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Edit File : mt_allocator.h \| Size : 22.96 KB
// MT-optimized allocator -- C++ -- // Copyright (C) 2003-2018 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 3, 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. // Under Section 7 of GPL version 3, you are granted additional // permissions described in the GCC Runtime Library Exception, version // 3.1, as published by the Free Software Foundation. // You should have received a copy of the GNU General Public License and // a copy of the GCC Runtime Library Exception along with this program; // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see // <http://www.gnu.org/licenses/>. /** @file ext/mt_allocator.h * This file is a GNU extension to the Standard C++ Library. / #ifndef _MT_ALLOCATOR_H #define _MT_ALLOCATOR_H 1 #include <new> #include <cstdlib> #include <bits/functexcept.h> #include <ext/atomicity.h> #include <bits/move.h> #if __cplusplus >= 201103L #include <type_traits> #endif namespace __gnu_cxx _GLIBCXX_VISIBILITY(default) { _GLIBCXX_BEGIN_NAMESPACE_VERSION using std::size_t; using std::ptrdiff_t; typedef void (__destroy_handler)(void); /// Base class for pool object. struct __pool_base { // Using short int as type for the binmap implies we are never // caching blocks larger than 32768 with this allocator. typedef unsigned short int _Binmap_type; // Variables used to configure the behavior of the allocator, // assigned and explained in detail below. struct _Tune { // Compile time constants for the default _Tune values. enum { _S_align = 8 }; enum { _S_max_bytes = 128 }; enum { _S_min_bin = 8 }; enum { _S_chunk_size = 4096 - 4 sizeof(void) }; enum { _S_max_threads = 4096 }; enum { _S_freelist_headroom = 10 }; // Alignment needed. // NB: In any case must be >= sizeof(_Block_record), that // is 4 on 32 bit machines and 8 on 64 bit machines. size_t _M_align; // Allocation requests (after round-up to power of 2) below // this value will be handled by the allocator. A raw new/ // call will be used for requests larger than this value. // NB: Must be much smaller than _M_chunk_size and in any // case <= 32768. size_t _M_max_bytes; // Size in bytes of the smallest bin. // NB: Must be a power of 2 and >= _M_align (and of course // much smaller than _M_max_bytes). size_t _M_min_bin; // In order to avoid fragmenting and minimize the number of // new() calls we always request new memory using this // value. Based on previous discussions on the libstdc++ // mailing list we have chosen the value below. // See http://gcc.gnu.org/ml/libstdc++/2001-07/msg00077.html // NB: At least one order of magnitude > _M_max_bytes. size_t _M_chunk_size; // The maximum number of supported threads. For // single-threaded operation, use one. Maximum values will // vary depending on details of the underlying system. (For // instance, Linux 2.4.18 reports 4070 in // /proc/sys/kernel/threads-max, while Linux 2.6.6 reports // 65534) size_t _M_max_threads; // Each time a deallocation occurs in a threaded application // we make sure that there are no more than // _M_freelist_headroom % of used memory on the freelist. If // the number of additional records is more than // _M_freelist_headroom % of the freelist, we move these // records back to the global pool. size_t _M_freelist_headroom; // Set to true forces all allocations to use new(). bool _M_force_new; explicit _Tune() : _M_align(_S_align), _M_max_bytes(_S_max_bytes), _M_min_bin(_S_min_bin), _M_chunk_size(_S_chunk_size), _M_max_threads(_S_max_threads), _M_freelist_headroom(_S_freelist_headroom), _M_force_new(std::getenv("GLIBCXX_FORCE_NEW") ? true : false) { } explicit _Tune(size_t __align, size_t __maxb, size_t __minbin, size_t __chunk, size_t __maxthreads, size_t __headroom, bool __force) : _M_align(__align), _M_max_bytes(__maxb), _M_min_bin(__minbin), _M_chunk_size(__chunk), _M_max_threads(__maxthreads), _M_freelist_headroom(__headroom), _M_force_new(__force) { } }; struct _Block_address { void _M_initial; _Block_address* _M_next; }; const _Tune& _M_get_options() const { return _M_options; } void _M_set_options(_Tune __t) { if (!_M_init) _M_options = __t; } bool _M_check_threshold(size_t __bytes) { return __bytes > _M_options._M_max_bytes \|\| _M_options._M_force_new; } size_t _M_get_binmap(size_t __bytes) { return _M_binmap[__bytes]; } size_t _M_get_align() { return _M_options._M_align; } explicit __pool_base() : _M_options(_Tune()), _M_binmap(0), _M_init(false) { } explicit __pool_base(const _Tune& __options) : _M_options(__options), _M_binmap(0), _M_init(false) { } private: explicit __pool_base(const __pool_base&); __pool_base& operator=(const __pool_base&); protected: // Configuration options. _Tune _M_options; _Binmap_type* _M_binmap; // Configuration of the pool object via _M_options can happen // after construction but before initialization. After // initialization is complete, this variable is set to true. bool _M_init; }; /** * @brief Data describing the underlying memory pool, parameterized on * threading support. / template<bool _Thread> class __pool; /// Specialization for single thread. template<> class __pool<false> : public __pool_base { public: union _Block_record { // Points to the block_record of the next free block. _Block_record _M_next; }; struct _Bin_record { // An "array" of pointers to the first free block. _Block_record** _M_first; // A list of the initial addresses of all allocated blocks. _Block_address* _M_address; }; void _M_initialize_once() { if (__builtin_expect(_M_init == false, false)) _M_initialize(); } void _M_destroy() throw(); char* _M_reserve_block(size_t __bytes, const size_t __thread_id); void _M_reclaim_block(char* __p, size_t __bytes) throw (); size_t _M_get_thread_id() { return 0; } const _Bin_record& _M_get_bin(size_t __which) { return _M_bin[__which]; } void _M_adjust_freelist(const _Bin_record&, _Block_record, size_t) { } explicit __pool() : _M_bin(0), _M_bin_size(1) { } explicit __pool(const __pool_base::_Tune& __tune) : __pool_base(__tune), _M_bin(0), _M_bin_size(1) { } private: // An "array" of bin_records each of which represents a specific // power of 2 size. Memory to this "array" is allocated in // _M_initialize(). _Bin_record _M_bin; // Actual value calculated in _M_initialize(). size_t _M_bin_size; void _M_initialize(); }; #ifdef __GTHREADS /// Specialization for thread enabled, via gthreads.h. template<> class __pool<true> : public __pool_base { public: // Each requesting thread is assigned an id ranging from 1 to // _S_max_threads. Thread id 0 is used as a global memory pool. // In order to get constant performance on the thread assignment // routine, we keep a list of free ids. When a thread first // requests memory we remove the first record in this list and // stores the address in a __gthread_key. When initializing the // __gthread_key we specify a destructor. When this destructor // (i.e. the thread dies) is called, we return the thread id to // the front of this list. struct _Thread_record { // Points to next free thread id record. NULL if last record in list. _Thread_record* _M_next; // Thread id ranging from 1 to _S_max_threads. size_t _M_id; }; union _Block_record { // Points to the block_record of the next free block. _Block_record* _M_next; // The thread id of the thread which has requested this block. size_t _M_thread_id; }; struct _Bin_record { // An "array" of pointers to the first free block for each // thread id. Memory to this "array" is allocated in // _S_initialize() for _S_max_threads + global pool 0. _Block_record** _M_first; // A list of the initial addresses of all allocated blocks. _Block_address* _M_address; // An "array" of counters used to keep track of the amount of // blocks that are on the freelist/used for each thread id. // - Note that the second part of the allocated _M_used "array" // actually hosts (atomic) counters of reclaimed blocks: in // _M_reserve_block and in _M_reclaim_block those numbers are // subtracted from the first ones to obtain the actual size // of the "working set" of the given thread. // - Memory to these "arrays" is allocated in _S_initialize() // for _S_max_threads + global pool 0. size_t* _M_free; size_t* _M_used; // Each bin has its own mutex which is used to ensure data // integrity while changing "ownership" on a block. The mutex // is initialized in _S_initialize(). __gthread_mutex_t* _M_mutex; }; // XXX GLIBCXX_ABI Deprecated void _M_initialize(__destroy_handler); void _M_initialize_once() { if (__builtin_expect(_M_init == false, false)) _M_initialize(); } void _M_destroy() throw(); char* _M_reserve_block(size_t __bytes, const size_t __thread_id); void _M_reclaim_block(char* __p, size_t __bytes) throw (); const _Bin_record& _M_get_bin(size_t __which) { return _M_bin[__which]; } void _M_adjust_freelist(const _Bin_record& __bin, _Block_record* __block, size_t __thread_id) { if (__gthread_active_p()) { __block->_M_thread_id = __thread_id; --__bin._M_free[__thread_id]; ++__bin._M_used[__thread_id]; } } // XXX GLIBCXX_ABI Deprecated void _M_destroy_thread_key(void) throw (); size_t _M_get_thread_id(); explicit __pool() : _M_bin(0), _M_bin_size(1), _M_thread_freelist(0) { } explicit __pool(const __pool_base::_Tune& __tune) : __pool_base(__tune), _M_bin(0), _M_bin_size(1), _M_thread_freelist(0) { } private: // An "array" of bin_records each of which represents a specific // power of 2 size. Memory to this "array" is allocated in // _M_initialize(). _Bin_record _M_bin; // Actual value calculated in _M_initialize(). size_t _M_bin_size; _Thread_record* _M_thread_freelist; void* _M_thread_freelist_initial; void _M_initialize(); }; #endif template<template <bool> class _PoolTp, bool _Thread> struct __common_pool { typedef _PoolTp<_Thread> pool_type; static pool_type& _S_get_pool() { static pool_type _S_pool; return _S_pool; } }; template<template <bool> class _PoolTp, bool _Thread> struct __common_pool_base; template<template <bool> class _PoolTp> struct __common_pool_base<_PoolTp, false> : public __common_pool<_PoolTp, false> { using __common_pool<_PoolTp, false>::_S_get_pool; static void _S_initialize_once() { static bool __init; if (__builtin_expect(__init == false, false)) { _S_get_pool()._M_initialize_once(); __init = true; } } }; #ifdef __GTHREADS template<template <bool> class _PoolTp> struct __common_pool_base<_PoolTp, true> : public __common_pool<_PoolTp, true> { using __common_pool<_PoolTp, true>::_S_get_pool; static void _S_initialize() { _S_get_pool()._M_initialize_once(); } static void _S_initialize_once() { static bool __init; if (__builtin_expect(__init == false, false)) { if (__gthread_active_p()) { // On some platforms, __gthread_once_t is an aggregate. static __gthread_once_t __once = __GTHREAD_ONCE_INIT; __gthread_once(&__once, _S_initialize); } // Double check initialization. May be necessary on some // systems for proper construction when not compiling with // thread flags. _S_get_pool()._M_initialize_once(); __init = true; } } }; #endif /// Policy for shared __pool objects. template<template <bool> class _PoolTp, bool _Thread> struct __common_pool_policy : public __common_pool_base<_PoolTp, _Thread> { template<typename _Tp1, template <bool> class _PoolTp1 = _PoolTp, bool _Thread1 = _Thread> struct _M_rebind { typedef __common_pool_policy<_PoolTp1, _Thread1> other; }; using __common_pool_base<_PoolTp, _Thread>::_S_get_pool; using __common_pool_base<_PoolTp, _Thread>::_S_initialize_once; }; template<typename _Tp, template <bool> class _PoolTp, bool _Thread> struct __per_type_pool { typedef _Tp value_type; typedef _PoolTp<_Thread> pool_type; static pool_type& _S_get_pool() { // Sane defaults for the _PoolTp. typedef typename pool_type::_Block_record _Block_record; const static size_t __a = (__alignof__(_Tp) >= sizeof(_Block_record) ? __alignof__(_Tp) : sizeof(_Block_record)); typedef typename __pool_base::_Tune _Tune; static _Tune _S_tune(__a, sizeof(_Tp) * 64, sizeof(_Tp) * 2 >= __a ? sizeof(_Tp) * 2 : __a, sizeof(_Tp) * size_t(_Tune::_S_chunk_size), _Tune::_S_max_threads, _Tune::_S_freelist_headroom, std::getenv("GLIBCXX_FORCE_NEW") ? true : false); static pool_type _S_pool(_S_tune); return _S_pool; } }; template<typename _Tp, template <bool> class _PoolTp, bool _Thread> struct __per_type_pool_base; template<typename _Tp, template <bool> class _PoolTp> struct __per_type_pool_base<_Tp, _PoolTp, false> : public __per_type_pool<_Tp, _PoolTp, false> { using __per_type_pool<_Tp, _PoolTp, false>::_S_get_pool; static void _S_initialize_once() { static bool __init; if (__builtin_expect(__init == false, false)) { _S_get_pool()._M_initialize_once(); __init = true; } } }; #ifdef __GTHREADS template<typename _Tp, template <bool> class _PoolTp> struct __per_type_pool_base<_Tp, _PoolTp, true> : public __per_type_pool<_Tp, _PoolTp, true> { using __per_type_pool<_Tp, _PoolTp, true>::_S_get_pool; static void _S_initialize() { _S_get_pool()._M_initialize_once(); } static void _S_initialize_once() { static bool __init; if (__builtin_expect(__init == false, false)) { if (__gthread_active_p()) { // On some platforms, __gthread_once_t is an aggregate. static __gthread_once_t __once = __GTHREAD_ONCE_INIT; __gthread_once(&__once, _S_initialize); } // Double check initialization. May be necessary on some // systems for proper construction when not compiling with // thread flags. _S_get_pool()._M_initialize_once(); __init = true; } } }; #endif /// Policy for individual __pool objects. template<typename _Tp, template <bool> class _PoolTp, bool _Thread> struct __per_type_pool_policy : public __per_type_pool_base<_Tp, _PoolTp, _Thread> { template<typename _Tp1, template <bool> class _PoolTp1 = _PoolTp, bool _Thread1 = _Thread> struct _M_rebind { typedef __per_type_pool_policy<_Tp1, _PoolTp1, _Thread1> other; }; using __per_type_pool_base<_Tp, _PoolTp, _Thread>::_S_get_pool; using __per_type_pool_base<_Tp, _PoolTp, _Thread>::_S_initialize_once; }; /// Base class for _Tp dependent member functions. template<typename _Tp> class __mt_alloc_base { public: typedef size_t size_type; typedef ptrdiff_t difference_type; typedef _Tp* pointer; typedef const _Tp* const_pointer; typedef _Tp& reference; typedef const _Tp& const_reference; typedef _Tp value_type; #if __cplusplus >= 201103L // _GLIBCXX_RESOLVE_LIB_DEFECTS // 2103. propagate_on_container_move_assignment typedef std::true_type propagate_on_container_move_assignment; #endif pointer address(reference __x) const _GLIBCXX_NOEXCEPT { return std::__addressof(__x); } const_pointer address(const_reference __x) const _GLIBCXX_NOEXCEPT { return std::__addressof(__x); } size_type max_size() const _GLIBCXX_USE_NOEXCEPT { return size_t(-1) / sizeof(_Tp); } #if __cplusplus >= 201103L template<typename _Up, typename... _Args> void construct(_Up* __p, _Args&&... __args) { ::new((void )__p) _Up(std::forward<_Args>(__args)...); } template<typename _Up> void destroy(_Up __p) { __p->~_Up(); } #else // _GLIBCXX_RESOLVE_LIB_DEFECTS // 402. wrong new expression in [some_] allocator::construct void construct(pointer __p, const _Tp& __val) { ::new((void )__p) _Tp(__val); } void destroy(pointer __p) { __p->~_Tp(); } #endif }; #ifdef __GTHREADS #define __thread_default true #else #define __thread_default false #endif /* * @brief This is a fixed size (power of 2) allocator which - when * compiled with thread support - will maintain one freelist per * size per thread plus a @a global one. Steps are taken to limit * the per thread freelist sizes (by returning excess back to * the @a global list). * @ingroup allocators * * Further details: * https://gcc.gnu.org/onlinedocs/libstdc++/manual/mt_allocator.html / template<typename _Tp, typename _Poolp = __common_pool_policy<__pool, __thread_default> > class __mt_alloc : public __mt_alloc_base<_Tp> { public: typedef size_t size_type; typedef ptrdiff_t difference_type; typedef _Tp pointer; typedef const _Tp* const_pointer; typedef _Tp& reference; typedef const _Tp& const_reference; typedef _Tp value_type; typedef _Poolp __policy_type; typedef typename _Poolp::pool_type __pool_type; template<typename _Tp1, typename _Poolp1 = _Poolp> struct rebind { typedef typename _Poolp1::template _M_rebind<_Tp1>::other pol_type; typedef __mt_alloc<_Tp1, pol_type> other; }; __mt_alloc() _GLIBCXX_USE_NOEXCEPT { } __mt_alloc(const __mt_alloc&) _GLIBCXX_USE_NOEXCEPT { } template<typename _Tp1, typename _Poolp1> __mt_alloc(const __mt_alloc<_Tp1, _Poolp1>&) _GLIBCXX_USE_NOEXCEPT { } ~__mt_alloc() _GLIBCXX_USE_NOEXCEPT { } pointer allocate(size_type __n, const void* = 0); void deallocate(pointer __p, size_type __n); const __pool_base::_Tune _M_get_options() { // Return a copy, not a reference, for external consumption. return __policy_type::_S_get_pool()._M_get_options(); } void _M_set_options(__pool_base::_Tune __t) { __policy_type::_S_get_pool()._M_set_options(__t); } }; template<typename _Tp, typename _Poolp> typename __mt_alloc<_Tp, _Poolp>::pointer __mt_alloc<_Tp, _Poolp>:: allocate(size_type __n, const void) { if (__n > this->max_size()) std::__throw_bad_alloc(); #if __cpp_aligned_new // Types with extended alignment are handled by operator new/delete. if (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__) { std::align_val_t __al = std::align_val_t(alignof(_Tp)); return static_cast<_Tp>(::operator new(__n * sizeof(_Tp), __al)); } #endif __policy_type::_S_initialize_once(); // Requests larger than _M_max_bytes are handled by operator // new/delete directly. __pool_type& __pool = __policy_type::_S_get_pool(); const size_t __bytes = __n * sizeof(_Tp); if (__pool._M_check_threshold(__bytes)) { void* __ret = ::operator new(__bytes); return static_cast<_Tp>(__ret); } // Round up to power of 2 and figure out which bin to use. const size_t __which = __pool._M_get_binmap(__bytes); const size_t __thread_id = __pool._M_get_thread_id(); // Find out if we have blocks on our freelist. If so, go ahead // and use them directly without having to lock anything. char __c; typedef typename __pool_type::_Bin_record _Bin_record; const _Bin_record& __bin = __pool._M_get_bin(__which); if (__bin._M_first[__thread_id]) { // Already reserved. typedef typename __pool_type::_Block_record _Block_record; _Block_record* __block = __bin._M_first[__thread_id]; __bin._M_first[__thread_id] = __block->_M_next; __pool._M_adjust_freelist(__bin, __block, __thread_id); __c = reinterpret_cast<char>(__block) + __pool._M_get_align(); } else { // Null, reserve. __c = __pool._M_reserve_block(__bytes, __thread_id); } return static_cast<_Tp>(static_cast<void>(__c)); } template<typename _Tp, typename _Poolp> void __mt_alloc<_Tp, _Poolp>:: deallocate(pointer __p, size_type __n) { if (__builtin_expect(__p != 0, true)) { #if __cpp_aligned_new // Types with extended alignment are handled by operator new/delete. if (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__) { ::operator delete(__p, std::align_val_t(alignof(_Tp))); return; } #endif // Requests larger than _M_max_bytes are handled by // operators new/delete directly. __pool_type& __pool = __policy_type::_S_get_pool(); const size_t __bytes = __n sizeof(_Tp); if (__pool._M_check_threshold(__bytes)) ::operator delete(__p); else __pool._M_reclaim_block(reinterpret_cast<char*>(__p), __bytes); } } template<typename _Tp, typename _Poolp> inline bool operator==(const __mt_alloc<_Tp, _Poolp>&, const __mt_alloc<_Tp, _Poolp>&) { return true; } template<typename _Tp, typename _Poolp> inline bool operator!=(const __mt_alloc<_Tp, _Poolp>&, const __mt_alloc<_Tp, _Poolp>&) { return false; } #undef __thread_default _GLIBCXX_END_NAMESPACE_VERSION } // namespace #endif Back

Edit File : mt_allocator.h | Size : 22.96 KB

// MT-optimized allocator -*- C++ -*-

// Copyright (C) 2003-2018 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 3, 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.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

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

#ifndef _MT_ALLOCATOR_H
#define _MT_ALLOCATOR_H 1

#include <new>
#include <cstdlib>
#include <bits/functexcept.h>
#include <ext/atomicity.h>
#include <bits/move.h>
#if __cplusplus >= 201103L
#include <type_traits>
#endif

namespace __gnu_cxx _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

using std::size_t;
  using std::ptrdiff_t;

typedef void (*__destroy_handler)(void*);

/// Base class for pool object.
  struct __pool_base
  {
    // Using short int as type for the binmap implies we are never
    // caching blocks larger than 32768 with this allocator.
    typedef unsigned short int _Binmap_type;

// Variables used to configure the behavior of the allocator,
    // assigned and explained in detail below.
    struct _Tune
     {
      // Compile time constants for the default _Tune values.
      enum { _S_align = 8 };
      enum { _S_max_bytes = 128 };
      enum { _S_min_bin = 8 };
      enum { _S_chunk_size = 4096 - 4 * sizeof(void*) };
      enum { _S_max_threads = 4096 };
      enum { _S_freelist_headroom = 10 };

// Alignment needed.
      // NB: In any case must be >= sizeof(_Block_record), that
      // is 4 on 32 bit machines and 8 on 64 bit machines.
      size_t	_M_align;
      
      // Allocation requests (after round-up to power of 2) below
      // this value will be handled by the allocator. A raw new/
      // call will be used for requests larger than this value.
      // NB: Must be much smaller than _M_chunk_size and in any
      // case <= 32768.
      size_t	_M_max_bytes;

// Size in bytes of the smallest bin.
      // NB: Must be a power of 2 and >= _M_align (and of course
      // much smaller than _M_max_bytes).
      size_t	_M_min_bin;

// In order to avoid fragmenting and minimize the number of
      // new() calls we always request new memory using this
      // value. Based on previous discussions on the libstdc++
      // mailing list we have chosen the value below.
      // See http://gcc.gnu.org/ml/libstdc++/2001-07/msg00077.html
      // NB: At least one order of magnitude > _M_max_bytes. 
      size_t	_M_chunk_size;

// The maximum number of supported threads. For
      // single-threaded operation, use one. Maximum values will
      // vary depending on details of the underlying system. (For
      // instance, Linux 2.4.18 reports 4070 in
      // /proc/sys/kernel/threads-max, while Linux 2.6.6 reports
      // 65534)
      size_t 	_M_max_threads;

// Each time a deallocation occurs in a threaded application
      // we make sure that there are no more than
      // _M_freelist_headroom % of used memory on the freelist. If
      // the number of additional records is more than
      // _M_freelist_headroom % of the freelist, we move these
      // records back to the global pool.
      size_t 	_M_freelist_headroom;
      
      // Set to true forces all allocations to use new().
      bool 	_M_force_new; 
      
      explicit
      _Tune()
      : _M_align(_S_align), _M_max_bytes(_S_max_bytes), _M_min_bin(_S_min_bin),
      _M_chunk_size(_S_chunk_size), _M_max_threads(_S_max_threads), 
      _M_freelist_headroom(_S_freelist_headroom), 
      _M_force_new(std::getenv("GLIBCXX_FORCE_NEW") ? true : false)
      { }

explicit
      _Tune(size_t __align, size_t __maxb, size_t __minbin, size_t __chunk, 
	    size_t __maxthreads, size_t __headroom, bool __force) 
      : _M_align(__align), _M_max_bytes(__maxb), _M_min_bin(__minbin),
      _M_chunk_size(__chunk), _M_max_threads(__maxthreads),
      _M_freelist_headroom(__headroom), _M_force_new(__force)
      { }
    };
    
    struct _Block_address
    {
      void* 			_M_initial;
      _Block_address* 		_M_next;
    };
    
    const _Tune&
    _M_get_options() const
    { return _M_options; }

void
    _M_set_options(_Tune __t)
    { 
      if (!_M_init)
	_M_options = __t;
    }

bool
    _M_check_threshold(size_t __bytes)
    { return __bytes > _M_options._M_max_bytes || _M_options._M_force_new; }

size_t
    _M_get_binmap(size_t __bytes)
    { return _M_binmap[__bytes]; }

size_t
    _M_get_align()
    { return _M_options._M_align; }

explicit 
    __pool_base() 
    : _M_options(_Tune()), _M_binmap(0), _M_init(false) { }

explicit 
    __pool_base(const _Tune& __options)
    : _M_options(__options), _M_binmap(0), _M_init(false) { }

private:
    explicit 
    __pool_base(const __pool_base&);

__pool_base&
    operator=(const __pool_base&);

protected:
    // Configuration options.
    _Tune 	       		_M_options;
    
    _Binmap_type* 		_M_binmap;

// Configuration of the pool object via _M_options can happen
    // after construction but before initialization. After
    // initialization is complete, this variable is set to true.
    bool 			_M_init;
  };

/**
   *  @brief  Data describing the underlying memory pool, parameterized on
   *  threading support.
   */
  template<bool _Thread>
    class __pool;

/// Specialization for single thread.
  template<>
    class __pool<false> : public __pool_base
    {
    public:
      union _Block_record
      {
	// Points to the block_record of the next free block.
	_Block_record* 			_M_next;
      };

struct _Bin_record
      {
	// An "array" of pointers to the first free block.
	_Block_record**			_M_first;

// A list of the initial addresses of all allocated blocks.
	_Block_address*		     	_M_address;
      };
      
      void
      _M_initialize_once()
      {
	if (__builtin_expect(_M_init == false, false))
	  _M_initialize();
      }

void
      _M_destroy() throw();

char* 
      _M_reserve_block(size_t __bytes, const size_t __thread_id);
    
      void
      _M_reclaim_block(char* __p, size_t __bytes) throw ();
    
      size_t 
      _M_get_thread_id() { return 0; }
      
      const _Bin_record&
      _M_get_bin(size_t __which)
      { return _M_bin[__which]; }
      
      void
      _M_adjust_freelist(const _Bin_record&, _Block_record*, size_t)
      { }

explicit __pool() 
      : _M_bin(0), _M_bin_size(1) { }

explicit __pool(const __pool_base::_Tune& __tune) 
      : __pool_base(__tune), _M_bin(0), _M_bin_size(1) { }

private:
      // An "array" of bin_records each of which represents a specific
      // power of 2 size. Memory to this "array" is allocated in
      // _M_initialize().
      _Bin_record*		 _M_bin;
      
      // Actual value calculated in _M_initialize().
      size_t 	       	     	_M_bin_size;

void
      _M_initialize();
  };
 
#ifdef __GTHREADS
  /// Specialization for thread enabled, via gthreads.h.
  template<>
    class __pool<true> : public __pool_base
    {
    public:
      // Each requesting thread is assigned an id ranging from 1 to
      // _S_max_threads. Thread id 0 is used as a global memory pool.
      // In order to get constant performance on the thread assignment
      // routine, we keep a list of free ids. When a thread first
      // requests memory we remove the first record in this list and
      // stores the address in a __gthread_key. When initializing the
      // __gthread_key we specify a destructor. When this destructor
      // (i.e. the thread dies) is called, we return the thread id to
      // the front of this list.
      struct _Thread_record
      {
	// Points to next free thread id record. NULL if last record in list.
	_Thread_record*			_M_next;
	
	// Thread id ranging from 1 to _S_max_threads.
	size_t                          _M_id;
      };
      
      union _Block_record
      {
	// Points to the block_record of the next free block.
	_Block_record*			_M_next;
	
	// The thread id of the thread which has requested this block.
	size_t                          _M_thread_id;
      };
      
      struct _Bin_record
      {
	// An "array" of pointers to the first free block for each
	// thread id. Memory to this "array" is allocated in
	// _S_initialize() for _S_max_threads + global pool 0.
	_Block_record**			_M_first;
	
	// A list of the initial addresses of all allocated blocks.
	_Block_address*		     	_M_address;

// An "array" of counters used to keep track of the amount of
	// blocks that are on the freelist/used for each thread id.
	// - Note that the second part of the allocated _M_used "array"
	//   actually hosts (atomic) counters of reclaimed blocks:  in
	//   _M_reserve_block and in _M_reclaim_block those numbers are
	//   subtracted from the first ones to obtain the actual size
	//   of the "working set" of the given thread.
	// - Memory to these "arrays" is allocated in _S_initialize()
	//   for _S_max_threads + global pool 0.
	size_t*				_M_free;
	size_t*			        _M_used;
	
	// Each bin has its own mutex which is used to ensure data
	// integrity while changing "ownership" on a block.  The mutex
	// is initialized in _S_initialize().
	__gthread_mutex_t*              _M_mutex;
      };
      
      // XXX GLIBCXX_ABI Deprecated
      void
      _M_initialize(__destroy_handler);

void
      _M_initialize_once()
      {
	if (__builtin_expect(_M_init == false, false))
	  _M_initialize();
      }

void
      _M_destroy() throw();

char* 
      _M_reserve_block(size_t __bytes, const size_t __thread_id);
    
      void
      _M_reclaim_block(char* __p, size_t __bytes) throw ();
    
      const _Bin_record&
      _M_get_bin(size_t __which)
      { return _M_bin[__which]; }
      
      void
      _M_adjust_freelist(const _Bin_record& __bin, _Block_record* __block, 
			 size_t __thread_id)
      {
	if (__gthread_active_p())
	  {
	    __block->_M_thread_id = __thread_id;
	    --__bin._M_free[__thread_id];
	    ++__bin._M_used[__thread_id];
	  }
      }

// XXX GLIBCXX_ABI Deprecated
      void
      _M_destroy_thread_key(void*) throw ();

size_t 
      _M_get_thread_id();

explicit __pool() 
      : _M_bin(0), _M_bin_size(1), _M_thread_freelist(0) 
      { }

explicit __pool(const __pool_base::_Tune& __tune) 
      : __pool_base(__tune), _M_bin(0), _M_bin_size(1), 
	_M_thread_freelist(0) 
      { }

private:
      // An "array" of bin_records each of which represents a specific
      // power of 2 size. Memory to this "array" is allocated in
      // _M_initialize().
      _Bin_record*		_M_bin;

// Actual value calculated in _M_initialize().
      size_t 	       	     	_M_bin_size;

_Thread_record* 		_M_thread_freelist;
      void*			_M_thread_freelist_initial;

void
      _M_initialize();
    };
#endif

template<template <bool> class _PoolTp, bool _Thread>
    struct __common_pool
    {
      typedef _PoolTp<_Thread> 		pool_type;
      
      static pool_type&
      _S_get_pool()
      { 
	static pool_type _S_pool;
	return _S_pool;
      }
    };

template<template <bool> class _PoolTp, bool _Thread>
    struct __common_pool_base;

template<template <bool> class _PoolTp>
    struct __common_pool_base<_PoolTp, false> 
    : public __common_pool<_PoolTp, false>
    {
      using  __common_pool<_PoolTp, false>::_S_get_pool;

static void
      _S_initialize_once()
      {
	static bool __init;
	if (__builtin_expect(__init == false, false))
	  {
	    _S_get_pool()._M_initialize_once(); 
	    __init = true;
	  }
      }
    };

#ifdef __GTHREADS
  template<template <bool> class _PoolTp>
    struct __common_pool_base<_PoolTp, true>
    : public __common_pool<_PoolTp, true>
    {
      using  __common_pool<_PoolTp, true>::_S_get_pool;
      
      static void
      _S_initialize() 
      { _S_get_pool()._M_initialize_once(); }

static void
      _S_initialize_once()
      { 
	static bool __init;
	if (__builtin_expect(__init == false, false))
	  {
	    if (__gthread_active_p())
	      {
		// On some platforms, __gthread_once_t is an aggregate.
		static __gthread_once_t __once = __GTHREAD_ONCE_INIT;
		__gthread_once(&__once, _S_initialize);
	      }

// Double check initialization. May be necessary on some
	    // systems for proper construction when not compiling with
	    // thread flags.
	    _S_get_pool()._M_initialize_once(); 
	    __init = true;
	  }
      }
    };
#endif

/// Policy for shared __pool objects.
  template<template <bool> class _PoolTp, bool _Thread>
    struct __common_pool_policy : public __common_pool_base<_PoolTp, _Thread>
    {
      template<typename _Tp1, template <bool> class _PoolTp1 = _PoolTp, 
	       bool _Thread1 = _Thread>
        struct _M_rebind
        { typedef __common_pool_policy<_PoolTp1, _Thread1> other; };

using  __common_pool_base<_PoolTp, _Thread>::_S_get_pool;
      using  __common_pool_base<_PoolTp, _Thread>::_S_initialize_once;
  };

template<typename _Tp, template <bool> class _PoolTp, bool _Thread>
    struct __per_type_pool
    {
      typedef _Tp 			value_type;
      typedef _PoolTp<_Thread> 		pool_type;
      
      static pool_type&
      _S_get_pool()
      { 
	// Sane defaults for the _PoolTp.
	typedef typename pool_type::_Block_record _Block_record;
	const static size_t __a = (__alignof__(_Tp) >= sizeof(_Block_record)
				   ? __alignof__(_Tp) : sizeof(_Block_record));

typedef typename __pool_base::_Tune _Tune;
	static _Tune _S_tune(__a, sizeof(_Tp) * 64,
			     sizeof(_Tp) * 2 >= __a ? sizeof(_Tp) * 2 : __a,
			     sizeof(_Tp) * size_t(_Tune::_S_chunk_size),
			     _Tune::_S_max_threads,
			     _Tune::_S_freelist_headroom,
			     std::getenv("GLIBCXX_FORCE_NEW") ? true : false);
	static pool_type _S_pool(_S_tune);
	return _S_pool;
      }
    };

template<typename _Tp, template <bool> class _PoolTp, bool _Thread>
    struct __per_type_pool_base;

template<typename _Tp, template <bool> class _PoolTp>
    struct __per_type_pool_base<_Tp, _PoolTp, false> 
    : public __per_type_pool<_Tp, _PoolTp, false> 
    {
      using  __per_type_pool<_Tp, _PoolTp, false>::_S_get_pool;

#ifdef __GTHREADS
 template<typename _Tp, template <bool> class _PoolTp>
    struct __per_type_pool_base<_Tp, _PoolTp, true> 
    : public __per_type_pool<_Tp, _PoolTp, true> 
    {
      using  __per_type_pool<_Tp, _PoolTp, true>::_S_get_pool;

static void
      _S_initialize() 
      { _S_get_pool()._M_initialize_once(); }

/// Policy for individual __pool objects.
  template<typename _Tp, template <bool> class _PoolTp, bool _Thread>
    struct __per_type_pool_policy 
    : public __per_type_pool_base<_Tp, _PoolTp, _Thread>
    {
      template<typename _Tp1, template <bool> class _PoolTp1 = _PoolTp, 
	       bool _Thread1 = _Thread>
        struct _M_rebind
        { typedef __per_type_pool_policy<_Tp1, _PoolTp1, _Thread1> other; };

using  __per_type_pool_base<_Tp, _PoolTp, _Thread>::_S_get_pool;
      using  __per_type_pool_base<_Tp, _PoolTp, _Thread>::_S_initialize_once;
  };

/// Base class for _Tp dependent member functions.
  template<typename _Tp>
    class __mt_alloc_base 
    {
    public:
      typedef size_t                    size_type;
      typedef ptrdiff_t                 difference_type;
      typedef _Tp*                      pointer;
      typedef const _Tp*                const_pointer;
      typedef _Tp&                      reference;
      typedef const _Tp&                const_reference;
      typedef _Tp                       value_type;

#if __cplusplus >= 201103L
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 2103. propagate_on_container_move_assignment
      typedef std::true_type propagate_on_container_move_assignment;
#endif

pointer
      address(reference __x) const _GLIBCXX_NOEXCEPT
      { return std::__addressof(__x); }

const_pointer
      address(const_reference __x) const _GLIBCXX_NOEXCEPT
      { return std::__addressof(__x); }

size_type
      max_size() const _GLIBCXX_USE_NOEXCEPT 
      { return size_t(-1) / sizeof(_Tp); }

#if __cplusplus >= 201103L
      template<typename _Up, typename... _Args>
        void
        construct(_Up* __p, _Args&&... __args)
	{ ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }

template<typename _Up>
        void 
        destroy(_Up* __p) { __p->~_Up(); }
#else
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 402. wrong new expression in [some_] allocator::construct
      void 
      construct(pointer __p, const _Tp& __val) 
      { ::new((void *)__p) _Tp(__val); }

void 
      destroy(pointer __p) { __p->~_Tp(); }
#endif
    };

#ifdef __GTHREADS
#define __thread_default true
#else
#define __thread_default false
#endif

/**
   *  @brief  This is a fixed size (power of 2) allocator which - when
   *  compiled with thread support - will maintain one freelist per
   *  size per thread plus a @a global one. Steps are taken to limit
   *  the per thread freelist sizes (by returning excess back to
   *  the @a global list).
   *  @ingroup allocators
   *
   *  Further details:
   *  https://gcc.gnu.org/onlinedocs/libstdc++/manual/mt_allocator.html
   */
  template<typename _Tp, 
	   typename _Poolp = __common_pool_policy<__pool, __thread_default> >
    class __mt_alloc : public __mt_alloc_base<_Tp>
    {
    public:
      typedef size_t                    	size_type;
      typedef ptrdiff_t                 	difference_type;
      typedef _Tp*                      	pointer;
      typedef const _Tp*                	const_pointer;
      typedef _Tp&                      	reference;
      typedef const _Tp&                	const_reference;
      typedef _Tp                       	value_type;
      typedef _Poolp      			__policy_type;
      typedef typename _Poolp::pool_type	__pool_type;

template<typename _Tp1, typename _Poolp1 = _Poolp>
        struct rebind
        { 
	  typedef typename _Poolp1::template _M_rebind<_Tp1>::other pol_type;
	  typedef __mt_alloc<_Tp1, pol_type> other;
	};

__mt_alloc() _GLIBCXX_USE_NOEXCEPT { }

__mt_alloc(const __mt_alloc&) _GLIBCXX_USE_NOEXCEPT { }

template<typename _Tp1, typename _Poolp1>
        __mt_alloc(const __mt_alloc<_Tp1, _Poolp1>&) _GLIBCXX_USE_NOEXCEPT { }

~__mt_alloc() _GLIBCXX_USE_NOEXCEPT { }

pointer
      allocate(size_type __n, const void* = 0);

void
      deallocate(pointer __p, size_type __n);

const __pool_base::_Tune
      _M_get_options()
      { 
	// Return a copy, not a reference, for external consumption.
	return __policy_type::_S_get_pool()._M_get_options();
      }
      
      void
      _M_set_options(__pool_base::_Tune __t)
      { __policy_type::_S_get_pool()._M_set_options(__t); }
    };

template<typename _Tp, typename _Poolp>
    typename __mt_alloc<_Tp, _Poolp>::pointer
    __mt_alloc<_Tp, _Poolp>::
    allocate(size_type __n, const void*)
    {
      if (__n > this->max_size())
	std::__throw_bad_alloc();

#if __cpp_aligned_new
      // Types with extended alignment are handled by operator new/delete.
      if (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
	{
	  std::align_val_t __al = std::align_val_t(alignof(_Tp));
	  return static_cast<_Tp*>(::operator new(__n * sizeof(_Tp), __al));
	}
#endif

__policy_type::_S_initialize_once();

// Requests larger than _M_max_bytes are handled by operator
      // new/delete directly.
      __pool_type& __pool = __policy_type::_S_get_pool();
      const size_t __bytes = __n * sizeof(_Tp);
      if (__pool._M_check_threshold(__bytes))
	{
	  void* __ret = ::operator new(__bytes);
	  return static_cast<_Tp*>(__ret);
	}
      
      // Round up to power of 2 and figure out which bin to use.
      const size_t __which = __pool._M_get_binmap(__bytes);
      const size_t __thread_id = __pool._M_get_thread_id();
      
      // Find out if we have blocks on our freelist.  If so, go ahead
      // and use them directly without having to lock anything.
      char* __c;
      typedef typename __pool_type::_Bin_record _Bin_record;
      const _Bin_record& __bin = __pool._M_get_bin(__which);
      if (__bin._M_first[__thread_id])
	{
	  // Already reserved.
	  typedef typename __pool_type::_Block_record _Block_record;
	  _Block_record* __block = __bin._M_first[__thread_id];
	  __bin._M_first[__thread_id] = __block->_M_next;
	  
	  __pool._M_adjust_freelist(__bin, __block, __thread_id);
	  __c = reinterpret_cast<char*>(__block) + __pool._M_get_align();
	}
      else
	{
	  // Null, reserve.
	  __c = __pool._M_reserve_block(__bytes, __thread_id);
	}
      return static_cast<_Tp*>(static_cast<void*>(__c));
    }
  
  template<typename _Tp, typename _Poolp>
    void
    __mt_alloc<_Tp, _Poolp>::
    deallocate(pointer __p, size_type __n)
    {
      if (__builtin_expect(__p != 0, true))
	{
#if __cpp_aligned_new
	  // Types with extended alignment are handled by operator new/delete.
	  if (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
	    {
	      ::operator delete(__p, std::align_val_t(alignof(_Tp)));
	      return;
	    }
#endif

// Requests larger than _M_max_bytes are handled by
	  // operators new/delete directly.
	  __pool_type& __pool = __policy_type::_S_get_pool();
	  const size_t __bytes = __n * sizeof(_Tp);
	  if (__pool._M_check_threshold(__bytes))
	    ::operator delete(__p);
	  else
	    __pool._M_reclaim_block(reinterpret_cast<char*>(__p), __bytes);
	}
    }
  
  template<typename _Tp, typename _Poolp>
    inline bool
    operator==(const __mt_alloc<_Tp, _Poolp>&, const __mt_alloc<_Tp, _Poolp>&)
    { return true; }
  
  template<typename _Tp, typename _Poolp>
    inline bool
    operator!=(const __mt_alloc<_Tp, _Poolp>&, const __mt_alloc<_Tp, _Poolp>&)
    { return false; }

#undef __thread_default

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace

#endif

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