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bitmap_allocator.h
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1 // Bitmap Allocator. -*- C++ -*-
2 
3 // Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011
4 // Free Software Foundation, Inc.
5 //
6 // This file is part of the GNU ISO C++ Library. This library is free
7 // software; you can redistribute it and/or modify it under the
8 // terms of the GNU General Public License as published by the
9 // Free Software Foundation; either version 3, or (at your option)
10 // any later version.
11 
12 // This library is distributed in the hope that it will be useful,
13 // but WITHOUT ANY WARRANTY; without even the implied warranty of
14 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 // GNU General Public License for more details.
16 
17 // Under Section 7 of GPL version 3, you are granted additional
18 // permissions described in the GCC Runtime Library Exception, version
19 // 3.1, as published by the Free Software Foundation.
20 
21 // You should have received a copy of the GNU General Public License and
22 // a copy of the GCC Runtime Library Exception along with this program;
23 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
24 // <http://www.gnu.org/licenses/>.
25 
26 /** @file ext/bitmap_allocator.h
27  * This file is a GNU extension to the Standard C++ Library.
28  */
29 
30 #ifndef _BITMAP_ALLOCATOR_H
31 #define _BITMAP_ALLOCATOR_H 1
32 
33 #include <utility> // For std::pair.
34 #include <bits/functexcept.h> // For __throw_bad_alloc().
35 #include <functional> // For greater_equal, and less_equal.
36 #include <new> // For operator new.
37 #include <debug/debug.h> // _GLIBCXX_DEBUG_ASSERT
38 #include <ext/concurrence.h>
39 #include <bits/move.h>
40 
41 /** @brief The constant in the expression below is the alignment
42  * required in bytes.
43  */
44 #define _BALLOC_ALIGN_BYTES 8
45 
46 namespace __gnu_cxx _GLIBCXX_VISIBILITY(default)
47 {
48  using std::size_t;
49  using std::ptrdiff_t;
50 
51  namespace __detail
52  {
53  _GLIBCXX_BEGIN_NAMESPACE_VERSION
54  /** @class __mini_vector bitmap_allocator.h bitmap_allocator.h
55  *
56  * @brief __mini_vector<> is a stripped down version of the
57  * full-fledged std::vector<>.
58  *
59  * It is to be used only for built-in types or PODs. Notable
60  * differences are:
61  *
62  * @detail
63  * 1. Not all accessor functions are present.
64  * 2. Used ONLY for PODs.
65  * 3. No Allocator template argument. Uses ::operator new() to get
66  * memory, and ::operator delete() to free it.
67  * Caveat: The dtor does NOT free the memory allocated, so this a
68  * memory-leaking vector!
69  */
70  template<typename _Tp>
72  {
74  __mini_vector& operator=(const __mini_vector&);
75 
76  public:
77  typedef _Tp value_type;
78  typedef _Tp* pointer;
79  typedef _Tp& reference;
80  typedef const _Tp& const_reference;
81  typedef size_t size_type;
82  typedef ptrdiff_t difference_type;
83  typedef pointer iterator;
84 
85  private:
86  pointer _M_start;
87  pointer _M_finish;
88  pointer _M_end_of_storage;
89 
90  size_type
91  _M_space_left() const throw()
92  { return _M_end_of_storage - _M_finish; }
93 
94  pointer
95  allocate(size_type __n)
96  { return static_cast<pointer>(::operator new(__n * sizeof(_Tp))); }
97 
98  void
99  deallocate(pointer __p, size_type)
100  { ::operator delete(__p); }
101 
102  public:
103  // Members used: size(), push_back(), pop_back(),
104  // insert(iterator, const_reference), erase(iterator),
105  // begin(), end(), back(), operator[].
106 
107  __mini_vector()
108  : _M_start(0), _M_finish(0), _M_end_of_storage(0) { }
109 
110  size_type
111  size() const throw()
112  { return _M_finish - _M_start; }
113 
114  iterator
115  begin() const throw()
116  { return this->_M_start; }
117 
118  iterator
119  end() const throw()
120  { return this->_M_finish; }
121 
122  reference
123  back() const throw()
124  { return *(this->end() - 1); }
125 
126  reference
127  operator[](const size_type __pos) const throw()
128  { return this->_M_start[__pos]; }
129 
130  void
131  insert(iterator __pos, const_reference __x);
132 
133  void
134  push_back(const_reference __x)
135  {
136  if (this->_M_space_left())
137  {
138  *this->end() = __x;
139  ++this->_M_finish;
140  }
141  else
142  this->insert(this->end(), __x);
143  }
144 
145  void
146  pop_back() throw()
147  { --this->_M_finish; }
148 
149  void
150  erase(iterator __pos) throw();
151 
152  void
153  clear() throw()
154  { this->_M_finish = this->_M_start; }
155  };
156 
157  // Out of line function definitions.
158  template<typename _Tp>
160  insert(iterator __pos, const_reference __x)
161  {
162  if (this->_M_space_left())
163  {
164  size_type __to_move = this->_M_finish - __pos;
165  iterator __dest = this->end();
166  iterator __src = this->end() - 1;
167 
168  ++this->_M_finish;
169  while (__to_move)
170  {
171  *__dest = *__src;
172  --__dest; --__src; --__to_move;
173  }
174  *__pos = __x;
175  }
176  else
177  {
178  size_type __new_size = this->size() ? this->size() * 2 : 1;
179  iterator __new_start = this->allocate(__new_size);
180  iterator __first = this->begin();
181  iterator __start = __new_start;
182  while (__first != __pos)
183  {
184  *__start = *__first;
185  ++__start; ++__first;
186  }
187  *__start = __x;
188  ++__start;
189  while (__first != this->end())
190  {
191  *__start = *__first;
192  ++__start; ++__first;
193  }
194  if (this->_M_start)
195  this->deallocate(this->_M_start, this->size());
196 
197  this->_M_start = __new_start;
198  this->_M_finish = __start;
199  this->_M_end_of_storage = this->_M_start + __new_size;
200  }
201  }
202 
203  template<typename _Tp>
204  void __mini_vector<_Tp>::
205  erase(iterator __pos) throw()
206  {
207  while (__pos + 1 != this->end())
208  {
209  *__pos = __pos[1];
210  ++__pos;
211  }
212  --this->_M_finish;
213  }
214 
215 
216  template<typename _Tp>
217  struct __mv_iter_traits
218  {
219  typedef typename _Tp::value_type value_type;
220  typedef typename _Tp::difference_type difference_type;
221  };
222 
223  template<typename _Tp>
224  struct __mv_iter_traits<_Tp*>
225  {
226  typedef _Tp value_type;
227  typedef ptrdiff_t difference_type;
228  };
229 
230  enum
231  {
232  bits_per_byte = 8,
233  bits_per_block = sizeof(size_t) * size_t(bits_per_byte)
234  };
235 
236  template<typename _ForwardIterator, typename _Tp, typename _Compare>
237  _ForwardIterator
238  __lower_bound(_ForwardIterator __first, _ForwardIterator __last,
239  const _Tp& __val, _Compare __comp)
240  {
241  typedef typename __mv_iter_traits<_ForwardIterator>::value_type
242  _ValueType;
243  typedef typename __mv_iter_traits<_ForwardIterator>::difference_type
244  _DistanceType;
245 
246  _DistanceType __len = __last - __first;
247  _DistanceType __half;
248  _ForwardIterator __middle;
249 
250  while (__len > 0)
251  {
252  __half = __len >> 1;
253  __middle = __first;
254  __middle += __half;
255  if (__comp(*__middle, __val))
256  {
257  __first = __middle;
258  ++__first;
259  __len = __len - __half - 1;
260  }
261  else
262  __len = __half;
263  }
264  return __first;
265  }
266 
267  /** @brief The number of Blocks pointed to by the address pair
268  * passed to the function.
269  */
270  template<typename _AddrPair>
271  inline size_t
272  __num_blocks(_AddrPair __ap)
273  { return (__ap.second - __ap.first) + 1; }
274 
275  /** @brief The number of Bit-maps pointed to by the address pair
276  * passed to the function.
277  */
278  template<typename _AddrPair>
279  inline size_t
280  __num_bitmaps(_AddrPair __ap)
281  { return __num_blocks(__ap) / size_t(bits_per_block); }
282 
283  // _Tp should be a pointer type.
284  template<typename _Tp>
285  class _Inclusive_between
286  : public std::unary_function<typename std::pair<_Tp, _Tp>, bool>
287  {
288  typedef _Tp pointer;
289  pointer _M_ptr_value;
290  typedef typename std::pair<_Tp, _Tp> _Block_pair;
291 
292  public:
293  _Inclusive_between(pointer __ptr) : _M_ptr_value(__ptr)
294  { }
295 
296  bool
297  operator()(_Block_pair __bp) const throw()
298  {
299  if (std::less_equal<pointer>()(_M_ptr_value, __bp.second)
300  && std::greater_equal<pointer>()(_M_ptr_value, __bp.first))
301  return true;
302  else
303  return false;
304  }
305  };
306 
307  // Used to pass a Functor to functions by reference.
308  template<typename _Functor>
309  class _Functor_Ref
310  : public std::unary_function<typename _Functor::argument_type,
311  typename _Functor::result_type>
312  {
313  _Functor& _M_fref;
314 
315  public:
316  typedef typename _Functor::argument_type argument_type;
317  typedef typename _Functor::result_type result_type;
318 
319  _Functor_Ref(_Functor& __fref) : _M_fref(__fref)
320  { }
321 
322  result_type
323  operator()(argument_type __arg)
324  { return _M_fref(__arg); }
325  };
326 
327  /** @class _Ffit_finder bitmap_allocator.h bitmap_allocator.h
328  *
329  * @brief The class which acts as a predicate for applying the
330  * first-fit memory allocation policy for the bitmap allocator.
331  */
332  // _Tp should be a pointer type, and _Alloc is the Allocator for
333  // the vector.
334  template<typename _Tp>
336  : public std::unary_function<typename std::pair<_Tp, _Tp>, bool>
337  {
338  typedef typename std::pair<_Tp, _Tp> _Block_pair;
340  typedef typename _BPVector::difference_type _Counter_type;
341 
342  size_t* _M_pbitmap;
343  _Counter_type _M_data_offset;
344 
345  public:
346  _Ffit_finder() : _M_pbitmap(0), _M_data_offset(0)
347  { }
348 
349  bool
350  operator()(_Block_pair __bp) throw()
351  {
352  // Set the _rover to the last physical location bitmap,
353  // which is the bitmap which belongs to the first free
354  // block. Thus, the bitmaps are in exact reverse order of
355  // the actual memory layout. So, we count down the bitmaps,
356  // which is the same as moving up the memory.
357 
358  // If the used count stored at the start of the Bit Map headers
359  // is equal to the number of Objects that the current Block can
360  // store, then there is definitely no space for another single
361  // object, so just return false.
362  _Counter_type __diff = __detail::__num_bitmaps(__bp);
363 
364  if (*(reinterpret_cast<size_t*>
365  (__bp.first) - (__diff + 1)) == __detail::__num_blocks(__bp))
366  return false;
367 
368  size_t* __rover = reinterpret_cast<size_t*>(__bp.first) - 1;
369 
370  for (_Counter_type __i = 0; __i < __diff; ++__i)
371  {
372  _M_data_offset = __i;
373  if (*__rover)
374  {
375  _M_pbitmap = __rover;
376  return true;
377  }
378  --__rover;
379  }
380  return false;
381  }
382 
383  size_t*
384  _M_get() const throw()
385  { return _M_pbitmap; }
386 
387  _Counter_type
388  _M_offset() const throw()
389  { return _M_data_offset * size_t(bits_per_block); }
390  };
391 
392  /** @class _Bitmap_counter bitmap_allocator.h bitmap_allocator.h
393  *
394  * @brief The bitmap counter which acts as the bitmap
395  * manipulator, and manages the bit-manipulation functions and
396  * the searching and identification functions on the bit-map.
397  */
398  // _Tp should be a pointer type.
399  template<typename _Tp>
401  {
402  typedef typename
404  typedef typename _BPVector::size_type _Index_type;
405  typedef _Tp pointer;
406 
407  _BPVector& _M_vbp;
408  size_t* _M_curr_bmap;
409  size_t* _M_last_bmap_in_block;
410  _Index_type _M_curr_index;
411 
412  public:
413  // Use the 2nd parameter with care. Make sure that such an
414  // entry exists in the vector before passing that particular
415  // index to this ctor.
416  _Bitmap_counter(_BPVector& Rvbp, long __index = -1) : _M_vbp(Rvbp)
417  { this->_M_reset(__index); }
418 
419  void
420  _M_reset(long __index = -1) throw()
421  {
422  if (__index == -1)
423  {
424  _M_curr_bmap = 0;
425  _M_curr_index = static_cast<_Index_type>(-1);
426  return;
427  }
428 
429  _M_curr_index = __index;
430  _M_curr_bmap = reinterpret_cast<size_t*>
431  (_M_vbp[_M_curr_index].first) - 1;
432 
433  _GLIBCXX_DEBUG_ASSERT(__index <= (long)_M_vbp.size() - 1);
434 
435  _M_last_bmap_in_block = _M_curr_bmap
436  - ((_M_vbp[_M_curr_index].second
437  - _M_vbp[_M_curr_index].first + 1)
438  / size_t(bits_per_block) - 1);
439  }
440 
441  // Dangerous Function! Use with extreme care. Pass to this
442  // function ONLY those values that are known to be correct,
443  // otherwise this will mess up big time.
444  void
445  _M_set_internal_bitmap(size_t* __new_internal_marker) throw()
446  { _M_curr_bmap = __new_internal_marker; }
447 
448  bool
449  _M_finished() const throw()
450  { return(_M_curr_bmap == 0); }
451 
453  operator++() throw()
454  {
455  if (_M_curr_bmap == _M_last_bmap_in_block)
456  {
457  if (++_M_curr_index == _M_vbp.size())
458  _M_curr_bmap = 0;
459  else
460  this->_M_reset(_M_curr_index);
461  }
462  else
463  --_M_curr_bmap;
464  return *this;
465  }
466 
467  size_t*
468  _M_get() const throw()
469  { return _M_curr_bmap; }
470 
471  pointer
472  _M_base() const throw()
473  { return _M_vbp[_M_curr_index].first; }
474 
475  _Index_type
476  _M_offset() const throw()
477  {
478  return size_t(bits_per_block)
479  * ((reinterpret_cast<size_t*>(this->_M_base())
480  - _M_curr_bmap) - 1);
481  }
482 
483  _Index_type
484  _M_where() const throw()
485  { return _M_curr_index; }
486  };
487 
488  /** @brief Mark a memory address as allocated by re-setting the
489  * corresponding bit in the bit-map.
490  */
491  inline void
492  __bit_allocate(size_t* __pbmap, size_t __pos) throw()
493  {
494  size_t __mask = 1 << __pos;
495  __mask = ~__mask;
496  *__pbmap &= __mask;
497  }
498 
499  /** @brief Mark a memory address as free by setting the
500  * corresponding bit in the bit-map.
501  */
502  inline void
503  __bit_free(size_t* __pbmap, size_t __pos) throw()
504  {
505  size_t __mask = 1 << __pos;
506  *__pbmap |= __mask;
507  }
508 
509  _GLIBCXX_END_NAMESPACE_VERSION
510  } // namespace __detail
511 
512 _GLIBCXX_BEGIN_NAMESPACE_VERSION
513 
514  /** @brief Generic Version of the bsf instruction.
515  */
516  inline size_t
517  _Bit_scan_forward(size_t __num)
518  { return static_cast<size_t>(__builtin_ctzl(__num)); }
519 
520  /** @class free_list bitmap_allocator.h bitmap_allocator.h
521  *
522  * @brief The free list class for managing chunks of memory to be
523  * given to and returned by the bitmap_allocator.
524  */
525  class free_list
526  {
527  public:
528  typedef size_t* value_type;
530  typedef vector_type::iterator iterator;
531  typedef __mutex __mutex_type;
532 
533  private:
534  struct _LT_pointer_compare
535  {
536  bool
537  operator()(const size_t* __pui,
538  const size_t __cui) const throw()
539  { return *__pui < __cui; }
540  };
541 
542 #if defined __GTHREADS
543  __mutex_type&
544  _M_get_mutex()
545  {
546  static __mutex_type _S_mutex;
547  return _S_mutex;
548  }
549 #endif
550 
551  vector_type&
552  _M_get_free_list()
553  {
554  static vector_type _S_free_list;
555  return _S_free_list;
556  }
557 
558  /** @brief Performs validation of memory based on their size.
559  *
560  * @param __addr The pointer to the memory block to be
561  * validated.
562  *
563  * @detail Validates the memory block passed to this function and
564  * appropriately performs the action of managing the free list of
565  * blocks by adding this block to the free list or deleting this
566  * or larger blocks from the free list.
567  */
568  void
569  _M_validate(size_t* __addr) throw()
570  {
571  vector_type& __free_list = _M_get_free_list();
572  const vector_type::size_type __max_size = 64;
573  if (__free_list.size() >= __max_size)
574  {
575  // Ok, the threshold value has been reached. We determine
576  // which block to remove from the list of free blocks.
577  if (*__addr >= *__free_list.back())
578  {
579  // Ok, the new block is greater than or equal to the
580  // last block in the list of free blocks. We just free
581  // the new block.
582  ::operator delete(static_cast<void*>(__addr));
583  return;
584  }
585  else
586  {
587  // Deallocate the last block in the list of free lists,
588  // and insert the new one in its correct position.
589  ::operator delete(static_cast<void*>(__free_list.back()));
590  __free_list.pop_back();
591  }
592  }
593 
594  // Just add the block to the list of free lists unconditionally.
595  iterator __temp = __detail::__lower_bound
596  (__free_list.begin(), __free_list.end(),
597  *__addr, _LT_pointer_compare());
598 
599  // We may insert the new free list before _temp;
600  __free_list.insert(__temp, __addr);
601  }
602 
603  /** @brief Decides whether the wastage of memory is acceptable for
604  * the current memory request and returns accordingly.
605  *
606  * @param __block_size The size of the block available in the free
607  * list.
608  *
609  * @param __required_size The required size of the memory block.
610  *
611  * @return true if the wastage incurred is acceptable, else returns
612  * false.
613  */
614  bool
615  _M_should_i_give(size_t __block_size,
616  size_t __required_size) throw()
617  {
618  const size_t __max_wastage_percentage = 36;
619  if (__block_size >= __required_size &&
620  (((__block_size - __required_size) * 100 / __block_size)
621  < __max_wastage_percentage))
622  return true;
623  else
624  return false;
625  }
626 
627  public:
628  /** @brief This function returns the block of memory to the
629  * internal free list.
630  *
631  * @param __addr The pointer to the memory block that was given
632  * by a call to the _M_get function.
633  */
634  inline void
635  _M_insert(size_t* __addr) throw()
636  {
637 #if defined __GTHREADS
638  __scoped_lock __bfl_lock(_M_get_mutex());
639 #endif
640  // Call _M_validate to decide what should be done with
641  // this particular free list.
642  this->_M_validate(reinterpret_cast<size_t*>(__addr) - 1);
643  // See discussion as to why this is 1!
644  }
645 
646  /** @brief This function gets a block of memory of the specified
647  * size from the free list.
648  *
649  * @param __sz The size in bytes of the memory required.
650  *
651  * @return A pointer to the new memory block of size at least
652  * equal to that requested.
653  */
654  size_t*
655  _M_get(size_t __sz) throw(std::bad_alloc);
656 
657  /** @brief This function just clears the internal Free List, and
658  * gives back all the memory to the OS.
659  */
660  void
661  _M_clear();
662  };
663 
664 
665  // Forward declare the class.
666  template<typename _Tp>
668 
669  // Specialize for void:
670  template<>
671  class bitmap_allocator<void>
672  {
673  public:
674  typedef void* pointer;
675  typedef const void* const_pointer;
676 
677  // Reference-to-void members are impossible.
678  typedef void value_type;
679  template<typename _Tp1>
680  struct rebind
681  {
682  typedef bitmap_allocator<_Tp1> other;
683  };
684  };
685 
686  /**
687  * @brief Bitmap Allocator, primary template.
688  * @ingroup allocators
689  */
690  template<typename _Tp>
691  class bitmap_allocator : private free_list
692  {
693  public:
694  typedef size_t size_type;
695  typedef ptrdiff_t difference_type;
696  typedef _Tp* pointer;
697  typedef const _Tp* const_pointer;
698  typedef _Tp& reference;
699  typedef const _Tp& const_reference;
700  typedef _Tp value_type;
701  typedef free_list::__mutex_type __mutex_type;
702 
703  template<typename _Tp1>
704  struct rebind
705  {
706  typedef bitmap_allocator<_Tp1> other;
707  };
708 
709  private:
710  template<size_t _BSize, size_t _AlignSize>
711  struct aligned_size
712  {
713  enum
714  {
715  modulus = _BSize % _AlignSize,
716  value = _BSize + (modulus ? _AlignSize - (modulus) : 0)
717  };
718  };
719 
720  struct _Alloc_block
721  {
722  char __M_unused[aligned_size<sizeof(value_type),
723  _BALLOC_ALIGN_BYTES>::value];
724  };
725 
726 
727  typedef typename std::pair<_Alloc_block*, _Alloc_block*> _Block_pair;
728 
729  typedef typename __detail::__mini_vector<_Block_pair> _BPVector;
730  typedef typename _BPVector::iterator _BPiter;
731 
732  template<typename _Predicate>
733  static _BPiter
734  _S_find(_Predicate __p)
735  {
736  _BPiter __first = _S_mem_blocks.begin();
737  while (__first != _S_mem_blocks.end() && !__p(*__first))
738  ++__first;
739  return __first;
740  }
741 
742 #if defined _GLIBCXX_DEBUG
743  // Complexity: O(lg(N)). Where, N is the number of block of size
744  // sizeof(value_type).
745  void
746  _S_check_for_free_blocks() throw()
747  {
748  typedef typename __detail::_Ffit_finder<_Alloc_block*> _FFF;
749  _BPiter __bpi = _S_find(_FFF());
750 
751  _GLIBCXX_DEBUG_ASSERT(__bpi == _S_mem_blocks.end());
752  }
753 #endif
754 
755  /** @brief Responsible for exponentially growing the internal
756  * memory pool.
757  *
758  * @throw std::bad_alloc. If memory can not be allocated.
759  *
760  * @detail Complexity: O(1), but internally depends upon the
761  * complexity of the function free_list::_M_get. The part where
762  * the bitmap headers are written has complexity: O(X),where X
763  * is the number of blocks of size sizeof(value_type) within
764  * the newly acquired block. Having a tight bound.
765  */
766  void
767  _S_refill_pool() throw(std::bad_alloc)
768  {
769 #if defined _GLIBCXX_DEBUG
770  _S_check_for_free_blocks();
771 #endif
772 
773  const size_t __num_bitmaps = (_S_block_size
774  / size_t(__detail::bits_per_block));
775  const size_t __size_to_allocate = sizeof(size_t)
776  + _S_block_size * sizeof(_Alloc_block)
777  + __num_bitmaps * sizeof(size_t);
778 
779  size_t* __temp =
780  reinterpret_cast<size_t*>(this->_M_get(__size_to_allocate));
781  *__temp = 0;
782  ++__temp;
783 
784  // The Header information goes at the Beginning of the Block.
785  _Block_pair __bp =
786  std::make_pair(reinterpret_cast<_Alloc_block*>
787  (__temp + __num_bitmaps),
788  reinterpret_cast<_Alloc_block*>
789  (__temp + __num_bitmaps)
790  + _S_block_size - 1);
791 
792  // Fill the Vector with this information.
793  _S_mem_blocks.push_back(__bp);
794 
795  for (size_t __i = 0; __i < __num_bitmaps; ++__i)
796  __temp[__i] = ~static_cast<size_t>(0); // 1 Indicates all Free.
797 
798  _S_block_size *= 2;
799  }
800 
801  static _BPVector _S_mem_blocks;
802  static size_t _S_block_size;
803  static __detail::_Bitmap_counter<_Alloc_block*> _S_last_request;
804  static typename _BPVector::size_type _S_last_dealloc_index;
805 #if defined __GTHREADS
806  static __mutex_type _S_mut;
807 #endif
808 
809  public:
810 
811  /** @brief Allocates memory for a single object of size
812  * sizeof(_Tp).
813  *
814  * @throw std::bad_alloc. If memory can not be allocated.
815  *
816  * @detail Complexity: Worst case complexity is O(N), but that
817  * is hardly ever hit. If and when this particular case is
818  * encountered, the next few cases are guaranteed to have a
819  * worst case complexity of O(1)! That's why this function
820  * performs very well on average. You can consider this
821  * function to have a complexity referred to commonly as:
822  * Amortized Constant time.
823  */
824  pointer
825  _M_allocate_single_object() throw(std::bad_alloc)
826  {
827 #if defined __GTHREADS
828  __scoped_lock __bit_lock(_S_mut);
829 #endif
830 
831  // The algorithm is something like this: The last_request
832  // variable points to the last accessed Bit Map. When such a
833  // condition occurs, we try to find a free block in the
834  // current bitmap, or succeeding bitmaps until the last bitmap
835  // is reached. If no free block turns up, we resort to First
836  // Fit method.
837 
838  // WARNING: Do not re-order the condition in the while
839  // statement below, because it relies on C++'s short-circuit
840  // evaluation. The return from _S_last_request->_M_get() will
841  // NOT be dereference able if _S_last_request->_M_finished()
842  // returns true. This would inevitably lead to a NULL pointer
843  // dereference if tinkered with.
844  while (_S_last_request._M_finished() == false
845  && (*(_S_last_request._M_get()) == 0))
846  _S_last_request.operator++();
847 
848  if (__builtin_expect(_S_last_request._M_finished() == true, false))
849  {
850  // Fall Back to First Fit algorithm.
851  typedef typename __detail::_Ffit_finder<_Alloc_block*> _FFF;
852  _FFF __fff;
853  _BPiter __bpi = _S_find(__detail::_Functor_Ref<_FFF>(__fff));
854 
855  if (__bpi != _S_mem_blocks.end())
856  {
857  // Search was successful. Ok, now mark the first bit from
858  // the right as 0, meaning Allocated. This bit is obtained
859  // by calling _M_get() on __fff.
860  size_t __nz_bit = _Bit_scan_forward(*__fff._M_get());
861  __detail::__bit_allocate(__fff._M_get(), __nz_bit);
862 
863  _S_last_request._M_reset(__bpi - _S_mem_blocks.begin());
864 
865  // Now, get the address of the bit we marked as allocated.
866  pointer __ret = reinterpret_cast<pointer>
867  (__bpi->first + __fff._M_offset() + __nz_bit);
868  size_t* __puse_count =
869  reinterpret_cast<size_t*>
870  (__bpi->first) - (__detail::__num_bitmaps(*__bpi) + 1);
871 
872  ++(*__puse_count);
873  return __ret;
874  }
875  else
876  {
877  // Search was unsuccessful. We Add more memory to the
878  // pool by calling _S_refill_pool().
879  _S_refill_pool();
880 
881  // _M_Reset the _S_last_request structure to the first
882  // free block's bit map.
883  _S_last_request._M_reset(_S_mem_blocks.size() - 1);
884 
885  // Now, mark that bit as allocated.
886  }
887  }
888 
889  // _S_last_request holds a pointer to a valid bit map, that
890  // points to a free block in memory.
891  size_t __nz_bit = _Bit_scan_forward(*_S_last_request._M_get());
892  __detail::__bit_allocate(_S_last_request._M_get(), __nz_bit);
893 
894  pointer __ret = reinterpret_cast<pointer>
895  (_S_last_request._M_base() + _S_last_request._M_offset() + __nz_bit);
896 
897  size_t* __puse_count = reinterpret_cast<size_t*>
898  (_S_mem_blocks[_S_last_request._M_where()].first)
899  - (__detail::
900  __num_bitmaps(_S_mem_blocks[_S_last_request._M_where()]) + 1);
901 
902  ++(*__puse_count);
903  return __ret;
904  }
905 
906  /** @brief Deallocates memory that belongs to a single object of
907  * size sizeof(_Tp).
908  *
909  * @detail Complexity: O(lg(N)), but the worst case is not hit
910  * often! This is because containers usually deallocate memory
911  * close to each other and this case is handled in O(1) time by
912  * the deallocate function.
913  */
914  void
915  _M_deallocate_single_object(pointer __p) throw()
916  {
917 #if defined __GTHREADS
918  __scoped_lock __bit_lock(_S_mut);
919 #endif
920  _Alloc_block* __real_p = reinterpret_cast<_Alloc_block*>(__p);
921 
922  typedef typename _BPVector::iterator _Iterator;
923  typedef typename _BPVector::difference_type _Difference_type;
924 
925  _Difference_type __diff;
926  long __displacement;
927 
928  _GLIBCXX_DEBUG_ASSERT(_S_last_dealloc_index >= 0);
929 
930  __detail::_Inclusive_between<_Alloc_block*> __ibt(__real_p);
931  if (__ibt(_S_mem_blocks[_S_last_dealloc_index]))
932  {
933  _GLIBCXX_DEBUG_ASSERT(_S_last_dealloc_index
934  <= _S_mem_blocks.size() - 1);
935 
936  // Initial Assumption was correct!
937  __diff = _S_last_dealloc_index;
938  __displacement = __real_p - _S_mem_blocks[__diff].first;
939  }
940  else
941  {
942  _Iterator _iter = _S_find(__ibt);
943 
944  _GLIBCXX_DEBUG_ASSERT(_iter != _S_mem_blocks.end());
945 
946  __diff = _iter - _S_mem_blocks.begin();
947  __displacement = __real_p - _S_mem_blocks[__diff].first;
948  _S_last_dealloc_index = __diff;
949  }
950 
951  // Get the position of the iterator that has been found.
952  const size_t __rotate = (__displacement
953  % size_t(__detail::bits_per_block));
954  size_t* __bitmapC =
955  reinterpret_cast<size_t*>
956  (_S_mem_blocks[__diff].first) - 1;
957  __bitmapC -= (__displacement / size_t(__detail::bits_per_block));
958 
959  __detail::__bit_free(__bitmapC, __rotate);
960  size_t* __puse_count = reinterpret_cast<size_t*>
961  (_S_mem_blocks[__diff].first)
962  - (__detail::__num_bitmaps(_S_mem_blocks[__diff]) + 1);
963 
964  _GLIBCXX_DEBUG_ASSERT(*__puse_count != 0);
965 
966  --(*__puse_count);
967 
968  if (__builtin_expect(*__puse_count == 0, false))
969  {
970  _S_block_size /= 2;
971 
972  // We can safely remove this block.
973  // _Block_pair __bp = _S_mem_blocks[__diff];
974  this->_M_insert(__puse_count);
975  _S_mem_blocks.erase(_S_mem_blocks.begin() + __diff);
976 
977  // Reset the _S_last_request variable to reflect the
978  // erased block. We do this to protect future requests
979  // after the last block has been removed from a particular
980  // memory Chunk, which in turn has been returned to the
981  // free list, and hence had been erased from the vector,
982  // so the size of the vector gets reduced by 1.
983  if ((_Difference_type)_S_last_request._M_where() >= __diff--)
984  _S_last_request._M_reset(__diff);
985 
986  // If the Index into the vector of the region of memory
987  // that might hold the next address that will be passed to
988  // deallocated may have been invalidated due to the above
989  // erase procedure being called on the vector, hence we
990  // try to restore this invariant too.
991  if (_S_last_dealloc_index >= _S_mem_blocks.size())
992  {
993  _S_last_dealloc_index =(__diff != -1 ? __diff : 0);
994  _GLIBCXX_DEBUG_ASSERT(_S_last_dealloc_index >= 0);
995  }
996  }
997  }
998 
999  public:
1000  bitmap_allocator() throw()
1001  { }
1002 
1003  bitmap_allocator(const bitmap_allocator&)
1004  { }
1005 
1006  template<typename _Tp1>
1007  bitmap_allocator(const bitmap_allocator<_Tp1>&) throw()
1008  { }
1009 
1010  ~bitmap_allocator() throw()
1011  { }
1012 
1013  pointer
1014  allocate(size_type __n)
1015  {
1016  if (__n > this->max_size())
1017  std::__throw_bad_alloc();
1018 
1019  if (__builtin_expect(__n == 1, true))
1020  return this->_M_allocate_single_object();
1021  else
1022  {
1023  const size_type __b = __n * sizeof(value_type);
1024  return reinterpret_cast<pointer>(::operator new(__b));
1025  }
1026  }
1027 
1028  pointer
1029  allocate(size_type __n, typename bitmap_allocator<void>::const_pointer)
1030  { return allocate(__n); }
1031 
1032  void
1033  deallocate(pointer __p, size_type __n) throw()
1034  {
1035  if (__builtin_expect(__p != 0, true))
1036  {
1037  if (__builtin_expect(__n == 1, true))
1038  this->_M_deallocate_single_object(__p);
1039  else
1040  ::operator delete(__p);
1041  }
1042  }
1043 
1044  pointer
1045  address(reference __r) const
1046  { return std::__addressof(__r); }
1047 
1048  const_pointer
1049  address(const_reference __r) const
1050  { return std::__addressof(__r); }
1051 
1052  size_type
1053  max_size() const throw()
1054  { return size_type(-1) / sizeof(value_type); }
1055 
1056  void
1057  construct(pointer __p, const_reference __data)
1058  { ::new((void *)__p) value_type(__data); }
1059 
1060 #ifdef __GXX_EXPERIMENTAL_CXX0X__
1061  template<typename... _Args>
1062  void
1063  construct(pointer __p, _Args&&... __args)
1064  { ::new((void *)__p) _Tp(std::forward<_Args>(__args)...); }
1065 #endif
1066 
1067  void
1068  destroy(pointer __p)
1069  { __p->~value_type(); }
1070  };
1071 
1072  template<typename _Tp1, typename _Tp2>
1073  bool
1074  operator==(const bitmap_allocator<_Tp1>&,
1075  const bitmap_allocator<_Tp2>&) throw()
1076  { return true; }
1077 
1078  template<typename _Tp1, typename _Tp2>
1079  bool
1080  operator!=(const bitmap_allocator<_Tp1>&,
1081  const bitmap_allocator<_Tp2>&) throw()
1082  { return false; }
1083 
1084  // Static member definitions.
1085  template<typename _Tp>
1086  typename bitmap_allocator<_Tp>::_BPVector
1087  bitmap_allocator<_Tp>::_S_mem_blocks;
1088 
1089  template<typename _Tp>
1090  size_t bitmap_allocator<_Tp>::_S_block_size =
1091  2 * size_t(__detail::bits_per_block);
1092 
1093  template<typename _Tp>
1094  typename bitmap_allocator<_Tp>::_BPVector::size_type
1095  bitmap_allocator<_Tp>::_S_last_dealloc_index = 0;
1096 
1097  template<typename _Tp>
1098  __detail::_Bitmap_counter
1099  <typename bitmap_allocator<_Tp>::_Alloc_block*>
1100  bitmap_allocator<_Tp>::_S_last_request(_S_mem_blocks);
1101 
1102 #if defined __GTHREADS
1103  template<typename _Tp>
1104  typename bitmap_allocator<_Tp>::__mutex_type
1105  bitmap_allocator<_Tp>::_S_mut;
1106 #endif
1107 
1108 _GLIBCXX_END_NAMESPACE_VERSION
1109 } // namespace __gnu_cxx
1110 
1111 #endif
1112 
Struct holding two objects of arbitrary type.
Definition: stl_pair.h:87
_T1 first
second_type is the second bound type
Definition: stl_pair.h:92
constexpr const _Tp * begin(initializer_list< _Tp > __ils)
Return an iterator pointing to the first element of the initilizer_list.
void __bit_allocate(size_t *__pbmap, size_t __pos)
Mark a memory address as allocated by re-setting the corresponding bit in the bit-map.
size_t __num_blocks(_AddrPair __ap)
The number of Blocks pointed to by the address pair passed to the function.
size_t _Bit_scan_forward(size_t __num)
Generic Version of the bsf instruction.
void __rotate(_ForwardIterator __first, _ForwardIterator __middle, _ForwardIterator __last, forward_iterator_tag)
This is a helper function for the rotate algorithm.
Definition: stl_algo.h:1505
Exception possibly thrown by new.bad_alloc (or classes derived from it) is used to report allocation ...
Definition: new:56
__mini_vector<> is a stripped down version of the full-fledged std::vector<>.
#define _BALLOC_ALIGN_BYTES
The constant in the expression below is the alignment required in bytes.
One of the comparison functors.
Definition: stl_function.h:250
pair< typename __decay_and_strip< _T1 >::__type, typename __decay_and_strip< _T2 >::__type > make_pair(_T1 &&__x, _T2 &&__y)
A convenience wrapper for creating a pair from two objects.
Definition: stl_pair.h:262
void __bit_free(size_t *__pbmap, size_t __pos)
Mark a memory address as free by setting the corresponding bit in the bit-map.
constexpr size_t size() const
Returns the total number of bits.
Definition: bitset:1275
One of the comparison functors.
Definition: stl_function.h:241
pointer _M_allocate_single_object()
Allocates memory for a single object of size sizeof(_Tp).
The free list class for managing chunks of memory to be given to and returned by the bitmap_allocator...
Bitmap Allocator, primary template.
size_t __num_bitmaps(_AddrPair __ap)
The number of Bit-maps pointed to by the address pair passed to the function.
constexpr const _Tp * end(initializer_list< _Tp > __ils)
Return an iterator pointing to one past the last element of the initilizer_list.
void _M_deallocate_single_object(pointer __p)
Deallocates memory that belongs to a single object of size sizeof(_Tp).
The class which acts as a predicate for applying the first-fit memory allocation policy for the bitma...
Common iterator class.
void _M_clear()
This function just clears the internal Free List, and gives back all the memory to the OS...
_Tp && forward(typename std::remove_reference< _Tp >::type &__t)
forward (as per N3143)
Definition: move.h:62
void _M_insert(size_t *__addr)
This function returns the block of memory to the internal free list.
Scoped lock idiom.
Definition: concurrence.h:311
size_t * _M_get(size_t __sz)
This function gets a block of memory of the specified size from the free list.
The bitmap counter which acts as the bitmap manipulator, and manages the bit-manipulation functions a...