libstdc++
bitmap_allocator.h
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1 // Bitmap Allocator. -*- C++ -*-
2 
3 // Copyright (C) 2004-2019 Free Software Foundation, Inc.
4 //
5 // This file is part of the GNU ISO C++ Library. This library is free
6 // software; you can redistribute it and/or modify it under the
7 // terms of the GNU General Public License as published by the
8 // Free Software Foundation; either version 3, or (at your option)
9 // any later version.
10 
11 // This library is distributed in the hope that it will be useful,
12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 // GNU General Public License for more details.
15 
16 // Under Section 7 of GPL version 3, you are granted additional
17 // permissions described in the GCC Runtime Library Exception, version
18 // 3.1, as published by the Free Software Foundation.
19 
20 // You should have received a copy of the GNU General Public License and
21 // a copy of the GCC Runtime Library Exception along with this program;
22 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
23 // <http://www.gnu.org/licenses/>.
24 
25 /** @file ext/bitmap_allocator.h
26  * This file is a GNU extension to the Standard C++ Library.
27  */
28 
29 #ifndef _BITMAP_ALLOCATOR_H
30 #define _BITMAP_ALLOCATOR_H 1
31 
32 #include <utility> // For std::pair.
33 #include <bits/functexcept.h> // For __throw_bad_alloc().
34 #include <functional> // For greater_equal, and less_equal.
35 #include <new> // For operator new.
36 #include <debug/debug.h> // _GLIBCXX_DEBUG_ASSERT
37 #include <ext/concurrence.h>
38 #include <bits/move.h>
39 
40 /** @brief The constant in the expression below is the alignment
41  * required in bytes.
42  */
43 #define _BALLOC_ALIGN_BYTES 8
44 
45 namespace __gnu_cxx _GLIBCXX_VISIBILITY(default)
46 {
47 _GLIBCXX_BEGIN_NAMESPACE_VERSION
48 
49  using std::size_t;
50  using std::ptrdiff_t;
51 
52  namespace __detail
53  {
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  * 1. Not all accessor functions are present.
63  * 2. Used ONLY for PODs.
64  * 3. No Allocator template argument. Uses ::operator new() to get
65  * memory, and ::operator delete() to free it.
66  * Caveat: The dtor does NOT free the memory allocated, so this a
67  * memory-leaking vector!
68  */
69  template<typename _Tp>
71  {
73  __mini_vector& operator=(const __mini_vector&);
74 
75  public:
76  typedef _Tp value_type;
77  typedef _Tp* pointer;
78  typedef _Tp& reference;
79  typedef const _Tp& const_reference;
80  typedef size_t size_type;
81  typedef ptrdiff_t difference_type;
82  typedef pointer iterator;
83 
84  private:
85  pointer _M_start;
86  pointer _M_finish;
87  pointer _M_end_of_storage;
88 
89  size_type
90  _M_space_left() const throw()
91  { return _M_end_of_storage - _M_finish; }
92 
93  _GLIBCXX_NODISCARD pointer
94  allocate(size_type __n)
95  { return static_cast<pointer>(::operator new(__n * sizeof(_Tp))); }
96 
97  void
98  deallocate(pointer __p, size_type)
99  { ::operator delete(__p); }
100 
101  public:
102  // Members used: size(), push_back(), pop_back(),
103  // insert(iterator, const_reference), erase(iterator),
104  // begin(), end(), back(), operator[].
105 
106  __mini_vector()
107  : _M_start(0), _M_finish(0), _M_end_of_storage(0) { }
108 
109  size_type
110  size() const throw()
111  { return _M_finish - _M_start; }
112 
113  iterator
114  begin() const throw()
115  { return this->_M_start; }
116 
117  iterator
118  end() const throw()
119  { return this->_M_finish; }
120 
121  reference
122  back() const throw()
123  { return *(this->end() - 1); }
124 
125  reference
126  operator[](const size_type __pos) const throw()
127  { return this->_M_start[__pos]; }
128 
129  void
130  insert(iterator __pos, const_reference __x);
131 
132  void
133  push_back(const_reference __x)
134  {
135  if (this->_M_space_left())
136  {
137  *this->end() = __x;
138  ++this->_M_finish;
139  }
140  else
141  this->insert(this->end(), __x);
142  }
143 
144  void
145  pop_back() throw()
146  { --this->_M_finish; }
147 
148  void
149  erase(iterator __pos) throw();
150 
151  void
152  clear() throw()
153  { this->_M_finish = this->_M_start; }
154  };
155 
156  // Out of line function definitions.
157  template<typename _Tp>
159  insert(iterator __pos, const_reference __x)
160  {
161  if (this->_M_space_left())
162  {
163  size_type __to_move = this->_M_finish - __pos;
164  iterator __dest = this->end();
165  iterator __src = this->end() - 1;
166 
167  ++this->_M_finish;
168  while (__to_move)
169  {
170  *__dest = *__src;
171  --__dest; --__src; --__to_move;
172  }
173  *__pos = __x;
174  }
175  else
176  {
177  size_type __new_size = this->size() ? this->size() * 2 : 1;
178  iterator __new_start = this->allocate(__new_size);
179  iterator __first = this->begin();
180  iterator __start = __new_start;
181  while (__first != __pos)
182  {
183  *__start = *__first;
184  ++__start; ++__first;
185  }
186  *__start = __x;
187  ++__start;
188  while (__first != this->end())
189  {
190  *__start = *__first;
191  ++__start; ++__first;
192  }
193  if (this->_M_start)
194  this->deallocate(this->_M_start, this->size());
195 
196  this->_M_start = __new_start;
197  this->_M_finish = __start;
198  this->_M_end_of_storage = this->_M_start + __new_size;
199  }
200  }
201 
202  template<typename _Tp>
203  void __mini_vector<_Tp>::
204  erase(iterator __pos) throw()
205  {
206  while (__pos + 1 != this->end())
207  {
208  *__pos = __pos[1];
209  ++__pos;
210  }
211  --this->_M_finish;
212  }
213 
214 
215  template<typename _Tp>
216  struct __mv_iter_traits
217  {
218  typedef typename _Tp::value_type value_type;
219  typedef typename _Tp::difference_type difference_type;
220  };
221 
222  template<typename _Tp>
223  struct __mv_iter_traits<_Tp*>
224  {
225  typedef _Tp value_type;
226  typedef ptrdiff_t difference_type;
227  };
228 
229  enum
230  {
231  bits_per_byte = 8,
232  bits_per_block = sizeof(size_t) * size_t(bits_per_byte)
233  };
234 
235  template<typename _ForwardIterator, typename _Tp, typename _Compare>
236  _ForwardIterator
237  __lower_bound(_ForwardIterator __first, _ForwardIterator __last,
238  const _Tp& __val, _Compare __comp)
239  {
240  typedef typename __mv_iter_traits<_ForwardIterator>::difference_type
241  _DistanceType;
242 
243  _DistanceType __len = __last - __first;
244  _DistanceType __half;
245  _ForwardIterator __middle;
246 
247  while (__len > 0)
248  {
249  __half = __len >> 1;
250  __middle = __first;
251  __middle += __half;
252  if (__comp(*__middle, __val))
253  {
254  __first = __middle;
255  ++__first;
256  __len = __len - __half - 1;
257  }
258  else
259  __len = __half;
260  }
261  return __first;
262  }
263 
264  /** @brief The number of Blocks pointed to by the address pair
265  * passed to the function.
266  */
267  template<typename _AddrPair>
268  inline size_t
269  __num_blocks(_AddrPair __ap)
270  { return (__ap.second - __ap.first) + 1; }
271 
272  /** @brief The number of Bit-maps pointed to by the address pair
273  * passed to the function.
274  */
275  template<typename _AddrPair>
276  inline size_t
277  __num_bitmaps(_AddrPair __ap)
278  { return __num_blocks(__ap) / size_t(bits_per_block); }
279 
280  // _Tp should be a pointer type.
281  template<typename _Tp>
282  class _Inclusive_between
283  : public std::unary_function<typename std::pair<_Tp, _Tp>, bool>
284  {
285  typedef _Tp pointer;
286  pointer _M_ptr_value;
287  typedef typename std::pair<_Tp, _Tp> _Block_pair;
288 
289  public:
290  _Inclusive_between(pointer __ptr) : _M_ptr_value(__ptr)
291  { }
292 
293  bool
294  operator()(_Block_pair __bp) const throw()
295  {
296  if (std::less_equal<pointer>()(_M_ptr_value, __bp.second)
297  && std::greater_equal<pointer>()(_M_ptr_value, __bp.first))
298  return true;
299  else
300  return false;
301  }
302  };
303 
304  // Used to pass a Functor to functions by reference.
305  template<typename _Functor>
306  class _Functor_Ref
307  : public std::unary_function<typename _Functor::argument_type,
308  typename _Functor::result_type>
309  {
310  _Functor& _M_fref;
311 
312  public:
313  typedef typename _Functor::argument_type argument_type;
314  typedef typename _Functor::result_type result_type;
315 
316  _Functor_Ref(_Functor& __fref) : _M_fref(__fref)
317  { }
318 
319  result_type
320  operator()(argument_type __arg)
321  { return _M_fref(__arg); }
322  };
323 
324  /** @class _Ffit_finder bitmap_allocator.h bitmap_allocator.h
325  *
326  * @brief The class which acts as a predicate for applying the
327  * first-fit memory allocation policy for the bitmap allocator.
328  */
329  // _Tp should be a pointer type, and _Alloc is the Allocator for
330  // the vector.
331  template<typename _Tp>
333  : public std::unary_function<typename std::pair<_Tp, _Tp>, bool>
334  {
335  typedef typename std::pair<_Tp, _Tp> _Block_pair;
337  typedef typename _BPVector::difference_type _Counter_type;
338 
339  size_t* _M_pbitmap;
340  _Counter_type _M_data_offset;
341 
342  public:
343  _Ffit_finder() : _M_pbitmap(0), _M_data_offset(0)
344  { }
345 
346  bool
347  operator()(_Block_pair __bp) throw()
348  {
349  // Set the _rover to the last physical location bitmap,
350  // which is the bitmap which belongs to the first free
351  // block. Thus, the bitmaps are in exact reverse order of
352  // the actual memory layout. So, we count down the bitmaps,
353  // which is the same as moving up the memory.
354 
355  // If the used count stored at the start of the Bit Map headers
356  // is equal to the number of Objects that the current Block can
357  // store, then there is definitely no space for another single
358  // object, so just return false.
359  _Counter_type __diff = __detail::__num_bitmaps(__bp);
360 
361  if (*(reinterpret_cast<size_t*>
362  (__bp.first) - (__diff + 1)) == __detail::__num_blocks(__bp))
363  return false;
364 
365  size_t* __rover = reinterpret_cast<size_t*>(__bp.first) - 1;
366 
367  for (_Counter_type __i = 0; __i < __diff; ++__i)
368  {
369  _M_data_offset = __i;
370  if (*__rover)
371  {
372  _M_pbitmap = __rover;
373  return true;
374  }
375  --__rover;
376  }
377  return false;
378  }
379 
380  size_t*
381  _M_get() const throw()
382  { return _M_pbitmap; }
383 
384  _Counter_type
385  _M_offset() const throw()
386  { return _M_data_offset * size_t(bits_per_block); }
387  };
388 
389  /** @class _Bitmap_counter bitmap_allocator.h bitmap_allocator.h
390  *
391  * @brief The bitmap counter which acts as the bitmap
392  * manipulator, and manages the bit-manipulation functions and
393  * the searching and identification functions on the bit-map.
394  */
395  // _Tp should be a pointer type.
396  template<typename _Tp>
398  {
399  typedef typename
401  typedef typename _BPVector::size_type _Index_type;
402  typedef _Tp pointer;
403 
404  _BPVector& _M_vbp;
405  size_t* _M_curr_bmap;
406  size_t* _M_last_bmap_in_block;
407  _Index_type _M_curr_index;
408 
409  public:
410  // Use the 2nd parameter with care. Make sure that such an
411  // entry exists in the vector before passing that particular
412  // index to this ctor.
413  _Bitmap_counter(_BPVector& Rvbp, long __index = -1) : _M_vbp(Rvbp)
414  { this->_M_reset(__index); }
415 
416  void
417  _M_reset(long __index = -1) throw()
418  {
419  if (__index == -1)
420  {
421  _M_curr_bmap = 0;
422  _M_curr_index = static_cast<_Index_type>(-1);
423  return;
424  }
425 
426  _M_curr_index = __index;
427  _M_curr_bmap = reinterpret_cast<size_t*>
428  (_M_vbp[_M_curr_index].first) - 1;
429 
430  _GLIBCXX_DEBUG_ASSERT(__index <= (long)_M_vbp.size() - 1);
431 
432  _M_last_bmap_in_block = _M_curr_bmap
433  - ((_M_vbp[_M_curr_index].second
434  - _M_vbp[_M_curr_index].first + 1)
435  / size_t(bits_per_block) - 1);
436  }
437 
438  // Dangerous Function! Use with extreme care. Pass to this
439  // function ONLY those values that are known to be correct,
440  // otherwise this will mess up big time.
441  void
442  _M_set_internal_bitmap(size_t* __new_internal_marker) throw()
443  { _M_curr_bmap = __new_internal_marker; }
444 
445  bool
446  _M_finished() const throw()
447  { return(_M_curr_bmap == 0); }
448 
450  operator++() throw()
451  {
452  if (_M_curr_bmap == _M_last_bmap_in_block)
453  {
454  if (++_M_curr_index == _M_vbp.size())
455  _M_curr_bmap = 0;
456  else
457  this->_M_reset(_M_curr_index);
458  }
459  else
460  --_M_curr_bmap;
461  return *this;
462  }
463 
464  size_t*
465  _M_get() const throw()
466  { return _M_curr_bmap; }
467 
468  pointer
469  _M_base() const throw()
470  { return _M_vbp[_M_curr_index].first; }
471 
472  _Index_type
473  _M_offset() const throw()
474  {
475  return size_t(bits_per_block)
476  * ((reinterpret_cast<size_t*>(this->_M_base())
477  - _M_curr_bmap) - 1);
478  }
479 
480  _Index_type
481  _M_where() const throw()
482  { return _M_curr_index; }
483  };
484 
485  /** @brief Mark a memory address as allocated by re-setting the
486  * corresponding bit in the bit-map.
487  */
488  inline void
489  __bit_allocate(size_t* __pbmap, size_t __pos) throw()
490  {
491  size_t __mask = 1 << __pos;
492  __mask = ~__mask;
493  *__pbmap &= __mask;
494  }
495 
496  /** @brief Mark a memory address as free by setting the
497  * corresponding bit in the bit-map.
498  */
499  inline void
500  __bit_free(size_t* __pbmap, size_t __pos) throw()
501  {
502  size_t __mask = 1 << __pos;
503  *__pbmap |= __mask;
504  }
505  } // namespace __detail
506 
507  /** @brief Generic Version of the bsf instruction.
508  */
509  inline size_t
510  _Bit_scan_forward(size_t __num)
511  { return static_cast<size_t>(__builtin_ctzl(__num)); }
512 
513  /** @class free_list bitmap_allocator.h bitmap_allocator.h
514  *
515  * @brief The free list class for managing chunks of memory to be
516  * given to and returned by the bitmap_allocator.
517  */
518  class free_list
519  {
520  public:
521  typedef size_t* value_type;
523  typedef vector_type::iterator iterator;
524  typedef __mutex __mutex_type;
525 
526  private:
527  struct _LT_pointer_compare
528  {
529  bool
530  operator()(const size_t* __pui,
531  const size_t __cui) const throw()
532  { return *__pui < __cui; }
533  };
534 
535 #if defined __GTHREADS
536  __mutex_type&
537  _M_get_mutex()
538  {
539  static __mutex_type _S_mutex;
540  return _S_mutex;
541  }
542 #endif
543 
544  vector_type&
545  _M_get_free_list()
546  {
547  static vector_type _S_free_list;
548  return _S_free_list;
549  }
550 
551  /** @brief Performs validation of memory based on their size.
552  *
553  * @param __addr The pointer to the memory block to be
554  * validated.
555  *
556  * Validates the memory block passed to this function and
557  * appropriately performs the action of managing the free list of
558  * blocks by adding this block to the free list or deleting this
559  * or larger blocks from the free list.
560  */
561  void
562  _M_validate(size_t* __addr) throw()
563  {
564  vector_type& __free_list = _M_get_free_list();
565  const vector_type::size_type __max_size = 64;
566  if (__free_list.size() >= __max_size)
567  {
568  // Ok, the threshold value has been reached. We determine
569  // which block to remove from the list of free blocks.
570  if (*__addr >= *__free_list.back())
571  {
572  // Ok, the new block is greater than or equal to the
573  // last block in the list of free blocks. We just free
574  // the new block.
575  ::operator delete(static_cast<void*>(__addr));
576  return;
577  }
578  else
579  {
580  // Deallocate the last block in the list of free lists,
581  // and insert the new one in its correct position.
582  ::operator delete(static_cast<void*>(__free_list.back()));
583  __free_list.pop_back();
584  }
585  }
586 
587  // Just add the block to the list of free lists unconditionally.
588  iterator __temp = __detail::__lower_bound
589  (__free_list.begin(), __free_list.end(),
590  *__addr, _LT_pointer_compare());
591 
592  // We may insert the new free list before _temp;
593  __free_list.insert(__temp, __addr);
594  }
595 
596  /** @brief Decides whether the wastage of memory is acceptable for
597  * the current memory request and returns accordingly.
598  *
599  * @param __block_size The size of the block available in the free
600  * list.
601  *
602  * @param __required_size The required size of the memory block.
603  *
604  * @return true if the wastage incurred is acceptable, else returns
605  * false.
606  */
607  bool
608  _M_should_i_give(size_t __block_size,
609  size_t __required_size) throw()
610  {
611  const size_t __max_wastage_percentage = 36;
612  if (__block_size >= __required_size &&
613  (((__block_size - __required_size) * 100 / __block_size)
614  < __max_wastage_percentage))
615  return true;
616  else
617  return false;
618  }
619 
620  public:
621  /** @brief This function returns the block of memory to the
622  * internal free list.
623  *
624  * @param __addr The pointer to the memory block that was given
625  * by a call to the _M_get function.
626  */
627  inline void
628  _M_insert(size_t* __addr) throw()
629  {
630 #if defined __GTHREADS
631  __scoped_lock __bfl_lock(_M_get_mutex());
632 #endif
633  // Call _M_validate to decide what should be done with
634  // this particular free list.
635  this->_M_validate(reinterpret_cast<size_t*>(__addr) - 1);
636  // See discussion as to why this is 1!
637  }
638 
639  /** @brief This function gets a block of memory of the specified
640  * size from the free list.
641  *
642  * @param __sz The size in bytes of the memory required.
643  *
644  * @return A pointer to the new memory block of size at least
645  * equal to that requested.
646  */
647  size_t*
648  _M_get(size_t __sz) _GLIBCXX_THROW(std::bad_alloc);
649 
650  /** @brief This function just clears the internal Free List, and
651  * gives back all the memory to the OS.
652  */
653  void
654  _M_clear();
655  };
656 
657 
658  // Forward declare the class.
659  template<typename _Tp>
661 
662  // Specialize for void:
663  template<>
664  class bitmap_allocator<void>
665  {
666  public:
667  typedef void* pointer;
668  typedef const void* const_pointer;
669 
670  // Reference-to-void members are impossible.
671  typedef void value_type;
672  template<typename _Tp1>
673  struct rebind
674  {
675  typedef bitmap_allocator<_Tp1> other;
676  };
677  };
678 
679  /**
680  * @brief Bitmap Allocator, primary template.
681  * @ingroup allocators
682  */
683  template<typename _Tp>
684  class bitmap_allocator : private free_list
685  {
686  public:
687  typedef size_t size_type;
688  typedef ptrdiff_t difference_type;
689  typedef _Tp* pointer;
690  typedef const _Tp* const_pointer;
691  typedef _Tp& reference;
692  typedef const _Tp& const_reference;
693  typedef _Tp value_type;
694  typedef free_list::__mutex_type __mutex_type;
695 
696  template<typename _Tp1>
697  struct rebind
698  {
699  typedef bitmap_allocator<_Tp1> other;
700  };
701 
702 #if __cplusplus >= 201103L
703  // _GLIBCXX_RESOLVE_LIB_DEFECTS
704  // 2103. propagate_on_container_move_assignment
705  typedef std::true_type propagate_on_container_move_assignment;
706 #endif
707 
708  private:
709  template<size_t _BSize, size_t _AlignSize>
710  struct aligned_size
711  {
712  enum
713  {
714  modulus = _BSize % _AlignSize,
715  value = _BSize + (modulus ? _AlignSize - (modulus) : 0)
716  };
717  };
718 
719  struct _Alloc_block
720  {
721  char __M_unused[aligned_size<sizeof(value_type),
722  _BALLOC_ALIGN_BYTES>::value];
723  };
724 
725 
726  typedef typename std::pair<_Alloc_block*, _Alloc_block*> _Block_pair;
727 
728  typedef typename __detail::__mini_vector<_Block_pair> _BPVector;
729  typedef typename _BPVector::iterator _BPiter;
730 
731  template<typename _Predicate>
732  static _BPiter
733  _S_find(_Predicate __p)
734  {
735  _BPiter __first = _S_mem_blocks.begin();
736  while (__first != _S_mem_blocks.end() && !__p(*__first))
737  ++__first;
738  return __first;
739  }
740 
741 #if defined _GLIBCXX_DEBUG
742  // Complexity: O(lg(N)). Where, N is the number of block of size
743  // sizeof(value_type).
744  void
745  _S_check_for_free_blocks() throw()
746  {
747  typedef typename __detail::_Ffit_finder<_Alloc_block*> _FFF;
748  _BPiter __bpi = _S_find(_FFF());
749 
750  _GLIBCXX_DEBUG_ASSERT(__bpi == _S_mem_blocks.end());
751  }
752 #endif
753 
754  /** @brief Responsible for exponentially growing the internal
755  * memory pool.
756  *
757  * @throw std::bad_alloc. If memory cannot be allocated.
758  *
759  * Complexity: O(1), but internally depends upon the
760  * complexity of the function free_list::_M_get. The part where
761  * the bitmap headers are written has complexity: O(X),where X
762  * is the number of blocks of size sizeof(value_type) within
763  * the newly acquired block. Having a tight bound.
764  */
765  void
766  _S_refill_pool() _GLIBCXX_THROW(std::bad_alloc)
767  {
768 #if defined _GLIBCXX_DEBUG
769  _S_check_for_free_blocks();
770 #endif
771 
772  const size_t __num_bitmaps = (_S_block_size
773  / size_t(__detail::bits_per_block));
774  const size_t __size_to_allocate = sizeof(size_t)
775  + _S_block_size * sizeof(_Alloc_block)
776  + __num_bitmaps * sizeof(size_t);
777 
778  size_t* __temp =
779  reinterpret_cast<size_t*>(this->_M_get(__size_to_allocate));
780  *__temp = 0;
781  ++__temp;
782 
783  // The Header information goes at the Beginning of the Block.
784  _Block_pair __bp =
785  std::make_pair(reinterpret_cast<_Alloc_block*>
786  (__temp + __num_bitmaps),
787  reinterpret_cast<_Alloc_block*>
788  (__temp + __num_bitmaps)
789  + _S_block_size - 1);
790 
791  // Fill the Vector with this information.
792  _S_mem_blocks.push_back(__bp);
793 
794  for (size_t __i = 0; __i < __num_bitmaps; ++__i)
795  __temp[__i] = ~static_cast<size_t>(0); // 1 Indicates all Free.
796 
797  _S_block_size *= 2;
798  }
799 
800  static _BPVector _S_mem_blocks;
801  static size_t _S_block_size;
802  static __detail::_Bitmap_counter<_Alloc_block*> _S_last_request;
803  static typename _BPVector::size_type _S_last_dealloc_index;
804 #if defined __GTHREADS
805  static __mutex_type _S_mut;
806 #endif
807 
808  public:
809 
810  /** @brief Allocates memory for a single object of size
811  * sizeof(_Tp).
812  *
813  * @throw std::bad_alloc. If memory cannot be allocated.
814  *
815  * Complexity: Worst case complexity is O(N), but that
816  * is hardly ever hit. If and when this particular case is
817  * encountered, the next few cases are guaranteed to have a
818  * worst case complexity of O(1)! That's why this function
819  * performs very well on average. You can consider this
820  * function to have a complexity referred to commonly as:
821  * Amortized Constant time.
822  */
823  pointer
824  _M_allocate_single_object() _GLIBCXX_THROW(std::bad_alloc)
825  {
826 #if defined __GTHREADS
827  __scoped_lock __bit_lock(_S_mut);
828 #endif
829 
830  // The algorithm is something like this: The last_request
831  // variable points to the last accessed Bit Map. When such a
832  // condition occurs, we try to find a free block in the
833  // current bitmap, or succeeding bitmaps until the last bitmap
834  // is reached. If no free block turns up, we resort to First
835  // Fit method.
836 
837  // WARNING: Do not re-order the condition in the while
838  // statement below, because it relies on C++'s short-circuit
839  // evaluation. The return from _S_last_request->_M_get() will
840  // NOT be dereference able if _S_last_request->_M_finished()
841  // returns true. This would inevitably lead to a NULL pointer
842  // dereference if tinkered with.
843  while (_S_last_request._M_finished() == false
844  && (*(_S_last_request._M_get()) == 0))
845  _S_last_request.operator++();
846 
847  if (__builtin_expect(_S_last_request._M_finished() == true, false))
848  {
849  // Fall Back to First Fit algorithm.
850  typedef typename __detail::_Ffit_finder<_Alloc_block*> _FFF;
851  _FFF __fff;
852  _BPiter __bpi = _S_find(__detail::_Functor_Ref<_FFF>(__fff));
853 
854  if (__bpi != _S_mem_blocks.end())
855  {
856  // Search was successful. Ok, now mark the first bit from
857  // the right as 0, meaning Allocated. This bit is obtained
858  // by calling _M_get() on __fff.
859  size_t __nz_bit = _Bit_scan_forward(*__fff._M_get());
860  __detail::__bit_allocate(__fff._M_get(), __nz_bit);
861 
862  _S_last_request._M_reset(__bpi - _S_mem_blocks.begin());
863 
864  // Now, get the address of the bit we marked as allocated.
865  pointer __ret = reinterpret_cast<pointer>
866  (__bpi->first + __fff._M_offset() + __nz_bit);
867  size_t* __puse_count =
868  reinterpret_cast<size_t*>
869  (__bpi->first) - (__detail::__num_bitmaps(*__bpi) + 1);
870 
871  ++(*__puse_count);
872  return __ret;
873  }
874  else
875  {
876  // Search was unsuccessful. We Add more memory to the
877  // pool by calling _S_refill_pool().
878  _S_refill_pool();
879 
880  // _M_Reset the _S_last_request structure to the first
881  // free block's bit map.
882  _S_last_request._M_reset(_S_mem_blocks.size() - 1);
883 
884  // Now, mark that bit as allocated.
885  }
886  }
887 
888  // _S_last_request holds a pointer to a valid bit map, that
889  // points to a free block in memory.
890  size_t __nz_bit = _Bit_scan_forward(*_S_last_request._M_get());
891  __detail::__bit_allocate(_S_last_request._M_get(), __nz_bit);
892 
893  pointer __ret = reinterpret_cast<pointer>
894  (_S_last_request._M_base() + _S_last_request._M_offset() + __nz_bit);
895 
896  size_t* __puse_count = reinterpret_cast<size_t*>
897  (_S_mem_blocks[_S_last_request._M_where()].first)
898  - (__detail::
899  __num_bitmaps(_S_mem_blocks[_S_last_request._M_where()]) + 1);
900 
901  ++(*__puse_count);
902  return __ret;
903  }
904 
905  /** @brief Deallocates memory that belongs to a single object of
906  * size sizeof(_Tp).
907  *
908  * Complexity: O(lg(N)), but the worst case is not hit
909  * often! This is because containers usually deallocate memory
910  * close to each other and this case is handled in O(1) time by
911  * the deallocate function.
912  */
913  void
914  _M_deallocate_single_object(pointer __p) throw()
915  {
916 #if defined __GTHREADS
917  __scoped_lock __bit_lock(_S_mut);
918 #endif
919  _Alloc_block* __real_p = reinterpret_cast<_Alloc_block*>(__p);
920 
921  typedef typename _BPVector::iterator _Iterator;
922  typedef typename _BPVector::difference_type _Difference_type;
923 
924  _Difference_type __diff;
925  long __displacement;
926 
927  _GLIBCXX_DEBUG_ASSERT(_S_last_dealloc_index >= 0);
928 
929  __detail::_Inclusive_between<_Alloc_block*> __ibt(__real_p);
930  if (__ibt(_S_mem_blocks[_S_last_dealloc_index]))
931  {
932  _GLIBCXX_DEBUG_ASSERT(_S_last_dealloc_index
933  <= _S_mem_blocks.size() - 1);
934 
935  // Initial Assumption was correct!
936  __diff = _S_last_dealloc_index;
937  __displacement = __real_p - _S_mem_blocks[__diff].first;
938  }
939  else
940  {
941  _Iterator _iter = _S_find(__ibt);
942 
943  _GLIBCXX_DEBUG_ASSERT(_iter != _S_mem_blocks.end());
944 
945  __diff = _iter - _S_mem_blocks.begin();
946  __displacement = __real_p - _S_mem_blocks[__diff].first;
947  _S_last_dealloc_index = __diff;
948  }
949 
950  // Get the position of the iterator that has been found.
951  const size_t __rotate = (__displacement
952  % size_t(__detail::bits_per_block));
953  size_t* __bitmapC =
954  reinterpret_cast<size_t*>
955  (_S_mem_blocks[__diff].first) - 1;
956  __bitmapC -= (__displacement / size_t(__detail::bits_per_block));
957 
958  __detail::__bit_free(__bitmapC, __rotate);
959  size_t* __puse_count = reinterpret_cast<size_t*>
960  (_S_mem_blocks[__diff].first)
961  - (__detail::__num_bitmaps(_S_mem_blocks[__diff]) + 1);
962 
963  _GLIBCXX_DEBUG_ASSERT(*__puse_count != 0);
964 
965  --(*__puse_count);
966 
967  if (__builtin_expect(*__puse_count == 0, false))
968  {
969  _S_block_size /= 2;
970 
971  // We can safely remove this block.
972  // _Block_pair __bp = _S_mem_blocks[__diff];
973  this->_M_insert(__puse_count);
974  _S_mem_blocks.erase(_S_mem_blocks.begin() + __diff);
975 
976  // Reset the _S_last_request variable to reflect the
977  // erased block. We do this to protect future requests
978  // after the last block has been removed from a particular
979  // memory Chunk, which in turn has been returned to the
980  // free list, and hence had been erased from the vector,
981  // so the size of the vector gets reduced by 1.
982  if ((_Difference_type)_S_last_request._M_where() >= __diff--)
983  _S_last_request._M_reset(__diff);
984 
985  // If the Index into the vector of the region of memory
986  // that might hold the next address that will be passed to
987  // deallocated may have been invalidated due to the above
988  // erase procedure being called on the vector, hence we
989  // try to restore this invariant too.
990  if (_S_last_dealloc_index >= _S_mem_blocks.size())
991  {
992  _S_last_dealloc_index =(__diff != -1 ? __diff : 0);
993  _GLIBCXX_DEBUG_ASSERT(_S_last_dealloc_index >= 0);
994  }
995  }
996  }
997 
998  public:
999  bitmap_allocator() _GLIBCXX_USE_NOEXCEPT
1000  { }
1001 
1002  bitmap_allocator(const bitmap_allocator&) _GLIBCXX_USE_NOEXCEPT
1003  { }
1004 
1005  template<typename _Tp1>
1006  bitmap_allocator(const bitmap_allocator<_Tp1>&) _GLIBCXX_USE_NOEXCEPT
1007  { }
1008 
1009  ~bitmap_allocator() _GLIBCXX_USE_NOEXCEPT
1010  { }
1011 
1012  _GLIBCXX_NODISCARD pointer
1013  allocate(size_type __n)
1014  {
1015  if (__n > this->max_size())
1016  std::__throw_bad_alloc();
1017 
1018 #if __cpp_aligned_new
1019  if (alignof(value_type) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
1020  {
1021  const size_type __b = __n * sizeof(value_type);
1022  std::align_val_t __al = std::align_val_t(alignof(value_type));
1023  return static_cast<pointer>(::operator new(__b, __al));
1024  }
1025 #endif
1026 
1027  if (__builtin_expect(__n == 1, true))
1028  return this->_M_allocate_single_object();
1029  else
1030  {
1031  const size_type __b = __n * sizeof(value_type);
1032  return reinterpret_cast<pointer>(::operator new(__b));
1033  }
1034  }
1035 
1036  _GLIBCXX_NODISCARD pointer
1037  allocate(size_type __n, typename bitmap_allocator<void>::const_pointer)
1038  { return allocate(__n); }
1039 
1040  void
1041  deallocate(pointer __p, size_type __n) throw()
1042  {
1043  if (__builtin_expect(__p != 0, true))
1044  {
1045 #if __cpp_aligned_new
1046  // Types with extended alignment are handled by operator delete.
1047  if (alignof(value_type) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
1048  {
1049  ::operator delete(__p, std::align_val_t(alignof(value_type)));
1050  return;
1051  }
1052 #endif
1053 
1054  if (__builtin_expect(__n == 1, true))
1055  this->_M_deallocate_single_object(__p);
1056  else
1057  ::operator delete(__p);
1058  }
1059  }
1060 
1061  pointer
1062  address(reference __r) const _GLIBCXX_NOEXCEPT
1063  { return std::__addressof(__r); }
1064 
1065  const_pointer
1066  address(const_reference __r) const _GLIBCXX_NOEXCEPT
1067  { return std::__addressof(__r); }
1068 
1069  size_type
1070  max_size() const _GLIBCXX_USE_NOEXCEPT
1071  { return size_type(-1) / sizeof(value_type); }
1072 
1073 #if __cplusplus >= 201103L
1074  template<typename _Up, typename... _Args>
1075  void
1076  construct(_Up* __p, _Args&&... __args)
1077  { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
1078 
1079  template<typename _Up>
1080  void
1081  destroy(_Up* __p)
1082  { __p->~_Up(); }
1083 #else
1084  void
1085  construct(pointer __p, const_reference __data)
1086  { ::new((void *)__p) value_type(__data); }
1087 
1088  void
1089  destroy(pointer __p)
1090  { __p->~value_type(); }
1091 #endif
1092  };
1093 
1094  template<typename _Tp1, typename _Tp2>
1095  bool
1096  operator==(const bitmap_allocator<_Tp1>&,
1097  const bitmap_allocator<_Tp2>&) throw()
1098  { return true; }
1099 
1100  template<typename _Tp1, typename _Tp2>
1101  bool
1102  operator!=(const bitmap_allocator<_Tp1>&,
1103  const bitmap_allocator<_Tp2>&) throw()
1104  { return false; }
1105 
1106  // Static member definitions.
1107  template<typename _Tp>
1108  typename bitmap_allocator<_Tp>::_BPVector
1109  bitmap_allocator<_Tp>::_S_mem_blocks;
1110 
1111  template<typename _Tp>
1112  size_t bitmap_allocator<_Tp>::_S_block_size =
1113  2 * size_t(__detail::bits_per_block);
1114 
1115  template<typename _Tp>
1116  typename bitmap_allocator<_Tp>::_BPVector::size_type
1117  bitmap_allocator<_Tp>::_S_last_dealloc_index = 0;
1118 
1119  template<typename _Tp>
1120  __detail::_Bitmap_counter
1121  <typename bitmap_allocator<_Tp>::_Alloc_block*>
1122  bitmap_allocator<_Tp>::_S_last_request(_S_mem_blocks);
1123 
1124 #if defined __GTHREADS
1125  template<typename _Tp>
1126  typename bitmap_allocator<_Tp>::__mutex_type
1127  bitmap_allocator<_Tp>::_S_mut;
1128 #endif
1129 
1130 _GLIBCXX_END_NAMESPACE_VERSION
1131 } // namespace __gnu_cxx
1132 
1133 #endif
Scoped lock idiom.
Definition: concurrence.h:228
constexpr const _Tp * end(initializer_list< _Tp > __ils) noexcept
Return an iterator pointing to one past the last element of the initializer_list. ...
Bitmap Allocator, primary template.
The bitmap counter which acts as the bitmap manipulator, and manages the bit-manipulation functions a...
ISO C++ entities toplevel namespace is std.
void _M_clear()
This function just clears the internal Free List, and gives back all the memory to the OS...
size_t * _M_get(size_t __sz)
This function gets a block of memory of the specified size from the free list.
void _M_deallocate_single_object(pointer __p)
Deallocates memory that belongs to a single object of size sizeof(_Tp).
_T1 first
second_type is the second bound type
Definition: stl_pair.h:214
constexpr const _Tp * begin(initializer_list< _Tp > __ils) noexcept
Return an iterator pointing to the first element of the initializer_list.
integral_constant
Definition: type_traits:57
Exception possibly thrown by new.bad_alloc (or classes derived from it) is used to report allocation ...
Definition: new:54
Common iterator class.
size_t _Bit_scan_forward(size_t __num)
Generic Version of the bsf instruction.
pointer _M_allocate_single_object()
Allocates memory for a single object of size sizeof(_Tp).
void _M_insert(size_t *__addr)
This function returns the block of memory to the internal free 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.
One of the comparison functors.
Definition: stl_function.h:343
__mini_vector<> is a stripped down version of the full-fledged std::vector<>.
constexpr _Tp * __addressof(_Tp &__r) noexcept
Same as C++11 std::addressof.
Definition: move.h:47
GNU extensions for public use.
void __bit_free(size_t *__pbmap, size_t __pos)
Mark a memory address as free by setting the corresponding bit in the bit-map.
#define _BALLOC_ALIGN_BYTES
The constant in the expression below is the alignment required in bytes.
constexpr 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:524
size_t __num_blocks(_AddrPair __ap)
The number of Blocks pointed to by the address pair passed to the function.
The class which acts as a predicate for applying the first-fit memory allocation policy for the bitma...
Struct holding two objects of arbitrary type.
Definition: stl_pair.h:208
The free list class for managing chunks of memory to be given to and returned by the bitmap_allocator...
size_t __num_bitmaps(_AddrPair __ap)
The number of Bit-maps pointed to by the address pair passed to the function.
One of the comparison functors.
Definition: stl_function.h:346