1 // -*- C++ -*-
2
3 // Copyright (C) 2007-2023 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 */