libstdc++: queue.h Source File

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queue.h
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1// -*- C++ -*-
2
3// Copyright (C) 2007-2026 Free Software Foundation, Inc.
4//
5// This file is part of the GNU ISO C++ Library. This library is free
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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.
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11// This library is distributed in the hope that it will be useful, but
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14// General Public License for more details.
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16// Under Section 7 of GPL version 3, you are granted additional
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18// 3.1, as published by the Free Software Foundation.
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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
42namespace __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 */
base.h
Sequential helper functions. This file is a GNU parallel extension to the Standard C++ Library.
_GLIBCXX_VOLATILE
#define _GLIBCXX_VOLATILE
Decide whether to declare certain variables volatile.
Definition partition.h:43
types.h
Basic types and typedefs. This file is a GNU parallel extension to the Standard C++ Library.
__gnu_parallel
GNU parallel code for public use.
Definition balanced_quicksort.h:62
__gnu_parallel::_SequenceIndex
uint64_t _SequenceIndex
Unsigned integer to index __elements. The total number of elements for each algorithm must fit into t...
Definition types.h:117
__gnu_parallel::__fetch_and_add
_Tp __fetch_and_add(volatile _Tp *__ptr, _Tp __addend)
Add a value to a variable, atomically.
Definition parallel/compatibility.h:74
__gnu_parallel::__encode2
_CASable __encode2(int __a, int __b)
Encode two integers into one gnu_parallel::_CASable.
Definition base.h:119
__gnu_parallel::_CASable
int64_t _CASable
Longest compare-and-swappable integer type on this platform.
Definition types.h:127
__gnu_parallel::__compare_and_swap
bool __compare_and_swap(volatile _Tp *__ptr, _Tp __comparand, _Tp __replacement)
Compare-and-swap.
Definition parallel/compatibility.h:108
__gnu_parallel::__decode2
void __decode2(_CASable __x, int &__a, int &__b)
Decode two integers from one gnu_parallel::_CASable.
Definition base.h:133
__gnu_parallel::_RestrictedBoundedConcurrentQueue::~_RestrictedBoundedConcurrentQueue
~_RestrictedBoundedConcurrentQueue()
Destructor. Not to be called concurrent, of course.
Definition queue.h:77
__gnu_parallel::_RestrictedBoundedConcurrentQueue::push_front
void push_front(const _Tp &__t)
Pushes one element into the queue at the front end. Must not be called concurrently with pop_front().
Definition queue.h:83
__gnu_parallel::_RestrictedBoundedConcurrentQueue::pop_front
bool pop_front(_Tp &__t)
Pops one element from the queue at the front end. Must not be called concurrently with pop_front().
Definition queue.h:100
__gnu_parallel::_RestrictedBoundedConcurrentQueue::_RestrictedBoundedConcurrentQueue
_RestrictedBoundedConcurrentQueue(_SequenceIndex __max_size)
Constructor. Not to be called concurrent, of course.
Definition queue.h:68
__gnu_parallel::_RestrictedBoundedConcurrentQueue::pop_back
bool pop_back(_Tp &__t)
Pops one element from the queue at the front end. Must not be called concurrently with pop_front().
Definition queue.h:127
compatibility.h
Compatibility layer, mostly concerned with atomic operations.