libstdc++-v3/include/parallel/queue.h

   1 // -*- C++ -*-
   2
   3 // Copyright (C) 2007-2024 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 terms
   7 // of the GNU General Public License as published by the Free Software
   8 // Foundation; either version 3, or (at your option) any later
   9 // version.
  10
  11 // This library is distributed in the hope that it will be useful, but
  12 // WITHOUT ANY WARRANTY; without even the implied warranty of
  13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14 // 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 parallel/queue.h
  26  *  @brief Lock-free double-ended queue.
  27  *  This file is a GNU parallel extension to the Standard C++ Library.
  28  */
  29
  30 // Written by Johannes Singler.
  31
  32 #ifndef _GLIBCXX_PARALLEL_QUEUE_H
  33 #define _GLIBCXX_PARALLEL_QUEUE_H 1
  34
  35 #include <parallel/types.h>
  36 #include <parallel/base.h>
  37 #include <parallel/compatibility.h>
  38
  39 /** @brief Decide whether to declare certain variable volatile in this file. */
  40 #define _GLIBCXX_VOLATILE volatile
  41
  42 namespace __gnu_parallel
  43 {
  44   /**@brief Double-ended queue of bounded size, allowing lock-free
  45    *  atomic access.  push_front() and pop_front() must not be called
  46    *  concurrently to each other, while pop_back() can be called
  47    *  concurrently at all times.
  48    *  @c empty(), @c size(), and @c top() are intentionally not provided.
  49    *  Calling them would not make sense in a concurrent setting.
  50    *  @param _Tp Contained element type. */
  51   template<typename _Tp>
  52     class _RestrictedBoundedConcurrentQueue
  53     {
  54     private:
  55       /** @brief Array of elements, seen as cyclic buffer. */
  56       _Tp* _M_base;
  57
  58       /** @brief Maximal number of elements contained at the same time. */
  59       _SequenceIndex _M_max_size;
  60
  61       /** @brief Cyclic __begin and __end pointers contained in one
  62           atomically changeable value. */
  63       _GLIBCXX_VOLATILE _CASable _M_borders;
  64
  65     public:
  66       /** @brief Constructor. Not to be called concurrent, of course.
  67        *  @param __max_size Maximal number of elements to be contained. */
  68       _RestrictedBoundedConcurrentQueue(_SequenceIndex __max_size)
  69       {
  70         _M_max_size = __max_size;
  71         _M_base = new _Tp[__max_size];
  72         _M_borders = __encode2(0, 0);
  73 #pragma omp flush
  74       }
  75
  76       /** @brief Destructor. Not to be called concurrent, of course. */
  77       ~_RestrictedBoundedConcurrentQueue()
  78       { delete[] _M_base; }
  79
  80       /** @brief Pushes one element into the queue at the front end.
  81        *  Must not be called concurrently with pop_front(). */
  82       void
  83       push_front(const _Tp& __t)
  84       {
  85         _CASable __former_borders = _M_borders;
  86         int __former_front, __former_back;
  87         __decode2(__former_borders, __former_front, __former_back);
  88         *(_M_base + __former_front % _M_max_size) = __t;
  89 #if _GLIBCXX_PARALLEL_ASSERTIONS
  90         // Otherwise: front - back > _M_max_size eventually.
  91         _GLIBCXX_PARALLEL_ASSERT(((__former_front + 1) - __former_back)
  92                                  <= _M_max_size);
  93 #endif
  94         __fetch_and_add(&_M_borders, __encode2(1, 0));
  95       }
  96
  97       /** @brief Pops one element from the queue at the front end.
  98        *  Must not be called concurrently with pop_front(). */
  99       bool
 100       pop_front(_Tp& __t)
 101       {
 102         int __former_front, __former_back;
 103 #pragma omp flush
 104         __decode2(_M_borders, __former_front, __former_back);
 105         while (__former_front > __former_back)
 106           {
 107             // Chance.
 108             _CASable __former_borders = __encode2(__former_front,
 109                                                   __former_back);
 110             _CASable __new_borders = __encode2(__former_front - 1,
 111                                                __former_back);
 112             if (__compare_and_swap(&_M_borders, __former_borders,
 113                                    __new_borders))
 114               {
 115                 __t = *(_M_base + (__former_front - 1) % _M_max_size);
 116                 return true;
 117               }
 118 #pragma omp flush
 119             __decode2(_M_borders, __former_front, __former_back);
 120           }
 121         return false;
 122       }
 123
 124       /** @brief Pops one element from the queue at the front end.
 125        *  Must not be called concurrently with pop_front(). */
 126       bool
 127       pop_back(_Tp& __t)        //queue behavior
 128       {
 129         int __former_front, __former_back;
 130 #pragma omp flush
 131         __decode2(_M_borders, __former_front, __former_back);
 132         while (__former_front > __former_back)
 133           {
 134             // Chance.
 135             _CASable __former_borders = __encode2(__former_front,
 136                                                   __former_back);
 137             _CASable __new_borders = __encode2(__former_front,
 138                                                __former_back + 1);
 139             if (__compare_and_swap(&_M_borders, __former_borders,
 140                                    __new_borders))
 141               {
 142                 __t = *(_M_base + __former_back % _M_max_size);
 143                 return true;
 144               }
 145 #pragma omp flush
 146             __decode2(_M_borders, __former_front, __former_back);
 147           }
 148         return false;
 149       }
 150   };
 151 }       //namespace __gnu_parallel
 152
 153 #undef _GLIBCXX_VOLATILE
 154
 155 #endif /* _GLIBCXX_PARALLEL_QUEUE_H */