1 // -*- C++ -*-
2 
3 // Copyright (C) 2007-2018 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. */
_RestrictedBoundedConcurrentQueue(_SequenceIndex __max_size)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. */
~_RestrictedBoundedConcurrentQueue()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
push_front(const _Tp & __t)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
pop_front(_Tp & __t)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
pop_back(_Tp & __t)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 */
156