doxygroups.cc: Add std::tr1 namespace.

[thirdparty/gcc.git] / libstdc++-v3 / include / ext / bitmap_allocator.h

// Bitmap Allocator. -*- C++ -*-

// Copyright (C) 2004 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library 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 2, or (at your option)
// any later version.

// This library 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 library; see the file COPYING.  If not, write to the Free
// Software Foundation, 59 Temple Place - Suite 330, Boston, MA 02111-1307,
// USA.

// As a special exception, you may use this file as part of a free software
// library without restriction.  Specifically, if other files instantiate
// templates or use macros or inline functions from this file, or you compile
// this file and link it with other files to produce an executable, this
// file does not by itself cause the resulting executable to be covered by
// the GNU General Public License.  This exception does not however
// invalidate any other reasons why the executable file might be covered by
// the GNU General Public License.

/** @file ext/bitmap_allocator.h
 *  This file is a GNU extension to the Standard C++ Library.
 */

#ifndef _BITMAP_ALLOCATOR_H
#define _BITMAP_ALLOCATOR_H 1

// For std::size_t, and ptrdiff_t.
#include <cstddef>

// For __throw_bad_alloc().
#include <bits/functexcept.h>

// For std::pair.
#include <utility>

// For greater_equal, and less_equal.
#include <functional>

// For operator new.
#include <new>

// For __gthread_mutex_t, __gthread_mutex_lock and __gthread_mutex_unlock.
#include <bits/gthr.h>

// Define this to enable error checking withing the allocator
// itself(to debug the allocator itself).
//#define _BALLOC_SANITY_CHECK

// The constant in the expression below is the alignment required in
// bytes.
#define _BALLOC_ALIGN_BYTES 8

#if defined _BALLOC_SANITY_CHECK
#include <cassert>
#define _BALLOC_ASSERT(_EXPR) assert(_EXPR)
#else
#define _BALLOC_ASSERT(_EXPR)
#endif


namespace __gnu_cxx
{
#if defined __GTHREADS
  namespace
  {
    // If true, then the application being compiled will be using
    // threads, so use mutexes as a synchronization primitive, else do
    // no use any synchronization primitives.
    bool const __threads_enabled = __gthread_active_p();
  }
#endif

#if defined __GTHREADS
  // _Mutex is an OO-Wrapper for __gthread_mutex_t. It does not allow
  // you to copy or assign an already initialized mutex. This is used
  // merely as a convenience for the locking classes.
  class _Mutex 
  {
    __gthread_mutex_t _M_mut;

    // Prevent Copying and assignment.
    _Mutex(_Mutex const&);
    _Mutex& operator=(_Mutex const&);

  public:
    _Mutex()
    {
      if (__threads_enabled)
        {
#if !defined __GTHREAD_MUTEX_INIT
          __GTHREAD_MUTEX_INIT_FUNCTION(&_M_mut);
#else
          __gthread_mutex_t __mtemp = __GTHREAD_MUTEX_INIT;
          _M_mut = __mtemp;
#endif
        }
    }

    ~_Mutex()
    {
      // Gthreads does not define a Mutex Destruction Function.
    }

    __gthread_mutex_t*
    _M_get() { return &_M_mut; }
  };

  // _Lock is a simple manual lokcing class which allows you to
  // manually lock and unlock a mutex associated with the lock. There
  // is not automatic locking or unlocking happening without the
  // programmer's explicit instructions. This class unlocks the mutex
  // ONLY if it has not been locked. However, this check does not
  // apply for lokcing, and wayward use may cause dead-locks.
  class _Lock 
  {
    _Mutex* _M_pmt;
    bool _M_locked;

    // Prevent Copying and assignment.
    _Lock(_Lock const&);
    _Lock& operator=(_Lock const&);

  public:
    _Lock(_Mutex* __mptr)
    : _M_pmt(__mptr), _M_locked(false)
    { }

    void
    _M_lock()
    {
      if (__threads_enabled)
        {
          _M_locked = true;
          __gthread_mutex_lock(_M_pmt->_M_get());
        }
    }

    void
    _M_unlock()
    {
      if (__threads_enabled)
        {
          if (__builtin_expect(_M_locked, true))
            {
              __gthread_mutex_unlock(_M_pmt->_M_get());
              _M_locked = false;
            }
        }
    }
    
    ~_Lock() { }
  };

  // _Auto_Lock locks the associated mutex on construction, and
  // unlocks on it's destruction. There are no checks performed, and
  // this calss follows the RAII principle.
  class _Auto_Lock 
  {
    _Mutex* _M_pmt;
    // Prevent Copying and assignment.
    _Auto_Lock(_Auto_Lock const&);
    _Auto_Lock& operator=(_Auto_Lock const&);

    void
    _M_lock()
    {
      if (__threads_enabled)
        __gthread_mutex_lock(_M_pmt->_M_get());
    }

    void
    _M_unlock()
    {
      if (__threads_enabled)
        __gthread_mutex_unlock(_M_pmt->_M_get());
    }

  public:
    _Auto_Lock(_Mutex* __mptr) : _M_pmt(__mptr)
    { this->_M_lock(); }

    ~_Auto_Lock() { this->_M_unlock(); }
  };
#endif 

  namespace balloc
  {
    // __mini_vector<> is to be used only for built-in types or
    // PODs. It is a stripped down version of the full-fledged
    // std::vector<>. Noteable differences are: 
    // 
    // 1. Not all accessor functions are present.
    // 2. Used ONLY for PODs.
    // 3. No Allocator template argument. Uses ::operator new() to get
    // memory, and ::operator delete() to free it.
    template<typename _Tp>
      class __mini_vector
      {
        __mini_vector(const __mini_vector&);
        __mini_vector& operator=(const __mini_vector&);

      public:
        typedef _Tp value_type;
        typedef _Tp* pointer;
        typedef _Tp& reference;
        typedef const _Tp& const_reference;
        typedef std::size_t size_type;
        typedef std::ptrdiff_t difference_type;
        typedef pointer iterator;

      private:
        pointer _M_start;
        pointer _M_finish;
        pointer _M_end_of_storage;

        size_type
        _M_space_left() const throw()
        { return _M_end_of_storage - _M_finish; }

        pointer
        allocate(size_type __n)
        { return static_cast<pointer>(::operator new(__n * sizeof(_Tp))); }

        void
        deallocate(pointer __p, size_type)
        { ::operator delete(__p); }

      public:
        // Members used: size(), push_back(), pop_back(),
        // insert(iterator, const_reference), erase(iterator),
        // begin(), end(), back(), operator[].

        __mini_vector() : _M_start(0), _M_finish(0), 
                          _M_end_of_storage(0)
        { }

#if 0
        ~__mini_vector()
        {
          if (this->_M_start)
            {
              this->deallocate(this->_M_start, this->_M_end_of_storage 
                               - this->_M_start);
            }
        }
#endif

        size_type
        size() const throw()
        { return _M_finish - _M_start; }

        iterator
        begin() const throw()
        { return this->_M_start; }

        iterator
        end() const throw()
        { return this->_M_finish; }

        reference
        back() const throw()
        { return *(this->end() - 1); }

        reference
        operator[](const size_type __pos) const throw()
        { return this->_M_start[__pos]; }

        void
        insert(iterator __pos, const_reference __x);

        void
        push_back(const_reference __x)
        {
          if (this->_M_space_left())
            {
              *this->end() = __x;
              ++this->_M_finish;
            }
          else
            this->insert(this->end(), __x);
        }

        void
        pop_back() throw()
        { --this->_M_finish; }

        void
        erase(iterator __pos) throw();

        void
        clear() throw()
        { this->_M_finish = this->_M_start; }
      };

    // Out of line function definitions.
    template<typename _Tp>
      void __mini_vector<_Tp>::
      insert(iterator __pos, const_reference __x)
      {
        if (this->_M_space_left())
          {
            size_type __to_move = this->_M_finish - __pos;
            iterator __dest = this->end();
            iterator __src = this->end() - 1;

            ++this->_M_finish;
            while (__to_move)
              {
                *__dest = *__src;
                --__dest; --__src; --__to_move;
              }
            *__pos = __x;
          }
        else
          {
            size_type __new_size = this->size() ? this->size() * 2 : 1;
            iterator __new_start = this->allocate(__new_size);
            iterator __first = this->begin();
            iterator __start = __new_start;
            while (__first != __pos)
              {
                *__start = *__first;
                ++__start; ++__first;
              }
            *__start = __x;
            ++__start;
            while (__first != this->end())
              {
                *__start = *__first;
                ++__start; ++__first;
              }
            if (this->_M_start)
              this->deallocate(this->_M_start, this->size());

            this->_M_start = __new_start;
            this->_M_finish = __start;
            this->_M_end_of_storage = this->_M_start + __new_size;
          }
      }

    template<typename _Tp>
      void __mini_vector<_Tp>::
      erase(iterator __pos) throw()
      {
        while (__pos + 1 != this->end())
          {
            *__pos = __pos[1];
            ++__pos;
          }
        --this->_M_finish;
      }


    template<typename _Tp>
      struct __mv_iter_traits
      {
        typedef typename _Tp::value_type value_type;
        typedef typename _Tp::difference_type difference_type;
      };

    template<typename _Tp>
      struct __mv_iter_traits<_Tp*>
      {
        typedef _Tp value_type;
        typedef std::ptrdiff_t difference_type;
      };

    enum 
      { 
        bits_per_byte = 8, 
        bits_per_block = sizeof(size_t) * bits_per_byte 
      };

    template<typename _ForwardIterator, typename _Tp, typename _Compare>
      _ForwardIterator
      __lower_bound(_ForwardIterator __first, _ForwardIterator __last,
                    const _Tp& __val, _Compare __comp)
      {
        typedef typename __mv_iter_traits<_ForwardIterator>::value_type
          _ValueType;
        typedef typename __mv_iter_traits<_ForwardIterator>::difference_type
          _DistanceType;

        _DistanceType __len = __last - __first;
        _DistanceType __half;
        _ForwardIterator __middle;

        while (__len > 0)
          {
            __half = __len >> 1;
            __middle = __first;
            __middle += __half;
            if (__comp(*__middle, __val))
              {
                __first = __middle;
                ++__first;
                __len = __len - __half - 1;
              }
            else
              __len = __half;
          }
        return __first;
      }

    template<typename _InputIterator, typename _Predicate>
      inline _InputIterator
      __find_if(_InputIterator __first, _InputIterator __last, _Predicate __p)
      {
        while (__first != __last && !__p(*__first))
          ++__first;
        return __first;
      }

    template<typename _AddrPair>
      inline size_t
      __num_blocks(_AddrPair __ap)
      { return (__ap.second - __ap.first) + 1; }

    template<typename _AddrPair>
      inline size_t
      __num_bitmaps(_AddrPair __ap)
      { return __num_blocks(__ap) / bits_per_block; }

    // _Tp should be a pointer type.
    template<typename _Tp>
      class _Inclusive_between 
      : public std::unary_function<typename std::pair<_Tp, _Tp>, bool>
      {
        typedef _Tp pointer;
        pointer _M_ptr_value;
        typedef typename std::pair<_Tp, _Tp> _Block_pair;
        
      public:
        _Inclusive_between(pointer __ptr) : _M_ptr_value(__ptr) 
        { }
        
        bool 
        operator()(_Block_pair __bp) const throw()
        {
          if (std::less_equal<pointer>()(_M_ptr_value, __bp.second) 
              && std::greater_equal<pointer>()(_M_ptr_value, __bp.first))
            return true;
          else
            return false;
        }
      };
  
    // Used to pass a Functor to functions by reference.
    template<typename _Functor>
      class _Functor_Ref 
      : public std::unary_function<typename _Functor::argument_type, 
                                   typename _Functor::result_type>
      {
        _Functor& _M_fref;
        
      public:
        typedef typename _Functor::argument_type argument_type;
        typedef typename _Functor::result_type result_type;

        _Functor_Ref(_Functor& __fref) : _M_fref(__fref) 
        { }

        result_type 
        operator()(argument_type __arg) 
        { return _M_fref(__arg); }
      };

    // _Tp should be a pointer type, and _Alloc is the Allocator for
    // the vector.
    template<typename _Tp>
      class _Ffit_finder 
      : public std::unary_function<typename std::pair<_Tp, _Tp>, bool>
      {
        typedef typename std::pair<_Tp, _Tp> _Block_pair;
        typedef typename balloc::__mini_vector<_Block_pair> _BPVector;
        typedef typename _BPVector::difference_type _Counter_type;

        size_t* _M_pbitmap;
        _Counter_type _M_data_offset;

      public:
        _Ffit_finder() : _M_pbitmap(0), _M_data_offset(0)
        { }

        bool 
        operator()(_Block_pair __bp) throw()
        {
          // Set the _rover to the last physical location bitmap,
          // which is the bitmap which belongs to the first free
          // block. Thus, the bitmaps are in exact reverse order of
          // the actual memory layout. So, we count down the bimaps,
          // which is the same as moving up the memory.

          // If the used count stored at the start of the Bit Map headers
          // is equal to the number of Objects that the current Block can
          // store, then there is definitely no space for another single
          // object, so just return false.
          _Counter_type __diff = 
            __gnu_cxx::balloc::__num_bitmaps(__bp);

          if (*(reinterpret_cast<size_t*>
                (__bp.first) - (__diff + 1))
              == __gnu_cxx::balloc::__num_blocks(__bp))
            return false;

          size_t* __rover = reinterpret_cast<size_t*>(__bp.first) - 1;

          for (_Counter_type __i = 0; __i < __diff; ++__i)
            {
              _M_data_offset = __i;
              if (*__rover)
                {
                  _M_pbitmap = __rover;
                  return true;
                }
              --__rover;
            }
          return false;
        }

    
        size_t*
        _M_get() const throw()
        { return _M_pbitmap; }

        _Counter_type
        _M_offset() const throw()
        { return _M_data_offset * bits_per_block; }
      };


  
    // _Tp should be a pointer type.
    template<typename _Tp>
      class _Bitmap_counter
      {
        typedef typename balloc::__mini_vector<typename std::pair<_Tp, _Tp> > 
        _BPVector;
        typedef typename _BPVector::size_type _Index_type;
        typedef _Tp pointer;
    
        _BPVector& _M_vbp;
        size_t* _M_curr_bmap;
        size_t* _M_last_bmap_in_block;
        _Index_type _M_curr_index;
    
      public:
        // Use the 2nd parameter with care. Make sure that such an
        // entry exists in the vector before passing that particular
        // index to this ctor.
        _Bitmap_counter(_BPVector& Rvbp, long __index = -1) : _M_vbp(Rvbp)
        { this->_M_reset(__index); }
    
        void 
        _M_reset(long __index = -1) throw()
        {
          if (__index == -1)
            {
              _M_curr_bmap = 0;
              _M_curr_index = static_cast<_Index_type>(-1);
              return;
            }

          _M_curr_index = __index;
          _M_curr_bmap = reinterpret_cast<size_t*>
            (_M_vbp[_M_curr_index].first) - 1;
          
          _BALLOC_ASSERT(__index <= (long)_M_vbp.size() - 1);
        
          _M_last_bmap_in_block = _M_curr_bmap
            - ((_M_vbp[_M_curr_index].second 
                - _M_vbp[_M_curr_index].first + 1) 
               / bits_per_block - 1);
        }
    
        // Dangerous Function! Use with extreme care. Pass to this
        // function ONLY those values that are known to be correct,
        // otherwise this will mess up big time.
        void
        _M_set_internal_bitmap(size_t* __new_internal_marker) throw()
        { _M_curr_bmap = __new_internal_marker; }
    
        bool
        _M_finished() const throw()
        { return(_M_curr_bmap == 0); }
    
        _Bitmap_counter&
        operator++() throw()
        {
          if (_M_curr_bmap == _M_last_bmap_in_block)
            {
              if (++_M_curr_index == _M_vbp.size())
                _M_curr_bmap = 0;
              else
                this->_M_reset(_M_curr_index);
            }
          else
            --_M_curr_bmap;
          return *this;
        }
    
        size_t*
        _M_get() const throw()
        { return _M_curr_bmap; }
    
        pointer 
        _M_base() const throw()
        { return _M_vbp[_M_curr_index].first; }

        _Index_type
        _M_offset() const throw()
        {
          return bits_per_block
            * ((reinterpret_cast<size_t*>(this->_M_base()) 
                - _M_curr_bmap) - 1);
        }
    
        _Index_type
        _M_where() const throw()
        { return _M_curr_index; }
      };

    inline void 
    __bit_allocate(size_t* __pbmap, size_t __pos) throw()
    {
      size_t __mask = 1 << __pos;
      __mask = ~__mask;
      *__pbmap &= __mask;
    }
  
    inline void 
    __bit_free(size_t* __pbmap, size_t __pos) throw()
    {
      size_t __mask = 1 << __pos;
      *__pbmap |= __mask;
    }
  } // namespace balloc

  // Generic Version of the bsf instruction.
  inline size_t 
  _Bit_scan_forward(size_t __num)
  { return static_cast<size_t>(__builtin_ctzl(__num)); }

  class free_list
  {
    typedef size_t* value_type;
    typedef balloc::__mini_vector<value_type> vector_type;
    typedef vector_type::iterator iterator;

    struct _LT_pointer_compare
    {
      bool
      operator()(const size_t* __pui, 
                 const size_t __cui) const throw()
      { return *__pui < __cui; }
    };

#if defined __GTHREADS 
    static _Mutex _S_bfl_mutex;
#endif
    static vector_type _S_free_list;
    
    void
    _M_validate(size_t* __addr) throw()
    {
      const vector_type::size_type __max_size = 64;
      if (_S_free_list.size() >= __max_size)
        {
          // Ok, the threshold value has been reached.  We determine
          // which block to remove from the list of free blocks.
          if (*__addr >= *_S_free_list.back())
            {
              // Ok, the new block is greater than or equal to the
              // last block in the list of free blocks. We just free
              // the new block.
              ::operator delete(static_cast<void*>(__addr));
              return;
            }
          else
            {
              // Deallocate the last block in the list of free lists,
              // and insert the new one in it's correct position.
              ::operator delete(static_cast<void*>(_S_free_list.back()));
              _S_free_list.pop_back();
            }
        }
          
      // Just add the block to the list of free lists unconditionally.
      iterator __temp = __gnu_cxx::balloc::__lower_bound
        (_S_free_list.begin(), _S_free_list.end(), 
         *__addr, _LT_pointer_compare());

      // We may insert the new free list before _temp;
      _S_free_list.insert(__temp, __addr);
    }

    bool 
    _M_should_i_give(size_t __block_size, 
                     size_t __required_size) throw()
    {
      const size_t __max_wastage_percentage = 36;
      if (__block_size >= __required_size && 
          (((__block_size - __required_size) * 100 / __block_size)
           < __max_wastage_percentage))
        return true;
      else
        return false;
    }

  public:
    inline void 
    _M_insert(size_t* __addr) throw()
    {
#if defined __GTHREADS
      _Auto_Lock __bfl_lock(&_S_bfl_mutex);
#endif
      // Call _M_validate to decide what should be done with
      // this particular free list.
      this->_M_validate(reinterpret_cast<size_t*>(__addr) - 1);
      // See discussion as to why this is 1!
    }
    
    size_t*
    _M_get(size_t __sz) throw(std::bad_alloc);

    // This function just clears the internal Free List, and gives back
    // all the memory to the OS.
    void 
    _M_clear();
  };


  // Forward declare the class.
  template<typename _Tp> 
    class bitmap_allocator;

  // Specialize for void:
  template<>
    class bitmap_allocator<void>
    {
    public:
      typedef void*       pointer;
      typedef const void* const_pointer;

      // Reference-to-void members are impossible.
      typedef void  value_type;
      template<typename _Tp1>
        struct rebind
        {
          typedef bitmap_allocator<_Tp1> other;
        };
    };

  template<typename _Tp>
    class bitmap_allocator : private free_list
    {
    public:
      typedef std::size_t    size_type;
      typedef std::ptrdiff_t difference_type;
      typedef _Tp*        pointer;
      typedef const _Tp*  const_pointer;
      typedef _Tp&        reference;
      typedef const _Tp&  const_reference;
      typedef _Tp         value_type;
      template<typename _Tp1>
        struct rebind
        {
          typedef bitmap_allocator<_Tp1> other;
        };

    private:
      template<size_t _BSize, size_t _AlignSize>
        struct aligned_size
        {
          enum
            { 
              modulus = _BSize % _AlignSize,
              value = _BSize + (modulus ? _AlignSize - (modulus) : 0)
            };
        };

      struct _Alloc_block
      {
        char __M_unused[aligned_size<sizeof(value_type),
                        _BALLOC_ALIGN_BYTES>::value];
      };


      typedef typename std::pair<_Alloc_block*, _Alloc_block*> _Block_pair;

      typedef typename 
      balloc::__mini_vector<_Block_pair> _BPVector;

#if defined _BALLOC_SANITY_CHECK
      // Complexity: O(lg(N)). Where, N is the number of block of size
      // sizeof(value_type).
      void 
      _S_check_for_free_blocks() throw()
      {
        typedef typename 
          __gnu_cxx::balloc::_Ffit_finder<_Alloc_block*> _FFF;
        _FFF __fff;
        typedef typename _BPVector::iterator _BPiter;
        _BPiter __bpi = 
          __gnu_cxx::balloc::__find_if
          (_S_mem_blocks.begin(), _S_mem_blocks.end(), 
           __gnu_cxx::balloc::_Functor_Ref<_FFF>(__fff));

        _BALLOC_ASSERT(__bpi == _S_mem_blocks.end());
      }
#endif

      // Complexity: O(1), but internally depends upon the complexity
      // of the function free_list::_M_get. The
      // part where the bitmap headers are written is of worst case
      // complexity: O(X),where X is the number of blocks of size
      // sizeof(value_type) within the newly acquired block. Having a
      // tight bound.
      void 
      _S_refill_pool() throw(std::bad_alloc)
      {
#if defined _BALLOC_SANITY_CHECK
        _S_check_for_free_blocks();
#endif

        const size_t __num_bitmaps = _S_block_size / balloc::bits_per_block;
        const size_t __size_to_allocate = sizeof(size_t) 
          + _S_block_size * sizeof(_Alloc_block) 
          + __num_bitmaps * sizeof(size_t);

        size_t* __temp = 
          reinterpret_cast<size_t*>
          (this->_M_get(__size_to_allocate));
        *__temp = 0;
        ++__temp;

        // The Header information goes at the Beginning of the Block.
        _Block_pair __bp = 
          std::make_pair(reinterpret_cast<_Alloc_block*>
                         (__temp + __num_bitmaps), 
                         reinterpret_cast<_Alloc_block*>
                         (__temp + __num_bitmaps) 
                         + _S_block_size - 1);
        
        // Fill the Vector with this information.
        _S_mem_blocks.push_back(__bp);

        size_t __bit_mask = 0; // 0 Indicates all Allocated.
        __bit_mask = ~__bit_mask; // 1 Indicates all Free.

        for (size_t __i = 0; __i < __num_bitmaps; ++__i)
          __temp[__i] = __bit_mask;

        _S_block_size *= 2;
      }


      static _BPVector _S_mem_blocks;
      static size_t _S_block_size;
      static __gnu_cxx::balloc::
      _Bitmap_counter<_Alloc_block*> _S_last_request;
      static typename _BPVector::size_type _S_last_dealloc_index;
#if defined __GTHREADS
      static _Mutex _S_mut;
#endif

    public:

      // Complexity: Worst case complexity is O(N), but that is hardly
      // ever hit. if and when this particular case is encountered,
      // the next few cases are guaranteed to have a worst case
      // complexity of O(1)!  That's why this function performs very
      // well on the average. you can consider this function to be
      // having a complexity referred to commonly as: Amortized
      // Constant time.
      pointer 
      _M_allocate_single_object() throw(std::bad_alloc)
      {
#if defined __GTHREADS
        _Auto_Lock __bit_lock(&_S_mut);
#endif

        // The algorithm is something like this: The last_request
        // variable points to the last accessed Bit Map. When such a
        // condition occurs, we try to find a free block in the
        // current bitmap, or succeeding bitmaps until the last bitmap
        // is reached. If no free block turns up, we resort to First
        // Fit method.

        // WARNING: Do not re-order the condition in the while
        // statement below, because it relies on C++'s short-circuit
        // evaluation. The return from _S_last_request->_M_get() will
        // NOT be dereference able if _S_last_request->_M_finished()
        // returns true. This would inevitably lead to a NULL pointer
        // dereference if tinkered with.
        while (_S_last_request._M_finished() == false
               && (*(_S_last_request._M_get()) == 0))
          {
            _S_last_request.operator++();
          }

        if (__builtin_expect(_S_last_request._M_finished() == true, false))
          {
            // Fall Back to First Fit algorithm.
            typedef typename 
              __gnu_cxx::balloc::_Ffit_finder<_Alloc_block*> _FFF;
            _FFF __fff;
            typedef typename _BPVector::iterator _BPiter;
            _BPiter __bpi = 
              __gnu_cxx::balloc::__find_if
              (_S_mem_blocks.begin(), _S_mem_blocks.end(), 
               __gnu_cxx::balloc::_Functor_Ref<_FFF>(__fff));

            if (__bpi != _S_mem_blocks.end())
              {
                // Search was successful. Ok, now mark the first bit from
                // the right as 0, meaning Allocated. This bit is obtained
                // by calling _M_get() on __fff.
                size_t __nz_bit = _Bit_scan_forward(*__fff._M_get());
                balloc::__bit_allocate(__fff._M_get(), __nz_bit);

                _S_last_request._M_reset(__bpi - _S_mem_blocks.begin());

                // Now, get the address of the bit we marked as allocated.
                pointer __ret = reinterpret_cast<pointer>
                  (__bpi->first + __fff._M_offset() + __nz_bit);
                size_t* __puse_count = 
                  reinterpret_cast<size_t*>
                  (__bpi->first) 
                  - (__gnu_cxx::balloc::__num_bitmaps(*__bpi) + 1);
                
                ++(*__puse_count);
                return __ret;
              }
            else
              {
                // Search was unsuccessful. We Add more memory to the
                // pool by calling _S_refill_pool().
                _S_refill_pool();

                // _M_Reset the _S_last_request structure to the first
                // free block's bit map.
                _S_last_request._M_reset(_S_mem_blocks.size() - 1);

                // Now, mark that bit as allocated.
              }
          }

        // _S_last_request holds a pointer to a valid bit map, that
        // points to a free block in memory.
        size_t __nz_bit = _Bit_scan_forward(*_S_last_request._M_get());
        balloc::__bit_allocate(_S_last_request._M_get(), __nz_bit);

        pointer __ret = reinterpret_cast<pointer>
          (_S_last_request._M_base() + _S_last_request._M_offset() + __nz_bit);

        size_t* __puse_count = reinterpret_cast<size_t*>
          (_S_mem_blocks[_S_last_request._M_where()].first)
          - (__gnu_cxx::balloc::
             __num_bitmaps(_S_mem_blocks[_S_last_request._M_where()]) + 1);

        ++(*__puse_count);
        return __ret;
      }

      // Complexity: O(lg(N)), but the worst case is hit quite often!
      // I need to do something about this. I'll be able to work on
      // it, only when I have some solid figures from a few real apps.
      void 
      _M_deallocate_single_object(pointer __p) throw()
      {
#if defined __GTHREADS
        _Auto_Lock __bit_lock(&_S_mut);
#endif
        _Alloc_block* __real_p = reinterpret_cast<_Alloc_block*>(__p);

        typedef typename _BPVector::iterator _Iterator;
        typedef typename _BPVector::difference_type _Difference_type;

        _Difference_type __diff;
        long __displacement;

        _BALLOC_ASSERT(_S_last_dealloc_index >= 0);

        
        if (__gnu_cxx::balloc::_Inclusive_between<_Alloc_block*>
            (__real_p)
            (_S_mem_blocks[_S_last_dealloc_index]))
          {
            _BALLOC_ASSERT(_S_last_dealloc_index <= _S_mem_blocks.size() - 1);

            // Initial Assumption was correct!
            __diff = _S_last_dealloc_index;
            __displacement = __real_p - _S_mem_blocks[__diff].first;
          }
        else
          {
            _Iterator _iter = 
              __gnu_cxx::balloc::
              __find_if(_S_mem_blocks.begin(), 
                        _S_mem_blocks.end(), 
                        __gnu_cxx::balloc::
                        _Inclusive_between<_Alloc_block*>(__real_p));

            _BALLOC_ASSERT(_iter != _S_mem_blocks.end());

            __diff = _iter - _S_mem_blocks.begin();
            __displacement = __real_p - _S_mem_blocks[__diff].first;
            _S_last_dealloc_index = __diff;
          }

        // Get the position of the iterator that has been found.
        const size_t __rotate = __displacement % balloc::bits_per_block;
        size_t* __bitmapC = 
          reinterpret_cast<size_t*>
          (_S_mem_blocks[__diff].first) - 1;
        __bitmapC -= (__displacement / balloc::bits_per_block);
      
        balloc::__bit_free(__bitmapC, __rotate);
        size_t* __puse_count = reinterpret_cast<size_t*>
          (_S_mem_blocks[__diff].first)
          - (__gnu_cxx::balloc::__num_bitmaps(_S_mem_blocks[__diff]) + 1);
        
        _BALLOC_ASSERT(*__puse_count != 0);

        --(*__puse_count);

        if (__builtin_expect(*__puse_count == 0, false))
          {
            _S_block_size /= 2;
          
            // We can safely remove this block.
            // _Block_pair __bp = _S_mem_blocks[__diff];
            this->_M_insert(__puse_count);
            _S_mem_blocks.erase(_S_mem_blocks.begin() + __diff);

            // Reset the _S_last_request variable to reflect the
            // erased block. We do this to protect future requests
            // after the last block has been removed from a particular
            // memory Chunk, which in turn has been returned to the
            // free list, and hence had been erased from the vector,
            // so the size of the vector gets reduced by 1.
            if ((_Difference_type)_S_last_request._M_where() >= __diff--)
              _S_last_request._M_reset(__diff); 

            // If the Index into the vector of the region of memory
            // that might hold the next address that will be passed to
            // deallocated may have been invalidated due to the above
            // erase procedure being called on the vector, hence we
            // try to restore this invariant too.
            if (_S_last_dealloc_index >= _S_mem_blocks.size())
              {
                _S_last_dealloc_index =(__diff != -1 ? __diff : 0);
                _BALLOC_ASSERT(_S_last_dealloc_index >= 0);
              }
          }
      }

    public:
      bitmap_allocator() throw()
      { }

      bitmap_allocator(const bitmap_allocator&)
      { }

      template<typename _Tp1>
        bitmap_allocator(const bitmap_allocator<_Tp1>&) throw()
        { }

      ~bitmap_allocator() throw()
      { }

      // Complexity: O(1), but internally the complexity depends upon the
      // complexity of the function(s) _S_allocate_single_object and
      // operator new.
      pointer 
      allocate(size_type __n)
      {
        if (__builtin_expect(__n > this->max_size(), false))
          std::__throw_bad_alloc();

        if (__builtin_expect(__n == 1, true))
          return this->_M_allocate_single_object();
        else
          { 
            const size_type __b = __n * sizeof(value_type);
            return reinterpret_cast<pointer>(::operator new(__b));
          }
      }

      pointer 
      allocate(size_type __n, typename bitmap_allocator<void>::const_pointer)
      { return allocate(__n); }

      void 
      deallocate(pointer __p, size_type __n) throw()
      {
        if (__builtin_expect(__p != 0, true))
          {
            if (__builtin_expect(__n == 1, true))
              this->_M_deallocate_single_object(__p);
            else
              ::operator delete(__p);
          }
      }

      pointer 
      address(reference __r) const
      { return &__r; }

      const_pointer 
      address(const_reference __r) const
      { return &__r; }

      size_type 
      max_size() const throw()
      { return size_type(-1) / sizeof(value_type); }

      void 
      construct(pointer __p, const_reference __data)
      { ::new(__p) value_type(__data); }

      void 
      destroy(pointer __p)
      { __p->~value_type(); }
    };

  template<typename _Tp1, typename _Tp2>
    bool 
    operator==(const bitmap_allocator<_Tp1>&, 
               const bitmap_allocator<_Tp2>&) throw()
    { return true; }
  
  template<typename _Tp1, typename _Tp2>
    bool 
    operator!=(const bitmap_allocator<_Tp1>&, 
               const bitmap_allocator<_Tp2>&) throw() 
  { return false; }

  // Static member definitions.
  template<typename _Tp>
    typename bitmap_allocator<_Tp>::_BPVector
    bitmap_allocator<_Tp>::_S_mem_blocks;

  template<typename _Tp>
    size_t bitmap_allocator<_Tp>::_S_block_size = 
    2 * balloc::bits_per_block;

  template<typename _Tp>
    typename __gnu_cxx::bitmap_allocator<_Tp>::_BPVector::size_type 
    bitmap_allocator<_Tp>::_S_last_dealloc_index = 0;

  template<typename _Tp>
    __gnu_cxx::balloc::_Bitmap_counter 
  <typename bitmap_allocator<_Tp>::_Alloc_block*>
    bitmap_allocator<_Tp>::_S_last_request(_S_mem_blocks);

#if defined __GTHREADS
  template<typename _Tp>
    __gnu_cxx::_Mutex
    bitmap_allocator<_Tp>::_S_mut;
#endif


}

#endif 

//  LocalWords:  namespace GTHREADS bool const gthread endif Mutex mutex