCC does not attempt to
deposit the value of EDOM into errno directly. Look in
/usr/include/errno.h to find the value of EDOM on your
system.
If you do not define TARGET_EDOM, then compiled code reports
domain errors by calling the library function and letting it report the
error. If mathematical functions on your system use matherr when
there is an error, then you should leave TARGET_EDOM undefined so
that matherr is used normally.
Define this macro as a C expression to create an rtl expression that
refers to the global “variable” errno. (On certain systems,
errno may not actually be a variable.) If you don’t define this
macro, a reasonable default is used.
When this macro is nonzero, GCC will implicitly optimize sin calls into
sinf and similarly for other functions defined by C99 standard. The
default is zero because a number of existing systems lack support for these
functions in their runtime so this macro needs to be redefined to one on
systems that do support the C99 runtime.
When this macro is nonzero, GCC will implicitly optimize calls to sin
and cos with the same argument to a call to sincos. The
default is zero. The target has to provide the following functions:
void sincos(double x, double *sin, double *cos); void sincosf(float x, float *sin, float *cos); void sincosl(long double x, long double *sin, long double *cos);
Define this macro to generate code for Objective-C message sending using the calling convention of the NeXT system. This calling convention involves passing the object, the selector and the method arguments all at once to the method-lookup library function.
The default calling convention passes just the object and the selector to the lookup function, which returns a pointer to the method.
Next: Anchored Addresses, Previous: Library Calls, Up: Target Macros [Contents][Index]
This is about addressing modes.
A C expression that is nonzero if the machine supports pre-increment, pre-decrement, post-increment, or post-decrement addressing respectively.
A C expression that is nonzero if the machine supports pre- or post-address side-effect generation involving constants other than the size of the memory operand.
A C expression that is nonzero if the machine supports pre- or post-address side-effect generation involving a register displacement.
A C expression that is 1 if the RTX x is a constant which
is a valid address. On most machines the default definition of
(CONSTANT_P (x) && GET_CODE (x) != CONST_DOUBLE)
is acceptable, but a few machines are more restrictive as to which
constant addresses are supported.
CONSTANT_P, which is defined by target-independent code,
accepts integer-values expressions whose values are not explicitly
known, such as symbol_ref, label_ref, and high
expressions and const arithmetic expressions, in addition to
const_int and const_double expressions.
A number, the maximum number of registers that can appear in a valid
memory address. Note that it is up to you to specify a value equal to
the maximum number that TARGET_LEGITIMATE_ADDRESS_P would ever
accept.
A function that returns whether x (an RTX) is a legitimate memory address on the target machine for a memory operand of mode mode.
Legitimate addresses are defined in two variants: a strict variant and a non-strict one. The strict parameter chooses which variant is desired by the caller.
The strict variant is used in the reload pass. It must be defined so
that any pseudo-register that has not been allocated a hard register is
considered a memory reference. This is because in contexts where some
kind of register is required, a pseudo-register with no hard register
must be rejected. For non-hard registers, the strict variant should look
up the reg_renumber array; it should then proceed using the hard
register number in the array, or treat the pseudo as a memory reference
if the array holds -1.
The non-strict variant is used in other passes. It must be defined to accept all pseudo-registers in every context where some kind of register is required.
Normally, constant addresses which are the sum of a symbol_ref
and an integer are stored inside a const RTX to mark them as
constant. Therefore, there is no need to recognize such sums
specifically as legitimate addresses. Normally you would simply
recognize any const as legitimate.
Usually PRINT_OPERAND_ADDRESS is not prepared to handle constant
sums that are not marked with const. It assumes that a naked
plus indicates indexing. If so, then you must reject such
naked constant sums as illegitimate addresses, so that none of them will
be given to PRINT_OPERAND_ADDRESS.
On some machines, whether a symbolic address is legitimate depends on
the section that the address refers to. On these machines, define the
target hook TARGET_ENCODE_SECTION_INFO to store the information
into the symbol_ref, and then check for it here. When you see a
const, you will have to look inside it to find the
symbol_ref in order to determine the section. See Assembler Format.
Some ports are still using a deprecated legacy substitute for
this hook, the GO_IF_LEGITIMATE_ADDRESS macro. This macro
has this syntax:
#define GO_IF_LEGITIMATE_ADDRESS (mode, x, label)
and should goto label if the address x is a valid
address on the target machine for a memory operand of mode mode.
Compiler source files that want to use the strict variant of this
macro define the macro REG_OK_STRICT. You should use an
#ifdef REG_OK_STRICT conditional to define the strict variant in
that case and the non-strict variant otherwise.
Using the hook is usually simpler because it limits the number of files that are recompiled when changes are made.
A single character to be used instead of the default 'm'
character for general memory addresses. This defines the constraint
letter which matches the memory addresses accepted by
TARGET_LEGITIMATE_ADDRESS_P. Define this macro if you want to
support new address formats in your back end without changing the
semantics of the 'm' constraint. This is necessary in order to
preserve functionality of inline assembly constructs using the
'm' constraint.
A C expression to determine the base term of address x,
or to provide a simplified version of x from which alias.c
can easily find the base term. This macro is used in only two places:
find_base_value and find_base_term in alias.c.
It is always safe for this macro to not be defined. It exists so that alias analysis can understand machine-dependent addresses.
The typical use of this macro is to handle addresses containing a label_ref or symbol_ref within an UNSPEC.
This hook is given an invalid memory address x for an operand of mode mode and should try to return a valid memory address.
x will always be the result of a call to break_out_memory_refs,
and oldx will be the operand that was given to that function to produce
x.
The code of the hook should not alter the substructure of x. If it transforms x into a more legitimate form, it should return the new x.
It is not necessary for this hook to come up with a legitimate address. The compiler has standard ways of doing so in all cases. In fact, it is safe to omit this hook or make it return x if it cannot find a valid way to legitimize the address. But often a machine-dependent strategy can generate better code.
A C compound statement that attempts to replace x, which is an address that needs reloading, with a valid memory address for an operand of mode mode. win will be a C statement label elsewhere in the code. It is not necessary to define this macro, but it might be useful for performance reasons.
For example, on the i386, it is sometimes possible to use a single
reload register instead of two by reloading a sum of two pseudo
registers into a register. On the other hand, for number of RISC
processors offsets are limited so that often an intermediate address
needs to be generated in order to address a stack slot. By defining
LEGITIMIZE_RELOAD_ADDRESS appropriately, the intermediate addresses
generated for adjacent some stack slots can be made identical, and thus
be shared.
Note: This macro should be used with caution. It is necessary to know something of how reload works in order to effectively use this, and it is quite easy to produce macros that build in too much knowledge of reload internals.
Note: This macro must be able to reload an address created by a previous invocation of this macro. If it fails to handle such addresses then the compiler may generate incorrect code or abort.
The macro definition should use push_reload to indicate parts that
need reloading; opnum, type and ind_levels are usually
suitable to be passed unaltered to push_reload.
The code generated by this macro must not alter the substructure of
x. If it transforms x into a more legitimate form, it
should assign x (which will always be a C variable) a new value.
This also applies to parts that you change indirectly by calling
push_reload.
The macro definition may use strict_memory_address_p to test if
the address has become legitimate.
If you want to change only a part of x, one standard way of doing
this is to use copy_rtx. Note, however, that it unshares only a
single level of rtl. Thus, if the part to be changed is not at the
top level, you’ll need to replace first the top level.
It is not necessary for this macro to come up with a legitimate
address; but often a machine-dependent strategy can generate better code.
This hook returns true if memory address addr can have
different meanings depending on the machine mode of the memory
reference it is used for or if the address is valid for some modes
but not others.
Autoincrement and autodecrement addresses typically have mode-dependent effects because the amount of the increment or decrement is the size of the operand being addressed. Some machines have other mode-dependent addresses. Many RISC machines have no mode-dependent addresses.
You may assume that addr is a valid address for the machine.
The default version of this hook returns false.
A C statement or compound statement with a conditional goto
label; executed if memory address x (an RTX) can have
different meanings depending on the machine mode of the memory
reference it is used for or if the address is valid for some modes
but not others.
Autoincrement and autodecrement addresses typically have mode-dependent effects because the amount of the increment or decrement is the size of the operand being addressed. Some machines have other mode-dependent addresses. Many RISC machines have no mode-dependent addresses.
You may assume that addr is a valid address for the machine.
These are obsolete macros, replaced by the
TARGET_MODE_DEPENDENT_ADDRESS_P target hook.
This hook returns true if x is a legitimate constant for a
mode-mode immediate operand on the target machine. You can assume that
x satisfies CONSTANT_P, so you need not check this.
The default definition returns true.
This hook is used to undo the possibly obfuscating effects of the
LEGITIMIZE_ADDRESS and LEGITIMIZE_RELOAD_ADDRESS target
macros. Some backend implementations of these macros wrap symbol
references inside an UNSPEC rtx to represent PIC or similar
addressing modes. This target hook allows GCC’s optimizers to understand
the semantics of these opaque UNSPECs by converting them back
into their original form.
This hook should return true if x is of a form that cannot (or should not) be spilled to the constant pool. The default version of this hook returns false.
The primary reason to define this hook is to prevent reload from deciding that a non-legitimate constant would be better reloaded from the constant pool instead of spilling and reloading a register holding the constant. This restriction is often true of addresses of TLS symbols for various targets.
This hook should return true if pool entries for constant x can
be placed in an object_block structure. mode is the mode
of x.
The default version returns false for all constants.
This hook should return the DECL of a function that implements reciprocal of
the builtin function with builtin function code fn, or
NULL_TREE if such a function is not available. md_fn is true
when fn is a code of a machine-dependent builtin function. When
sqrt is true, additional optimizations that apply only to the reciprocal
of a square root function are performed, and only reciprocals of sqrt
function are valid.
This hook should return the DECL of a function f that given an address addr as an argument returns a mask m that can be used to extract from two vectors the relevant data that resides in addr in case addr is not properly aligned.
The autovectorizer, when vectorizing a load operation from an address
addr that may be unaligned, will generate two vector loads from
the two aligned addresses around addr. It then generates a
REALIGN_LOAD operation to extract the relevant data from the
two loaded vectors. The first two arguments to REALIGN_LOAD,
v1 and v2, are the two vectors, each of size VS, and
the third argument, OFF, defines how the data will be extracted
from these two vectors: if OFF is 0, then the returned vector is
v2; otherwise, the returned vector is composed from the last
VS-OFF elements of v1 concatenated to the first
OFF elements of v2.
If this hook is defined, the autovectorizer will generate a call
to f (using the DECL tree that this hook returns) and will
use the return value of f as the argument OFF to
REALIGN_LOAD. Therefore, the mask m returned by f
should comply with the semantics expected by REALIGN_LOAD
described above.
If this hook is not defined, then addr will be used as
the argument OFF to REALIGN_LOAD, in which case the low
log2(VS) - 1 bits of addr will be considered.
This hook should return the DECL of a function f that implements widening multiplication of the even elements of two input vectors of type x.
If this hook is defined, the autovectorizer will use it along with the
TARGET_VECTORIZE_BUILTIN_MUL_WIDEN_ODD target hook when vectorizing
widening multiplication in cases that the order of the results does not have to be
preserved (e.g. used only by a reduction computation). Otherwise, the
widen_mult_hi/lo idioms will be used.
This hook should return the DECL of a function f that implements widening multiplication of the odd elements of two input vectors of type x.
If this hook is defined, the autovectorizer will use it along with the
TARGET_VECTORIZE_BUILTIN_MUL_WIDEN_EVEN target hook when vectorizing
widening multiplication in cases that the order of the results does not have to be
preserved (e.g. used only by a reduction computation). Otherwise, the
widen_mult_hi/lo idioms will be used.
Returns cost of different scalar or vector statements for vectorization cost model. For vector memory operations the cost may depend on type (vectype) and misalignment value (misalign).
Return true if vector alignment is reachable (by peeling N iterations) for the given type.
Target builtin that implements vector permute.
Return true if a vector created for builtin_vec_perm is valid.
This hook should return the DECL of a function that implements conversion of the
input vector of type src_type to type dest_type.
The value of code is one of the enumerators in enum tree_code and
specifies how the conversion is to be applied
(truncation, rounding, etc.).
If this hook is defined, the autovectorizer will use the
TARGET_VECTORIZE_BUILTIN_CONVERSION target hook when vectorizing
conversion. Otherwise, it will return NULL_TREE.
This hook should return the decl of a function that implements the
vectorized variant of the builtin function with builtin function code
code or NULL_TREE if such a function is not available.
The value of fndecl is the builtin function declaration. The
return type of the vectorized function shall be of vector type
vec_type_out and the argument types should be vec_type_in.
This hook should return true if the target supports misaligned vector store/load of a specific factor denoted in the misalignment parameter. The vector store/load should be of machine mode mode and the elements in the vectors should be of type type. is_packed parameter is true if the memory access is defined in a packed struct.
This hook should return the preferred mode for vectorizing scalar
mode mode. The default is
equal to word_mode, because the vectorizer can do some
transformations even in absence of specialized SIMD hardware.
This hook should return a mask of sizes that should be iterated over
after trying to autovectorize using the vector size derived from the
mode returned by TARGET_VECTORIZE_PREFERRED_SIMD_MODE.
The default is zero which means to not iterate over other vector sizes.
Next: Condition Code, Previous: Addressing Modes, Up: Target Macros [Contents][Index]
GCC usually addresses every static object as a separate entity. For example, if we have:
static int a, b, c;
int foo (void) { return a + b + c; }
the code for foo will usually calculate three separate symbolic
addresses: those of a, b and c. On some targets,
it would be better to calculate just one symbolic address and access
the three variables relative to it. The equivalent pseudocode would
be something like:
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
(which isn’t valid C). We refer to shared addresses like x as
“section anchors”. Their use is controlled by -fsection-anchors.
The hooks below describe the target properties that GCC needs to know
in order to make effective use of section anchors. It won’t use
section anchors at all unless either TARGET_MIN_ANCHOR_OFFSET
or TARGET_MAX_ANCHOR_OFFSET is set to a nonzero value.
The minimum offset that should be applied to a section anchor. On most targets, it should be the smallest offset that can be applied to a base register while still giving a legitimate address for every mode. The default value is 0.
Like TARGET_MIN_ANCHOR_OFFSET, but the maximum (inclusive)
offset that should be applied to section anchors. The default
value is 0.
Write the assembly code to define section anchor x, which is a
SYMBOL_REF for which ‘SYMBOL_REF_ANCHOR_P (x)’ is true.
The hook is called with the assembly output position set to the beginning
of SYMBOL_REF_BLOCK (x).
If ASM_OUTPUT_DEF is available, the hook’s default definition uses
it to define the symbol as ‘. + SYMBOL_REF_BLOCK_OFFSET (x)’.
If ASM_OUTPUT_DEF is not available, the hook’s default definition
is NULL, which disables the use of section anchors altogether.
Return true if GCC should attempt to use anchors to access SYMBOL_REF
x. You can assume ‘SYMBOL_REF_HAS_BLOCK_INFO_P (x)’ and
‘!SYMBOL_REF_ANCHOR_P (x)’.
The default version is correct for most targets, but you might need to intercept this hook to handle things like target-specific attributes or target-specific sections.
Next: Costs, Previous: Anchored Addresses, Up: Target Macros [Contents][Index]
The macros in this section can be split in two families, according to the two ways of representing condition codes in GCC.
The first representation is the so called (cc0) representation
(see Jump Patterns), where all instructions can have an implicit
clobber of the condition codes. The second is the condition code
register representation, which provides better schedulability for
architectures that do have a condition code register, but on which
most instructions do not affect it. The latter category includes
most RISC machines.
The implicit clobbering poses a strong restriction on the placement of
the definition and use of the condition code, which need to be in adjacent
insns for machines using (cc0). This can prevent important
optimizations on some machines. For example, on the IBM RS/6000, there
is a delay for taken branches unless the condition code register is set
three instructions earlier than the conditional branch. The instruction
scheduler cannot perform this optimization if it is not permitted to
separate the definition and use of the condition code register.
For this reason, it is possible and suggested to use a register to
represent the condition code for new ports. If there is a specific
condition code register in the machine, use a hard register. If the
condition code or comparison result can be placed in any general register,
or if there are multiple condition registers, use a pseudo register.
Registers used to store the condition code value will usually have a mode
that is in class MODE_CC.
Alternatively, you can use BImode if the comparison operator is
specified already in the compare instruction. In this case, you are not
interested in most macros in this section.
| • CC0 Condition Codes: | Old style representation of condition codes. | |
| • MODE_CC Condition Codes: | Modern representation of condition codes. | |
| • Cond Exec Macros: | Macros to control conditional execution. |
Next: MODE_CC Condition Codes, Up: Condition Code [Contents][Index]
(cc0)The file conditions.h defines a variable cc_status to
describe how the condition code was computed (in case the interpretation of
the condition code depends on the instruction that it was set by). This
variable contains the RTL expressions on which the condition code is
currently based, and several standard flags.
Sometimes additional machine-specific flags must be defined in the machine
description header file. It can also add additional machine-specific
information by defining CC_STATUS_MDEP.
C code for a data type which is used for declaring the mdep
component of cc_status. It defaults to int.
This macro is not used on machines that do not use cc0.
A C expression to initialize the mdep field to “empty”.
The default definition does nothing, since most machines don’t use
the field anyway. If you want to use the field, you should probably
define this macro to initialize it.
This macro is not used on machines that do not use cc0.
A C compound statement to set the components of cc_status
appropriately for an insn insn whose body is exp. It is
this macro’s responsibility to recognize insns that set the condition
code as a byproduct of other activity as well as those that explicitly
set (cc0).
This macro is not used on machines that do not use cc0.
If there are insns that do not set the condition code but do alter
other machine registers, this macro must check to see whether they
invalidate the expressions that the condition code is recorded as
reflecting. For example, on the 68000, insns that store in address
registers do not set the condition code, which means that usually
NOTICE_UPDATE_CC can leave cc_status unaltered for such
insns. But suppose that the previous insn set the condition code
based on location ‘a4@(102)’ and the current insn stores a new
value in ‘a4’. Although the condition code is not changed by
this, it will no longer be true that it reflects the contents of
‘a4@(102)’. Therefore, NOTICE_UPDATE_CC must alter
cc_status in this case to say that nothing is known about the
condition code value.
The definition of NOTICE_UPDATE_CC must be prepared to deal
with the results of peephole optimization: insns whose patterns are
parallel RTXs containing various reg, mem or
constants which are just the operands. The RTL structure of these
insns is not sufficient to indicate what the insns actually do. What
NOTICE_UPDATE_CC should do when it sees one is just to run
CC_STATUS_INIT.
A possible definition of NOTICE_UPDATE_CC is to call a function
that looks at an attribute (see Insn Attributes) named, for example,
‘cc’. This avoids having detailed information about patterns in
two places, the md file and in NOTICE_UPDATE_CC.
Next: Cond Exec Macros, Previous: CC0 Condition Codes, Up: Condition Code [Contents][Index]
On many machines, the condition code may be produced by other instructions than compares, for example the branch can use directly the condition code set by a subtract instruction. However, on some machines when the condition code is set this way some bits (such as the overflow bit) are not set in the same way as a test instruction, so that a different branch instruction must be used for some conditional branches. When this happens, use the machine mode of the condition code register to record different formats of the condition code register. Modes can also be used to record which compare instruction (e.g. a signed or an unsigned comparison) produced the condition codes.
If other modes than CCmode are required, add them to
machine-modes.def and define SELECT_CC_MODE to choose
a mode given an operand of a compare. This is needed because the modes
have to be chosen not only during RTL generation but also, for example,
by instruction combination. The result of SELECT_CC_MODE should
be consistent with the mode used in the patterns; for example to support
the case of the add on the SPARC discussed above, we have the pattern
(define_insn ""
[(set (reg:CC_NOOV 0)
(compare:CC_NOOV
(plus:SI (match_operand:SI 0 "register_operand" "%r")
(match_operand:SI 1 "arith_operand" "rI"))
(const_int 0)))]
""
"…")
together with a SELECT_CC_MODE that returns CC_NOOVmode
for comparisons whose argument is a plus:
#define SELECT_CC_MODE(OP,X,Y) \
(GET_MODE_CLASS (GET_MODE (X)) == MODE_FLOAT \
? ((OP == EQ || OP == NE) ? CCFPmode : CCFPEmode) \
: ((GET_CODE (X) == PLUS || GET_CODE (X) == MINUS \
|| GET_CODE (X) == NEG) \
? CC_NOOVmode : CCmode))
Another reason to use modes is to retain information on which operands
were used by the comparison; see REVERSIBLE_CC_MODE later in
this section.
You should define this macro if and only if you define extra CC modes in machine-modes.def.
On some machines not all possible comparisons are defined, but you can
convert an invalid comparison into a valid one. For example, the Alpha
does not have a GT comparison, but you can use an LT
comparison instead and swap the order of the operands.
On such machines, define this macro to be a C statement to do any required conversions. code is the initial comparison code and op0 and op1 are the left and right operands of the comparison, respectively. You should modify code, op0, and op1 as required.
GCC will not assume that the comparison resulting from this macro is valid but will see if the resulting insn matches a pattern in the md file.
You need not define this macro if it would never change the comparison code or operands.
A C expression whose value is one if it is always safe to reverse a
comparison whose mode is mode. If SELECT_CC_MODE
can ever return mode for a floating-point inequality comparison,
then REVERSIBLE_CC_MODE (mode) must be zero.
You need not define this macro if it would always returns zero or if the
floating-point format is anything other than IEEE_FLOAT_FORMAT.
For example, here is the definition used on the SPARC, where floating-point
inequality comparisons are always given CCFPEmode:
#define REVERSIBLE_CC_MODE(MODE) ((MODE) != CCFPEmode)
A C expression whose value is reversed condition code of the code for
comparison done in CC_MODE mode. The macro is used only in case
REVERSIBLE_CC_MODE (mode) is nonzero. Define this macro in case
machine has some non-standard way how to reverse certain conditionals. For
instance in case all floating point conditions are non-trapping, compiler may
freely convert unordered compares to ordered one. Then definition may look
like:
#define REVERSE_CONDITION(CODE, MODE) \
((MODE) != CCFPmode ? reverse_condition (CODE) \
: reverse_condition_maybe_unordered (CODE))
On targets which do not use (cc0), and which use a hard
register rather than a pseudo-register to hold condition codes, the
regular CSE passes are often not able to identify cases in which the
hard register is set to a common value. Use this hook to enable a
small pass which optimizes such cases. This hook should return true
to enable this pass, and it should set the integers to which its
arguments point to the hard register numbers used for condition codes.
When there is only one such register, as is true on most systems, the
integer pointed to by p2 should be set to
INVALID_REGNUM.
The default version of this hook returns false.
On targets which use multiple condition code modes in class
MODE_CC, it is sometimes the case that a comparison can be
validly done in more than one mode. On such a system, define this
target hook to take two mode arguments and to return a mode in which
both comparisons may be validly done. If there is no such mode,
return VOIDmode.
The default version of this hook checks whether the modes are the
same. If they are, it returns that mode. If they are different, it
returns VOIDmode.
Previous: MODE_CC Condition Codes, Up: Condition Code [Contents][Index]
There is one macro that may need to be defined for targets supporting conditional execution, independent of how they represent conditional branches.
A C expression that returns true if the conditional execution predicate op1, a comparison operation, is the inverse of op2 and vice versa. Define this to return 0 if the target has conditional execution predicates that cannot be reversed safely. There is no need to validate that the arguments of op1 and op2 are the same, this is done separately. If no expansion is specified, this macro is defined as follows:
#define REVERSE_CONDEXEC_PREDICATES_P (x, y) \ (GET_CODE ((x)) == reversed_comparison_code ((y), NULL))
Next: Scheduling, Previous: Condition Code, Up: Target Macros [Contents][Index]
These macros let you describe the relative speed of various operations on the target machine.
A C expression for the cost of moving data of mode mode from a
register in class from to one in class to. The classes are
expressed using the enumeration values such as GENERAL_REGS. A
value of 2 is the default; other values are interpreted relative to
that.
It is not required that the cost always equal 2 when from is the same as to; on some machines it is expensive to move between registers if they are not general registers.
If reload sees an insn consisting of a single set between two
hard registers, and if REGISTER_MOVE_COST applied to their
classes returns a value of 2, reload does not check to ensure that the
constraints of the insn are met. Setting a cost of other than 2 will
allow reload to verify that the constraints are met. You should do this
if the ‘movm’ pattern’s constraints do not allow such copying.
These macros are obsolete, new ports should use the target hook
TARGET_REGISTER_MOVE_COST instead.
This target hook should return the cost of moving data of mode mode
from a register in class from to one in class to. The classes
are expressed using the enumeration values such as GENERAL_REGS.
A value of 2 is the default; other values are interpreted relative to
that.
It is not required that the cost always equal 2 when from is the same as to; on some machines it is expensive to move between registers if they are not general registers.
If reload sees an insn consisting of a single set between two
hard registers, and if TARGET_REGISTER_MOVE_COST applied to their
classes returns a value of 2, reload does not check to ensure that the
constraints of the insn are met. Setting a cost of other than 2 will
allow reload to verify that the constraints are met. You should do this
if the ‘movm’ pattern’s constraints do not allow such copying.
The default version of this function returns 2.
A C expression for the cost of moving data of mode mode between a
register of class class and memory; in is zero if the value
is to be written to memory, nonzero if it is to be read in. This cost
is relative to those in REGISTER_MOVE_COST. If moving between
registers and memory is more expensive than between two registers, you
should define this macro to express the relative cost.
If you do not define this macro, GCC uses a default cost of 4 plus the cost of copying via a secondary reload register, if one is needed. If your machine requires a secondary reload register to copy between memory and a register of class but the reload mechanism is more complex than copying via an intermediate, define this macro to reflect the actual cost of the move.
GCC defines the function memory_move_secondary_cost if
secondary reloads are needed. It computes the costs due to copying via
a secondary register. If your machine copies from memory using a
secondary register in the conventional way but the default base value of
4 is not correct for your machine, define this macro to add some other
value to the result of that function. The arguments to that function
are the same as to this macro.
These macros are obsolete, new ports should use the target hook
TARGET_MEMORY_MOVE_COST instead.
This target hook should return the cost of moving data of mode mode
between a register of class rclass and memory; in is false
if the value is to be written to memory, true if it is to be read in.
This cost is relative to those in TARGET_REGISTER_MOVE_COST.
If moving between registers and memory is more expensive than between two
registers, you should add this target hook to express the relative cost.
If you do not add this target hook, GCC uses a default cost of 4 plus the cost of copying via a secondary reload register, if one is needed. If your machine requires a secondary reload register to copy between memory and a register of rclass but the reload mechanism is more complex than copying via an intermediate, use this target hook to reflect the actual cost of the move.
GCC defines the function memory_move_secondary_cost if
secondary reloads are needed. It computes the costs due to copying via
a secondary register. If your machine copies from memory using a
secondary register in the conventional way but the default base value of
4 is not correct for your machine, use this target hook to add some other
value to the result of that function. The arguments to that function
are the same as to this target hook.
A C expression for the cost of a branch instruction. A value of 1 is
the default; other values are interpreted relative to that. Parameter
speed_p is true when the branch in question should be optimized
for speed. When it is false, BRANCH_COST should return a value
optimal for code size rather than performance. predictable_p is
true for well-predicted branches. On many architectures the
BRANCH_COST can be reduced then.
Here are additional macros which do not specify precise relative costs, but only that certain actions are more expensive than GCC would ordinarily expect.
Define this macro as a C expression which is nonzero if accessing less
than a word of memory (i.e. a char or a short) is no
faster than accessing a word of memory, i.e., if such access
require more than one instruction or if there is no difference in cost
between byte and (aligned) word loads.
When this macro is not defined, the compiler will access a field by finding the smallest containing object; when it is defined, a fullword load will be used if alignment permits. Unless bytes accesses are faster than word accesses, using word accesses is preferable since it may eliminate subsequent memory access if subsequent accesses occur to other fields in the same word of the structure, but to different bytes.
Define this macro to be the value 1 if memory accesses described by the mode and alignment parameters have a cost many times greater than aligned accesses, for example if they are emulated in a trap handler.
When this macro is nonzero, the compiler will act as if
STRICT_ALIGNMENT were nonzero when generating code for block
moves. This can cause significantly more instructions to be produced.
Therefore, do not set this macro nonzero if unaligned accesses only add a
cycle or two to the time for a memory access.
If the value of this macro is always zero, it need not be defined. If
this macro is defined, it should produce a nonzero value when
STRICT_ALIGNMENT is nonzero.
The threshold of number of scalar memory-to-memory move insns, below which a sequence of insns should be generated instead of a string move insn or a library call. Increasing the value will always make code faster, but eventually incurs high cost in increased code size.
Note that on machines where the corresponding move insn is a
define_expand that emits a sequence of insns, this macro counts
the number of such sequences.
The parameter speed is true if the code is currently being optimized for speed rather than size.
If you don’t define this, a reasonable default is used.
A C expression used to determine whether move_by_pieces will be used to
copy a chunk of memory, or whether some other block move mechanism
will be used. Defaults to 1 if move_by_pieces_ninsns returns less
than MOVE_RATIO.
A C expression used by move_by_pieces to determine the largest unit
a load or store used to copy memory is. Defaults to MOVE_MAX.
The threshold of number of scalar move insns, below which a sequence of insns should be generated to clear memory instead of a string clear insn or a library call. Increasing the value will always make code faster, but eventually incurs high cost in increased code size.
The parameter speed is true if the code is currently being optimized for speed rather than size.
If you don’t define this, a reasonable default is used.
A C expression used to determine whether clear_by_pieces will be used
to clear a chunk of memory, or whether some other block clear mechanism
will be used. Defaults to 1 if move_by_pieces_ninsns returns less
than CLEAR_RATIO.
The threshold of number of scalar move insns, below which a sequence of insns should be generated to set memory to a constant value, instead of a block set insn or a library call. Increasing the value will always make code faster, but eventually incurs high cost in increased code size.
The parameter speed is true if the code is currently being optimized for speed rather than size.
If you don’t define this, it defaults to the value of MOVE_RATIO.
A C expression used to determine whether store_by_pieces will be
used to set a chunk of memory to a constant value, or whether some
other mechanism will be used. Used by __builtin_memset when
storing values other than constant zero.
Defaults to 1 if move_by_pieces_ninsns returns less
than SET_RATIO.
A C expression used to determine whether store_by_pieces will be
used to set a chunk of memory to a constant string value, or whether some
other mechanism will be used. Used by __builtin_strcpy when
called with a constant source string.
Defaults to 1 if move_by_pieces_ninsns returns less
than MOVE_RATIO.
A C expression used to determine whether a load postincrement is a good
thing to use for a given mode. Defaults to the value of
HAVE_POST_INCREMENT.
A C expression used to determine whether a load postdecrement is a good
thing to use for a given mode. Defaults to the value of
HAVE_POST_DECREMENT.
A C expression used to determine whether a load preincrement is a good
thing to use for a given mode. Defaults to the value of
HAVE_PRE_INCREMENT.
A C expression used to determine whether a load predecrement is a good
thing to use for a given mode. Defaults to the value of
HAVE_PRE_DECREMENT.
A C expression used to determine whether a store postincrement is a good
thing to use for a given mode. Defaults to the value of
HAVE_POST_INCREMENT.
A C expression used to determine whether a store postdecrement is a good
thing to use for a given mode. Defaults to the value of
HAVE_POST_DECREMENT.
This macro is used to determine whether a store preincrement is a good
thing to use for a given mode. Defaults to the value of
HAVE_PRE_INCREMENT.
This macro is used to determine whether a store predecrement is a good
thing to use for a given mode. Defaults to the value of
HAVE_PRE_DECREMENT.
Define this macro if it is as good or better to call a constant function address than to call an address kept in a register.
Define this macro if a non-short-circuit operation produced by
‘fold_range_test ()’ is optimal. This macro defaults to true if
BRANCH_COST is greater than or equal to the value 2.
This target hook describes the relative costs of RTL expressions.
The cost may depend on the precise form of the expression, which is
available for examination in x, and the rtx code of the expression
in which it is contained, found in outer_code. code is the
expression code—redundant, since it can be obtained with
GET_CODE (x).
In implementing this hook, you can use the construct
COSTS_N_INSNS (n) to specify a cost equal to n fast
instructions.
On entry to the hook, *total contains a default estimate
for the cost of the expression. The hook should modify this value as
necessary. Traditionally, the default costs are COSTS_N_INSNS (5)
for multiplications, COSTS_N_INSNS (7) for division and modulus
operations, and COSTS_N_INSNS (1) for all other operations.
When optimizing for code size, i.e. when speed is
false, this target hook should be used to estimate the relative
size cost of an expression, again relative to COSTS_N_INSNS.
The hook returns true when all subexpressions of x have been
processed, and false when rtx_cost should recurse.
This hook computes the cost of an addressing mode that contains
address. If not defined, the cost is computed from
the address expression and the TARGET_RTX_COST hook.
For most CISC machines, the default cost is a good approximation of the true cost of the addressing mode. However, on RISC machines, all instructions normally have the same length and execution time. Hence all addresses will have equal costs.
In cases where more than one form of an address is known, the form with the lowest cost will be used. If multiple forms have the same, lowest, cost, the one that is the most complex will be used.
For example, suppose an address that is equal to the sum of a register and a constant is used twice in the same basic block. When this macro is not defined, the address will be computed in a register and memory references will be indirect through that register. On machines where the cost of the addressing mode containing the sum is no higher than that of a simple indirect reference, this will produce an additional instruction and possibly require an additional register. Proper specification of this macro eliminates this overhead for such machines.
This hook is never called with an invalid address.
On machines where an address involving more than one register is as
cheap as an address computation involving only one register, defining
TARGET_ADDRESS_COST to reflect this can cause two registers to
be live over a region of code where only one would have been if
TARGET_ADDRESS_COST were not defined in that manner. This effect
should be considered in the definition of this macro. Equivalent costs
should probably only be given to addresses with different numbers of
registers on machines with lots of registers.
Next: Sections, Previous: Costs, Up: Target Macros [Contents][Index]
The instruction scheduler may need a fair amount of machine-specific adjustment in order to produce good code. GCC provides several target hooks for this purpose. It is usually enough to define just a few of them: try the first ones in this list first.
This hook returns the maximum number of instructions that can ever issue at the same time on the target machine. The default is one. Although the insn scheduler can define itself the possibility of issue an insn on the same cycle, the value can serve as an additional constraint to issue insns on the same simulated processor cycle (see hooks ‘TARGET_SCHED_REORDER’ and ‘TARGET_SCHED_REORDER2’). This value must be constant over the entire compilation. If you need it to vary depending on what the instructions are, you must use ‘TARGET_SCHED_VARIABLE_ISSUE’.
This hook is executed by the scheduler after it has scheduled an insn
from the ready list. It should return the number of insns which can
still be issued in the current cycle. The default is
‘more - 1’ for insns other than CLOBBER and
USE, which normally are not counted against the issue rate.
You should define this hook if some insns take more machine resources
than others, so that fewer insns can follow them in the same cycle.
file is either a null pointer, or a stdio stream to write any
debug output to. verbose is the verbose level provided by
-fsched-verbose-n. insn is the instruction that
was scheduled.
This function corrects the value of cost based on the relationship between insn and dep_insn through the dependence link. It should return the new value. The default is to make no adjustment to cost. This can be used for example to specify to the scheduler using the traditional pipeline description that an output- or anti-dependence does not incur the same cost as a data-dependence. If the scheduler using the automaton based pipeline description, the cost of anti-dependence is zero and the cost of output-dependence is maximum of one and the difference of latency times of the first and the second insns. If these values are not acceptable, you could use the hook to modify them too. See also see Processor pipeline description.
This hook adjusts the integer scheduling priority priority of insn. It should return the new priority. Increase the priority to execute insn earlier, reduce the priority to execute insn later. Do not define this hook if you do not need to adjust the scheduling priorities of insns.
This hook is executed by the scheduler after it has scheduled the ready
list, to allow the machine description to reorder it (for example to
combine two small instructions together on ‘VLIW’ machines).
file is either a null pointer, or a stdio stream to write any
debug output to. verbose is the verbose level provided by
-fsched-verbose-n. ready is a pointer to the ready
list of instructions that are ready to be scheduled. n_readyp is
a pointer to the number of elements in the ready list. The scheduler
reads the ready list in reverse order, starting with
ready[*n_readyp - 1] and going to ready[0]. clock
is the timer tick of the scheduler. You may modify the ready list and
the number of ready insns. The return value is the number of insns that
can issue this cycle; normally this is just issue_rate. See also
‘TARGET_SCHED_REORDER2’.
Like ‘TARGET_SCHED_REORDER’, but called at a different time. That function is called whenever the scheduler starts a new cycle. This one is called once per iteration over a cycle, immediately after ‘TARGET_SCHED_VARIABLE_ISSUE’; it can reorder the ready list and return the number of insns to be scheduled in the same cycle. Defining this hook can be useful if there are frequent situations where scheduling one insn causes other insns to become ready in the same cycle. These other insns can then be taken into account properly.
This hook is called after evaluation forward dependencies of insns in chain given by two parameter values (head and tail correspondingly) but before insns scheduling of the insn chain. For example, it can be used for better insn classification if it requires analysis of dependencies. This hook can use backward and forward dependencies of the insn scheduler because they are already calculated.
This hook is executed by the scheduler at the beginning of each block of instructions that are to be scheduled. file is either a null pointer, or a stdio stream to write any debug output to. verbose is the verbose level provided by -fsched-verbose-n. max_ready is the maximum number of insns in the current scheduling region that can be live at the same time. This can be used to allocate scratch space if it is needed, e.g. by ‘TARGET_SCHED_REORDER’.
This hook is executed by the scheduler at the end of each block of instructions that are to be scheduled. It can be used to perform cleanup of any actions done by the other scheduling hooks. file is either a null pointer, or a stdio stream to write any debug output to. verbose is the verbose level provided by -fsched-verbose-n.
This hook is executed by the scheduler after function level initializations. file is either a null pointer, or a stdio stream to write any debug output to. verbose is the verbose level provided by -fsched-verbose-n. old_max_uid is the maximum insn uid when scheduling begins.
This is the cleanup hook corresponding to TARGET_SCHED_INIT_GLOBAL.
file is either a null pointer, or a stdio stream to write any debug output to.
verbose is the verbose level provided by -fsched-verbose-n.
The hook returns an RTL insn. The automaton state used in the pipeline hazard recognizer is changed as if the insn were scheduled when the new simulated processor cycle starts. Usage of the hook may simplify the automaton pipeline description for some VLIW processors. If the hook is defined, it is used only for the automaton based pipeline description. The default is not to change the state when the new simulated processor cycle starts.
The hook can be used to initialize data used by the previous hook.
The hook is analogous to ‘TARGET_SCHED_DFA_PRE_CYCLE_INSN’ but used to changed the state as if the insn were scheduled when the new simulated processor cycle finishes.
The hook is analogous to ‘TARGET_SCHED_INIT_DFA_PRE_CYCLE_INSN’ but used to initialize data used by the previous hook.
The hook to notify target that the current simulated cycle is about to finish. The hook is analogous to ‘TARGET_SCHED_DFA_PRE_CYCLE_INSN’ but used to change the state in more complicated situations - e.g., when advancing state on a single insn is not enough.
The hook to notify target that new simulated cycle has just started. The hook is analogous to ‘TARGET_SCHED_DFA_POST_CYCLE_INSN’ but used to change the state in more complicated situations - e.g., when advancing state on a single insn is not enough.
This hook controls better choosing an insn from the ready insn queue for the DFA-based insn scheduler. Usually the scheduler chooses the first insn from the queue. If the hook returns a positive value, an additional scheduler code tries all permutations of ‘TARGET_SCHED_FIRST_CYCLE_MULTIPASS_DFA_LOOKAHEAD ()’ subsequent ready insns to choose an insn whose issue will result in maximal number of issued insns on the same cycle. For the VLIW processor, the code could actually solve the problem of packing simple insns into the VLIW insn. Of course, if the rules of VLIW packing are described in the automaton.
This code also could be used for superscalar RISC processors. Let us consider a superscalar RISC processor with 3 pipelines. Some insns can be executed in pipelines A or B, some insns can be executed only in pipelines B or C, and one insn can be executed in pipeline B. The processor may issue the 1st insn into A and the 2nd one into B. In this case, the 3rd insn will wait for freeing B until the next cycle. If the scheduler issues the 3rd insn the first, the processor could issue all 3 insns per cycle.
Actually this code demonstrates advantages of the automaton based pipeline hazard recognizer. We try quickly and easy many insn schedules to choose the best one.
The default is no multipass scheduling.
This hook controls what insns from the ready insn queue will be considered for the multipass insn scheduling. If the hook returns zero for insn, the insn will be not chosen to be issued.
The default is that any ready insns can be chosen to be issued.
This hook prepares the target backend for a new round of multipass scheduling.
This hook is called when multipass scheduling evaluates instruction INSN.
This is called when multipass scheduling backtracks from evaluation of an instruction.
This hook notifies the target about the result of the concluded current round of multipass scheduling.
This hook initializes target-specific data used in multipass scheduling.
This hook finalizes target-specific data used in multipass scheduling.
This hook is called by the insn scheduler before issuing insn on cycle clock. If the hook returns nonzero, insn is not issued on this processor cycle. Instead, the processor cycle is advanced. If *sort_p is zero, the insn ready queue is not sorted on the new cycle start as usually. dump and verbose specify the file and verbosity level to use for debugging output. last_clock and clock are, respectively, the processor cycle on which the previous insn has been issued, and the current processor cycle.
This hook is used to define which dependences are considered costly by
the target, so costly that it is not advisable to schedule the insns that
are involved in the dependence too close to one another. The parameters
to this hook are as follows: The first parameter _dep is the dependence
being evaluated. The second parameter cost is the cost of the
dependence as estimated by the scheduler, and the third
parameter distance is the distance in cycles between the two insns.
The hook returns true if considering the distance between the two
insns the dependence between them is considered costly by the target,
and false otherwise.
Defining this hook can be useful in multiple-issue out-of-order machines, where (a) it’s practically hopeless to predict the actual data/resource delays, however: (b) there’s a better chance to predict the actual grouping that will be formed, and (c) correctly emulating the grouping can be very important. In such targets one may want to allow issuing dependent insns closer to one another—i.e., closer than the dependence distance; however, not in cases of “costly dependences”, which this hooks allows to define.
This hook is called by the insn scheduler after emitting a new instruction to the instruction stream. The hook notifies a target backend to extend its per instruction data structures.
Return a pointer to a store large enough to hold target scheduling context.
Initialize store pointed to by tc to hold target scheduling context. It clean_p is true then initialize tc as if scheduler is at the beginning of the block. Otherwise, copy the current context into tc.
Copy target scheduling context pointed to by tc to the current context.
Deallocate internal data in target scheduling context pointed to by tc.
Deallocate a store for target scheduling context pointed to by tc.
This hook is called by the insn scheduler when insn has only speculative dependencies and therefore can be scheduled speculatively. The hook is used to check if the pattern of insn has a speculative version and, in case of successful check, to generate that speculative pattern. The hook should return 1, if the instruction has a speculative form, or -1, if it doesn’t. request describes the type of requested speculation. If the return value equals 1 then new_pat is assigned the generated speculative pattern.
This hook is called by the insn scheduler during generation of recovery code
for insn. It should return true, if the corresponding check
instruction should branch to recovery code, or false otherwise.
This hook is called by the insn scheduler to generate a pattern for recovery check instruction. If mutate_p is zero, then insn is a speculative instruction for which the check should be generated. label is either a label of a basic block, where recovery code should be emitted, or a null pointer, when requested check doesn’t branch to recovery code (a simple check). If mutate_p is nonzero, then a pattern for a branchy check corresponding to a simple check denoted by insn should be generated. In this case label can’t be null.
This hook is used as a workaround for
‘TARGET_SCHED_FIRST_CYCLE_MULTIPASS_DFA_LOOKAHEAD_GUARD’ not being
called on the first instruction of the ready list. The hook is used to
discard speculative instructions that stand first in the ready list from
being scheduled on the current cycle. If the hook returns false,
insn will not be chosen to be issued.
For non-speculative instructions,
the hook should always return true. For example, in the ia64 backend
the hook is used to cancel data speculative insns when the ALAT table
is nearly full.
This hook is used by the insn scheduler to find out what features should be enabled/used. The structure *spec_info should be filled in by the target. The structure describes speculation types that can be used in the scheduler.
This hook is called by the swing modulo scheduler to calculate a resource-based lower bound which is based on the resources available in the machine and the resources required by each instruction. The target backend can use g to calculate such bound. A very simple lower bound will be used in case this hook is not implemented: the total number of instructions divided by the issue rate.
This hook is called by Haifa Scheduler. It returns true if dispatch scheduling is supported in hardware and the condition specified in the parameter is true.
This hook is called by Haifa Scheduler. It performs the operation specified in its second parameter.
Next: PIC, Previous: Scheduling, Up: Target Macros [Contents][Index]
An object file is divided into sections containing different types of data. In the most common case, there are three sections: the text section, which holds instructions and read-only data; the data section, which holds initialized writable data; and the bss section, which holds uninitialized data. Some systems have other kinds of sections.
varasm.c provides several well-known sections, such as
text_section, data_section and bss_section.
The normal way of controlling a foo_section variable
is to define the associated FOO_SECTION_ASM_OP macro,
as described below. The macros are only read once, when varasm.c
initializes itself, so their values must be run-time constants.
They may however depend on command-line flags.
Note: Some run-time files, such crtstuff.c, also make
use of the FOO_SECTION_ASM_OP macros, and expect them
to be string literals.
Some assemblers require a different string to be written every time a
section is selected. If your assembler falls into this category, you
should define the TARGET_ASM_INIT_SECTIONS hook and use
get_unnamed_section to set up the sections.
You must always create a text_section, either by defining
TEXT_SECTION_ASM_OP or by initializing text_section
in TARGET_ASM_INIT_SECTIONS. The same is true of
data_section and DATA_SECTION_ASM_OP. If you do not
create a distinct readonly_data_section, the default is to
reuse text_section.
All the other varasm.c sections are optional, and are null if the target does not provide them.
A C expression whose value is a string, including spacing, containing the
assembler operation that should precede instructions and read-only data.
Normally "\t.text" is right.
If defined, a C string constant for the name of the section containing most frequently executed functions of the program. If not defined, GCC will provide a default definition if the target supports named sections.
If defined, a C string constant for the name of the section containing unlikely executed functions in the program.
A C expression whose value is a string, including spacing, containing the
assembler operation to identify the following data as writable initialized
data. Normally "\t.data" is right.
If defined, a C expression whose value is a string, including spacing, containing the assembler operation to identify the following data as initialized, writable small data.
A C expression whose value is a string, including spacing, containing the assembler operation to identify the following data as read-only initialized data.
If defined, a C expression whose value is a string, including spacing,
containing the assembler operation to identify the following data as
uninitialized global data. If not defined, and neither
ASM_OUTPUT_BSS nor ASM_OUTPUT_ALIGNED_BSS are defined,
uninitialized global data will be output in the data section if
-fno-common is passed, otherwise ASM_OUTPUT_COMMON will be
used.
If defined, a C expression whose value is a string, including spacing, containing the assembler operation to identify the following data as uninitialized, writable small data.
If defined, a C expression whose value is a string containing the
assembler operation to identify the following data as thread-local
common data. The default is ".tls_common".
If defined, a C expression whose value is a character constant
containing the flag used to mark a section as a TLS section. The
default is 'T'.
If defined, a C expression whose value is a string, including spacing,
containing the assembler operation to identify the following data as
initialization code. If not defined, GCC will assume such a section does
not exist. This section has no corresponding init_section
variable; it is used entirely in runtime code.
If defined, a C expression whose value is a string, including spacing,
containing the assembler operation to identify the following data as
finalization code. If not defined, GCC will assume such a section does
not exist. This section has no corresponding fini_section
variable; it is used entirely in runtime code.
If defined, a C expression whose value is a string, including spacing,
containing the assembler operation to identify the following data as
part of the .init_array (or equivalent) section. If not
defined, GCC will assume such a section does not exist. Do not define
both this macro and INIT_SECTION_ASM_OP.
If defined, a C expression whose value is a string, including spacing,
containing the assembler operation to identify the following data as
part of the .fini_array (or equivalent) section. If not
defined, GCC will assume such a section does not exist. Do not define
both this macro and FINI_SECTION_ASM_OP.
If defined, an ASM statement that switches to a different section
via section_op, calls function, and switches back to
the text section. This is used in crtstuff.c if
INIT_SECTION_ASM_OP or FINI_SECTION_ASM_OP to calls
to initialization and finalization functions from the init and fini
sections. By default, this macro uses a simple function call. Some
ports need hand-crafted assembly code to avoid dependencies on
registers initialized in the function prologue or to ensure that
constant pools don’t end up too far way in the text section.
If defined, a string which names the section into which small
variables defined in crtstuff and libgcc should go. This is useful
when the target has options for optimizing access to small data, and
you want the crtstuff and libgcc routines to be conservative in what
they expect of your application yet liberal in what your application
expects. For example, for targets with a .sdata section (like
MIPS), you could compile crtstuff with -G 0 so that it doesn’t
require small data support from your application, but use this macro
to put small data into .sdata so that your application can
access these variables whether it uses small data or not.
If defined, an ASM statement that aligns a code section to some
arbitrary boundary. This is used to force all fragments of the
.init and .fini sections to have to same alignment
and thus prevent the linker from having to add any padding.
Define this macro to be an expression with a nonzero value if jump
tables (for tablejump insns) should be output in the text
section, along with the assembler instructions. Otherwise, the
readonly data section is used.
This macro is irrelevant if there is no separate readonly data section.
Define this hook if you need to do something special to set up the varasm.c sections, or if your target has some special sections of its own that you need to create.
GCC calls this hook after processing the command line, but before writing any assembly code, and before calling any of the section-returning hooks described below.
Return a mask describing how relocations should be treated when selecting sections. Bit 1 should be set if global relocations should be placed in a read-write section; bit 0 should be set if local relocations should be placed in a read-write section.
The default version of this function returns 3 when -fpic is in effect, and 0 otherwise. The hook is typically redefined when the target cannot support (some kinds of) dynamic relocations in read-only sections even in executables.
Return the section into which exp should be placed. You can
assume that exp is either a VAR_DECL node or a constant of
some sort. reloc indicates whether the initial value of exp
requires link-time relocations. Bit 0 is set when variable contains
local relocations only, while bit 1 is set for global relocations.
align is the constant alignment in bits.
The default version of this function takes care of putting read-only
variables in readonly_data_section.
See also USE_SELECT_SECTION_FOR_FUNCTIONS.
Define this macro if you wish TARGET_ASM_SELECT_SECTION to be called
for FUNCTION_DECLs as well as for variables and constants.
In the case of a FUNCTION_DECL, reloc will be zero if the
function has been determined to be likely to be called, and nonzero if
it is unlikely to be called.
Build up a unique section name, expressed as a STRING_CST node,
and assign it to ‘DECL_SECTION_NAME (decl)’.
As with TARGET_ASM_SELECT_SECTION, reloc indicates whether
the initial value of exp requires link-time relocations.
The default version of this function appends the symbol name to the
ELF section name that would normally be used for the symbol. For
example, the function foo would be placed in .text.foo.
Whatever the actual target object format, this is often good enough.
Return the readonly data section associated with
‘DECL_SECTION_NAME (decl)’.
The default version of this function selects .gnu.linkonce.r.name if
the function’s section is .gnu.linkonce.t.name, .rodata.name
if function is in .text.name, and the normal readonly-data section
otherwise.
Return the section into which a constant x, of mode mode,
should be placed. You can assume that x is some kind of
constant in RTL. The argument mode is redundant except in the
case of a const_int rtx. align is the constant alignment
in bits.
The default version of this function takes care of putting symbolic
constants in flag_pic mode in data_section and everything
else in readonly_data_section.
Define this hook if you need to postprocess the assembler name generated
by target-independent code. The id provided to this hook will be
the computed name (e.g., the macro DECL_NAME of the decl in C,
or the mangled name of the decl in C++). The return value of the
hook is an IDENTIFIER_NODE for the appropriate mangled name on
your target system. The default implementation of this hook just
returns the id provided.
Define this hook if references to a symbol or a constant must be treated differently depending on something about the variable or function named by the symbol (such as what section it is in).
The hook is executed immediately after rtl has been created for
decl, which may be a variable or function declaration or
an entry in the constant pool. In either case, rtl is the
rtl in question. Do not use DECL_RTL (decl)
in this hook; that field may not have been initialized yet.
In the case of a constant, it is safe to assume that the rtl is
a mem whose address is a symbol_ref. Most decls
will also have this form, but that is not guaranteed. Global
register variables, for instance, will have a reg for their
rtl. (Normally the right thing to do with such unusual rtl is
leave it alone.)
The new_decl_p argument will be true if this is the first time
that TARGET_ENCODE_SECTION_INFO has been invoked on this decl. It will
be false for subsequent invocations, which will happen for duplicate
declarations. Whether or not anything must be done for the duplicate
declaration depends on whether the hook examines DECL_ATTRIBUTES.
new_decl_p is always true when the hook is called for a constant.
The usual thing for this hook to do is to record flags in the
symbol_ref, using SYMBOL_REF_FLAG or SYMBOL_REF_FLAGS.
Historically, the name string was modified if it was necessary to
encode more than one bit of information, but this practice is now
discouraged; use SYMBOL_REF_FLAGS.
The default definition of this hook, default_encode_section_info
in varasm.c, sets a number of commonly-useful bits in
SYMBOL_REF_FLAGS. Check whether the default does what you need
before overriding it.
Decode name and return the real name part, sans
the characters that TARGET_ENCODE_SECTION_INFO
may have added.
Returns true if exp should be placed into a “small data” section. The default version of this hook always returns false.
Contains the value true if the target places read-only “small data” into a separate section. The default value is false.
It returns true if target wants profile code emitted before prologue.
The default version of this hook use the target macro
PROFILE_BEFORE_PROLOGUE.
Returns true if exp names an object for which name resolution rules must resolve to the current “module” (dynamic shared library or executable image).
The default version of this hook implements the name resolution rules for ELF, which has a looser model of global name binding than other currently supported object file formats.
Contains the value true if the target supports thread-local storage. The default value is false.
Next: Assembler Format, Previous: Sections, Up: Target Macros [Contents][Index]
This section describes macros that help implement generation of position
independent code. Simply defining these macros is not enough to
generate valid PIC; you must also add support to the hook
TARGET_LEGITIMATE_ADDRESS_P and to the macro
PRINT_OPERAND_ADDRESS, as well as LEGITIMIZE_ADDRESS. You
must modify the definition of ‘movsi’ to do something appropriate
when the source operand contains a symbolic address. You may also
need to alter the handling of switch statements so that they use
relative addresses.
The register number of the register used to address a table of static
data addresses in memory. In some cases this register is defined by a
processor’s “application binary interface” (ABI). When this macro
is defined, RTL is generated for this register once, as with the stack
pointer and frame pointer registers. If this macro is not defined, it
is up to the machine-dependent files to allocate such a register (if
necessary). Note that this register must be fixed when in use (e.g.
when flag_pic is true).
A C expression that is nonzero if the register defined by
PIC_OFFSET_TABLE_REGNUM is clobbered by calls. If not defined,
the default is zero. Do not define
this macro if PIC_OFFSET_TABLE_REGNUM is not defined.
A C expression that is nonzero if x is a legitimate immediate
operand on the target machine when generating position independent code.
You can assume that x satisfies CONSTANT_P, so you need not
check this. You can also assume flag_pic is true, so you need not
check it either. You need not define this macro if all constants
(including SYMBOL_REF) can be immediate operands when generating
position independent code.
Next: Debugging Info, Previous: PIC, Up: Target Macros [Contents][Index]
This section describes macros whose principal purpose is to describe how to write instructions in assembler language—rather than what the instructions do.
| • File Framework: | Structural information for the assembler file. | |
| • Data Output: | Output of constants (numbers, strings, addresses). | |
| • Uninitialized Data: | Output of uninitialized variables. | |
| • Label Output: | Output and generation of labels. | |
| • Initialization: | General principles of initialization and termination routines. | |
| • Macros for Initialization: | Specific macros that control the handling of initialization and termination routines. | |
| • Instruction Output: | Output of actual instructions. | |
| • Dispatch Tables: | Output of jump tables. | |
| • Exception Region Output: | Output of exception region code. | |
| • Alignment Output: | Pseudo ops for alignment and skipping data. |
Next: Data Output, Up: Assembler Format [Contents][Index]
This describes the overall framework of an assembly file.
Output to asm_out_file any text which the assembler expects to
find at the beginning of a file. The default behavior is controlled
by two flags, documented below. Unless your target’s assembler is
quite unusual, if you override the default, you should call
default_file_start at some point in your target hook. This
lets other target files rely on these variables.
If this flag is true, the text of the macro ASM_APP_OFF will be
printed as the very first line in the assembly file, unless
-fverbose-asm is in effect. (If that macro has been defined
to the empty string, this variable has no effect.) With the normal
definition of ASM_APP_OFF, the effect is to notify the GNU
assembler that it need not bother stripping comments or extra
whitespace from its input. This allows it to work a bit faster.
The default is false. You should not set it to true unless you have verified that your port does not generate any extra whitespace or comments that will cause GAS to issue errors in NO_APP mode.
If this flag is true, output_file_directive will be called
for the primary source file, immediately after printing
ASM_APP_OFF (if that is enabled). Most ELF assemblers expect
this to be done. The default is false.
Output to asm_out_file any text which the assembler expects
to find at the end of a file. The default is to output nothing.
Some systems use a common convention, the ‘.note.GNU-stack’
special section, to indicate whether or not an object file relies on
the stack being executable. If your system uses this convention, you
should define TARGET_ASM_FILE_END to this function. If you
need to do other things in that hook, have your hook function call
this function.
Output to asm_out_file any text which the assembler expects
to find at the start of an LTO section. The default is to output
nothing.
Output to asm_out_file any text which the assembler expects
to find at the end of an LTO section. The default is to output
nothing.
Output to asm_out_file any text which is needed before emitting
unwind info and debug info at the end of a file. Some targets emit
here PIC setup thunks that cannot be emitted at the end of file,
because they couldn’t have unwind info then. The default is to output
nothing.
A C string constant describing how to begin a comment in the target assembler language. The compiler assumes that the comment will end at the end of the line.
A C string constant for text to be output before each asm
statement or group of consecutive ones. Normally this is
"#APP", which is a comment that has no effect on most
assemblers but tells the GNU assembler that it must check the lines
that follow for all valid assembler constructs.
A C string constant for text to be output after each asm
statement or group of consecutive ones. Normally this is
"#NO_APP", which tells the GNU assembler to resume making the
time-saving assumptions that are valid for ordinary compiler output.
A C statement to output COFF information or DWARF debugging information which indicates that filename name is the current source file to the stdio stream stream.
This macro need not be defined if the standard form of output for the file format in use is appropriate.
Output COFF information or DWARF debugging information which indicates that filename name is the current source file to the stdio stream file.
This target hook need not be defined if the standard form of output for the file format in use is appropriate.
A C statement to output the string string to the stdio stream
stream. If you do not call the function output_quoted_string
in your config files, GCC will only call it to output filenames to
the assembler source. So you can use it to canonicalize the format
of the filename using this macro.
A C statement to output something to the assembler file to handle a ‘#ident’ directive containing the text string. If this macro is not defined, nothing is output for a ‘#ident’ directive.
Output assembly directives to switch to section name. The section
should have attributes as specified by flags, which is a bit mask
of the SECTION_* flags defined in output.h. If decl
is non-NULL, it is the VAR_DECL or FUNCTION_DECL with which
this section is associated.
Return preferred text (sub)section for function decl.
Main purpose of this function is to separate cold, normal and hot
functions. startup is true when function is known to be used only
at startup (from static constructors or it is main()).
exit is true when function is known to be used only at exit
(from static destructors).
Return NULL if function should go to default text section.
Used by the target to emit any assembler directives or additional labels needed when a function is partitioned between different sections. Output should be written to file. The function decl is available as decl and the new section is ‘cold’ if new_is_cold is true.
This flag is true if the target supports TARGET_ASM_NAMED_SECTION.
It must not be modified by command-line option processing.
This flag is true if we can create zeroed data by switching to a BSS
section and then using ASM_OUTPUT_SKIP to allocate the space.
This is true on most ELF targets.
Choose a set of section attributes for use by TARGET_ASM_NAMED_SECTION
based on a variable or function decl, a section name, and whether or not the
declaration’s initializer may contain runtime relocations. decl may be
null, in which case read-write data should be assumed.
The default version of this function handles choosing code vs data,
read-only vs read-write data, and flag_pic. You should only
need to override this if your target has special flags that might be
set via __attribute__.
Provides the target with the ability to record the gcc command line switches that have been passed to the compiler, and options that are enabled. The type argument specifies what is being recorded. It can take the following values:
SWITCH_TYPE_PASSEDtext is a command line switch that has been set by the user.
SWITCH_TYPE_ENABLEDtext is an option which has been enabled. This might be as a direct result of a command line switch, or because it is enabled by default or because it has been enabled as a side effect of a different command line switch. For example, the -O2 switch enables various different individual optimization passes.
SWITCH_TYPE_DESCRIPTIVEtext is either NULL or some descriptive text which should be ignored. If text is NULL then it is being used to warn the target hook that either recording is starting or ending. The first time type is SWITCH_TYPE_DESCRIPTIVE and text is NULL, the warning is for start up and the second time the warning is for wind down. This feature is to allow the target hook to make any necessary preparations before it starts to record switches and to perform any necessary tidying up after it has finished recording switches.
SWITCH_TYPE_LINE_STARTThis option can be ignored by this target hook.
SWITCH_TYPE_LINE_ENDThis option can be ignored by this target hook.
The hook’s return value must be zero. Other return values may be supported in the future.
By default this hook is set to NULL, but an example implementation is
provided for ELF based targets. Called elf_record_gcc_switches,
it records the switches as ASCII text inside a new, string mergeable
section in the assembler output file. The name of the new section is
provided by the TARGET_ASM_RECORD_GCC_SWITCHES_SECTION target
hook.
This is the name of the section that will be created by the example
ELF implementation of the TARGET_ASM_RECORD_GCC_SWITCHES target
hook.
Next: Uninitialized Data, Previous: File Framework, Up: Assembler Format [Contents][Index]
These hooks specify assembly directives for creating certain kinds
of integer object. The TARGET_ASM_BYTE_OP directive creates a
byte-sized object, the TARGET_ASM_ALIGNED_HI_OP one creates an
aligned two-byte object, and so on. Any of the hooks may be
NULL, indicating that no suitable directive is available.
The compiler will print these strings at the start of a new line, followed immediately by the object’s initial value. In most cases, the string should contain a tab, a pseudo-op, and then another tab.
The assemble_integer function uses this hook to output an
integer object. x is the object’s value, size is its size
in bytes and aligned_p indicates whether it is aligned. The
function should return true if it was able to output the
object. If it returns false, assemble_integer will try to
split the object into smaller parts.
The default implementation of this hook will use the
TARGET_ASM_BYTE_OP family of strings, returning false
when the relevant string is NULL.
A target hook to recognize rtx patterns that output_addr_const
can’t deal with, and output assembly code to file corresponding to
the pattern x. This may be used to allow machine-dependent
UNSPECs to appear within constants.
If target hook fails to recognize a pattern, it must return false,
so that a standard error message is printed. If it prints an error message
itself, by calling, for example, output_operand_lossage, it may just
return true.
A C statement to recognize rtx patterns that
output_addr_const can’t deal with, and output assembly code to
stream corresponding to the pattern x. This may be used to
allow machine-dependent UNSPECs to appear within constants.
If OUTPUT_ADDR_CONST_EXTRA fails to recognize a pattern, it must
goto fail, so that a standard error message is printed. If it
prints an error message itself, by calling, for example,
output_operand_lossage, it may just complete normally.
A C statement to output to the stdio stream stream an assembler
instruction to assemble a string constant containing the len
bytes at ptr. ptr will be a C expression of type
char * and len a C expression of type int.
If the assembler has a .ascii pseudo-op as found in the
Berkeley Unix assembler, do not define the macro
ASM_OUTPUT_ASCII.
A C statement to output word n of a function descriptor for
decl. This must be defined if TARGET_VTABLE_USES_DESCRIPTORS
is defined, and is otherwise unused.
You may define this macro as a C expression. You should define the expression to have a nonzero value if GCC should output the constant pool for a function before the code for the function, or a zero value if GCC should output the constant pool after the function. If you do not define this macro, the usual case, GCC will output the constant pool before the function.
A C statement to output assembler commands to define the start of the constant pool for a function. funname is a string giving the name of the function. Should the return type of the function be required, it can be obtained via fundecl. size is the size, in bytes, of the constant pool that will be written immediately after this call.
If no constant-pool prefix is required, the usual case, this macro need not be defined.
A C statement (with or without semicolon) to output a constant in the constant pool, if it needs special treatment. (This macro need not do anything for RTL expressions that can be output normally.)
The argument file is the standard I/O stream to output the assembler code on. x is the RTL expression for the constant to output, and mode is the machine mode (in case x is a ‘const_int’). align is the required alignment for the value x; you should output an assembler directive to force this much alignment.
The argument labelno is a number to use in an internal label for the address of this pool entry. The definition of this macro is responsible for outputting the label definition at the proper place. Here is how to do this:
(*targetm.asm_out.internal_label) (file, "LC", labelno);
When you output a pool entry specially, you should end with a
goto to the label jumpto. This will prevent the same pool
entry from being output a second time in the usual manner.
You need not define this macro if it would do nothing.
A C statement to output assembler commands to at the end of the constant pool for a function. funname is a string giving the name of the function. Should the return type of the function be required, you can obtain it via fundecl. size is the size, in bytes, of the constant pool that GCC wrote immediately before this call.
If no constant-pool epilogue is required, the usual case, you need not define this macro.
Define this macro as a C expression which is nonzero if C is used as a logical line separator by the assembler. STR points to the position in the string where C was found; this can be used if a line separator uses multiple characters.
If you do not define this macro, the default is that only the character ‘;’ is treated as a logical line separator.
These target hooks are C string constants, describing the syntax in the assembler for grouping arithmetic expressions. If not overridden, they default to normal parentheses, which is correct for most assemblers.
These macros are provided by real.h for writing the definitions
of ASM_OUTPUT_DOUBLE and the like:
These translate x, of type REAL_VALUE_TYPE, to the
target’s floating point representation, and store its bit pattern in
the variable l. For REAL_VALUE_TO_TARGET_SINGLE and
REAL_VALUE_TO_TARGET_DECIMAL32, this variable should be a
simple long int. For the others, it should be an array of
long int. The number of elements in this array is determined
by the size of the desired target floating point data type: 32 bits of
it go in each long int array element. Each array element holds
32 bits of the result, even if long int is wider than 32 bits
on the host machine.
The array element values are designed so that you can print them out
using fprintf in the order they should appear in the target
machine’s memory.
Next: Label Output, Previous: Data Output, Up: Assembler Format [Contents][Index]
Each of the macros in this section is used to do the whole job of outputting a single uninitialized variable.
A C statement (sans semicolon) to output to the stdio stream stream the assembler definition of a common-label named name whose size is size bytes. The variable rounded is the size rounded up to whatever alignment the caller wants. It is possible that size may be zero, for instance if a struct with no other member than a zero-length array is defined. In this case, the backend must output a symbol definition that allocates at least one byte, both so that the address of the resulting object does not compare equal to any other, and because some object formats cannot even express the concept of a zero-sized common symbol, as that is how they represent an ordinary undefined external.
Use the expression assemble_name (stream, name) to
output the name itself; before and after that, output the additional
assembler syntax for defining the name, and a newline.
This macro controls how the assembler definitions of uninitialized common global variables are output.
Like ASM_OUTPUT_COMMON except takes the required alignment as a
separate, explicit argument. If you define this macro, it is used in
place of ASM_OUTPUT_COMMON, and gives you more flexibility in
handling the required alignment of the variable. The alignment is specified
as the number of bits.
Like ASM_OUTPUT_ALIGNED_COMMON except that decl of the
variable to be output, if there is one, or NULL_TREE if there
is no corresponding variable. If you define this macro, GCC will use it
in place of both ASM_OUTPUT_COMMON and
ASM_OUTPUT_ALIGNED_COMMON. Define this macro when you need to see
the variable’s decl in order to chose what to output.
A C statement (sans semicolon) to output to the stdio stream stream the assembler definition of uninitialized global decl named name whose size is size bytes. The variable rounded is the size rounded up to whatever alignment the caller wants.
Try to use function asm_output_bss defined in varasm.c when
defining this macro. If unable, use the expression
assemble_name (stream, name) to output the name itself;
before and after that, output the additional assembler syntax for defining
the name, and a newline.
There are two ways of handling global BSS. One is to define either
this macro or its aligned counterpart, ASM_OUTPUT_ALIGNED_BSS.
The other is to have TARGET_ASM_SELECT_SECTION return a
switchable BSS section (see TARGET_HAVE_SWITCHABLE_BSS_SECTIONS).
You do not need to do both.
Some languages do not have common data, and require a
non-common form of global BSS in order to handle uninitialized globals
efficiently. C++ is one example of this. However, if the target does
not support global BSS, the front end may choose to make globals
common in order to save space in the object file.
Like ASM_OUTPUT_BSS except takes the required alignment as a
separate, explicit argument. If you define this macro, it is used in
place of ASM_OUTPUT_BSS, and gives you more flexibility in
handling the required alignment of the variable. The alignment is specified
as the number of bits.
Try to use function asm_output_aligned_bss defined in file
varasm.c when defining this macro.
A C statement (sans semicolon) to output to the stdio stream stream the assembler definition of a local-common-label named name whose size is size bytes. The variable rounded is the size rounded up to whatever alignment the caller wants.
Use the expression assemble_name (stream, name) to
output the name itself; before and after that, output the additional
assembler syntax for defining the name, and a newline.
This macro controls how the assembler definitions of uninitialized static variables are output.
Like ASM_OUTPUT_LOCAL except takes the required alignment as a
separate, explicit argument. If you define this macro, it is used in
place of ASM_OUTPUT_LOCAL, and gives you more flexibility in
handling the required alignment of the variable. The alignment is specified
as the number of bits.
Like ASM_OUTPUT_ALIGNED_DECL except that decl of the
variable to be output, if there is one, or NULL_TREE if there
is no corresponding variable. If you define this macro, GCC will use it
in place of both ASM_OUTPUT_DECL and
ASM_OUTPUT_ALIGNED_DECL. Define this macro when you need to see
the variable’s decl in order to chose what to output.
Next: Initialization, Previous: Uninitialized Data, Up: Assembler Format [Contents][Index]
This is about outputting labels.
A C statement (sans semicolon) to output to the stdio stream
stream the assembler definition of a label named name.
Use the expression assemble_name (stream, name) to
output the name itself; before and after that, output the additional
assembler syntax for defining the name, and a newline. A default
definition of this macro is provided which is correct for most systems.
A C statement (sans semicolon) to output to the stdio stream
stream the assembler definition of a label named name of
a function.
Use the expression assemble_name (stream, name) to
output the name itself; before and after that, output the additional
assembler syntax for defining the name, and a newline. A default
definition of this macro is provided which is correct for most systems.
If this macro is not defined, then the function name is defined in the
usual manner as a label (by means of ASM_OUTPUT_LABEL).
Identical to ASM_OUTPUT_LABEL, except that name is known
to refer to a compiler-generated label. The default definition uses
assemble_name_raw, which is like assemble_name except
that it is more efficient.
A C string containing the appropriate assembler directive to specify the size of a symbol, without any arguments. On systems that use ELF, the default (in config/elfos.h) is ‘"\t.size\t"’; on other systems, the default is not to define this macro.
Define this macro only if it is correct to use the default definitions
of ASM_OUTPUT_SIZE_DIRECTIVE and ASM_OUTPUT_MEASURED_SIZE
for your system. If you need your own custom definitions of those
macros, or if you do not need explicit symbol sizes at all, do not
define this macro.
A C statement (sans semicolon) to output to the stdio stream
stream a directive telling the assembler that the size of the
symbol name is size. size is a HOST_WIDE_INT.
If you define SIZE_ASM_OP, a default definition of this macro is
provided.
A C statement (sans semicolon) to output to the stdio stream stream a directive telling the assembler to calculate the size of the symbol name by subtracting its address from the current address.
If you define SIZE_ASM_OP, a default definition of this macro is
provided. The default assumes that the assembler recognizes a special
‘.’ symbol as referring to the current address, and can calculate
the difference between this and another symbol. If your assembler does
not recognize ‘.’ or cannot do calculations with it, you will need
to redefine ASM_OUTPUT_MEASURED_SIZE to use some other technique.
A C string containing the appropriate assembler directive to specify the type of a symbol, without any arguments. On systems that use ELF, the default (in config/elfos.h) is ‘"\t.type\t"’; on other systems, the default is not to define this macro.
Define this macro only if it is correct to use the default definition of
ASM_OUTPUT_TYPE_DIRECTIVE for your system. If you need your own
custom definition of this macro, or if you do not need explicit symbol
types at all, do not define this macro.
A C string which specifies (using printf syntax) the format of
the second operand to TYPE_ASM_OP. On systems that use ELF, the
default (in config/elfos.h) is ‘"@%s"’; on other systems,
the default is not to define this macro.
Define this macro only if it is correct to use the default definition of
ASM_OUTPUT_TYPE_DIRECTIVE for your system. If you need your own
custom definition of this macro, or if you do not need explicit symbol
types at all, do not define this macro.
A C statement (sans semicolon) to output to the stdio stream stream a directive telling the assembler that the type of the symbol name is type. type is a C string; currently, that string is always either ‘"function"’ or ‘"object"’, but you should not count on this.
If you define TYPE_ASM_OP and TYPE_OPERAND_FMT, a default
definition of this macro is provided.
A C statement (sans semicolon) to output to the stdio stream
stream any text necessary for declaring the name name of a
function which is being defined. This macro is responsible for
outputting the label definition (perhaps using
ASM_OUTPUT_FUNCTION_LABEL). The argument decl is the
FUNCTION_DECL tree node representing the function.
If this macro is not defined, then the function name is defined in the
usual manner as a label (by means of ASM_OUTPUT_FUNCTION_LABEL).
You may wish to use ASM_OUTPUT_TYPE_DIRECTIVE in the definition
of this macro.
A C statement (sans semicolon) to output to the stdio stream
stream any text necessary for declaring the size of a function
which is being defined. The argument name is the name of the
function. The argument decl is the FUNCTION_DECL tree node
representing the function.
If this macro is not defined, then the function size is not defined.
You may wish to use ASM_OUTPUT_MEASURED_SIZE in the definition
of this macro.
A C statement (sans semicolon) to output to the stdio stream
stream any text necessary for declaring the name name of an
initialized variable which is being defined. This macro must output the
label definition (perhaps using ASM_OUTPUT_LABEL). The argument
decl is the VAR_DECL tree node representing the variable.
If this macro is not defined, then the variable name is defined in the
usual manner as a label (by means of ASM_OUTPUT_LABEL).
You may wish to use ASM_OUTPUT_TYPE_DIRECTIVE and/or
ASM_OUTPUT_SIZE_DIRECTIVE in the definition of this macro.
A target hook to output to the stdio stream file any text necessary
for declaring the name name of a constant which is being defined. This
target hook is responsible for outputting the label definition (perhaps using
assemble_label). The argument exp is the value of the constant,
and size is the size of the constant in bytes. The name
will be an internal label.
The default version of this target hook, define the name in the
usual manner as a label (by means of assemble_label).
You may wish to use ASM_OUTPUT_TYPE_DIRECTIVE in this target hook.
A C statement (sans semicolon) to output to the stdio stream stream any text necessary for claiming a register regno for a global variable decl with name name.
If you don’t define this macro, that is equivalent to defining it to do nothing.
A C statement (sans semicolon) to finish up declaring a variable name once the compiler has processed its initializer fully and thus has had a chance to determine the size of an array when controlled by an initializer. This is used on systems where it’s necessary to declare something about the size of the object.
If you don’t define this macro, that is equivalent to defining it to do nothing.
You may wish to use ASM_OUTPUT_SIZE_DIRECTIVE and/or
ASM_OUTPUT_MEASURED_SIZE in the definition of this macro.
This target hook is a function to output to the stdio stream stream some commands that will make the label name global; that is, available for reference from other files.
The default implementation relies on a proper definition of
GLOBAL_ASM_OP.
This target hook is a function to output to the stdio stream stream some commands that will make the name associated with decl global; that is, available for reference from other files.
The default implementation uses the TARGET_ASM_GLOBALIZE_LABEL target hook.
A C statement (sans semicolon) to output to the stdio stream
stream some commands that will make the label name weak;
that is, available for reference from other files but only used if
no other definition is available. Use the expression
assemble_name (stream, name) to output the name
itself; before and after that, output the additional assembler syntax
for making that name weak, and a newline.
If you don’t define this macro or ASM_WEAKEN_DECL, GCC will not
support weak symbols and you should not define the SUPPORTS_WEAK
macro.
Combines (and replaces) the function of ASM_WEAKEN_LABEL and
ASM_OUTPUT_WEAK_ALIAS, allowing access to the associated function
or variable decl. If value is not NULL, this C statement
should output to the stdio stream stream assembler code which
defines (equates) the weak symbol name to have the value
value. If value is NULL, it should output commands
to make name weak.
Outputs a directive that enables name to be used to refer to
symbol value with weak-symbol semantics. decl is the
declaration of name.
A preprocessor constant expression which evaluates to true if the target supports weak symbols.
If you don’t define this macro, defaults.h provides a default
definition. If either ASM_WEAKEN_LABEL or ASM_WEAKEN_DECL
is defined, the default definition is ‘1’; otherwise, it is ‘0’.
A C expression which evaluates to true if the target supports weak symbols.
If you don’t define this macro, defaults.h provides a default definition. The default definition is ‘(SUPPORTS_WEAK)’. Define this macro if you want to control weak symbol support with a compiler flag such as -melf.
A C statement (sans semicolon) to mark decl to be emitted as a public symbol such that extra copies in multiple translation units will be discarded by the linker. Define this macro if your object file format provides support for this concept, such as the ‘COMDAT’ section flags in the Microsoft Windows PE/COFF format, and this support requires changes to decl, such as putting it in a separate section.
A C expression which evaluates to true if the target supports one-only semantics.
If you don’t define this macro, varasm.c provides a default
definition. If MAKE_DECL_ONE_ONLY is defined, the default
definition is ‘1’; otherwise, it is ‘0’. Define this macro if
you want to control one-only symbol support with a compiler flag, or if
setting the DECL_ONE_ONLY flag is enough to mark a declaration to
be emitted as one-only.
This target hook is a function to output to asm_out_file some commands that will make the symbol(s) associated with decl have hidden, protected or internal visibility as specified by visibility.
A C expression that evaluates to true if the target’s linker expects
that weak symbols do not appear in a static archive’s table of contents.
The default is 0.
Leaving weak symbols out of an archive’s table of contents means that, if a symbol will only have a definition in one translation unit and will have undefined references from other translation units, that symbol should not be weak. Defining this macro to be nonzero will thus have the effect that certain symbols that would normally be weak (explicit template instantiations, and vtables for polymorphic classes with noninline key methods) will instead be nonweak.
The C++ ABI requires this macro to be zero. Define this macro for targets where full C++ ABI compliance is impossible and where linker restrictions require weak symbols to be left out of a static archive’s table of contents.
A C statement (sans semicolon) to output to the stdio stream stream any text necessary for declaring the name of an external symbol named name which is referenced in this compilation but not defined. The value of decl is the tree node for the declaration.
This macro need not be defined if it does not need to output anything. The GNU assembler and most Unix assemblers don’t require anything.
This target hook is a function to output to asm_out_file an assembler
pseudo-op to declare a library function name external. The name of the
library function is given by symref, which is a symbol_ref.
This target hook is a function to output to asm_out_file an assembler directive to annotate symbol as used. The Darwin target uses the .no_dead_code_strip directive.
A C statement (sans semicolon) to output to the stdio stream
stream a reference in assembler syntax to a label named
name. This should add ‘_’ to the front of the name, if that
is customary on your operating system, as it is in most Berkeley Unix
systems. This macro is used in assemble_name.
Given a symbol name, perform same mangling as varasm.c’s assemble_name, but in memory rather than to a file stream, returning result as an IDENTIFIER_NODE. Required for correct LTO symtabs. The default implementation calls the TARGET_STRIP_NAME_ENCODING hook and then prepends the USER_LABEL_PREFIX, if any.
A C statement (sans semicolon) to output a reference to
SYMBOL_REF sym. If not defined, assemble_name
will be used to output the name of the symbol. This macro may be used
to modify the way a symbol is referenced depending on information
encoded by TARGET_ENCODE_SECTION_INFO.
A C statement (sans semicolon) to output a reference to buf, the
result of ASM_GENERATE_INTERNAL_LABEL. If not defined,
assemble_name will be used to output the name of the symbol.
This macro is not used by output_asm_label, or the %l
specifier that calls it; the intention is that this macro should be set
when it is necessary to output a label differently when its address is
being taken.
A function to output to the stdio stream stream a label whose name is made from the string prefix and the number labelno.
It is absolutely essential that these labels be distinct from the labels used for user-level functions and variables. Otherwise, certain programs will have name conflicts with internal labels.
It is desirable to exclude internal labels from the symbol table of the object file. Most assemblers have a naming convention for labels that should be excluded; on many systems, the letter ‘L’ at the beginning of a label has this effect. You should find out what convention your system uses, and follow it.
The default version of this function utilizes ASM_GENERATE_INTERNAL_LABEL.
A C statement to output to the stdio stream stream a debug info label whose name is made from the string prefix and the number num. This is useful for VLIW targets, where debug info labels may need to be treated differently than branch target labels. On some systems, branch target labels must be at the beginning of instruction bundles, but debug info labels can occur in the middle of instruction bundles.
If this macro is not defined, then (*targetm.asm_out.internal_label) will be
used.
A C statement to store into the string string a label whose name is made from the string prefix and the number num.
This string, when output subsequently by assemble_name, should
produce the output that (*targetm.asm_out.internal_label) would produce
with the same prefix and num.
If the string begins with ‘*’, then assemble_name will
output the rest of the string unchanged. It is often convenient for
ASM_GENERATE_INTERNAL_LABEL to use ‘*’ in this way. If the
string doesn’t start with ‘*’, then ASM_OUTPUT_LABELREF gets
to output the string, and may change it. (Of course,
ASM_OUTPUT_LABELREF is also part of your machine description, so
you should know what it does on your machine.)
A C expression to assign to outvar (which is a variable of type
char *) a newly allocated string made from the string
name and the number number, with some suitable punctuation
added. Use alloca to get space for the string.
The string will be used as an argument to ASM_OUTPUT_LABELREF to
produce an assembler label for an internal static variable whose name is
name. Therefore, the string must be such as to result in valid
assembler code. The argument number is different each time this
macro is executed; it prevents conflicts between similarly-named
internal static variables in different scopes.
Ideally this string should not be a valid C identifier, to prevent any conflict with the user’s own symbols. Most assemblers allow periods or percent signs in assembler symbols; putting at least one of these between the name and the number will suffice.
If this macro is not defined, a default definition will be provided which is correct for most systems.
A C statement to output to the stdio stream stream assembler code which defines (equates) the symbol name to have the value value.
If SET_ASM_OP is defined, a default definition is provided which is
correct for most systems.
A C statement to output to the stdio stream stream assembler code which defines (equates) the symbol whose tree node is decl_of_name to have the value of the tree node decl_of_value. This macro will be used in preference to ‘ASM_OUTPUT_DEF’ if it is defined and if the tree nodes are available.
If SET_ASM_OP is defined, a default definition is provided which is
correct for most systems.
A C statement that evaluates to true if the assembler code which defines (equates) the symbol whose tree node is decl_of_name to have the value of the tree node decl_of_value should be emitted near the end of the current compilation unit. The default is to not defer output of defines. This macro affects defines output by ‘ASM_OUTPUT_DEF’ and ‘ASM_OUTPUT_DEF_FROM_DECLS’.
A C statement to output to the stdio stream stream assembler code
which defines (equates) the weak symbol name to have the value
value. If value is NULL, it defines name as
an undefined weak symbol.
Define this macro if the target only supports weak aliases; define
ASM_OUTPUT_DEF instead if possible.
Define this macro to override the default assembler names used for Objective-C methods.
The default name is a unique method number followed by the name of the class (e.g. ‘_1_Foo’). For methods in categories, the name of the category is also included in the assembler name (e.g. ‘_1_Foo_Bar’).
These names are safe on most systems, but make debugging difficult since the method’s selector is not present in the name. Therefore, particular systems define other ways of computing names.
buf is an expression of type char * which gives you a
buffer in which to store the name; its length is as long as
class_name, cat_name and sel_name put together, plus
50 characters extra.
The argument is_inst specifies whether the method is an instance
method or a class method; class_name is the name of the class;
cat_name is the name of the category (or NULL if the method is not
in a category); and sel_name is the name of the selector.
On systems where the assembler can handle quoted names, you can use this macro to provide more human-readable names.
A C statement (sans semicolon) to output to the stdio stream stream commands to declare that the label name is an Objective-C class reference. This is only needed for targets whose linkers have special support for NeXT-style runtimes.
A C statement (sans semicolon) to output to the stdio stream stream commands to declare that the label name is an unresolved Objective-C class reference. This is only needed for targets whose linkers have special support for NeXT-style runtimes.
Next: Macros for Initialization, Previous: Label Output, Up: Assembler Format [Contents][Index]
The compiled code for certain languages includes constructors
(also called initialization routines)—functions to initialize
data in the program when the program is started. These functions need
to be called before the program is “started”—that is to say, before
main is called.
Compiling some languages generates destructors (also called termination routines) that should be called when the program terminates.
To make the initialization and termination functions work, the compiler must output something in the assembler code to cause those functions to be called at the appropriate time. When you port the compiler to a new system, you need to specify how to do this.
There are two major ways that GCC currently supports the execution of initialization and termination functions. Each way has two variants. Much of the structure is common to all four variations.
The linker must build two lists of these functions—a list of
initialization functions, called __CTOR_LIST__, and a list of
termination functions, called __DTOR_LIST__.
Each list always begins with an ignored function pointer (which may hold 0, -1, or a count of the function pointers after it, depending on the environment). This is followed by a series of zero or more function pointers to constructors (or destructors), followed by a function pointer containing zero.
Depending on the operating system and its executable file format, either crtstuff.c or libgcc2.c traverses these lists at startup time and exit time. Constructors are called in reverse order of the list; destructors in forward order.
The best way to handle static constructors works only for object file formats which provide arbitrarily-named sections. A section is set aside for a list of constructors, and another for a list of destructors. Traditionally these are called ‘.ctors’ and ‘.dtors’. Each object file that defines an initialization function also puts a word in the constructor section to point to that function. The linker accumulates all these words into one contiguous ‘.ctors’ section. Termination functions are handled similarly.
This method will be chosen as the default by target-def.h if
TARGET_ASM_NAMED_SECTION is defined. A target that does not
support arbitrary sections, but does support special designated
constructor and destructor sections may define CTORS_SECTION_ASM_OP
and DTORS_SECTION_ASM_OP to achieve the same effect.
When arbitrary sections are available, there are two variants, depending
upon how the code in crtstuff.c is called. On systems that
support a .init section which is executed at program startup,
parts of crtstuff.c are compiled into that section. The
program is linked by the gcc driver like this:
ld -o output_file crti.o crtbegin.o … -lgcc crtend.o crtn.o
The prologue of a function (__init) appears in the .init
section of crti.o; the epilogue appears in crtn.o. Likewise
for the function __fini in the .fini section. Normally these
files are provided by the operating system or by the GNU C library, but
are provided by GCC for a few targets.
The objects crtbegin.o and crtend.o are (for most targets)
compiled from crtstuff.c. They contain, among other things, code
fragments within the .init and .fini sections that branch
to routines in the .text section. The linker will pull all parts
of a section together, which results in a complete __init function
that invokes the routines we need at startup.
To use this variant, you must define the INIT_SECTION_ASM_OP
macro properly.
If no init section is available, when GCC compiles any function called
main (or more accurately, any function designated as a program
entry point by the language front end calling expand_main_function),
it inserts a procedure call to __main as the first executable code
after the function prologue. The __main function is defined
in libgcc2.c and runs the global constructors.
In file formats that don’t support arbitrary sections, there are again
two variants. In the simplest variant, the GNU linker (GNU ld)
and an ‘a.out’ format must be used. In this case,
TARGET_ASM_CONSTRUCTOR is defined to produce a .stabs
entry of type ‘N_SETT’, referencing the name __CTOR_LIST__,
and with the address of the void function containing the initialization
code as its value. The GNU linker recognizes this as a request to add
the value to a set; the values are accumulated, and are eventually
placed in the executable as a vector in the format described above, with
a leading (ignored) count and a trailing zero element.
TARGET_ASM_DESTRUCTOR is handled similarly. Since no init
section is available, the absence of INIT_SECTION_ASM_OP causes
the compilation of main to call __main as above, starting
the initialization process.
The last variant uses neither arbitrary sections nor the GNU linker.
This is preferable when you want to do dynamic linking and when using
file formats which the GNU linker does not support, such as ‘ECOFF’. In
this case, TARGET_HAVE_CTORS_DTORS is false, initialization and
termination functions are recognized simply by their names. This requires
an extra program in the linkage step, called collect2. This program
pretends to be the linker, for use with GCC; it does its job by running
the ordinary linker, but also arranges to include the vectors of
initialization and termination functions. These functions are called
via __main as described above. In order to use this method,
use_collect2 must be defined in the target in config.gcc.
Next: Instruction Output, Previous: Initialization, Up: Assembler Format [Contents][Index]
Here are the macros that control how the compiler handles initialization and termination functions:
If defined, a C string constant, including spacing, for the assembler operation to identify the following data as initialization code. If not defined, GCC will assume such a section does not exist. When you are using special sections for initialization and termination functions, this macro also controls how crtstuff.c and libgcc2.c arrange to run the initialization functions.
If defined, main will not call __main as described above.
This macro should be defined for systems that control start-up code
on a symbol-by-symbol basis, such as OSF/1, and should not
be defined explicitly for systems that support INIT_SECTION_ASM_OP.
If defined, a C string constant for a switch that tells the linker that the following symbol is an initialization routine.
If defined, a C string constant for a switch that tells the linker that the following symbol is a finalization routine.
If defined, a C statement that will write a function that can be
automatically called when a shared library is loaded. The function
should call func, which takes no arguments. If not defined, and
the object format requires an explicit initialization function, then a
function called _GLOBAL__DI will be generated.
This function and the following one are used by collect2 when linking a shared library that needs constructors or destructors, or has DWARF2 exception tables embedded in the code.
If defined, a C statement that will write a function that can be
automatically called when a shared library is unloaded. The function
should call func, which takes no arguments. If not defined, and
the object format requires an explicit finalization function, then a
function called _GLOBAL__DD will be generated.
If defined, main will call __main despite the presence of
INIT_SECTION_ASM_OP. This macro should be defined for systems
where the init section is not actually run automatically, but is still
useful for collecting the lists of constructors and destructors.
If nonzero, the C++ init_priority attribute is supported and the
compiler should emit instructions to control the order of initialization
of objects. If zero, the compiler will issue an error message upon
encountering an init_priority attribute.
This value is true if the target supports some “native” method of
collecting constructors and destructors to be run at startup and exit.
It is false if we must use collect2.
If defined, a function that outputs assembler code to arrange to call the function referenced by symbol at initialization time.
Assume that symbol is a SYMBOL_REF for a function taking
no arguments and with no return value. If the target supports initialization
priorities, priority is a value between 0 and MAX_INIT_PRIORITY;
otherwise it must be DEFAULT_INIT_PRIORITY.
If this macro is not defined by the target, a suitable default will
be chosen if (1) the target supports arbitrary section names, (2) the
target defines CTORS_SECTION_ASM_OP, or (3) USE_COLLECT2
is not defined.
This is like TARGET_ASM_CONSTRUCTOR but used for termination
functions rather than initialization functions.
If TARGET_HAVE_CTORS_DTORS is true, the initialization routine
generated for the generated object file will have static linkage.
If your system uses collect2 as the means of processing
constructors, then that program normally uses nm to scan
an object file for constructor functions to be called.
On certain kinds of systems, you can define this macro to make
collect2 work faster (and, in some cases, make it work at all):
Define this macro if the system uses COFF (Common Object File Format)
object files, so that collect2 can assume this format and scan
object files directly for dynamic constructor/destructor functions.
This macro is effective only in a native compiler; collect2 as
part of a cross compiler always uses nm for the target machine.
Define this macro as a C string constant containing the file name to use
to execute nm. The default is to search the path normally for
nm.
collect2 calls nm to scan object files for static
constructors and destructors and LTO info. By default, -n is
passed. Define NM_FLAGS to a C string constant if other options
are needed to get the same output format as GNU nm -n
produces.
If your system supports shared libraries and has a program to list the dynamic dependencies of a given library or executable, you can define these macros to enable support for running initialization and termination functions in shared libraries:
Define this macro to a C string constant containing the name of the program
which lists dynamic dependencies, like ldd under SunOS 4.
Define this macro to be C code that extracts filenames from the output
of the program denoted by LDD_SUFFIX. ptr is a variable
of type char * that points to the beginning of a line of output
from LDD_SUFFIX. If the line lists a dynamic dependency, the
code must advance ptr to the beginning of the filename on that
line. Otherwise, it must set ptr to NULL.
Define this macro to a C string constant containing the default shared
library extension of the target (e.g., ‘".so"’). collect2
strips version information after this suffix when generating global
constructor and destructor names. This define is only needed on targets
that use collect2 to process constructors and destructors.
Next: Dispatch Tables, Previous: Macros for Initialization, Up: Assembler Format [Contents][Index]
This describes assembler instruction output.
A C initializer containing the assembler’s names for the machine registers, each one as a C string constant. This is what translates register numbers in the compiler into assembler language.
If defined, a C initializer for an array of structures containing a name
and a register number. This macro defines additional names for hard
registers, thus allowing the asm option in declarations to refer
to registers using alternate names.
If defined, a C initializer for an array of structures containing a
name, a register number and a count of the number of consecutive
machine registers the name overlaps. This macro defines additional
names for hard registers, thus allowing the asm option in
declarations to refer to registers using alternate names. Unlike
ADDITIONAL_REGISTER_NAMES, this macro should be used when the
register name implies multiple underlying registers.
This macro should be used when it is important that a clobber in an
asm statement clobbers all the underlying values implied by the
register name. For example, on ARM, clobbering the double-precision
VFP register “d0” implies clobbering both single-precision registers
“s0” and “s1”.
Define this macro if you are using an unusual assembler that requires different names for the machine instructions.
The definition is a C statement or statements which output an
assembler instruction opcode to the stdio stream stream. The
macro-operand ptr is a variable of type char * which
points to the opcode name in its “internal” form—the form that is
written in the machine description. The definition should output the
opcode name to stream, performing any translation you desire, and
increment the variable ptr to point at the end of the opcode
so that it will not be output twice.
In fact, your macro definition may process less than the entire opcode name, or more than the opcode name; but if you want to process text that includes ‘%’-sequences to substitute operands, you must take care of the substitution yourself. Just be sure to increment ptr over whatever text should not be output normally.
If you need to look at the operand values, they can be found as the
elements of recog_data.operand.
If the macro definition does nothing, the instruction is output in the usual way.
If defined, a C statement to be executed just prior to the output of assembler code for insn, to modify the extracted operands so they will be output differently.
Here the argument opvec is the vector containing the operands extracted from insn, and noperands is the number of elements of the vector which contain meaningful data for this insn. The contents of this vector are what will be used to convert the insn template into assembler code, so you can change the assembler output by changing the contents of the vector.
This macro is useful when various assembler syntaxes share a single file of instruction patterns; by defining this macro differently, you can cause a large class of instructions to be output differently (such as with rearranged operands). Naturally, variations in assembler syntax affecting individual insn patterns ought to be handled by writing conditional output routines in those patterns.
If this macro is not defined, it is equivalent to a null statement.
If defined, this target hook is a function which is executed just after the output of assembler code for insn, to change the mode of the assembler if necessary.
Here the argument opvec is the vector containing the operands extracted from insn, and noperands is the number of elements of the vector which contain meaningful data for this insn. The contents of this vector are what was used to convert the insn template into assembler code, so you can change the assembler mode by checking the contents of the vector.
A C compound statement to output to stdio stream stream the assembler syntax for an instruction operand x. x is an RTL expression.
code is a value that can be used to specify one of several ways of printing the operand. It is used when identical operands must be printed differently depending on the context. code comes from the ‘%’ specification that was used to request printing of the operand. If the specification was just ‘%digit’ then code is 0; if the specification was ‘%ltr digit’ then code is the ASCII code for ltr.
If x is a register, this macro should print the register’s name.
The names can be found in an array reg_names whose type is
char *[]. reg_names is initialized from
REGISTER_NAMES.
When the machine description has a specification ‘%punct’ (a ‘%’ followed by a punctuation character), this macro is called with a null pointer for x and the punctuation character for code.
A C expression which evaluates to true if code is a valid
punctuation character for use in the PRINT_OPERAND macro. If
PRINT_OPERAND_PUNCT_VALID_P is not defined, it means that no
punctuation characters (except for the standard one, ‘%’) are used
in this way.
A C compound statement to output to stdio stream stream the assembler syntax for an instruction operand that is a memory reference whose address is x. x is an RTL expression.
On some machines, the syntax for a symbolic address depends on the
section that the address refers to. On these machines, define the hook
TARGET_ENCODE_SECTION_INFO to store the information into the
symbol_ref, and then check for it here. See Assembler Format.
A C statement, to be executed after all slot-filler instructions have
been output. If necessary, call dbr_sequence_length to
determine the number of slots filled in a sequence (zero if not
currently outputting a sequence), to decide how many no-ops to output,
or whatever.
Don’t define this macro if it has nothing to do, but it is helpful in reading assembly output if the extent of the delay sequence is made explicit (e.g. with white space).
Note that output routines for instructions with delay slots must be
prepared to deal with not being output as part of a sequence
(i.e. when the scheduling pass is not run, or when no slot fillers could be
found.) The variable final_sequence is null when not
processing a sequence, otherwise it contains the sequence rtx
being output.
If defined, C string expressions to be used for the ‘%R’, ‘%L’,
‘%U’, and ‘%I’ options of asm_fprintf (see
final.c). These are useful when a single md file must
support multiple assembler formats. In that case, the various tm.h
files can define these macros differently.
If defined this macro should expand to a series of case
statements which will be parsed inside the switch statement of
the asm_fprintf function. This allows targets to define extra
printf formats which may useful when generating their assembler
statements. Note that uppercase letters are reserved for future
generic extensions to asm_fprintf, and so are not available to target
specific code. The output file is given by the parameter file.
The varargs input pointer is argptr and the rest of the format
string, starting the character after the one that is being switched
upon, is pointed to by format.
If your target supports multiple dialects of assembler language (such as different opcodes), define this macro as a C expression that gives the numeric index of the assembler language dialect to use, with zero as the first variant.
If this macro is defined, you may use constructs of the form
‘{option0|option1|option2…}’
in the output templates of patterns (see Output Template) or in the
first argument of asm_fprintf. This construct outputs
‘option0’, ‘option1’, ‘option2’, etc., if the value of
ASSEMBLER_DIALECT is zero, one, two, etc. Any special characters
within these strings retain their usual meaning. If there are fewer
alternatives within the braces than the value of
ASSEMBLER_DIALECT, the construct outputs nothing.
If you do not define this macro, the characters ‘{’, ‘|’ and
‘}’ do not have any special meaning when used in templates or
operands to asm_fprintf.
Define the macros REGISTER_PREFIX, LOCAL_LABEL_PREFIX,
USER_LABEL_PREFIX and IMMEDIATE_PREFIX if you can express
the variations in assembler language syntax with that mechanism. Define
ASSEMBLER_DIALECT and use the ‘{option0|option1}’ syntax
if the syntax variant are larger and involve such things as different
opcodes or operand order.
A C expression to output to stream some assembler code which will push hard register number regno onto the stack. The code need not be optimal, since this macro is used only when profiling.
A C expression to output to stream some assembler code which will pop hard register number regno off of the stack. The code need not be optimal, since this macro is used only when profiling.
Next: Exception Region Output, Previous: Instruction Output, Up: Assembler Format [Contents][Index]
This concerns dispatch tables.
A C statement to output to the stdio stream stream an assembler
pseudo-instruction to generate a difference between two labels.
value and rel are the numbers of two internal labels. The
definitions of these labels are output using
(*targetm.asm_out.internal_label), and they must be printed in the same
way here. For example,
fprintf (stream, "\t.word L%d-L%d\n",
value, rel)
You must provide this macro on machines where the addresses in a
dispatch table are relative to the table’s own address. If defined, GCC
will also use this macro on all machines when producing PIC.
body is the body of the ADDR_DIFF_VEC; it is provided so that the
mode and flags can be read.
This macro should be provided on machines where the addresses in a dispatch table are absolute.
The definition should be a C statement to output to the stdio stream
stream an assembler pseudo-instruction to generate a reference to
a label. value is the number of an internal label whose
definition is output using (*targetm.asm_out.internal_label).
For example,
fprintf (stream, "\t.word L%d\n", value)
Define this if the label before a jump-table needs to be output
specially. The first three arguments are the same as for
(*targetm.asm_out.internal_label); the fourth argument is the
jump-table which follows (a jump_insn containing an
addr_vec or addr_diff_vec).
This feature is used on system V to output a swbeg statement
for the table.
If this macro is not defined, these labels are output with
(*targetm.asm_out.internal_label).
Define this if something special must be output at the end of a jump-table. The definition should be a C statement to be executed after the assembler code for the table is written. It should write the appropriate code to stdio stream stream. The argument table is the jump-table insn, and num is the label-number of the preceding label.
If this macro is not defined, nothing special is output at the end of the jump-table.
This target hook emits a label at the beginning of each FDE. It should be defined on targets where FDEs need special labels, and it should write the appropriate label, for the FDE associated with the function declaration decl, to the stdio stream stream. The third argument, for_eh, is a boolean: true if this is for an exception table. The fourth argument, empty, is a boolean: true if this is a placeholder label for an omitted FDE.
The default is that FDEs are not given nonlocal labels.
This target hook emits a label at the beginning of the exception table. It should be defined on targets where it is desirable for the table to be broken up according to function.
The default is that no label is emitted.
If the target implements TARGET_ASM_UNWIND_EMIT, this hook may be used to emit a directive to install a personality hook into the unwind info. This hook should not be used if dwarf2 unwind info is used.
This target hook emits assembly directives required to unwind the
given instruction. This is only used when TARGET_EXCEPT_UNWIND_INFO
returns UI_TARGET.
True if the TARGET_ASM_UNWIND_EMIT hook should be called before the assembly for insn has been emitted, false if the hook should be called afterward.
Next: Alignment Output, Previous: Dispatch Tables, Up: Assembler Format [Contents][Index]
This describes commands marking the start and the end of an exception region.
If defined, a C string constant for the name of the section containing exception handling frame unwind information. If not defined, GCC will provide a default definition if the target supports named sections. crtstuff.c uses this macro to switch to the appropriate section.
You should define this symbol if your target supports DWARF 2 frame unwind information and the default definition does not work.
If defined, DWARF 2 frame unwind information will be placed in the data section even though the target supports named sections. This might be necessary, for instance, if the system linker does garbage collection and sections cannot be marked as not to be collected.
Do not define this macro unless TARGET_ASM_NAMED_SECTION is
also defined.
Define this macro to 1 if your target is such that no frame unwind information encoding used with non-PIC code will ever require a runtime relocation, but the linker may not support merging read-only and read-write sections into a single read-write section.
An rtx used to mask the return address found via RETURN_ADDR_RTX, so
that it does not contain any extraneous set bits in it.
Define this macro to 0 if your target supports DWARF 2 frame unwind
information, but it does not yet work with exception handling.
Otherwise, if your target supports this information (if it defines
INCOMING_RETURN_ADDR_RTX and either UNALIGNED_INT_ASM_OP
or OBJECT_FORMAT_ELF), GCC will provide a default definition of 1.
This hook defines the mechanism that will be used for exception handling
by the target. If the target has ABI specified unwind tables, the hook
should return UI_TARGET. If the target is to use the
setjmp/longjmp-based exception handling scheme, the hook
should return UI_SJLJ. If the target supports DWARF 2 frame unwind
information, the hook should return UI_DWARF2.
A target may, if exceptions are disabled, choose to return UI_NONE.
This may end up simplifying other parts of target-specific code. The
default implementation of this hook never returns UI_NONE.
Note that the value returned by this hook should be constant. It should
not depend on anything except the command-line switches described by
opts. In particular, the
setting UI_SJLJ must be fixed at compiler start-up as C pre-processor
macros and builtin functions related to exception handling are set up
depending on this setting.
The default implementation of the hook first honors the
--enable-sjlj-exceptions configure option, then
DWARF2_UNWIND_INFO, and finally defaults to UI_SJLJ. If
DWARF2_UNWIND_INFO depends on command-line options, the target
must define this hook so that opts is used correctly.
This variable should be set to true if the target ABI requires unwinding
tables even when exceptions are not used. It must not be modified by
command-line option processing.
Define this macro to 1 if the setjmp/longjmp-based scheme
should use the setjmp/longjmp functions from the C library
instead of the __builtin_setjmp/__builtin_longjmp machinery.
This macro need only be defined if the target might save registers in the
function prologue at an offset to the stack pointer that is not aligned to
UNITS_PER_WORD. The definition should be the negative minimum
alignment if STACK_GROWS_DOWNWARD is defined, and the positive
minimum alignment otherwise. See SDB and DWARF. Only applicable if
the target supports DWARF 2 frame unwind information.
Contains the value true if the target should add a zero word onto the
end of a Dwarf-2 frame info section when used for exception handling.
Default value is false if EH_FRAME_SECTION_NAME is defined, and
true otherwise.
Given a register, this hook should return a parallel of registers to
represent where to find the register pieces. Define this hook if the
register and its mode are represented in Dwarf in non-contiguous
locations, or if the register should be represented in more than one
register in Dwarf. Otherwise, this hook should return NULL_RTX.
If not defined, the default is to return NULL_RTX.
If some registers are represented in Dwarf-2 unwind information in
multiple pieces, define this hook to fill in information about the
sizes of those pieces in the table used by the unwinder at runtime.
It will be called by expand_builtin_init_dwarf_reg_sizes after
filling in a single size corresponding to each hard register;
address is the address of the table.
This hook is used to output a reference from a frame unwinding table to
the type_info object identified by sym. It should return true
if the reference was output. Returning false will cause the
reference to be output using the normal Dwarf2 routines.
This flag should be set to true on targets that use an ARM EABI
based unwinding library, and false on other targets. This effects
the format of unwinding tables, and how the unwinder in entered after
running a cleanup. The default is false.
Previous: Exception Region Output, Up: Assembler Format [Contents][Index]
This describes commands for alignment.
The alignment (log base 2) to put in front of label, which is a common destination of jumps and has no fallthru incoming edge.
This macro need not be defined if you don’t want any special alignment to be done at such a time. Most machine descriptions do not currently define the macro.
Unless it’s necessary to inspect the label parameter, it is better
to set the variable align_jumps in the target’s
TARGET_OPTION_OVERRIDE. Otherwise, you should try to honor the user’s
selection in align_jumps in a JUMP_ALIGN implementation.
The maximum number of bytes to skip before label when applying
JUMP_ALIGN. This works only if
ASM_OUTPUT_MAX_SKIP_ALIGN is defined.
The alignment (log base 2) to put in front of label, which follows
a BARRIER.
This macro need not be defined if you don’t want any special alignment to be done at such a time. Most machine descriptions do not currently define the macro.
The maximum number of bytes to skip before label when applying
LABEL_ALIGN_AFTER_BARRIER. This works only if
ASM_OUTPUT_MAX_SKIP_ALIGN is defined.
The alignment (log base 2) to put in front of label, which follows
a NOTE_INSN_LOOP_BEG note.
This macro need not be defined if you don’t want any special alignment to be done at such a time. Most machine descriptions do not currently define the macro.
Unless it’s necessary to inspect the label parameter, it is better
to set the variable align_loops in the target’s
TARGET_OPTION_OVERRIDE. Otherwise, you should try to honor the user’s
selection in align_loops in a LOOP_ALIGN implementation.
The maximum number of bytes to skip when applying LOOP_ALIGN to
label. This works only if ASM_OUTPUT_MAX_SKIP_ALIGN is
defined.
The alignment (log base 2) to put in front of label.
If LABEL_ALIGN_AFTER_BARRIER / LOOP_ALIGN specify a different alignment,
the maximum of the specified values is used.
Unless it’s necessary to inspect the label parameter, it is better
to set the variable align_labels in the target’s
TARGET_OPTION_OVERRIDE. Otherwise, you should try to honor the user’s
selection in align_labels in a LABEL_ALIGN implementation.
The maximum number of bytes to skip when applying LABEL_ALIGN
to label. This works only if ASM_OUTPUT_MAX_SKIP_ALIGN
is defined.
A C statement to output to the stdio stream stream an assembler
instruction to advance the location counter by nbytes bytes.
Those bytes should be zero when loaded. nbytes will be a C
expression of type unsigned HOST_WIDE_INT.
Define this macro if ASM_OUTPUT_SKIP should not be used in the
text section because it fails to put zeros in the bytes that are skipped.
This is true on many Unix systems, where the pseudo–op to skip bytes
produces no-op instructions rather than zeros when used in the text
section.
A C statement to output to the stdio stream stream an assembler
command to advance the location counter to a multiple of 2 to the
power bytes. power will be a C expression of type int.
Like ASM_OUTPUT_ALIGN, except that the “nop” instruction is used
for padding, if necessary.
A C statement to output to the stdio stream stream an assembler
command to advance the location counter to a multiple of 2 to the
power bytes, but only if max_skip or fewer bytes are needed to
satisfy the alignment request. power and max_skip will be
a C expression of type int.
Next: Floating Point, Previous: Assembler Format, Up: Target Macros [Contents][Index]
This describes how to specify debugging information.
| • All Debuggers: | Macros that affect all debugging formats uniformly. | |
| • DBX Options: | Macros enabling specific options in DBX format. | |
| • DBX Hooks: | Hook macros for varying DBX format. | |
| • File Names and DBX: | Macros controlling output of file names in DBX format. | |
| • SDB and DWARF: | Macros for SDB (COFF) and DWARF formats. | |
| • VMS Debug: | Macros for VMS debug format. |
Next: DBX Options, Up: Debugging Info [Contents][Index]
These macros affect all debugging formats.
A C expression that returns the DBX register number for the compiler register number regno. In the default macro provided, the value of this expression will be regno itself. But sometimes there are some registers that the compiler knows about and DBX does not, or vice versa. In such cases, some register may need to have one number in the compiler and another for DBX.
If two registers have consecutive numbers inside GCC, and they can be
used as a pair to hold a multiword value, then they must have
consecutive numbers after renumbering with DBX_REGISTER_NUMBER.
Otherwise, debuggers will be unable to access such a pair, because they
expect register pairs to be consecutive in their own numbering scheme.
If you find yourself defining DBX_REGISTER_NUMBER in way that
does not preserve register pairs, then what you must do instead is
redefine the actual register numbering scheme.
A C expression that returns the integer offset value for an automatic variable having address x (an RTL expression). The default computation assumes that x is based on the frame-pointer and gives the offset from the frame-pointer. This is required for targets that produce debugging output for DBX or COFF-style debugging output for SDB and allow the frame-pointer to be eliminated when the -g options is used.
A C expression that returns the integer offset value for an argument having address x (an RTL expression). The nominal offset is offset.
A C expression that returns the type of debugging output GCC should
produce when the user specifies just -g. Define
this if you have arranged for GCC to support more than one format of
debugging output. Currently, the allowable values are DBX_DEBUG,
SDB_DEBUG, DWARF_DEBUG, DWARF2_DEBUG,
XCOFF_DEBUG, VMS_DEBUG, and VMS_AND_DWARF2_DEBUG.
When the user specifies -ggdb, GCC normally also uses the
value of this macro to select the debugging output format, but with two
exceptions. If DWARF2_DEBUGGING_INFO is defined, GCC uses the
value DWARF2_DEBUG. Otherwise, if DBX_DEBUGGING_INFO is
defined, GCC uses DBX_DEBUG.
The value of this macro only affects the default debugging output; the user can always get a specific type of output by using -gstabs, -gcoff, -gdwarf-2, -gxcoff, or -gvms.
Next: DBX Hooks, Previous: All Debuggers, Up: Debugging Info [Contents][Index]
These are specific options for DBX output.
Define this macro if GCC should produce debugging output for DBX in response to the -g option.
Define this macro if GCC should produce XCOFF format debugging output in response to the -g option. This is a variant of DBX format.
Define this macro to control whether GCC should by default generate GDB’s extended version of DBX debugging information (assuming DBX-format debugging information is enabled at all). If you don’t define the macro, the default is 1: always generate the extended information if there is any occasion to.
Define this macro if all .stabs commands should be output while
in the text section.
A C string constant, including spacing, naming the assembler pseudo op to
use instead of "\t.stabs\t" to define an ordinary debugging symbol.
If you don’t define this macro, "\t.stabs\t" is used. This macro
applies only to DBX debugging information format.
A C string constant, including spacing, naming the assembler pseudo op to
use instead of "\t.stabd\t" to define a debugging symbol whose
value is the current location. If you don’t define this macro,
"\t.stabd\t" is used. This macro applies only to DBX debugging
information format.
A C string constant, including spacing, naming the assembler pseudo op to
use instead of "\t.stabn\t" to define a debugging symbol with no
name. If you don’t define this macro, "\t.stabn\t" is used. This
macro applies only to DBX debugging information format.
Define this macro if DBX on your system does not support the construct ‘xstagname’. On some systems, this construct is used to describe a forward reference to a structure named tagname. On other systems, this construct is not supported at all.
A symbol name in DBX-format debugging information is normally
continued (split into two separate .stabs directives) when it
exceeds a certain length (by default, 80 characters). On some
operating systems, DBX requires this splitting; on others, splitting
must not be done. You can inhibit splitting by defining this macro
with the value zero. You can override the default splitting-length by
defining this macro as an expression for the length you desire.
Normally continuation is indicated by adding a ‘\’ character to
the end of a .stabs string when a continuation follows. To use
a different character instead, define this macro as a character
constant for the character you want to use. Do not define this macro
if backslash is correct for your system.
Define this macro if it is necessary to go to the data section before outputting the ‘.stabs’ pseudo-op for a non-global static variable.
The value to use in the “code” field of the .stabs directive
for a typedef. The default is N_LSYM.
The value to use in the “code” field of the .stabs directive
for a static variable located in the text section. DBX format does not
provide any “right” way to do this. The default is N_FUN.
The value to use in the “code” field of the .stabs directive
for a parameter passed in registers. DBX format does not provide any
“right” way to do this. The default is N_RSYM.
The letter to use in DBX symbol data to identify a symbol as a parameter
passed in registers. DBX format does not customarily provide any way to
do this. The default is 'P'.
Define this macro if the DBX information for a function and its arguments should precede the assembler code for the function. Normally, in DBX format, the debugging information entirely follows the assembler code.
Define this macro, with value 1, if the value of a symbol describing
the scope of a block (N_LBRAC or N_RBRAC) should be
relative to the start of the enclosing function. Normally, GCC uses
an absolute address.
Define this macro, with value 1, if the value of a symbol indicating
the current line number (N_SLINE) should be relative to the
start of the enclosing function. Normally, GCC uses an absolute address.
Define this macro if GCC should generate N_BINCL and
N_EINCL stabs for included header files, as on Sun systems. This
macro also directs GCC to output a type number as a pair of a file
number and a type number within the file. Normally, GCC does not
generate N_BINCL or N_EINCL stabs, and it outputs a single
number for a type number.
Next: File Names and DBX, Previous: DBX Options, Up: Debugging Info [Contents][Index]
These are hooks for DBX format.
Define this macro to say how to output to stream the debugging
information for the start of a scope level for variable names. The
argument name is the name of an assembler symbol (for use with
assemble_name) whose value is the address where the scope begins.
Like DBX_OUTPUT_LBRAC, but for the end of a scope level.
Define this macro if the target machine requires special handling to
output an N_FUN entry for the function decl.
A C statement to output DBX debugging information before code for line number line of the current source file to the stdio stream stream. counter is the number of time the macro was invoked, including the current invocation; it is intended to generate unique labels in the assembly output.
This macro should not be defined if the default output is correct, or
if it can be made correct by defining DBX_LINES_FUNCTION_RELATIVE.
Some stabs encapsulation formats (in particular ECOFF), cannot handle the
.stabs "",N_FUN,,0,0,Lscope-function-1 gdb dbx extension construct.
On those machines, define this macro to turn this feature off without
disturbing the rest of the gdb extensions.
Some assemblers cannot handle the .stabd BNSYM/ENSYM,0,0 gdb dbx
extension construct. On those machines, define this macro to turn this
feature off without disturbing the rest of the gdb extensions.
Next: SDB and DWARF, Previous: DBX Hooks, Up: Debugging Info [Contents][Index]
This describes file names in DBX format.
A C statement to output DBX debugging information to the stdio stream stream, which indicates that file name is the main source file—the file specified as the input file for compilation. This macro is called only once, at the beginning of compilation.
This macro need not be defined if the standard form of output for DBX debugging information is appropriate.
It may be necessary to refer to a label equal to the beginning of the
text section. You can use ‘assemble_name (stream, ltext_label_name)’
to do so. If you do this, you must also set the variable
used_ltext_label_name to true.
Define this macro, with value 1, if GCC should not emit an indication of the current directory for compilation and current source language at the beginning of the file.
Define this macro, with value 1, if GCC should not emit an indication
that this object file was compiled by GCC. The default is to emit
an N_OPT stab at the beginning of every source file, with
‘gcc2_compiled.’ for the string and value 0.
A C statement to output DBX debugging information at the end of compilation of the main source file name. Output should be written to the stdio stream stream.
If you don’t define this macro, nothing special is output at the end of compilation, which is correct for most machines.
Define this macro instead of defining
DBX_OUTPUT_MAIN_SOURCE_FILE_END, if what needs to be output at
the end of compilation is an N_SO stab with an empty string,
whose value is the highest absolute text address in the file.
Next: VMS Debug, Previous: File Names and DBX, Up: Debugging Info [Contents][Index]
Here are macros for SDB and DWARF output.
Define this macro if GCC should produce COFF-style debugging output for SDB in response to the -g option.
Define this macro if GCC should produce dwarf version 2 format debugging output in response to the -g option.
Define this to enable the dwarf attribute DW_AT_calling_convention to
be emitted for each function. Instead of an integer return the enum
value for the DW_CC_ tag.
To support optional call frame debugging information, you must also
define INCOMING_RETURN_ADDR_RTX and either set
RTX_FRAME_RELATED_P on the prologue insns if you use RTL for the
prologue, or call dwarf2out_def_cfa and dwarf2out_reg_save
as appropriate from TARGET_ASM_FUNCTION_PROLOGUE if you don’t.
Define this macro to a nonzero value if GCC should always output
Dwarf 2 frame information. If TARGET_EXCEPT_UNWIND_INFO
(see Exception Region Output) returns UI_DWARF2, and
exceptions are enabled, GCC will output this information not matter
how you define DWARF2_FRAME_INFO.
This hook defines the mechanism that will be used for describing frame
unwind information to the debugger. Normally the hook will return
UI_DWARF2 if DWARF 2 debug information is enabled, and
return UI_NONE otherwise.
A target may return UI_DWARF2 even when DWARF 2 debug information
is disabled in order to always output DWARF 2 frame information.
A target may return UI_TARGET if it has ABI specified unwind tables.
This will suppress generation of the normal debug frame unwind information.
Define this macro to be a nonzero value if the assembler can generate Dwarf 2 line debug info sections. This will result in much more compact line number tables, and hence is desirable if it works.
True if the .debug_pubtypes and .debug_pubnames sections should be emitted. These sections are not used on most platforms, and in particular GDB does not use them.
True if sched2 is not to be run at its normal place. This usually means it will be run as part of machine-specific reorg.
True if vartrack is not to be run at its normal place. This usually means it will be run as part of machine-specific reorg.
A C statement to issue assembly directives that create a difference lab1 minus lab2, using an integer of the given size.
A C statement to issue assembly directives that create a difference between the two given labels in system defined units, e.g. instruction slots on IA64 VMS, using an integer of the given size.
A C statement to issue assembly directives that create a section-relative reference to the given label, using an integer of the given size. The label is known to be defined in the given section.
A C statement to issue assembly directives that create a self-relative reference to the given label, using an integer of the given size.
A C statement to issue assembly directives that create a reference to the DWARF table identifier label from the current section. This is used on some systems to avoid garbage collecting a DWARF table which is referenced by a function.
If defined, this target hook is a function which outputs a DTP-relative reference to the given TLS symbol of the specified size.
Define these macros to override the assembler syntax for the special SDB assembler directives. See sdbout.c for a list of these macros and their arguments. If the standard syntax is used, you need not define them yourself.
Some assemblers do not support a semicolon as a delimiter, even between
SDB assembler directives. In that case, define this macro to be the
delimiter to use (usually ‘\n’). It is not necessary to define
a new set of PUT_SDB_op macros if this is the only change
required.
Define this macro to allow references to unknown structure, union, or enumeration tags to be emitted. Standard COFF does not allow handling of unknown references, MIPS ECOFF has support for it.
Define this macro to allow references to structure, union, or enumeration tags that have not yet been seen to be handled. Some assemblers choke if forward tags are used, while some require it.
A C statement to output SDB debugging information before code for line
number line of the current source file to the stdio stream
stream. The default is to emit an .ln directive.
Previous: SDB and DWARF, Up: Debugging Info [Contents][Index]
Here are macros for VMS debug format.
Define this macro if GCC should produce debugging output for VMS
in response to the -g option. The default behavior for VMS
is to generate minimal debug info for a traceback in the absence of
-g unless explicitly overridden with -g0. This
behavior is controlled by TARGET_OPTION_OPTIMIZATION and
TARGET_OPTION_OVERRIDE.
Next: Mode Switching, Previous: Debugging Info, Up: Target Macros [Contents][Index]
While all modern machines use twos-complement representation for integers, there are a variety of representations for floating point numbers. This means that in a cross-compiler the representation of floating point numbers in the compiled program may be different from that used in the machine doing the compilation.
Because different representation systems may offer different amounts of range and precision, all floating point constants must be represented in the target machine’s format. Therefore, the cross compiler cannot safely use the host machine’s floating point arithmetic; it must emulate the target’s arithmetic. To ensure consistency, GCC always uses emulation to work with floating point values, even when the host and target floating point formats are identical.
The following macros are provided by real.h for the compiler to use. All parts of the compiler which generate or optimize floating-point calculations must use these macros. They may evaluate their operands more than once, so operands must not have side effects.
The C data type to be used to hold a floating point value in the target
machine’s format. Typically this is a struct containing an
array of HOST_WIDE_INT, but all code should treat it as an opaque
quantity.
Compares for equality the two values, x and y. If the target floating point format supports negative zeroes and/or NaNs, ‘REAL_VALUES_EQUAL (-0.0, 0.0)’ is true, and ‘REAL_VALUES_EQUAL (NaN, NaN)’ is false.
Tests whether x is less than y.
Truncates x to a signed integer, rounding toward zero.
Truncates x to an unsigned integer, rounding toward zero. If x is negative, returns zero.
Converts string into a floating point number in the target machine’s representation for mode mode. This routine can handle both decimal and hexadecimal floating point constants, using the syntax defined by the C language for both.
Returns 1 if x is negative (including negative zero), 0 otherwise.
Determines whether x represents infinity (positive or negative).
Determines whether x represents a “NaN” (not-a-number).
Calculates an arithmetic operation on the two floating point values x and y, storing the result in output (which must be a variable).
The operation to be performed is specified by code. Only the
following codes are supported: PLUS_EXPR, MINUS_EXPR,
MULT_EXPR, RDIV_EXPR, MAX_EXPR, MIN_EXPR.
If REAL_ARITHMETIC is asked to evaluate division by zero and the
target’s floating point format cannot represent infinity, it will call
abort. Callers should check for this situation first, using
MODE_HAS_INFINITIES. See Storage Layout.
Returns the negative of the floating point value x.
Returns the absolute value of x.
Truncates the floating point value x to fit in mode. The
return value is still a full-size REAL_VALUE_TYPE, but it has an
appropriate bit pattern to be output as a floating constant whose
precision accords with mode mode.
Converts a floating point value x into a double-precision integer which is then stored into low and high. If the value is not integral, it is truncated.
Converts a double-precision integer found in low and high, into a floating point value which is then stored into x. The value is truncated to fit in mode mode.
Next: Target Attributes, Previous: Floating Point, Up: Target Macros [Contents][Index]
The following macros control mode switching optimizations:
Define this macro if the port needs extra instructions inserted for mode switching in an optimizing compilation.
For an example, the SH4 can perform both single and double precision
floating point operations, but to perform a single precision operation,
the FPSCR PR bit has to be cleared, while for a double precision
operation, this bit has to be set. Changing the PR bit requires a general
purpose register as a scratch register, hence these FPSCR sets have to
be inserted before reload, i.e. you can’t put this into instruction emitting
or TARGET_MACHINE_DEPENDENT_REORG.
You can have multiple entities that are mode-switched, and select at run time
which entities actually need it. OPTIMIZE_MODE_SWITCHING should
return nonzero for any entity that needs mode-switching.
If you define this macro, you also have to define
NUM_MODES_FOR_MODE_SWITCHING, MODE_NEEDED,
MODE_PRIORITY_TO_MODE and EMIT_MODE_SET.
MODE_AFTER, MODE_ENTRY, and MODE_EXIT
are optional.
If you define OPTIMIZE_MODE_SWITCHING, you have to define this as
initializer for an array of integers. Each initializer element
N refers to an entity that needs mode switching, and specifies the number
of different modes that might need to be set for this entity.
The position of the initializer in the initializer—starting counting at
zero—determines the integer that is used to refer to the mode-switched
entity in question.
In macros that take mode arguments / yield a mode result, modes are
represented as numbers 0 … N - 1. N is used to specify that no mode
switch is needed / supplied.
entity is an integer specifying a mode-switched entity. If
OPTIMIZE_MODE_SWITCHING is defined, you must define this macro to
return an integer value not larger than the corresponding element in
NUM_MODES_FOR_MODE_SWITCHING, to denote the mode that entity must
be switched into prior to the execution of insn.
If this macro is defined, it is evaluated for every insn during mode switching. It determines the mode that an insn results in (if different from the incoming mode).
If this macro is defined, it is evaluated for every entity that needs
mode switching. It should evaluate to an integer, which is a mode that
entity is assumed to be switched to at function entry. If MODE_ENTRY
is defined then MODE_EXIT must be defined.
If this macro is defined, it is evaluated for every entity that needs
mode switching. It should evaluate to an integer, which is a mode that
entity is assumed to be switched to at function exit. If MODE_EXIT
is defined then MODE_ENTRY must be defined.
This macro specifies the order in which modes for entity are processed.
0 is the highest priority, NUM_MODES_FOR_MODE_SWITCHING[entity] - 1 the
lowest. The value of the macro should be an integer designating a mode
for entity. For any fixed entity, mode_priority_to_mode
(entity, n) shall be a bijection in 0 …
num_modes_for_mode_switching[entity] - 1.
Generate one or more insns to set entity to mode. hard_reg_live is the set of hard registers live at the point where the insn(s) are to be inserted.
Next: Emulated TLS, Previous: Mode Switching, Up: Target Macros [Contents][Index]
__attribute__Target-specific attributes may be defined for functions, data and types. These are described using the following target hooks; they also need to be documented in extend.texi.
If defined, this target hook points to an array of ‘struct attribute_spec’ (defined in tree.h) specifying the machine specific attributes for this target and some of the restrictions on the entities to which these attributes are applied and the arguments they take.
If defined, this target hook is a function which returns true if the machine-specific attribute named name expects an identifier given as its first argument to be passed on as a plain identifier, not subjected to name lookup. If this is not defined, the default is false for all machine-specific attributes.
If defined, this target hook is a function which returns zero if the attributes on type1 and type2 are incompatible, one if they are compatible, and two if they are nearly compatible (which causes a warning to be generated). If this is not defined, machine-specific attributes are supposed always to be compatible.
If defined, this target hook is a function which assigns default attributes to the newly defined type.
Define this target hook if the merging of type attributes needs special
handling. If defined, the result is a list of the combined
TYPE_ATTRIBUTES of type1 and type2. It is assumed
that comptypes has already been called and returned 1. This
function may call merge_attributes to handle machine-independent
merging.
Define this target hook if the merging of decl attributes needs special
handling. If defined, the result is a list of the combined
DECL_ATTRIBUTES of olddecl and newdecl.
newdecl is a duplicate declaration of olddecl. Examples of
when this is needed are when one attribute overrides another, or when an
attribute is nullified by a subsequent definition. This function may
call merge_attributes to handle machine-independent merging.
If the only target-specific handling you require is ‘dllimport’
for Microsoft Windows targets, you should define the macro
TARGET_DLLIMPORT_DECL_ATTRIBUTES to 1. The compiler
will then define a function called
merge_dllimport_decl_attributes which can then be defined as
the expansion of TARGET_MERGE_DECL_ATTRIBUTES. You can also
add handle_dll_attribute in the attribute table for your port
to perform initial processing of the ‘dllimport’ and
‘dllexport’ attributes. This is done in i386/cygwin.h and
i386/i386.c, for example.
decl is a variable or function with __attribute__((dllimport)) specified. Use this hook if the target needs to add extra validation checks to handle_dll_attribute.
Define this macro to a nonzero value if you want to treat
__declspec(X) as equivalent to __attribute((X)). By
default, this behavior is enabled only for targets that define
TARGET_DLLIMPORT_DECL_ATTRIBUTES. The current implementation
of __declspec is via a built-in macro, but you should not rely
on this implementation detail.
Define this target hook if you want to be able to add attributes to a decl
when it is being created. This is normally useful for back ends which
wish to implement a pragma by using the attributes which correspond to
the pragma’s effect. The node argument is the decl which is being
created. The attr_ptr argument is a pointer to the attribute list
for this decl. The list itself should not be modified, since it may be
shared with other decls, but attributes may be chained on the head of
the list and *attr_ptr modified to point to the new
attributes, or a copy of the list may be made if further changes are
needed.
This target hook returns true if it is ok to inline fndecl
into the current function, despite its having target-specific
attributes, false otherwise. By default, if a function has a
target specific attribute attached to it, it will not be inlined.
This hook is called to parse the attribute(option("...")), and
it allows the function to set different target machine compile time
options for the current function that might be different than the
options specified on the command line. The hook should return
true if the options are valid.
The hook should set the DECL_FUNCTION_SPECIFIC_TARGET field in the function declaration to hold a pointer to a target specific struct cl_target_option structure.
This hook is called to save any additional target specific information in the struct cl_target_option structure for function specific options. See Option file format.
This hook is called to restore any additional target specific information in the struct cl_target_option structure for function specific options.
This hook is called to print any additional target specific information in the struct cl_target_option structure for function specific options.
This target hook parses the options for #pragma GCC option to
set the machine specific options for functions that occur later in the
input stream. The options should be the same as handled by the
TARGET_OPTION_VALID_ATTRIBUTE_P hook.
Sometimes certain combinations of command options do not make sense on
a particular target machine. You can override the hook
TARGET_OPTION_OVERRIDE to take account of this. This hooks is called
once just after all the command options have been parsed.
Don’t use this hook to turn on various extra optimizations for
-O. That is what TARGET_OPTION_OPTIMIZATION is for.
If you need to do something whenever the optimization level is
changed via the optimize attribute or pragma, see
TARGET_OVERRIDE_OPTIONS_AFTER_CHANGE
This target hook returns false if the caller function
cannot inline callee, based on target specific information. By
default, inlining is not allowed if the callee function has function
specific target options and the caller does not use the same options.
Next: MIPS Coprocessors, Previous: Target Attributes, Up: Target Macros [Contents][Index]
For targets whose psABI does not provide Thread Local Storage via specific relocations and instruction sequences, an emulation layer is used. A set of target hooks allows this emulation layer to be configured for the requirements of a particular target. For instance the psABI may in fact specify TLS support in terms of an emulation layer.
The emulation layer works by creating a control object for every TLS object. To access the TLS object, a lookup function is provided which, when given the address of the control object, will return the address of the current thread’s instance of the TLS object.
Contains the name of the helper function that uses a TLS control object to locate a TLS instance. The default causes libgcc’s emulated TLS helper function to be used.
Contains the name of the helper function that should be used at
program startup to register TLS objects that are implicitly
initialized to zero. If this is NULL, all TLS objects will
have explicit initializers. The default causes libgcc’s emulated TLS
registration function to be used.
Contains the name of the section in which TLS control variables should
be placed. The default of NULL allows these to be placed in
any section.
Contains the name of the section in which TLS initializers should be
placed. The default of NULL allows these to be placed in any
section.
Contains the prefix to be prepended to TLS control variable names.
The default of NULL uses a target-specific prefix.
Contains the prefix to be prepended to TLS initializer objects. The
default of NULL uses a target-specific prefix.
Specifies a function that generates the FIELD_DECLs for a TLS control
object type. type is the RECORD_TYPE the fields are for and
name should be filled with the structure tag, if the default of
__emutls_object is unsuitable. The default creates a type suitable
for libgcc’s emulated TLS function.
Specifies a function that generates the CONSTRUCTOR to initialize a TLS control object. var is the TLS control object, decl is the TLS object and tmpl_addr is the address of the initializer. The default initializes libgcc’s emulated TLS control object.
Specifies whether the alignment of TLS control variable objects is fixed and should not be increased as some backends may do to optimize single objects. The default is false.
Specifies whether a DWARF DW_OP_form_tls_address location descriptor
may be used to describe emulated TLS control objects.
Next: PCH Target, Previous: Emulated TLS, Up: Target Macros [Contents][Index]
The MIPS specification allows MIPS implementations to have as many as 4 coprocessors, each with as many as 32 private registers. GCC supports accessing these registers and transferring values between the registers and memory using asm-ized variables. For example:
register unsigned int cp0count asm ("c0r1");
unsigned int d;
d = cp0count + 3;
(“c0r1” is the default name of register 1 in coprocessor 0; alternate
names may be added as described below, or the default names may be
overridden entirely in SUBTARGET_CONDITIONAL_REGISTER_USAGE.)
Coprocessor registers are assumed to be epilogue-used; sets to them will be preserved even if it does not appear that the register is used again later in the function.
Another note: according to the MIPS spec, coprocessor 1 (if present) is the FPU. One accesses COP1 registers through standard mips floating-point support; they are not included in this mechanism.
There is one macro used in defining the MIPS coprocessor interface which you may want to override in subtargets; it is described below.
A comma-separated list (with leading comma) of pairs describing the alternate names of coprocessor registers. The format of each entry should be
{ alternatename, register_number}
Default: empty.
Next: C++ ABI, Previous: MIPS Coprocessors, Up: Target Macros [Contents][Index]
This hook returns a pointer to the data needed by
TARGET_PCH_VALID_P and sets
‘*sz’ to the size of the data in bytes.
This hook checks whether the options used to create a PCH file are
compatible with the current settings. It returns NULL
if so and a suitable error message if not. Error messages will
be presented to the user and must be localized using ‘_(msg)’.
data is the data that was returned by TARGET_GET_PCH_VALIDITY
when the PCH file was created and sz is the size of that data in bytes.
It’s safe to assume that the data was created by the same version of the
compiler, so no format checking is needed.
The default definition of default_pch_valid_p should be
suitable for most targets.
If this hook is nonnull, the default implementation of
TARGET_PCH_VALID_P will use it to check for compatible values
of target_flags. pch_flags specifies the value that
target_flags had when the PCH file was created. The return
value is the same as for TARGET_PCH_VALID_P.
Next: Named Address Spaces, Previous: PCH Target, Up: Target Macros [Contents][Index]
Define this hook to override the integer type used for guard variables. These are used to implement one-time construction of static objects. The default is long_long_integer_type_node.
This hook determines how guard variables are used. It should return
false (the default) if the first byte should be used. A return value of
true indicates that only the least significant bit should be used.
This hook returns the size of the cookie to use when allocating an array
whose elements have the indicated type. Assumes that it is already
known that a cookie is needed. The default is
max(sizeof (size_t), alignof(type)), as defined in section 2.7 of the
IA64/Generic C++ ABI.
This hook should return true if the element size should be stored in
array cookies. The default is to return false.
If defined by a backend this hook allows the decision made to export class type to be overruled. Upon entry import_export will contain 1 if the class is going to be exported, -1 if it is going to be imported and 0 otherwise. This function should return the modified value and perform any other actions necessary to support the backend’s targeted operating system.
This hook should return true if constructors and destructors return
the address of the object created/destroyed. The default is to return
false.
This hook returns true if the key method for a class (i.e., the method
which, if defined in the current translation unit, causes the virtual
table to be emitted) may be an inline function. Under the standard
Itanium C++ ABI the key method may be an inline function so long as
the function is not declared inline in the class definition. Under
some variants of the ABI, an inline function can never be the key
method. The default is to return true.
decl is a virtual table, virtual table table, typeinfo object, or other similar implicit class data object that will be emitted with external linkage in this translation unit. No ELF visibility has been explicitly specified. If the target needs to specify a visibility other than that of the containing class, use this hook to set DECL_VISIBILITY and DECL_VISIBILITY_SPECIFIED.
This hook returns true (the default) if virtual tables and other similar implicit class data objects are always COMDAT if they have external linkage. If this hook returns false, then class data for classes whose virtual table will be emitted in only one translation unit will not be COMDAT.
This hook returns true (the default) if the RTTI information for the basic types which is defined in the C++ runtime should always be COMDAT, false if it should not be COMDAT.
This hook returns true if __aeabi_atexit (as defined by the ARM EABI)
should be used to register static destructors when -fuse-cxa-atexit
is in effect. The default is to return false to use __cxa_atexit.
This hook returns true if the target atexit function can be used
in the same manner as __cxa_atexit to register C++ static
destructors. This requires that atexit-registered functions in
shared libraries are run in the correct order when the libraries are
unloaded. The default is to return false.
type is a C++ class (i.e., RECORD_TYPE or UNION_TYPE) that has just been defined. Use this hook to make adjustments to the class (eg, tweak visibility or perform any other required target modifications).
Next: Misc, Previous: C++ ABI, Up: Target Macros [Contents][Index]
The draft technical report of the ISO/IEC JTC1 S22 WG14 N1275
standards committee, Programming Languages - C - Extensions to
support embedded processors, specifies a syntax for embedded
processors to specify alternate address spaces. You can configure a
GCC port to support section 5.1 of the draft report to add support for
address spaces other than the default address space. These address
spaces are new keywords that are similar to the volatile and
const type attributes.
Pointers to named address spaces can have a different size than pointers to the generic address space.
For example, the SPU port uses the __ea address space to refer
to memory in the host processor, rather than memory local to the SPU
processor. Access to memory in the __ea address space involves
issuing DMA operations to move data between the host processor and the
local processor memory address space. Pointers in the __ea
address space are either 32 bits or 64 bits based on the
-mea32 or -mea64 switches (native SPU pointers are
always 32 bits).
Internally, address spaces are represented as a small integer in the range 0 to 15 with address space 0 being reserved for the generic address space.
To register a named address space qualifier keyword with the C front end,
the target may call the c_register_addr_space routine. For example,
the SPU port uses the following to declare __ea as the keyword for
named address space #1:
#define ADDR_SPACE_EA 1
c_register_addr_space ("__ea", ADDR_SPACE_EA);
Define this to return the machine mode to use for pointers to
address_space if the target supports named address spaces.
The default version of this hook returns ptr_mode for the
generic address space only.
Define this to return the machine mode to use for addresses in
address_space if the target supports named address spaces.
The default version of this hook returns Pmode for the
generic address space only.
Define this to return nonzero if the port can handle pointers
with machine mode mode to address space as. This target
hook is the same as the TARGET_VALID_POINTER_MODE target hook,
except that it includes explicit named address space support. The default
version of this hook returns true for the modes returned by either the
TARGET_ADDR_SPACE_POINTER_MODE or TARGET_ADDR_SPACE_ADDRESS_MODE
target hooks for the given address space.
Define this to return true if exp is a valid address for mode
mode in the named address space as. The strict
parameter says whether strict addressing is in effect after reload has
finished. This target hook is the same as the
TARGET_LEGITIMATE_ADDRESS_P target hook, except that it includes
explicit named address space support.
Define this to modify an invalid address x to be a valid address
with mode mode in the named address space as. This target
hook is the same as the TARGET_LEGITIMIZE_ADDRESS target hook,
except that it includes explicit named address space support.
Define this to return whether the subset named address space is contained within the superset named address space. Pointers to a named address space that is a subset of another named address space will be converted automatically without a cast if used together in arithmetic operations. Pointers to a superset address space can be converted to pointers to a subset address space via explicit casts.
Define this to convert the pointer expression represented by the RTL
op with type from_type that points to a named address
space to a new pointer expression with type to_type that points
to a different named address space. When this hook it called, it is
guaranteed that one of the two address spaces is a subset of the other,
as determined by the TARGET_ADDR_SPACE_SUBSET_P target hook.
Previous: Named Address Spaces, Up: Target Macros [Contents][Index]
Here are several miscellaneous parameters.
Define this boolean macro to indicate whether or not your architecture has conditional branches that can span all of memory. It is used in conjunction with an optimization that partitions hot and cold basic blocks into separate sections of the executable. If this macro is set to false, gcc will convert any conditional branches that attempt to cross between sections into unconditional branches or indirect jumps.
Define this boolean macro to indicate whether or not your architecture has unconditional branches that can span all of memory. It is used in conjunction with an optimization that partitions hot and cold basic blocks into separate sections of the executable. If this macro is set to false, gcc will convert any unconditional branches that attempt to cross between sections into indirect jumps.
An alias for a machine mode name. This is the machine mode that elements of a jump-table should have.
Optional: return the preferred mode for an addr_diff_vec
when the minimum and maximum offset are known. If you define this,
it enables extra code in branch shortening to deal with addr_diff_vec.
To make this work, you also have to define INSN_ALIGN and
make the alignment for addr_diff_vec explicit.
The body argument is provided so that the offset_unsigned and scale
flags can be updated.
Define this macro to be a C expression to indicate when jump-tables should contain relative addresses. You need not define this macro if jump-tables never contain relative addresses, or jump-tables should contain relative addresses only when -fPIC or -fPIC is in effect.
This function return the smallest number of different values for which it
is best to use a jump-table instead of a tree of conditional branches.
The default is four for machines with a casesi instruction and
five otherwise. This is best for most machines.
Define this macro to be a C expression to indicate whether C switch
statements may be implemented by a sequence of bit tests. This is
advantageous on processors that can efficiently implement left shift
of 1 by the number of bits held in a register, but inappropriate on
targets that would require a loop. By default, this macro returns
true if the target defines an ashlsi3 pattern, and
false otherwise.
Define this macro if operations between registers with integral mode smaller than a word are always performed on the entire register. Most RISC machines have this property and most CISC machines do not.
Define this macro to be a C expression indicating when insns that read
memory in mem_mode, an integral mode narrower than a word, set the
bits outside of mem_mode to be either the sign-extension or the
zero-extension of the data read. Return SIGN_EXTEND for values
of mem_mode for which the
insn sign-extends, ZERO_EXTEND for which it zero-extends, and
UNKNOWN for other modes.
This macro is not called with mem_mode non-integral or with a width
greater than or equal to BITS_PER_WORD, so you may return any
value in this case. Do not define this macro if it would always return
UNKNOWN. On machines where this macro is defined, you will normally
define it as the constant SIGN_EXTEND or ZERO_EXTEND.
You may return a non-UNKNOWN value even if for some hard registers
the sign extension is not performed, if for the REGNO_REG_CLASS
of these hard registers CANNOT_CHANGE_MODE_CLASS returns nonzero
when the from mode is mem_mode and the to mode is any
integral mode larger than this but not larger than word_mode.
You must return UNKNOWN if for some hard registers that allow this
mode, CANNOT_CHANGE_MODE_CLASS says that they cannot change to
word_mode, but that they can change to another integral mode that
is larger then mem_mode but still smaller than word_mode.
Define this macro if loading short immediate values into registers sign extends.
Define this macro if the same instructions that convert a floating point number to a signed fixed point number also convert validly to an unsigned one.
When -ffast-math is in effect, GCC tries to optimize divisions by the same divisor, by turning them into multiplications by the reciprocal. This target hook specifies the minimum number of divisions that should be there for GCC to perform the optimization for a variable of mode mode. The default implementation returns 3 if the machine has an instruction for the division, and 2 if it does not.
The maximum number of bytes that a single instruction can move quickly between memory and registers or between two memory locations.
The maximum number of bytes that a single instruction can move quickly
between memory and registers or between two memory locations. If this
is undefined, the default is MOVE_MAX. Otherwise, it is the
constant value that is the largest value that MOVE_MAX can have
at run-time.
A C expression that is nonzero if on this machine the number of bits
actually used for the count of a shift operation is equal to the number
of bits needed to represent the size of the object being shifted. When
this macro is nonzero, the compiler will assume that it is safe to omit
a sign-extend, zero-extend, and certain bitwise ‘and’ instructions that
truncates the count of a shift operation. On machines that have
instructions that act on bit-fields at variable positions, which may
include ‘bit test’ instructions, a nonzero SHIFT_COUNT_TRUNCATED
also enables deletion of truncations of the values that serve as
arguments to bit-field instructions.
If both types of instructions truncate the count (for shifts) and position (for bit-field operations), or if no variable-position bit-field instructions exist, you should define this macro.
However, on some machines, such as the 80386 and the 680x0, truncation
only applies to shift operations and not the (real or pretended)
bit-field operations. Define SHIFT_COUNT_TRUNCATED to be zero on
such machines. Instead, add patterns to the md file that include
the implied truncation of the shift instructions.
You need not define this macro if it would always have the value of zero.
This function describes how the standard shift patterns for mode deal with shifts by negative amounts or by more than the width of the mode. See shift patterns.
On many machines, the shift patterns will apply a mask m to the shift count, meaning that a fixed-width shift of x by y is equivalent to an arbitrary-width shift of x by y & m. If this is true for mode mode, the function should return m, otherwise it should return 0. A return value of 0 indicates that no particular behavior is guaranteed.
Note that, unlike SHIFT_COUNT_TRUNCATED, this function does
not apply to general shift rtxes; it applies only to instructions
that are generated by the named shift patterns.
The default implementation of this function returns
GET_MODE_BITSIZE (mode) - 1 if SHIFT_COUNT_TRUNCATED
and 0 otherwise. This definition is always safe, but if
SHIFT_COUNT_TRUNCATED is false, and some shift patterns
nevertheless truncate the shift count, you may get better code
by overriding it.
A C expression which is nonzero if on this machine it is safe to “convert” an integer of inprec bits to one of outprec bits (where outprec is smaller than inprec) by merely operating on it as if it had only outprec bits.
On many machines, this expression can be 1.
When TRULY_NOOP_TRUNCATION returns 1 for a pair of sizes for
modes for which MODES_TIEABLE_P is 0, suboptimal code can result.
If this is the case, making TRULY_NOOP_TRUNCATION return 0 in
such cases may improve things.
The representation of an integral mode can be such that the values
are always extended to a wider integral mode. Return
SIGN_EXTEND if values of mode are represented in
sign-extended form to rep_mode. Return UNKNOWN
otherwise. (Currently, none of the targets use zero-extended
representation this way so unlike LOAD_EXTEND_OP,
TARGET_MODE_REP_EXTENDED is expected to return either
SIGN_EXTEND or UNKNOWN. Also no target extends
mode to rep_mode so that rep_mode is not the next
widest integral mode and currently we take advantage of this fact.)
Similarly to LOAD_EXTEND_OP you may return a non-UNKNOWN
value even if the extension is not performed on certain hard registers
as long as for the REGNO_REG_CLASS of these hard registers
CANNOT_CHANGE_MODE_CLASS returns nonzero.
Note that TARGET_MODE_REP_EXTENDED and LOAD_EXTEND_OP
describe two related properties. If you define
TARGET_MODE_REP_EXTENDED (mode, word_mode) you probably also want
to define LOAD_EXTEND_OP (mode) to return the same type of
extension.
In order to enforce the representation of mode,
TRULY_NOOP_TRUNCATION should return false when truncating to
mode.
A C expression describing the value returned by a comparison operator
with an integral mode and stored by a store-flag instruction
(‘cstoremode4’) when the condition is true. This description must
apply to all the ‘cstoremode4’ patterns and all the
comparison operators whose results have a MODE_INT mode.
A value of 1 or -1 means that the instruction implementing the
comparison operator returns exactly 1 or -1 when the comparison is true
and 0 when the comparison is false. Otherwise, the value indicates
which bits of the result are guaranteed to be 1 when the comparison is
true. This value is interpreted in the mode of the comparison
operation, which is given by the mode of the first operand in the
‘cstoremode4’ pattern. Either the low bit or the sign bit of
STORE_FLAG_VALUE be on. Presently, only those bits are used by
the compiler.
If STORE_FLAG_VALUE is neither 1 or -1, the compiler will
generate code that depends only on the specified bits. It can also
replace comparison operators with equivalent operations if they cause
the required bits to be set, even if the remaining bits are undefined.
For example, on a machine whose comparison operators return an
SImode value and where STORE_FLAG_VALUE is defined as
‘0x80000000’, saying that just the sign bit is relevant, the
expression
(ne:SI (and:SI x (const_int power-of-2)) (const_int 0))
can be converted to
(ashift:SI x (const_int n))
where n is the appropriate shift count to move the bit being tested into the sign bit.
There is no way to describe a machine that always sets the low-order bit for a true value, but does not guarantee the value of any other bits, but we do not know of any machine that has such an instruction. If you are trying to port GCC to such a machine, include an instruction to perform a logical-and of the result with 1 in the pattern for the comparison operators and let us know at gcc@gcc.gnu.org.
Often, a machine will have multiple instructions that obtain a value
from a comparison (or the condition codes). Here are rules to guide the
choice of value for STORE_FLAG_VALUE, and hence the instructions
to be used:
STORE_FLAG_VALUE. It is more efficient for the compiler to
“normalize” the value (convert it to, e.g., 1 or 0) than for the
comparison operators to do so because there may be opportunities to
combine the normalization with other operations.
Many machines can produce both the value chosen for
STORE_FLAG_VALUE and its negation in the same number of
instructions. On those machines, you should also define a pattern for
those cases, e.g., one matching
(set A (neg:m (ne:m B C)))
Some machines can also perform and or plus operations on
condition code values with less instructions than the corresponding
‘cstoremode4’ insn followed by and or plus. On those
machines, define the appropriate patterns. Use the names incscc
and decscc, respectively, for the patterns which perform
plus or minus operations on condition code values. See
rs6000.md for some examples. The GNU Superoptimizer can be used to
find such instruction sequences on other machines.
If this macro is not defined, the default value, 1, is used. You need
not define STORE_FLAG_VALUE if the machine has no store-flag
instructions, or if the value generated by these instructions is 1.
A C expression that gives a nonzero REAL_VALUE_TYPE value that is
returned when comparison operators with floating-point results are true.
Define this macro on machines that have comparison operations that return
floating-point values. If there are no such operations, do not define
this macro.
A C expression that gives a rtx representing the nonzero true element
for vector comparisons. The returned rtx should be valid for the inner
mode of mode which is guaranteed to be a vector mode. Define
this macro on machines that have vector comparison operations that
return a vector result. If there are no such operations, do not define
this macro. Typically, this macro is defined as const1_rtx or
constm1_rtx. This macro may return NULL_RTX to prevent
the compiler optimizing such vector comparison operations for the
given mode.
A C expression that indicates whether the architecture defines a value
for clz or ctz with a zero operand.
A result of 0 indicates the value is undefined.
If the value is defined for only the RTL expression, the macro should
evaluate to 1; if the value applies also to the corresponding optab
entry (which is normally the case if it expands directly into
the corresponding RTL), then the macro should evaluate to 2.
In the cases where the value is defined, value should be set to
this value.
If this macro is not defined, the value of clz or
ctz at zero is assumed to be undefined.
This macro must be defined if the target’s expansion for ffs
relies on a particular value to get correct results. Otherwise it
is not necessary, though it may be used to optimize some corner cases, and
to provide a default expansion for the ffs optab.
Note that regardless of this macro the “definedness” of clz
and ctz at zero do not extend to the builtin functions
visible to the user. Thus one may be free to adjust the value at will
to match the target expansion of these operations without fear of
breaking the API.
An alias for the machine mode for pointers. On most machines, define
this to be the integer mode corresponding to the width of a hardware
pointer; SImode on 32-bit machine or DImode on 64-bit machines.
On some machines you must define this to be one of the partial integer
modes, such as PSImode.
The width of Pmode must be at least as large as the value of
POINTER_SIZE. If it is not equal, you must define the macro
POINTERS_EXTEND_UNSIGNED to specify how pointers are extended
to Pmode.
An alias for the machine mode used for memory references to functions
being called, in call RTL expressions. On most CISC machines,
where an instruction can begin at any byte address, this should be
QImode. On most RISC machines, where all instructions have fixed
size and alignment, this should be a mode with the same size and alignment
as the machine instruction words - typically SImode or HImode.
In normal operation, the preprocessor expands __STDC__ to the
constant 1, to signify that GCC conforms to ISO Standard C. On some
hosts, like Solaris, the system compiler uses a different convention,
where __STDC__ is normally 0, but is 1 if the user specifies
strict conformance to the C Standard.
Defining STDC_0_IN_SYSTEM_HEADERS makes GNU CPP follows the host
convention when processing system header files, but when processing user
files __STDC__ will always expand to 1.
Define this macro if the system header files support C++ as well as C. This macro inhibits the usual method of using system header files in C++, which is to pretend that the file’s contents are enclosed in ‘extern "C" {…}’.
Define this macro if you want to implement any target-specific pragmas.
If defined, it is a C expression which makes a series of calls to
c_register_pragma or c_register_pragma_with_expansion
for each pragma. The macro may also do any
setup required for the pragmas.
The primary reason to define this macro is to provide compatibility with other compilers for the same target. In general, we discourage definition of target-specific pragmas for GCC.
If the pragma can be implemented by attributes then you should consider defining the target hook ‘TARGET_INSERT_ATTRIBUTES’ as well.
Preprocessor macros that appear on pragma lines are not expanded. All ‘#pragma’ directives that do not match any registered pragma are silently ignored, unless the user specifies -Wunknown-pragmas.
Each call to c_register_pragma or
c_register_pragma_with_expansion establishes one pragma. The
callback routine will be called when the preprocessor encounters a
pragma of the form
#pragma [space] name …
space is the case-sensitive namespace of the pragma, or
NULL to put the pragma in the global namespace. The callback
routine receives pfile as its first argument, which can be passed
on to cpplib’s functions if necessary. You can lex tokens after the
name by calling pragma_lex. Tokens that are not read by the
callback will be silently ignored. The end of the line is indicated by
a token of type CPP_EOF. Macro expansion occurs on the
arguments of pragmas registered with
c_register_pragma_with_expansion but not on the arguments of
pragmas registered with c_register_pragma.
Note that the use of pragma_lex is specific to the C and C++
compilers. It will not work in the Java or Fortran compilers, or any
other language compilers for that matter. Thus if pragma_lex is going
to be called from target-specific code, it must only be done so when
building the C and C++ compilers. This can be done by defining the
variables c_target_objs and cxx_target_objs in the
target entry in the config.gcc file. These variables should name
the target-specific, language-specific object file which contains the
code that uses pragma_lex. Note it will also be necessary to add a
rule to the makefile fragment pointed to by tmake_file that shows
how to build this object file.
Define this macro if macros should be expanded in the arguments of ‘#pragma pack’.
True if #pragma extern_prefix is to be supported.
If your target requires a structure packing default other than 0 (meaning the machine default), define this macro to the necessary value (in bytes). This must be a value that would also be valid to use with ‘#pragma pack()’ (that is, a small power of two).
Define this macro to control use of the character ‘$’ in identifier names for the C family of languages. 0 means ‘$’ is not allowed by default; 1 means it is allowed. 1 is the default; there is no need to define this macro in that case.
Define this macro if the assembler does not accept the character ‘$’ in label names. By default constructors and destructors in G++ have ‘$’ in the identifiers. If this macro is defined, ‘.’ is used instead.
Define this macro if the assembler does not accept the character ‘.’ in label names. By default constructors and destructors in G++ have names that use ‘.’. If this macro is defined, these names are rewritten to avoid ‘.’.
Define this macro as a C expression that is nonzero if it is safe for the
delay slot scheduler to place instructions in the delay slot of insn,
even if they appear to use a resource set or clobbered in insn.
insn is always a jump_insn or an insn; GCC knows that
every call_insn has this behavior. On machines where some insn
or jump_insn is really a function call and hence has this behavior,
you should define this macro.
You need not define this macro if it would always return zero.
Define this macro as a C expression that is nonzero if it is safe for the
delay slot scheduler to place instructions in the delay slot of insn,
even if they appear to set or clobber a resource referenced in insn.
insn is always a jump_insn or an insn. On machines where
some insn or jump_insn is really a function call and its operands
are registers whose use is actually in the subroutine it calls, you should
define this macro. Doing so allows the delay slot scheduler to move
instructions which copy arguments into the argument registers into the delay
slot of insn.
You need not define this macro if it would always return zero.
Define this macro as a C expression that is nonzero if, in some cases, global symbols from one translation unit may not be bound to undefined symbols in another translation unit without user intervention. For instance, under Microsoft Windows symbols must be explicitly imported from shared libraries (DLLs).
You need not define this macro if it would always evaluate to zero.
This target hook should add to clobbers STRING_CST trees for
any hard regs the port wishes to automatically clobber for an asm.
It should return the result of the last tree_cons used to add a
clobber. The outputs, inputs and clobber lists are the
corresponding parameters to the asm and may be inspected to avoid
clobbering a register that is an input or output of the asm. You can use
tree_overlaps_hard_reg_set, declared in tree.h, to test
for overlap with regards to asm-declared registers.
Define this macro as a C string constant for the linker argument to link in the system math library, minus the initial ‘"-l"’, or ‘""’ if the target does not have a separate math library.
You need only define this macro if the default of ‘"m"’ is wrong.
Define this macro as a C string constant for the environment variable that specifies where the linker should look for libraries.
You need only define this macro if the default of ‘"LIBRARY_PATH"’ is wrong.
Define this macro if the target supports the following POSIX file
functions, access, mkdir and file locking with fcntl / F_SETLKW.
Defining TARGET_POSIX_IO will enable the test coverage code
to use file locking when exiting a program, which avoids race conditions
if the program has forked. It will also create directories at run-time
for cross-profiling.
A C expression for the maximum number of instructions to execute via
conditional execution instructions instead of a branch. A value of
BRANCH_COST+1 is the default if the machine does not use cc0, and
1 if it does use cc0.
Used if the target needs to perform machine-dependent modifications on the
conditionals used for turning basic blocks into conditionally executed code.
ce_info points to a data structure, struct ce_if_block, which
contains information about the currently processed blocks. true_expr
and false_expr are the tests that are used for converting the
then-block and the else-block, respectively. Set either true_expr or
false_expr to a null pointer if the tests cannot be converted.
Like IFCVT_MODIFY_TESTS, but used when converting more complicated
if-statements into conditions combined by and and or operations.
bb contains the basic block that contains the test that is currently
being processed and about to be turned into a condition.
A C expression to modify the PATTERN of an INSN that is to
be converted to conditional execution format. ce_info points to
a data structure, struct ce_if_block, which contains information
about the currently processed blocks.
A C expression to perform any final machine dependent modifications in
converting code to conditional execution. The involved basic blocks
can be found in the struct ce_if_block structure that is pointed
to by ce_info.
A C expression to cancel any machine dependent modifications in
converting code to conditional execution. The involved basic blocks
can be found in the struct ce_if_block structure that is pointed
to by ce_info.
A C expression to initialize any extra fields in a struct ce_if_block
structure, which are defined by the IFCVT_EXTRA_FIELDS macro.
If defined, it should expand to a set of field declarations that will be
added to the struct ce_if_block structure. These should be initialized
by the IFCVT_INIT_EXTRA_FIELDS macro.
If non-null, this hook performs a target-specific pass over the instruction stream. The compiler will run it at all optimization levels, just before the point at which it normally does delayed-branch scheduling.
The exact purpose of the hook varies from target to target. Some use it to do transformations that are necessary for correctness, such as laying out in-function constant pools or avoiding hardware hazards. Others use it as an opportunity to do some machine-dependent optimizations.
You need not implement the hook if it has nothing to do. The default definition is null.
Define this hook if you have any machine-specific built-in functions that need to be defined. It should be a function that performs the necessary setup.
Machine specific built-in functions can be useful to expand special machine instructions that would otherwise not normally be generated because they have no equivalent in the source language (for example, SIMD vector instructions or prefetch instructions).
To create a built-in function, call the function
lang_hooks.builtin_function
which is defined by the language front end. You can use any type nodes set
up by build_common_tree_nodes and build_common_tree_nodes_2;
only language front ends that use those two functions will call
‘TARGET_INIT_BUILTINS’.
Define this hook if you have any machine-specific built-in functions
that need to be defined. It should be a function that returns the
builtin function declaration for the builtin function code code.
If there is no such builtin and it cannot be initialized at this time
if initialize_p is true the function should return NULL_TREE.
If code is out of range the function should return
error_mark_node.
Expand a call to a machine specific built-in function that was set up by ‘TARGET_INIT_BUILTINS’. exp is the expression for the function call; the result should go to target if that is convenient, and have mode mode if that is convenient. subtarget may be used as the target for computing one of exp’s operands. ignore is nonzero if the value is to be ignored. This function should return the result of the call to the built-in function.
Select a replacement for a machine specific built-in function that
was set up by ‘TARGET_INIT_BUILTINS’. This is done
before regular type checking, and so allows the target to
implement a crude form of function overloading. fndecl is the
declaration of the built-in function. arglist is the list of
arguments passed to the built-in function. The result is a
complete expression that implements the operation, usually
another CALL_EXPR.
arglist really has type ‘VEC(tree,gc)*’
Fold a call to a machine specific built-in function that was set up by ‘TARGET_INIT_BUILTINS’. fndecl is the declaration of the built-in function. n_args is the number of arguments passed to the function; the arguments themselves are pointed to by argp. The result is another tree containing a simplified expression for the call’s result. If ignore is true the value will be ignored.
Take an instruction in insn and return NULL if it is valid within a low-overhead loop, otherwise return a string explaining why doloop could not be applied.
Many targets use special registers for low-overhead looping. For any instruction that clobbers these this function should return a string indicating the reason why the doloop could not be applied. By default, the RTL loop optimizer does not use a present doloop pattern for loops containing function calls or branch on table instructions.
Take a branch insn in branch1 and another in branch2. Return true if redirecting branch1 to the destination of branch2 is possible.
On some targets, branches may have a limited range. Optimizing the filling of delay slots can result in branches being redirected, and this may in turn cause a branch offset to overflow.
This target hook returns true if x is considered to be commutative.
Usually, this is just COMMUTATIVE_P (x), but the HP PA doesn’t consider
PLUS to be commutative inside a MEM. outer_code is the rtx code
of the enclosing rtl, if known, otherwise it is UNKNOWN.
When the initial value of a hard register has been copied in a pseudo
register, it is often not necessary to actually allocate another register
to this pseudo register, because the original hard register or a stack slot
it has been saved into can be used. TARGET_ALLOCATE_INITIAL_VALUE
is called at the start of register allocation once for each hard register
that had its initial value copied by using
get_func_hard_reg_initial_val or get_hard_reg_initial_val.
Possible values are NULL_RTX, if you don’t want
to do any special allocation, a REG rtx—that would typically be
the hard register itself, if it is known not to be clobbered—or a
MEM.
If you are returning a MEM, this is only a hint for the allocator;
it might decide to use another register anyways.
You may use current_function_leaf_function in the hook, functions
that use REG_N_SETS, to determine if the hard
register in question will not be clobbered.
The default value of this hook is NULL, which disables any special
allocation.
This target hook returns nonzero if x, an unspec or
unspec_volatile operation, might cause a trap. Targets can use
this hook to enhance precision of analysis for unspec and
unspec_volatile operations. You may call may_trap_p_1
to analyze inner elements of x in which case flags should be
passed along.
The compiler invokes this hook whenever it changes its current function
context (cfun). You can define this function if
the back end needs to perform any initialization or reset actions on a
per-function basis. For example, it may be used to implement function
attributes that affect register usage or code generation patterns.
The argument decl is the declaration for the new function context,
and may be null to indicate that the compiler has left a function context
and is returning to processing at the top level.
The default hook function does nothing.
GCC sets cfun to a dummy function context during initialization of
some parts of the back end. The hook function is not invoked in this
situation; you need not worry about the hook being invoked recursively,
or when the back end is in a partially-initialized state.
cfun might be NULL to indicate processing at top level,
outside of any function scope.
Define this macro to be a C string representing the suffix for object files on your target machine. If you do not define this macro, GCC will use ‘.o’ as the suffix for object files.
Define this macro to be a C string representing the suffix to be automatically added to executable files on your target machine. If you do not define this macro, GCC will use the null string as the suffix for executable files.
If defined, collect2 will scan the individual object files
specified on its command line and create an export list for the linker.
Define this macro for systems like AIX, where the linker discards
object files that are not referenced from main and uses export
lists.
Define this macro to a C expression representing a variant of the
method call mdecl, if Java Native Interface (JNI) methods
must be invoked differently from other methods on your target.
For example, on 32-bit Microsoft Windows, JNI methods must be invoked using
the stdcall calling convention and this macro is then
defined as this expression:
build_type_attribute_variant (mdecl,
build_tree_list
(get_identifier ("stdcall"),
NULL))
This target hook returns true past the point in which new jump
instructions could be created. On machines that require a register for
every jump such as the SHmedia ISA of SH5, this point would typically be
reload, so this target hook should be defined to a function such as:
static bool
cannot_modify_jumps_past_reload_p ()
{
return (reload_completed || reload_in_progress);
}
This target hook returns a register class for which branch target register optimizations should be applied. All registers in this class should be usable interchangeably. After reload, registers in this class will be re-allocated and loads will be hoisted out of loops and be subjected to inter-block scheduling.
Branch target register optimization will by default exclude callee-saved
registers
that are not already live during the current function; if this target hook
returns true, they will be included. The target code must than make sure
that all target registers in the class returned by
‘TARGET_BRANCH_TARGET_REGISTER_CLASS’ that might need saving are
saved. after_prologue_epilogue_gen indicates if prologues and
epilogues have already been generated. Note, even if you only return
true when after_prologue_epilogue_gen is false, you still are likely
to have to make special provisions in INITIAL_ELIMINATION_OFFSET
to reserve space for caller-saved target registers.
This target hook returns true if the target supports conditional execution. This target hook is required only when the target has several different modes and they have different conditional execution capability, such as ARM.
This target hook returns a new value for the number of times loop should be unrolled. The parameter nunroll is the number of times the loop is to be unrolled. The parameter loop is a pointer to the loop, which is going to be checked for unrolling. This target hook is required only when the target has special constraints like maximum number of memory accesses.
If defined, this macro is interpreted as a signed integer C expression
that specifies the maximum number of floating point multiplications
that should be emitted when expanding exponentiation by an integer
constant inline. When this value is defined, exponentiation requiring
more than this number of multiplications is implemented by calling the
system library’s pow, powf or powl routines.
The default value places no upper bound on the multiplication count.
This target hook should register any extra include files for the target. The parameter stdinc indicates if normal include files are present. The parameter sysroot is the system root directory. The parameter iprefix is the prefix for the gcc directory.
This target hook should register any extra include files for the target before any standard headers. The parameter stdinc indicates if normal include files are present. The parameter sysroot is the system root directory. The parameter iprefix is the prefix for the gcc directory.
This target hook should register special include paths for the target. The parameter path is the include to register. On Darwin systems, this is used for Framework includes, which have semantics that are different from -I.
This target macro returns true if it is safe to use a local alias
for a virtual function fndecl when constructing thunks,
false otherwise. By default, the macro returns true for all
functions, if a target supports aliases (i.e. defines
ASM_OUTPUT_DEF), false otherwise,
If defined, this macro is the name of a global variable containing target-specific format checking information for the -Wformat option. The default is to have no target-specific format checks.
If defined, this macro is the number of entries in
TARGET_FORMAT_TYPES.
If defined, this macro is the name of a global variable containing
target-specific format overrides for the -Wformat option. The
default is to have no target-specific format overrides. If defined,
TARGET_FORMAT_TYPES must be defined, too.
If defined, this macro specifies the number of entries in
TARGET_OVERRIDES_FORMAT_ATTRIBUTES.
If defined, this macro specifies the optional initialization routine for target specific customizations of the system printf and scanf formatter settings.
If set to true, means that the target’s memory model does not
guarantee that loads which do not depend on one another will access
main memory in the order of the instruction stream; if ordering is
important, an explicit memory barrier must be used. This is true of
many recent processors which implement a policy of “relaxed,”
“weak,” or “release” memory consistency, such as Alpha, PowerPC,
and ia64. The default is false.
If defined, this macro returns the diagnostic message when it is illegal to pass argument val to function funcdecl with prototype typelist.
If defined, this macro returns the diagnostic message when it is
invalid to convert from fromtype to totype, or NULL
if validity should be determined by the front end.
If defined, this macro returns the diagnostic message when it is
invalid to apply operation op (where unary plus is denoted by
CONVERT_EXPR) to an operand of type type, or NULL
if validity should be determined by the front end.
If defined, this macro returns the diagnostic message when it is
invalid to apply operation op to operands of types type1
and type2, or NULL if validity should be determined by
the front end.
If defined, this macro returns the diagnostic message when it is
invalid for functions to include parameters of type type,
or NULL if validity should be determined by
the front end. This is currently used only by the C and C++ front ends.
If defined, this macro returns the diagnostic message when it is
invalid for functions to have return type type,
or NULL if validity should be determined by
the front end. This is currently used only by the C and C++ front ends.
If defined, this target hook returns the type to which values of
type should be promoted when they appear in expressions,
analogous to the integer promotions, or NULL_TREE to use the
front end’s normal promotion rules. This hook is useful when there are
target-specific types with special promotion rules.
This is currently used only by the C and C++ front ends.
If defined, this hook returns the result of converting expr to
type. It should return the converted expression,
or NULL_TREE to apply the front end’s normal conversion rules.
This hook is useful when there are target-specific types with special
conversion rules.
This is currently used only by the C and C++ front ends.
This macro determines whether to use the JCR section to register Java classes. By default, TARGET_USE_JCR_SECTION is defined to 1 if both SUPPORTS_WEAK and TARGET_HAVE_NAMED_SECTIONS are true, else 0.
This macro determines the size of the objective C jump buffer for the NeXT runtime. By default, OBJC_JBLEN is defined to an innocuous value.
Define this macro if any target-specific attributes need to be attached to the functions in libgcc that provide low-level support for call stack unwinding. It is used in declarations in unwind-generic.h and the associated definitions of those functions.
Define this macro to update the current function stack boundary if necessary.
This hook should return an rtx for Dynamic Realign Argument Pointer (DRAP) if a
different argument pointer register is needed to access the function’s
argument list due to stack realignment. Return NULL if no DRAP
is needed.
When optimization is disabled, this hook indicates whether or not
arguments should be allocated to stack slots. Normally, GCC allocates
stacks slots for arguments when not optimizing in order to make
debugging easier. However, when a function is declared with
__attribute__((naked)), there is no stack frame, and the compiler
cannot safely move arguments from the registers in which they are passed
to the stack. Therefore, this hook should return true in general, but
false for naked functions. The default implementation always returns true.
On some architectures it can take multiple instructions to synthesize
a constant. If there is another constant already in a register that
is close enough in value then it is preferable that the new constant
is computed from this register using immediate addition or
subtraction. We accomplish this through CSE. Besides the value of
the constant we also add a lower and an upper constant anchor to the
available expressions. These are then queried when encountering new
constants. The anchors are computed by rounding the constant up and
down to a multiple of the value of TARGET_CONST_ANCHOR.
TARGET_CONST_ANCHOR should be the maximum positive value
accepted by immediate-add plus one. We currently assume that the
value of TARGET_CONST_ANCHOR is a power of 2. For example, on
MIPS, where add-immediate takes a 16-bit signed value,
TARGET_CONST_ANCHOR is set to ‘0x8000’. The default value
is zero, which disables this optimization.
Next: Fragments, Previous: Target Macros, Up: Top [Contents][Index]
Most details about the machine and system on which the compiler is
actually running are detected by the configure script. Some
things are impossible for configure to detect; these are
described in two ways, either by macros defined in a file named
xm-machine.h or by hook functions in the file specified
by the out_host_hook_obj variable in config.gcc. (The
intention is that very few hosts will need a header file but nearly
every fully supported host will need to override some hooks.)
If you need to define only a few macros, and they have simple
definitions, consider using the xm_defines variable in your
config.gcc entry instead of creating a host configuration
header. See System Config.
| • Host Common: | Things every host probably needs implemented. | |
| • Filesystem: | Your host can’t have the letter ‘a’ in filenames? | |
| • Host Misc: | Rare configuration options for hosts. |
Next: Filesystem, Up: Host Config [Contents][Index]
Some things are just not portable, even between similar operating systems, and are too difficult for autoconf to detect. They get implemented using hook functions in the file specified by the host_hook_obj variable in config.gcc.
This host hook is used to set up handling for extra signals. The most common thing to do in this hook is to detect stack overflow.
This host hook returns the address of some space that is likely to be
free in some subsequent invocation of the compiler. We intend to load
the PCH data at this address such that the data need not be relocated.
The area should be able to hold size bytes. If the host uses
mmap, fd is an open file descriptor that can be used for
probing.
This host hook is called when a PCH file is about to be loaded.
We want to load size bytes from fd at offset
into memory at address. The given address will be the result of
a previous invocation of HOST_HOOKS_GT_PCH_GET_ADDRESS.
Return -1 if we couldn’t allocate size bytes at address.
Return 0 if the memory is allocated but the data is not loaded. Return 1
if the hook has performed everything.
If the implementation uses reserved address space, free any reserved space beyond size, regardless of the return value. If no PCH will be loaded, this hook may be called with size zero, in which case all reserved address space should be freed.
Do not try to handle values of address that could not have been returned by this executable; just return -1. Such values usually indicate an out-of-date PCH file (built by some other GCC executable), and such a PCH file won’t work.
This host hook returns the alignment required for allocating virtual memory. Usually this is the same as getpagesize, but on some hosts the alignment for reserving memory differs from the pagesize for committing memory.
Next: Host Misc, Previous: Host Common, Up: Host Config [Contents][Index]
GCC needs to know a number of things about the semantics of the host machine’s filesystem. Filesystems with Unix and MS-DOS semantics are automatically detected. For other systems, you can define the following macros in xm-machine.h.
HAVE_DOS_BASED_FILE_SYSTEM
This macro is automatically defined by system.h if the host file system obeys the semantics defined by MS-DOS instead of Unix. DOS file systems are case insensitive, file specifications may begin with a drive letter, and both forward slash and backslash (‘/’ and ‘\’) are directory separators.
DIR_SEPARATOR
DIR_SEPARATOR_2
If defined, these macros expand to character constants specifying separators for directory names within a file specification. system.h will automatically give them appropriate values on Unix and MS-DOS file systems. If your file system is neither of these, define one or both appropriately in xm-machine.h.
However, operating systems like VMS, where constructing a pathname is more complicated than just stringing together directory names separated by a special character, should not define either of these macros.
PATH_SEPARATOR
If defined, this macro should expand to a character constant specifying the separator for elements of search paths. The default value is a colon (‘:’). DOS-based systems usually, but not always, use semicolon (‘;’).
VMS
Define this macro if the host system is VMS.
HOST_OBJECT_SUFFIX
Define this macro to be a C string representing the suffix for object files on your host machine. If you do not define this macro, GCC will use ‘.o’ as the suffix for object files.
HOST_EXECUTABLE_SUFFIX
Define this macro to be a C string representing the suffix for executable files on your host machine. If you do not define this macro, GCC will use the null string as the suffix for executable files.
HOST_BIT_BUCKET
A pathname defined by the host operating system, which can be opened as a file and written to, but all the information written is discarded. This is commonly known as a bit bucket or null device. If you do not define this macro, GCC will use ‘/dev/null’ as the bit bucket. If the host does not support a bit bucket, define this macro to an invalid filename.
UPDATE_PATH_HOST_CANONICALIZE (path)
If defined, a C statement (sans semicolon) that performs host-dependent canonicalization when a path used in a compilation driver or preprocessor is canonicalized. path is a malloc-ed path to be canonicalized. If the C statement does canonicalize path into a different buffer, the old path should be freed and the new buffer should have been allocated with malloc.
DUMPFILE_FORMAT
Define this macro to be a C string representing the format to use for constructing the index part of debugging dump file names. The resultant string must fit in fifteen bytes. The full filename will be the concatenation of: the prefix of the assembler file name, the string resulting from applying this format to an index number, and a string unique to each dump file kind, e.g. ‘rtl’.
If you do not define this macro, GCC will use ‘.%02d.’. You should define this macro if using the default will create an invalid file name.
DELETE_IF_ORDINARY
Define this macro to be a C statement (sans semicolon) that performs host-dependent removal of ordinary temp files in the compilation driver.
If you do not define this macro, GCC will use the default version. You should define this macro if the default version does not reliably remove the temp file as, for example, on VMS which allows multiple versions of a file.
HOST_LACKS_INODE_NUMBERS
Define this macro if the host filesystem does not report meaningful inode numbers in struct stat.
Previous: Filesystem, Up: Host Config [Contents][Index]
FATAL_EXIT_CODE
A C expression for the status code to be returned when the compiler exits after serious errors. The default is the system-provided macro ‘EXIT_FAILURE’, or ‘1’ if the system doesn’t define that macro. Define this macro only if these defaults are incorrect.
SUCCESS_EXIT_CODE
A C expression for the status code to be returned when the compiler exits without serious errors. (Warnings are not serious errors.) The default is the system-provided macro ‘EXIT_SUCCESS’, or ‘0’ if the system doesn’t define that macro. Define this macro only if these defaults are incorrect.
USE_C_ALLOCA
Define this macro if GCC should use the C implementation of alloca
provided by libiberty.a. This only affects how some parts of the
compiler itself allocate memory. It does not change code generation.
When GCC is built with a compiler other than itself, the C alloca
is always used. This is because most other implementations have serious
bugs. You should define this macro only on a system where no
stack-based alloca can possibly work. For instance, if a system
has a small limit on the size of the stack, GCC’s builtin alloca
will not work reliably.
COLLECT2_HOST_INITIALIZATION
If defined, a C statement (sans semicolon) that performs host-dependent
initialization when collect2 is being initialized.
GCC_DRIVER_HOST_INITIALIZATION
If defined, a C statement (sans semicolon) that performs host-dependent initialization when a compilation driver is being initialized.
HOST_LONG_LONG_FORMAT
If defined, the string used to indicate an argument of type long
long to functions like printf. The default value is
"ll".
HOST_LONG_FORMAT
If defined, the string used to indicate an argument of type long
to functions like printf. The default value is "l".
HOST_PTR_PRINTF
If defined, the string used to indicate an argument of type void *
to functions like printf. The default value is "%p".
In addition, if configure generates an incorrect definition of
any of the macros in auto-host.h, you can override that
definition in a host configuration header. If you need to do this,
first see if it is possible to fix configure.
Next: Collect2, Previous: Host Config, Up: Top [Contents][Index]
When you configure GCC using the configure script, it will construct the file Makefile from the template file Makefile.in. When it does this, it can incorporate makefile fragments from the config directory. These are used to set Makefile parameters that are not amenable to being calculated by autoconf. The list of fragments to incorporate is set by config.gcc (and occasionally config.build and config.host); See System Config.
Fragments are named either t-target or x-host, depending on whether they are relevant to configuring GCC to produce code for a particular target, or to configuring GCC to run on a particular host. Here target and host are mnemonics which usually have some relationship to the canonical system name, but no formal connection.
If these files do not exist, it means nothing needs to be added for a given target or host. Most targets need a few t-target fragments, but needing x-host fragments is rare.
| • Target Fragment: | Writing t-target files. | |
| • Host Fragment: | Writing x-host files. |
Next: Host Fragment, Up: Fragments [Contents][Index]
Target makefile fragments can set these Makefile variables.
LIBGCC2_CFLAGSCompiler flags to use when compiling libgcc2.c.
LIB2FUNCS_EXTRAA list of source file names to be compiled or assembled and inserted into libgcc.a.
Floating Point EmulationTo have GCC include software floating point libraries in libgcc.a
define FPBIT and DPBIT along with a few rules as follows:
# We want fine grained libraries, so use the new code
# to build the floating point emulation libraries.
FPBIT = fp-bit.c
DPBIT = dp-bit.c
fp-bit.c: $(srcdir)/config/fp-bit.c
echo '#define FLOAT' > fp-bit.c
cat $(srcdir)/config/fp-bit.c >> fp-bit.c
dp-bit.c: $(srcdir)/config/fp-bit.c
cat $(srcdir)/config/fp-bit.c > dp-bit.c
You may need to provide additional #defines at the beginning of fp-bit.c and dp-bit.c to control target endianness and other options.
CRTSTUFF_T_CFLAGSSpecial flags used when compiling crtstuff.c. See Initialization.
CRTSTUFF_T_CFLAGS_SSpecial flags used when compiling crtstuff.c for shared
linking. Used if you use crtbeginS.o and crtendS.o
in EXTRA-PARTS.
See Initialization.
MULTILIB_OPTIONSFor some targets, invoking GCC in different ways produces objects that can not be linked together. For example, for some targets GCC produces both big and little endian code. For these targets, you must arrange for multiple versions of libgcc.a to be compiled, one for each set of incompatible options. When GCC invokes the linker, it arranges to link in the right version of libgcc.a, based on the command line options used.
The MULTILIB_OPTIONS macro lists the set of options for which
special versions of libgcc.a must be built. Write options that
are mutually incompatible side by side, separated by a slash. Write
options that may be used together separated by a space. The build
procedure will build all combinations of compatible options.
For example, if you set MULTILIB_OPTIONS to ‘m68000/m68020
msoft-float’, Makefile will build special versions of
libgcc.a using the following sets of options: -m68000,
-m68020, -msoft-float, ‘-m68000 -msoft-float’, and
‘-m68020 -msoft-float’.
MULTILIB_DIRNAMESIf MULTILIB_OPTIONS is used, this variable specifies the
directory names that should be used to hold the various libraries.
Write one element in MULTILIB_DIRNAMES for each element in
MULTILIB_OPTIONS. If MULTILIB_DIRNAMES is not used, the
default value will be MULTILIB_OPTIONS, with all slashes treated
as spaces.
MULTILIB_DIRNAMES describes the multilib directories using GCC
conventions and is applied to directories that are part of the GCC
installation. When multilib-enabled, the compiler will add a
subdirectory of the form prefix/multilib before each
directory in the search path for libraries and crt files.
For example, if MULTILIB_OPTIONS is set to ‘m68000/m68020
msoft-float’, then the default value of MULTILIB_DIRNAMES is
‘m68000 m68020 msoft-float’. You may specify a different value if
you desire a different set of directory names.
MULTILIB_MATCHESSometimes the same option may be written in two different ways. If an
option is listed in MULTILIB_OPTIONS, GCC needs to know about
any synonyms. In that case, set MULTILIB_MATCHES to a list of
items of the form ‘option=option’ to describe all relevant
synonyms. For example, ‘m68000=mc68000 m68020=mc68020’.
MULTILIB_EXCEPTIONSSometimes when there are multiple sets of MULTILIB_OPTIONS being
specified, there are combinations that should not be built. In that
case, set MULTILIB_EXCEPTIONS to be all of the switch exceptions
in shell case syntax that should not be built.
For example the ARM processor cannot execute both hardware floating
point instructions and the reduced size THUMB instructions at the same
time, so there is no need to build libraries with both of these
options enabled. Therefore MULTILIB_EXCEPTIONS is set to:
*mthumb/*mhard-float*
MULTILIB_EXTRA_OPTSSometimes it is desirable that when building multiple versions of
libgcc.a certain options should always be passed on to the
compiler. In that case, set MULTILIB_EXTRA_OPTS to be the list
of options to be used for all builds. If you set this, you should
probably set CRTSTUFF_T_CFLAGS to a dash followed by it.
NATIVE_SYSTEM_HEADER_DIRIf the default location for system headers is not /usr/include,
you must set this to the directory containing the headers. This value
should match the value of the SYSTEM_INCLUDE_DIR macro.
MULTILIB_OSDIRNAMESIf MULTILIB_OPTIONS is used, this variable specifies
a list of subdirectory names, that are used to modify the search
path depending on the chosen multilib. Unlike MULTILIB_DIRNAMES,
MULTILIB_OSDIRNAMES describes the multilib directories using
operating systems conventions, and is applied to the directories such as
lib or those in the LIBRARY_PATH environment variable.
The format is either the same as of
MULTILIB_DIRNAMES, or a set of mappings. When it is the same
as MULTILIB_DIRNAMES, it describes the multilib directories
using operating system conventions, rather than GCC conventions. When it is a set
of mappings of the form gccdir=osdir, the left side gives
the GCC convention and the right gives the equivalent OS defined
location. If the osdir part begins with a ‘!’,
GCC will not search in the non-multilib directory and use
exclusively the multilib directory. Otherwise, the compiler will
examine the search path for libraries and crt files twice; the first
time it will add multilib to each directory in the search path,
the second it will not.
For configurations that support both multilib and multiarch,
MULTILIB_OSDIRNAMES also encodes the multiarch name, thus
subsuming MULTIARCH_DIRNAME. The multiarch name is appended to
each directory name, separated by a colon (e.g.
‘../lib32:i386-linux-gnu’).
Each multiarch subdirectory will be searched before the corresponding OS
multilib directory, for example ‘/lib/i386-linux-gnu’ before
‘/lib/../lib32’. The multiarch name will also be used to modify the
system header search path, as explained for MULTIARCH_DIRNAME.
MULTIARCH_DIRNAMEThis variable specifies the multiarch name for configurations that are multiarch-enabled but not multilibbed configurations.
The multiarch name is used to augment the search path for libraries, crt
files and system header files with additional locations. The compiler
will add a multiarch subdirectory of the form
prefix/multiarch before each directory in the library and
crt search path. It will also add two directories
LOCAL_INCLUDE_DIR/multiarch and
NATIVE_SYSTEM_HEADER_DIR/multiarch) to the system header
search path, respectively before LOCAL_INCLUDE_DIR and
NATIVE_SYSTEM_HEADER_DIR.
MULTIARCH_DIRNAME is not used for configurations that support
both multilib and multiarch. In that case, multiarch names are encoded
in MULTILIB_OSDIRNAMES instead.
More documentation about multiarch can be found at http://wiki.debian.org/Multiarch.
SPECSUnfortunately, setting MULTILIB_EXTRA_OPTS is not enough, since
it does not affect the build of target libraries, at least not the
build of the default multilib. One possible work-around is to use
DRIVER_SELF_SPECS to bring options from the specs file
as if they had been passed in the compiler driver command line.
However, you don’t want to be adding these options after the toolchain
is installed, so you can instead tweak the specs file that will
be used during the toolchain build, while you still install the
original, built-in specs. The trick is to set SPECS to
some other filename (say specs.install), that will then be
created out of the built-in specs, and introduce a Makefile
rule to generate the specs file that’s going to be used at
build time out of your specs.install.
T_CFLAGSThese are extra flags to pass to the C compiler. They are used both when building GCC, and when compiling things with the just-built GCC. This variable is deprecated and should not be used.
Previous: Target Fragment, Up: Fragments [Contents][Index]
The use of x-host fragments is discouraged. You should only use it for makefile dependencies.
Next: Header Dirs, Previous: Fragments, Up: Top [Contents][Index]
collect2GCC uses a utility called collect2 on nearly all systems to arrange
to call various initialization functions at start time.
The program collect2 works by linking the program once and
looking through the linker output file for symbols with particular names
indicating they are constructor functions. If it finds any, it
creates a new temporary ‘.c’ file containing a table of them,
compiles it, and links the program a second time including that file.
The actual calls to the constructors are carried out by a subroutine
called __main, which is called (automatically) at the beginning
of the body of main (provided main was compiled with GNU
CC). Calling __main is necessary, even when compiling C code, to
allow linking C and C++ object code together. (If you use
-nostdlib, you get an unresolved reference to __main,
since it’s defined in the standard GCC library. Include -lgcc at
the end of your compiler command line to resolve this reference.)
The program collect2 is installed as ld in the directory
where the passes of the compiler are installed. When collect2
needs to find the real ld, it tries the following file
names:
PATH.
REAL_LD_FILE_NAME configuration macro,
if specified.
collect2 will not execute itself recursively.
PATH.
“The compiler’s search directories” means all the directories where
gcc searches for passes of the compiler. This includes
directories that you specify with -B.
Cross-compilers search a little differently:
PATH.
REAL_LD_FILE_NAME configuration macro,
if specified.
PATH.
collect2 explicitly avoids running ld using the file name
under which collect2 itself was invoked. In fact, it remembers
up a list of such names—in case one copy of collect2 finds
another copy (or version) of collect2 installed as ld in a
second place in the search path.
collect2 searches for the utilities nm and strip
using the same algorithm as above for ld.
Next: Type Information, Previous: Collect2, Up: Top [Contents][Index]
GCC_INCLUDE_DIR means the same thing for native and cross. It is
where GCC stores its private include files, and also where GCC
stores the fixed include files. A cross compiled GCC runs
fixincludes on the header files in $(tooldir)/include.
(If the cross compilation header files need to be fixed, they must be
installed before GCC is built. If the cross compilation header files
are already suitable for GCC, nothing special need be done).
GPLUSPLUS_INCLUDE_DIR means the same thing for native and cross. It
is where g++ looks first for header files. The C++ library
installs only target independent header files in that directory.
LOCAL_INCLUDE_DIR is used only by native compilers. GCC
doesn’t install anything there. It is normally
/usr/local/include. This is where local additions to a packaged
system should place header files.
CROSS_INCLUDE_DIR is used only by cross compilers. GCC
doesn’t install anything there.
TOOL_INCLUDE_DIR is used for both native and cross compilers. It
is the place for other packages to install header files that GCC will
use. For a cross-compiler, this is the equivalent of
/usr/include. When you build a cross-compiler,
fixincludes processes any header files in this directory.
Next: Plugins, Previous: Header Dirs, Up: Top [Contents][Index]
GCC uses some fairly sophisticated memory management techniques, which involve determining information about GCC’s data structures from GCC’s source code and using this information to perform garbage collection and implement precompiled headers.
A full C parser would be too complicated for this task, so a limited
subset of C is interpreted and special markers are used to determine
what parts of the source to look at. All struct and
union declarations that define data structures that are
allocated under control of the garbage collector must be marked. All
global variables that hold pointers to garbage-collected memory must
also be marked. Finally, all global variables that need to be saved
and restored by a precompiled header must be marked. (The precompiled
header mechanism can only save static variables if they’re scalar.
Complex data structures must be allocated in garbage-collected memory
to be saved in a precompiled header.)
The full format of a marker is
GTY (([option] [(param)], [option] [(param)] …))
but in most cases no options are needed. The outer double parentheses
are still necessary, though: GTY(()). Markers can appear:
static or
extern; and
Here are some examples of marking simple data structures and globals.
struct GTY(()) tag
{
fields…
};
typedef struct GTY(()) tag
{
fields…
} *typename;
static GTY(()) struct tag *list; /* points to GC memory */
static GTY(()) int counter; /* save counter in a PCH */
The parser understands simple typedefs such as
typedef struct tag *name; and
typedef int name;.
These don’t need to be marked.
| • GTY Options: | What goes inside a GTY(()).
| |
| • GGC Roots: | Making global variables GGC roots. | |
| • Files: | How the generated files work. | |
| • Invoking the garbage collector: | How to invoke the garbage collector. | |
| • Troubleshooting: | When something does not work as expected. |
Next: GGC Roots, Up: Type Information [Contents][Index]
GTY(())Sometimes the C code is not enough to fully describe the type
structure. Extra information can be provided with GTY options
and additional markers. Some options take a parameter, which may be
either a string or a type name, depending on the parameter. If an
option takes no parameter, it is acceptable either to omit the
parameter entirely, or to provide an empty string as a parameter. For
example, GTY ((skip)) and GTY ((skip (""))) are
equivalent.
When the parameter is a string, often it is a fragment of C code. Four special escapes may be used in these strings, to refer to pieces of the data structure being marked:
%hThe current structure.
%1The structure that immediately contains the current structure.
%0The outermost structure that contains the current structure.
%aA partial expression of the form [i1][i2]… that indexes
the array item currently being marked.
For instance, suppose that you have a structure of the form
struct A {
…
};
struct B {
struct A foo[12];
};
and b is a variable of type struct B. When marking
‘b.foo[11]’, %h would expand to ‘b.foo[11]’,
%0 and %1 would both expand to ‘b’, and %a
would expand to ‘[11]’.
As in ordinary C, adjacent strings will be concatenated; this is helpful when you have a complicated expression.
GTY ((chain_next ("TREE_CODE (&%h.generic) == INTEGER_TYPE"
" ? TYPE_NEXT_VARIANT (&%h.generic)"
" : TREE_CHAIN (&%h.generic)")))
The available options are:
length ("expression")There are two places the type machinery will need to be explicitly told the length of an array. The first case is when a structure ends in a variable-length array, like this:
struct GTY(()) rtvec_def {
int num_elem; /* number of elements */
rtx GTY ((length ("%h.num_elem"))) elem[1];
};
In this case, the length option is used to override the specified
array length (which should usually be 1). The parameter of the
option is a fragment of C code that calculates the length.
The second case is when a structure or a global variable contains a pointer to an array, like this:
struct gimple_omp_for_iter * GTY((length ("%h.collapse"))) iter;
In this case, iter has been allocated by writing something like
x->iter = ggc_alloc_cleared_vec_gimple_omp_for_iter (collapse);
and the collapse provides the length of the field.
This second use of length also works on global variables, like:
static GTY((length("reg_known_value_size"))) rtx *reg_known_value;
skipIf skip is applied to a field, the type machinery will ignore it.
This is somewhat dangerous; the only safe use is in a union when one
field really isn’t ever used.
desc ("expression")tag ("constant")defaultThe type machinery needs to be told which field of a union is
currently active. This is done by giving each field a constant
tag value, and then specifying a discriminator using desc.
The value of the expression given by desc is compared against
each tag value, each of which should be different. If no
tag is matched, the field marked with default is used if
there is one, otherwise no field in the union will be marked.
In the desc option, the “current structure” is the union that
it discriminates. Use %1 to mean the structure containing it.
There are no escapes available to the tag option, since it is a
constant.
For example,
struct GTY(()) tree_binding
{
struct tree_common common;
union tree_binding_u {
tree GTY ((tag ("0"))) scope;
struct cp_binding_level * GTY ((tag ("1"))) level;
} GTY ((desc ("BINDING_HAS_LEVEL_P ((tree)&%0)"))) xscope;
tree value;
};
In this example, the value of BINDING_HAS_LEVEL_P when applied to a
struct tree_binding * is presumed to be 0 or 1. If 1, the type
mechanism will treat the field level as being present and if 0,
will treat the field scope as being present.
param_is (type)use_paramSometimes it’s convenient to define some data structure to work on
generic pointers (that is, PTR) and then use it with a specific
type. param_is specifies the real type pointed to, and
use_param says where in the generic data structure that type
should be put.
For instance, to have a htab_t that points to trees, one would
write the definition of htab_t like this:
typedef struct GTY(()) {
…
void ** GTY ((use_param, …)) entries;
…
} htab_t;
and then declare variables like this:
static htab_t GTY ((param_is (union tree_node))) ict;
paramn_is (type)use_paramnIn more complicated cases, the data structure might need to work on
several different types, which might not necessarily all be pointers.
For this, param1_is through param9_is may be used to
specify the real type of a field identified by use_param1 through
use_param9.
use_paramsWhen a structure contains another structure that is parameterized,
there’s no need to do anything special, the inner structure inherits the
parameters of the outer one. When a structure contains a pointer to a
parameterized structure, the type machinery won’t automatically detect
this (it could, it just doesn’t yet), so it’s necessary to tell it that
the pointed-to structure should use the same parameters as the outer
structure. This is done by marking the pointer with the
use_params option.
deletabledeletable, when applied to a global variable, indicates that when
garbage collection runs, there’s no need to mark anything pointed to
by this variable, it can just be set to NULL instead. This is used
to keep a list of free structures around for re-use.
if_marked ("expression")Suppose you want some kinds of object to be unique, and so you put them
in a hash table. If garbage collection marks the hash table, these
objects will never be freed, even if the last other reference to them
goes away. GGC has special handling to deal with this: if you use the
if_marked option on a global hash table, GGC will call the
routine whose name is the parameter to the option on each hash table
entry. If the routine returns nonzero, the hash table entry will
be marked as usual. If the routine returns zero, the hash table entry
will be deleted.
The routine ggc_marked_p can be used to determine if an element
has been marked already; in fact, the usual case is to use
if_marked ("ggc_marked_p").
mark_hook ("hook-routine-name")If provided for a structure or union type, the given hook-routine-name (between double-quotes) is the name of a routine called when the garbage collector has just marked the data as reachable. This routine should not change the data, or call any ggc routine. Its only argument is a pointer to the just marked (const) structure or union.
maybe_undefWhen applied to a field, maybe_undef indicates that it’s OK if
the structure that this fields points to is never defined, so long as
this field is always NULL. This is used to avoid requiring
backends to define certain optional structures. It doesn’t work with
language frontends.
nested_ptr (type, "to expression", "from expression")The type machinery expects all pointers to point to the start of an
object. Sometimes for abstraction purposes it’s convenient to have
a pointer which points inside an object. So long as it’s possible to
convert the original object to and from the pointer, such pointers
can still be used. type is the type of the original object,
the to expression returns the pointer given the original object,
and the from expression returns the original object given
the pointer. The pointer will be available using the %h
escape.
chain_next ("expression")chain_prev ("expression")chain_circular ("expression")It’s helpful for the type machinery to know if objects are often
chained together in long lists; this lets it generate code that uses
less stack space by iterating along the list instead of recursing down
it. chain_next is an expression for the next item in the list,
chain_prev is an expression for the previous item. For singly
linked lists, use only chain_next; for doubly linked lists, use
both. The machinery requires that taking the next item of the
previous item gives the original item. chain_circular is similar
to chain_next, but can be used for circular single linked lists.
reorder ("function name")Some data structures depend on the relative ordering of pointers. If
the precompiled header machinery needs to change that ordering, it
will call the function referenced by the reorder option, before
changing the pointers in the object that’s pointed to by the field the
option applies to. The function must take four arguments, with the
signature ‘void *, void *, gt_pointer_operator, void *’.
The first parameter is a pointer to the structure that contains the
object being updated, or the object itself if there is no containing
structure. The second parameter is a cookie that should be ignored.
The third parameter is a routine that, given a pointer, will update it
to its correct new value. The fourth parameter is a cookie that must
be passed to the second parameter.
PCH cannot handle data structures that depend on the absolute values
of pointers. reorder functions can be expensive. When
possible, it is better to depend on properties of the data, like an ID
number or the hash of a string instead.
variable_sizeThe type machinery expects the types to be of constant size. When this
is not true, for example, with structs that have array fields or unions,
the type machinery cannot tell how many bytes need to be allocated at
each allocation. The variable_size is used to mark such types.
The type machinery then provides allocators that take a parameter
indicating an exact size of object being allocated. Note that the size
must be provided in bytes whereas the length option works with
array lengths in number of elements.
For example,
struct GTY((variable_size)) sorted_fields_type {
int len;
tree GTY((length ("%h.len"))) elts[1];
};
Then the objects of struct sorted_fields_type are allocated in GC
memory as follows:
field_vec = ggc_alloc_sorted_fields_type (size);
If field_vec->elts stores n elements, then size could be calculated as follows:
size_t size = sizeof (struct sorted_fields_type) + n * sizeof (tree);
special ("name")The special option is used to mark types that have to be dealt
with by special case machinery. The parameter is the name of the
special case. See gengtype.c for further details. Avoid
adding new special cases unless there is no other alternative.
Next: Files, Previous: GTY Options, Up: Type Information [Contents][Index]
In addition to keeping track of types, the type machinery also locates the global variables (roots) that the garbage collector starts at. Roots must be declared using one of the following syntaxes:
extern GTY(([options])) type name;
static GTY(([options])) type name;
The syntax
GTY(([options])) type name;
is not accepted. There should be an extern declaration
of such a variable in a header somewhere—mark that, not the
definition. Or, if the variable is only used in one file, make it
static.
Next: Invoking the garbage collector, Previous: GGC Roots, Up: Type Information [Contents][Index]
Whenever you add GTY markers to a source file that previously
had none, or create a new source file containing GTY markers,
there are three things you need to do:
target_gtfiles in
the appropriate port’s entries in config.gcc.
GTFILES variable in Makefile.in.
gtfiles variable defined in the appropriate
config-lang.in. For C, the file is c-config-lang.in.
Headers should appear before non-headers in this list.
gtfiles variable of all the front ends
that use it.
ifiles in open_base_file in gengtype.c.
For source files that aren’t header files, the machinery will generate a header file that should be included in the source file you just changed. The file will be called gt-path.h where path is the pathname relative to the gcc directory with slashes replaced by -, so for example the header file to be included in cp/parser.c is called gt-cp-parser.c. The generated header file should be included after everything else in the source file. Don’t forget to mention this file as a dependency in the Makefile!
For language frontends, there is another file that needs to be included somewhere. It will be called gtype-lang.h, where lang is the name of the subdirectory the language is contained in.
Plugins can add additional root tables. Run the gengtype
utility in plugin mode as gengtype -P pluginout.h source-dir
file-list plugin*.c with your plugin files
plugin*.c using GTY to generate the pluginout.h file.
The GCC build tree is needed to be present in that mode.
Next: Troubleshooting, Previous: Files, Up: Type Information [Contents][Index]
The GCC garbage collector GGC is only invoked explicitly. In contrast
with many other garbage collectors, it is not implicitly invoked by
allocation routines when a lot of memory has been consumed. So the
only way to have GGC reclaim storage it to call the ggc_collect
function explicitly. This call is an expensive operation, as it may
have to scan the entire heap. Beware that local variables (on the GCC
call stack) are not followed by such an invocation (as many other
garbage collectors do): you should reference all your data from static
or external GTY-ed variables, and it is advised to call
ggc_collect with a shallow call stack. The GGC is an exact mark
and sweep garbage collector (so it does not scan the call stack for
pointers). In practice GCC passes don’t often call ggc_collect
themselves, because it is called by the pass manager between passes.
At the time of the ggc_collect call all pointers in the GC-marked
structures must be valid or NULL. In practice this means that
there should not be uninitialized pointer fields in the structures even
if your code never reads or writes those fields at a particular
instance. One way to ensure this is to use cleared versions of
allocators unless all the fields are initialized manually immediately
after allocation.
Previous: Invoking the garbage collector, Up: Type Information [Contents][Index]
With the current garbage collector implementation, most issues should show up as GCC compilation errors. Some of the most commonly encountered issues are described below.
GTY-marked type.
Gengtype checks if there is at least one possible path from GC roots to
at least one instance of each type before outputting allocators. If
there is no such path, the GTY markers will be ignored and no
allocators will be output. Solve this by making sure that there exists
at least one such path. If creating it is unfeasible or raises a “code
smell”, consider if you really must use GC for allocating such type.
gt_ggc_r_foo_bar and
similarly-named symbols. Check if your foo_bar source file has
#include "gt-foo_bar.h" as its very last line.
Next: LTO, Previous: Type Information, Up: Top [Contents][Index]
Plugins are supported on platforms that support -ldl
-rdynamic. They are loaded by the compiler using dlopen
and invoked at pre-determined locations in the compilation
process.
Plugins are loaded with
-fplugin=/path/to/name.so -fplugin-arg-name-key1[=value1]
The plugin arguments are parsed by GCC and passed to respective plugins as key-value pairs. Multiple plugins can be invoked by specifying multiple -fplugin arguments.
A plugin can be simply given by its short name (no dots or slashes). When simply passing -fplugin=name, the plugin is loaded from the plugin directory, so -fplugin=name is the same as -fplugin=`gcc -print-file-name=plugin`/name.so, using backquote shell syntax to query the plugin directory.
Plugins are activated by the compiler at specific events as defined in
gcc-plugin.h. For each event of interest, the plugin should
call register_callback specifying the name of the event and
address of the callback function that will handle that event.
The header gcc-plugin.h must be the first gcc header to be included.
Every plugin should define the global symbol plugin_is_GPL_compatible
to assert that it has been licensed under a GPL-compatible license.
If this symbol does not exist, the compiler will emit a fatal error
and exit with the error message:
fatal error: plugin name is not licensed under a GPL-compatible license name: undefined symbol: plugin_is_GPL_compatible compilation terminated
The declared type of the symbol should be int, to match a forward declaration in gcc-plugin.h that suppresses C++ mangling. It does not need to be in any allocated section, though. The compiler merely asserts that the symbol exists in the global scope. Something like this is enough:
int plugin_is_GPL_compatible;
Every plugin should export a function called plugin_init that
is called right after the plugin is loaded. This function is
responsible for registering all the callbacks required by the plugin
and do any other required initialization.
This function is called from compile_file right before invoking
the parser. The arguments to plugin_init are:
plugin_info: Plugin invocation information.
version: GCC version.
The plugin_info struct is defined as follows:
struct plugin_name_args
{
char *base_name; /* Short name of the plugin
(filename without .so suffix). */
const char *full_name; /* Path to the plugin as specified with
-fplugin=. */
int argc; /* Number of arguments specified with
-fplugin-arg-.... */
struct plugin_argument *argv; /* Array of ARGC key-value pairs. */
const char *version; /* Version string provided by plugin. */
const char *help; /* Help string provided by plugin. */
}
If initialization fails, plugin_init must return a non-zero
value. Otherwise, it should return 0.
The version of the GCC compiler loading the plugin is described by the following structure:
struct plugin_gcc_version
{
const char *basever;
const char *datestamp;
const char *devphase;
const char *revision;
const char *configuration_arguments;
};
The function plugin_default_version_check takes two pointers to
such structure and compare them field by field. It can be used by the
plugin’s plugin_init function.
The version of GCC used to compile the plugin can be found in the symbol
gcc_version defined in the header plugin-version.h. The
recommended version check to perform looks like
#include "plugin-version.h"
...
int
plugin_init (struct plugin_name_args *plugin_info,
struct plugin_gcc_version *version)
{
if (!plugin_default_version_check (version, &gcc_version))
return 1;
}
but you can also check the individual fields if you want a less strict check.
Callback functions have the following prototype:
/* The prototype for a plugin callback function.
gcc_data - event-specific data provided by GCC
user_data - plugin-specific data provided by the plug-in. */
typedef void (*plugin_callback_func)(void *gcc_data, void *user_data);
Callbacks can be invoked at the following pre-determined events:
enum plugin_event
{
PLUGIN_PASS_MANAGER_SETUP, /* To hook into pass manager. */
PLUGIN_FINISH_TYPE, /* After finishing parsing a type. */
PLUGIN_FINISH_UNIT, /* Useful for summary processing. */
PLUGIN_PRE_GENERICIZE, /* Allows to see low level AST in C and C++ frontends. */
PLUGIN_FINISH, /* Called before GCC exits. */
PLUGIN_INFO, /* Information about the plugin. */
PLUGIN_GGC_START, /* Called at start of GCC Garbage Collection. */
PLUGIN_GGC_MARKING, /* Extend the GGC marking. */
PLUGIN_GGC_END, /* Called at end of GGC. */
PLUGIN_REGISTER_GGC_ROOTS, /* Register an extra GGC root table. */
PLUGIN_REGISTER_GGC_CACHES, /* Register an extra GGC cache table. */
PLUGIN_ATTRIBUTES, /* Called during attribute registration */
PLUGIN_START_UNIT, /* Called before processing a translation unit. */
PLUGIN_PRAGMAS, /* Called during pragma registration. */
/* Called before first pass from all_passes. */
PLUGIN_ALL_PASSES_START,
/* Called after last pass from all_passes. */
PLUGIN_ALL_PASSES_END,
/* Called before first ipa pass. */
PLUGIN_ALL_IPA_PASSES_START,
/* Called after last ipa pass. */
PLUGIN_ALL_IPA_PASSES_END,
/* Allows to override pass gate decision for current_pass. */
PLUGIN_OVERRIDE_GATE,
/* Called before executing a pass. */
PLUGIN_PASS_EXECUTION,
/* Called before executing subpasses of a GIMPLE_PASS in
execute_ipa_pass_list. */
PLUGIN_EARLY_GIMPLE_PASSES_START,
/* Called after executing subpasses of a GIMPLE_PASS in
execute_ipa_pass_list. */
PLUGIN_EARLY_GIMPLE_PASSES_END,
/* Called when a pass is first instantiated. */
PLUGIN_NEW_PASS,
PLUGIN_EVENT_FIRST_DYNAMIC /* Dummy event used for indexing callback
array. */
};
In addition, plugins can also look up the enumerator of a named event,
and / or generate new events dynamically, by calling the function
get_named_event_id.
To register a callback, the plugin calls register_callback with
the arguments:
char *name: Plugin name.
int event: The event code.
plugin_callback_func callback: The function that handles event.
void *user_data: Pointer to plugin-specific data.
For the PLUGIN_PASS_MANAGER_SETUP, PLUGIN_INFO, PLUGIN_REGISTER_GGC_ROOTS
and PLUGIN_REGISTER_GGC_CACHES pseudo-events the callback should be
null, and the user_data is specific.
When the PLUGIN_PRAGMAS event is triggered (with a null
pointer as data from GCC), plugins may register their own pragmas
using functions like c_register_pragma or
c_register_pragma_with_expansion.
There needs to be a way to add/reorder/remove passes dynamically. This is useful for both analysis plugins (plugging in after a certain pass such as CFG or an IPA pass) and optimization plugins.
Basic support for inserting new passes or replacing existing passes is
provided. A plugin registers a new pass with GCC by calling
register_callback with the PLUGIN_PASS_MANAGER_SETUP
event and a pointer to a struct register_pass_info object defined as follows
enum pass_positioning_ops
{
PASS_POS_INSERT_AFTER, // Insert after the reference pass.
PASS_POS_INSERT_BEFORE, // Insert before the reference pass.
PASS_POS_REPLACE // Replace the reference pass.
};
struct register_pass_info
{
struct opt_pass *pass; /* New pass provided by the plugin. */
const char *reference_pass_name; /* Name of the reference pass for hooking
up the new pass. */
int ref_pass_instance_number; /* Insert the pass at the specified
instance number of the reference pass. */
/* Do it for every instance if it is 0. */
enum pass_positioning_ops pos_op; /* how to insert the new pass. */
};
/* Sample plugin code that registers a new pass. */
int
plugin_init (struct plugin_name_args *plugin_info,
struct plugin_gcc_version *version)
{
struct register_pass_info pass_info;
...
/* Code to fill in the pass_info object with new pass information. */
...
/* Register the new pass. */
register_callback (plugin_info->base_name, PLUGIN_PASS_MANAGER_SETUP, NULL, &pass_info);
...
}
Some plugins may want to be informed when GGC (the GCC Garbage
Collector) is running. They can register callbacks for the
PLUGIN_GGC_START and PLUGIN_GGC_END events (for which
the callback is called with a null gcc_data) to be notified of
the start or end of the GCC garbage collection.
Some plugins may need to have GGC mark additional data. This can be
done by registering a callback (called with a null gcc_data)
for the PLUGIN_GGC_MARKING event. Such callbacks can call the
ggc_set_mark routine, preferably thru the ggc_mark macro
(and conversely, these routines should usually not be used in plugins
outside of the PLUGIN_GGC_MARKING event).
Some plugins may need to add extra GGC root tables, e.g. to handle their own
GTY-ed data. This can be done with the PLUGIN_REGISTER_GGC_ROOTS
pseudo-event with a null callback and the extra root table (of type struct
ggc_root_tab*) as user_data. Plugins that want to use the
if_marked hash table option can add the extra GGC cache tables generated
by gengtype using the PLUGIN_REGISTER_GGC_CACHES pseudo-event with
a null callback and the extra cache table (of type struct ggc_cache_tab*)
as user_data. Running the gengtype -p source-dir
file-list plugin*.c ... utility generates these extra root tables.
You should understand the details of memory management inside GCC
before using PLUGIN_GGC_MARKING, PLUGIN_REGISTER_GGC_ROOTS
or PLUGIN_REGISTER_GGC_CACHES.
A plugin should give some information to the user about itself. This uses the following structure:
struct plugin_info
{
const char *version;
const char *help;
};
Such a structure is passed as the user_data by the plugin’s
init routine using register_callback with the
PLUGIN_INFO pseudo-event and a null callback.
For analysis (or other) purposes it is useful to be able to add custom attributes or pragmas.
The PLUGIN_ATTRIBUTES callback is called during attribute
registration. Use the register_attribute function to register
custom attributes.
/* Attribute handler callback */
static tree
handle_user_attribute (tree *node, tree name, tree args,
int flags, bool *no_add_attrs)
{
return NULL_TREE;
}
/* Attribute definition */
static struct attribute_spec user_attr =
{ "user", 1, 1, false, false, false, handle_user_attribute };
/* Plugin callback called during attribute registration.
Registered with register_callback (plugin_name, PLUGIN_ATTRIBUTES, register_attributes, NULL)
*/
static void
register_attributes (void *event_data, void *data)
{
warning (0, G_("Callback to register attributes"));
register_attribute (&user_attr);
}
The PLUGIN_PRAGMAS callback is called during pragmas
registration. Use the c_register_pragma or
c_register_pragma_with_expansion functions to register custom
pragmas.
/* Plugin callback called during pragmas registration. Registered with
register_callback (plugin_name, PLUGIN_PRAGMAS,
register_my_pragma, NULL);
*/
static void
register_my_pragma (void *event_data, void *data)
{
warning (0, G_("Callback to register pragmas"));
c_register_pragma ("GCCPLUGIN", "sayhello", handle_pragma_sayhello);
}
It is suggested to pass "GCCPLUGIN" (or a short name identifying
your plugin) as the “space” argument of your pragma.
The event PLUGIN_PASS_EXECUTION passes the pointer to the executed pass
(the same as current_pass) as gcc_data to the callback. You can also
inspect cfun to find out about which function this pass is executed for.
Note that this event will only be invoked if the gate check (if
applicable, modified by PLUGIN_OVERRIDE_GATE) succeeds.
You can use other hooks, like PLUGIN_ALL_PASSES_START,
PLUGIN_ALL_PASSES_END, PLUGIN_ALL_IPA_PASSES_START,
PLUGIN_ALL_IPA_PASSES_END, PLUGIN_EARLY_GIMPLE_PASSES_START,
and/or PLUGIN_EARLY_GIMPLE_PASSES_END to manipulate global state
in your plugin(s) in order to get context for the pass execution.
After the original gate function for a pass is called, its result
- the gate status - is stored as an integer.
Then the event PLUGIN_OVERRIDE_GATE is invoked, with a pointer
to the gate status in the gcc_data parameter to the callback function.
A nonzero value of the gate status means that the pass is to be executed.
You can both read and write the gate status via the passed pointer.
When your plugin is loaded, you can inspect the various
pass lists to determine what passes are available. However, other
plugins might add new passes. Also, future changes to GCC might cause
generic passes to be added after plugin loading.
When a pass is first added to one of the pass lists, the event
PLUGIN_NEW_PASS is invoked, with the callback parameter
gcc_data pointing to the new pass.
If plugins are enabled, GCC installs the headers needed to build a plugin (somewhere in the installation tree, e.g. under /usr/local). In particular a plugin/include directory is installed, containing all the header files needed to build plugins.
On most systems, you can query this plugin directory by
invoking gcc -print-file-name=plugin (replace if needed
gcc with the appropriate program path).
Inside plugins, this plugin directory name can be queried by
calling default_plugin_dir_name ().
The following GNU Makefile excerpt shows how to build a simple plugin:
GCC=gcc PLUGIN_SOURCE_FILES= plugin1.c plugin2.c PLUGIN_OBJECT_FILES= $(patsubst %.c,%.o,$(PLUGIN_SOURCE_FILES)) GCCPLUGINS_DIR:= $(shell $(GCC) -print-file-name=plugin) CFLAGS+= -I$(GCCPLUGINS_DIR)/include -fPIC -O2 plugin.so: $(PLUGIN_OBJECT_FILES) $(GCC) -shared $^ -o $@
A single source file plugin may be built with gcc -I`gcc
-print-file-name=plugin`/include -fPIC -shared -O2 plugin.c -o
plugin.so, using backquote shell syntax to query the plugin
directory.
Plugins needing to use gengtype require a GCC build
directory for the same version of GCC that they will be linked
against.
Link time optimization is implemented as a GCC front end for a
bytecode representation of GIMPLE that is emitted in special sections
of .o files. Currently, LTO support is enabled in most
ELF-based systems, as well as darwin, cygwin and mingw systems.
Since GIMPLE bytecode is saved alongside final object code, object
files generated with LTO support are larger than regular object files.
This “fat” object format makes it easy to integrate LTO into
existing build systems, as one can, for instance, produce archives of
the files. Additionally, one might be able to ship one set of fat
objects which could be used both for development and the production of
optimized builds. A, perhaps surprising, side effect of this feature
is that any mistake in the toolchain that leads to LTO information not
being used (e.g. an older libtool calling ld directly).
This is both an advantage, as the system is more robust, and a
disadvantage, as the user is not informed that the optimization has
been disabled.
The current implementation only produces “fat” objects, effectively
doubling compilation time and increasing file sizes up to 5x the
original size. This hides the problem that some tools, such as
ar and nm, need to understand symbol tables of LTO
sections. These tools were extended to use the plugin infrastructure,
and with these problems solved, GCC will also support “slim” objects
consisting of the intermediate code alone.
At the highest level, LTO splits the compiler in two. The first half (the “writer”) produces a streaming representation of all the internal data structures needed to optimize and generate code. This includes declarations, types, the callgraph and the GIMPLE representation of function bodies.
When -flto is given during compilation of a source file, the
pass manager executes all the passes in all_lto_gen_passes.
Currently, this phase is composed of two IPA passes:
pass_ipa_lto_gimple_out
This pass executes the function lto_output in
lto-streamer-out.c, which traverses the call graph encoding
every reachable declaration, type and function. This generates a
memory representation of all the file sections described below.
pass_ipa_lto_finish_out
This pass executes the function produce_asm_for_decls in
lto-streamer-out.c, which takes the memory image built in the
previous pass and encodes it in the corresponding ELF file sections.
The second half of LTO support is the “reader”. This is implemented
as the GCC front end lto1 in lto/lto.c. When
collect2 detects a link set of .o/.a files with
LTO information and the -flto is enabled, it invokes
lto1 which reads the set of files and aggregates them into a
single translation unit for optimization. The main entry point for
the reader is lto/lto.c:lto_main.
One of the main goals of the GCC link-time infrastructure was to allow effective compilation of large programs. For this reason GCC implements two link-time compilation modes.
.o
files and distributes the compilation of the sub-graphs to different
CPUs.
Note that distributed compilation is not implemented yet, but since
the parallelism is facilitated via generating a Makefile, it
would be easy to implement.
WHOPR splits LTO into three main stages:
WHOPR can be seen as an extension of the usual LTO mode of compilation. In LTO, WPA and LTRANS are executed within a single execution of the compiler, after the whole program has been read into memory.
When compiling in WHOPR mode, the callgraph is partitioned during the WPA stage. The whole program is split into a given number of partitions of roughly the same size. The compiler tries to minimize the number of references which cross partition boundaries. The main advantage of WHOPR is to allow the parallel execution of LTRANS stages, which are the most time-consuming part of the compilation process. Additionally, it avoids the need to load the whole program into memory.
LTO information is stored in several ELF sections inside object files. Data structures and enum codes for sections are defined in lto-streamer.h.
These sections are emitted from lto-streamer-out.c and mapped
in all at once from lto/lto.c:lto_file_read. The
individual functions dealing with the reading/writing of each section
are described below.
.gnu.lto_.opts)
This section contains the command line options used to generate the object files. This is used at link time to determine the optimization level and other settings when they are not explicitly specified at the linker command line.
Currently, GCC does not support combining LTO object files compiled
with different set of the command line options into a single binary.
At link time, the options given on the command line and the options
saved on all the files in a link-time set are applied globally. No
attempt is made at validating the combination of flags (other than the
usual validation done by option processing). This is implemented in
lto/lto.c:lto_read_all_file_options.
.gnu.lto_.symtab)
This table replaces the ELF symbol table for functions and variables represented in the LTO IL. Symbols used and exported by the optimized assembly code of “fat” objects might not match the ones used and exported by the intermediate code. This table is necessary because the intermediate code is less optimized and thus requires a separate symbol table.
Additionally, the binary code in the “fat” object will lack a call to a function, since the call was optimized out at compilation time after the intermediate language was streamed out. In some special cases, the same optimization may not happen during link-time optimization. This would lead to an undefined symbol if only one symbol table was used.
The symbol table is emitted in
lto-streamer-out.c:produce_symtab.
.gnu.lto_.decls)
This section contains an intermediate language dump of all declarations and types required to represent the callgraph, static variables and top-level debug info.
The contents of this section are emitted in
lto-streamer-out.c:produce_asm_for_decls. Types and
symbols are emitted in a topological order that preserves the sharing
of pointers when the file is read back in
(lto.c:read_cgraph_and_symbols).
.gnu.lto_.cgraph)
This section contains the basic data structure used by the GCC
inter-procedural optimization infrastructure. This section stores an
annotated multi-graph which represents the functions and call sites as
well as the variables, aliases and top-level asm statements.
This section is emitted in
lto-streamer-out.c:output_cgraph and read in
lto-cgraph.c:input_cgraph.
.gnu.lto_.refs)
This section contains references between function and static
variables. It is emitted by lto-cgraph.c:output_refs
and read by lto-cgraph.c:input_refs.
.gnu.lto_.function_body.<name>)
This section contains function bodies in the intermediate language representation. Every function body is in a separate section to allow copying of the section independently to different object files or reading the function on demand.
Functions are emitted in
lto-streamer-out.c:output_function and read in
lto-streamer-in.c:input_function.
.gnu.lto_.vars)
This section contains all the symbols in the global variable pool. It
is emitted by lto-cgraph.c:output_varpool and read in
lto-cgraph.c:input_cgraph.
.gnu.lto_.<xxx>, where <xxx> is one of jmpfuncs,
pureconst or reference)
These sections are used by IPA passes that need to emit summary information during LTO generation to be read and aggregated at link time. Each pass is responsible for implementing two pass manager hooks: one for writing the summary and another for reading it in. The format of these sections is entirely up to each individual pass. The only requirement is that the writer and reader hooks agree on the format.
Programs are represented internally as a callgraph (a multi-graph where nodes are functions and edges are call sites) and a varpool (a list of static and external variables in the program).
The inter-procedural optimization is organized as a sequence of individual passes, which operate on the callgraph and the varpool. To make the implementation of WHOPR possible, every inter-procedural optimization pass is split into several stages that are executed at different times during WHOPR compilation:
generate_summary in
struct ipa_opt_pass_d). This stage analyzes every function
body and variable initializer is examined and stores relevant
information into a pass-specific data structure.
write_summary in
struct ipa_opt_pass_d). This stage writes all the
pass-specific information generated by generate_summary.
Summaries go into their own LTO_section_* sections that
have to be declared in lto-streamer.h:enum
lto_section_type. A new section is created by calling
create_output_block and data can be written using the
lto_output_* routines.
read_summary in
struct ipa_opt_pass_d). This stage reads all the
pass-specific information in exactly the same order that it was
written by write_summary.
execute in struct
opt_pass). This performs inter-procedural propagation. This
must be done without actual access to the individual function
bodies or variable initializers. Typically, this results in a
transitive closure operation over the summary information of all
the nodes in the callgraph.
write_optimization_summary in struct
ipa_opt_pass_d). This writes the result of the inter-procedural
propagation into the object file. This can use the same data
structures and helper routines used in write_summary.
read_optimization_summary in struct
ipa_opt_pass_d). The counterpart to
write_optimization_summary. This reads the interprocedural
optimization decisions in exactly the same format emitted by
write_optimization_summary.
function_transform and
variable_transform in struct ipa_opt_pass_d).
The actual function bodies and variable initializers are updated
based on the information passed down from the Execute stage.
The implementation of the inter-procedural passes are shared between LTO, WHOPR and classic non-LTO compilation.
To simplify development, the GCC pass manager differentiates
between normal inter-procedural passes and small inter-procedural
passes. A small inter-procedural pass
(SIMPLE_IPA_PASS) is a pass that does
everything at once and thus it can not be executed during WPA in
WHOPR mode. It defines only the Execute stage and during
this stage it accesses and modifies the function bodies. Such
passes are useful for optimization at LGEN or LTRANS time and are
used, for example, to implement early optimization before writing
object files. The simple inter-procedural passes can also be used
for easier prototyping and development of a new inter-procedural
pass.
One of the main challenges of introducing the WHOPR compilation mode was addressing the interactions between optimization passes. In LTO compilation mode, the passes are executed in a sequence, each of which consists of analysis (or Generate summary), propagation (or Execute) and Transform stages. Once the work of one pass is finished, the next pass sees the updated program representation and can execute. This makes the individual passes dependent on each other.
In WHOPR mode all passes first execute their Generate summary stage. Then summary writing marks the end of the LGEN stage. At WPA time, the summaries are read back into memory and all passes run the Execute stage. Optimization summaries are streamed and sent to LTRANS, where all the passes execute the Transform stage.
Most optimization passes split naturally into analysis, propagation and transformation stages. But some do not. The main problem arises when one pass performs changes and the following pass gets confused by seeing different callgraphs between the Transform stage and the Generate summary or Execute stage. This means that the passes are required to communicate their decisions with each other.
To facilitate this communication, the GCC callgraph infrastructure implements virtual clones, a method of representing the changes performed by the optimization passes in the callgraph without needing to update function bodies.
A virtual clone in the callgraph is a function that has no associated body, just a description of how to create its body based on a different function (which itself may be a virtual clone).
The description of function modifications includes adjustments to the function’s signature (which allows, for example, removing or adding function arguments), substitutions to perform on the function body, and, for inlined functions, a pointer to the function that it will be inlined into.
It is also possible to redirect any edge of the callgraph from a function to its virtual clone. This implies updating of the call site to adjust for the new function signature.
Most of the transformations performed by inter-procedural optimizations can be represented via virtual clones. For instance, a constant propagation pass can produce a virtual clone of the function which replaces one of its arguments by a constant. The inliner can represent its decisions by producing a clone of a function whose body will be later integrated into a given function.
Using virtual clones, the program can be easily updated during the Execute stage, solving most of pass interactions problems that would otherwise occur during Transform.
Virtual clones are later materialized in the LTRANS stage and turned into real functions. Passes executed after the virtual clone were introduced also perform their Transform stage on new functions, so for a pass there is no significant difference between operating on a real function or a virtual clone introduced before its Execute stage.
Optimization passes then work on virtual clones introduced before their Execute stage as if they were real functions. The only difference is that clones are not visible during the Generate Summary stage.
To keep function summaries updated, the callgraph interface allows an optimizer to register a callback that is called every time a new clone is introduced as well as when the actual function or variable is generated or when a function or variable is removed. These hooks are registered in the Generate summary stage and allow the pass to keep its information intact until the Execute stage. The same hooks can also be registered during the Execute stage to keep the optimization summaries updated for the Transform stage.
GCC represents IPA references in the callgraph. For a function
or variable A, the IPA reference is a list of all
locations where the address of A is taken and, when
A is a variable, a list of all direct stores and reads
to/from A. References represent an oriented multi-graph on
the union of nodes of the callgraph and the varpool. See
ipa-reference.c:ipa_reference_write_optimization_summary
and
ipa-reference.c:ipa_reference_read_optimization_summary
for details.
Suppose that an optimization pass sees a function A and it
knows the values of (some of) its arguments. The jump
function describes the value of a parameter of a given function
call in function A based on this knowledge.
Jump functions are used by several optimizations, such as the inter-procedural constant propagation pass and the devirtualization pass. The inliner also uses jump functions to perform inlining of callbacks.
Link-time optimization gives relatively minor benefits when used alone. The problem is that propagation of inter-procedural information does not work well across functions and variables that are called or referenced by other compilation units (such as from a dynamically linked library). We say that such functions are variables are externally visible.
To make the situation even more difficult, many applications
organize themselves as a set of shared libraries, and the default
ELF visibility rules allow one to overwrite any externally
visible symbol with a different symbol at runtime. This
basically disables any optimizations across such functions and
variables, because the compiler cannot be sure that the function
body it is seeing is the same function body that will be used at
runtime. Any function or variable not declared static in
the sources degrades the quality of inter-procedural
optimization.
To avoid this problem the compiler must assume that it sees the
whole program when doing link-time optimization. Strictly
speaking, the whole program is rarely visible even at link-time.
Standard system libraries are usually linked dynamically or not
provided with the link-time information. In GCC, the whole
program option (-fwhole-program) asserts that every
function and variable defined in the current compilation
unit is static, except for function main (note: at
link time, the current unit is the union of all objects compiled
with LTO). Since some functions and variables need to
be referenced externally, for example by another DSO or from an
assembler file, GCC also provides the function and variable
attribute externally_visible which can be used to disable
the effect of -fwhole-program on a specific symbol.
The whole program mode assumptions are slightly more complex in C++, where inline functions in headers are put into COMDAT sections. COMDAT function and variables can be defined by multiple object files and their bodies are unified at link-time and dynamic link-time. COMDAT functions are changed to local only when their address is not taken and thus un-sharing them with a library is not harmful. COMDAT variables always remain externally visible, however for readonly variables it is assumed that their initializers cannot be overwritten by a different value.
GCC provides the function and variable attribute
visibility that can be used to specify the visibility of
externally visible symbols (or alternatively an
-fdefault-visibility command line option). ELF defines
the default, protected, hidden and
internal visibilities.
The most commonly used is visibility is hidden. It
specifies that the symbol cannot be referenced from outside of
the current shared library. Unfortunately, this information
cannot be used directly by the link-time optimization in the
compiler since the whole shared library also might contain
non-LTO objects and those are not visible to the compiler.
GCC solves this problem using linker plugins. A linker plugin is an interface to the linker that allows an external program to claim the ownership of a given object file. The linker then performs the linking procedure by querying the plugin about the symbol table of the claimed objects and once the linking decisions are complete, the plugin is allowed to provide the final object file before the actual linking is made. The linker plugin obtains the symbol resolution information which specifies which symbols provided by the claimed objects are bound from the rest of a binary being linked.
Currently, the linker plugin works only in combination with the Gold linker, but a GNU ld implementation is under development.
GCC is designed to be independent of the rest of the toolchain
and aims to support linkers without plugin support. For this
reason it does not use the linker plugin by default. Instead,
the object files are examined by collect2 before being
passed to the linker and objects found to have LTO sections are
passed to lto1 first. This mode does not work for
library archives. The decision on what object files from the
archive are needed depends on the actual linking and thus GCC
would have to implement the linker itself. The resolution
information is missing too and thus GCC needs to make an educated
guess based on -fwhole-program. Without the linker
plugin GCC also assumes that symbols are declared hidden
and not referred by non-LTO code by default.
lto1The following flags are passed into lto1 and are not
meant to be used directly from the command line.
Next: GNU Project, Previous: LTO, Up: Top [Contents][Index]
If you want to have more free software a few years from now, it makes sense for you to help encourage people to contribute funds for its development. The most effective approach known is to encourage commercial redistributors to donate.
Users of free software systems can boost the pace of development by encouraging for-a-fee distributors to donate part of their selling price to free software developers—the Free Software Foundation, and others.
The way to convince distributors to do this is to demand it and expect it from them. So when you compare distributors, judge them partly by how much they give to free software development. Show distributors they must compete to be the one who gives the most.
To make this approach work, you must insist on numbers that you can compare, such as, “We will donate ten dollars to the Frobnitz project for each disk sold.” Don’t be satisfied with a vague promise, such as “A portion of the profits are donated,” since it doesn’t give a basis for comparison.
Even a precise fraction “of the profits from this disk” is not very meaningful, since creative accounting and unrelated business decisions can greatly alter what fraction of the sales price counts as profit. If the price you pay is $50, ten percent of the profit is probably less than a dollar; it might be a few cents, or nothing at all.
Some redistributors do development work themselves. This is useful too; but to keep everyone honest, you need to inquire how much they do, and what kind. Some kinds of development make much more long-term difference than others. For example, maintaining a separate version of a program contributes very little; maintaining the standard version of a program for the whole community contributes much. Easy new ports contribute little, since someone else would surely do them; difficult ports such as adding a new CPU to the GNU Compiler Collection contribute more; major new features or packages contribute the most.
By establishing the idea that supporting further development is “the proper thing to do” when distributing free software for a fee, we can assure a steady flow of resources into making more free software.
Copyright © 1994 Free Software Foundation, Inc. Verbatim copying and redistribution of this section is permitted without royalty; alteration is not permitted.
The GNU Project was launched in 1984 to develop a complete Unix-like operating system which is free software: the GNU system. (GNU is a recursive acronym for “GNU’s Not Unix”; it is pronounced “guh-NEW”.) Variants of the GNU operating system, which use the kernel Linux, are now widely used; though these systems are often referred to as “Linux”, they are more accurately called GNU/Linux systems.
For more information, see:
Next: GNU Free Documentation License, Previous: GNU Project, Up: Top [Contents][Index]
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If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively state the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found.
one line to give the program's name and a brief idea of what it does. Copyright (C) year name of author This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.
Also add information on how to contact you by electronic and paper mail.
If the program does terminal interaction, make it output a short notice like this when it starts in an interactive mode:
program Copyright (C) year name of author This program comes with ABSOLUTELY NO WARRANTY; for details type ‘show w’. This is free software, and you are welcome to redistribute it under certain conditions; type ‘show c’ for details.
The hypothetical commands ‘show w’ and ‘show c’ should show the appropriate parts of the General Public License. Of course, your program’s commands might be different; for a GUI interface, you would use an “about box”.
You should also get your employer (if you work as a programmer) or school, if any, to sign a “copyright disclaimer” for the program, if necessary. For more information on this, and how to apply and follow the GNU GPL, see http://www.gnu.org/licenses/.
The GNU General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Lesser General Public License instead of this License. But first, please read http://www.gnu.org/philosophy/why-not-lgpl.html.
Next: Contributors, Previous: Copying, Up: Top [Contents][Index]
Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. http://fsf.org/ Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
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Copyright (C) year your name. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:
with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
Next: Option Index, Previous: GNU Free Documentation License, Up: Top [Contents][Index]
The GCC project would like to thank its many contributors. Without them the project would not have been nearly as successful as it has been. Any omissions in this list are accidental. Feel free to contact law@redhat.com or gerald@pfeifer.com if you have been left out or some of your contributions are not listed. Please keep this list in alphabetical order.
valarray<>, complex<>, maintaining the numerics library
(including that pesky <limits> :-) and keeping up-to-date anything
to do with numbers.
complex<>, sanity checking and disbursement, configuration
architecture, libio maintenance, and early math work.
protoize and unprotoize
tools, the support for Dwarf symbolic debugging information, and much of
the support for System V Release 4. He has also worked heavily on the
Intel 386 and 860 support.
restrict support, and serving as release manager for GCC 3.x.
INTEGER*1, INTEGER*2, and
LOGICAL*1.
The following people are recognized for their contributions to GNAT, the Ada front end of GCC:
The following people are recognized for their contributions of new features, bug reports, testing and integration of classpath/libgcj for GCC version 4.1:
JTree implementation and lots Free Swing
additions and bug fixes.
GapContent bug fixes.
JList, Free Swing 1.5 updates and mouse event
fixes, lots of Free Swing work including JTable editing.
HTTPURLConnection fixes.
MessageFormat fixes.
Serialization fixes.
StAX
and DOM xml:id support.
TreePath and TreeSelection fixes.
URLClassLoader updates.
SocketTimeoutException.
BitSet bug fixes, HttpURLConnection
rewrite and improvements.
ClassLoader and nio cleanups, serialization fixes,
better Proxy support, bug fixes and IKVM integration.
AccessControlContext fixes.
VMClassLoader and AccessController
improvements.
basic and metal icon and plaf support
and lots of documenting, Lots of Free Swing and metal theme
additions. MetalIconFactory implementation.
MIDI framework, ALSA and DSSI
providers.
Serialization and URLClassLoader fixes,
gcj build speedups.
JFileChooser implementation.
Locale and net fixes, URI RFC2986
updates, Serialization fixes, Properties XML support and
generic branch work, VMIntegration guide update.
TimeZone bug fixing.
NetworkInterface implementation and updates.
BoxLayout, GrayFilter and
SplitPane, plus bug fixes all over. Lots of Free Swing work
including styled text.
String cleanups and optimization suggestions.
Locale updates, bug and
build fixes.
Pointer updates. Logger bug fixes.
Graphics2D upgraded to Cairo 0.5 and new regex
features.
TextLayout
fixes. GtkImage rewrite, 2D, awt, free swing and date/time fixes and
implementing the Qt4 peers.
FileChannel lock,
SystemLogger and FileHandler rotate implementations, NIO
FileChannel.map support, security and policy updates.
File locking fixes.
Image, Logger and URLClassLoader
updates.
MenuSelectionManager implementation.
BasicTreeUI and JTree fixes.
TreeNode enumerations and ActionCommand and various
fixes, XML and URL, AWT and Free Swing bug fixes.
CACAO integration, fdlibm updates.
VMClassLoader boot packages support suggestions.
Qt4
support for Darwin/OS X, Graphics2D support, gtk+
updates.
DEBUG support, build cleanups and
Kaffe integration. Qt4 build infrastructure, SHA1PRNG
and GdkPixbugDecoder updates.
Clipboard implementation, system call interrupts and network
timeouts and GdkPixpufDecoder fixes.
In addition to the above, all of which also contributed time and energy in testing GCC, we would like to thank the following for their contributions to testing:
And finally we’d like to thank everyone who uses the compiler, provides feedback and generally reminds us why we’re doing this work in the first place.
Next: Concept Index, Previous: Contributors, Up: Top [Contents][Index]
GCC’s command line options are indexed here without any initial ‘-’ or ‘--’. Where an option has both positive and negative forms (such as -foption and -fno-option), relevant entries in the manual are indexed under the most appropriate form; it may sometimes be useful to look up both forms.
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fltrans: | LTO | ||
fltrans-output-list: | LTO | ||
fwpa: | LTO | ||
| | |||
| M | |||
msoft-float: | Soft float library routines | ||
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Previous: Option Index, Up: Top [Contents][Index]
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Except if the compiler was buggy and miscompiled
some of the files that were not modified. In this case, it’s best
to use make restrap.
Customarily, the system compiler is also termed the stage0 GCC.
These restrictions are derived from those in Morgan 4.8.
note insns can separate them, though.
However, the size of the automaton depends on processor complexity. To limit this effect, machine descriptions can split orthogonal parts of the machine description among several automata: but then, since each of these must be stepped independently, this does cause a small decrease in the algorithm’s performance.