queue.h

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00001 // -*- C++ -*-
00002 
00003 // Copyright (C) 2007, 2008, 2009 Free Software Foundation, Inc.
00004 //
00005 // This file is part of the GNU ISO C++ Library.  This library is free
00006 // software; you can redistribute it and/or modify it under the terms
00007 // of the GNU General Public License as published by the Free Software
00008 // Foundation; either version 3, or (at your option) any later
00009 // version.
00010 
00011 // This library is distributed in the hope that it will be useful, but
00012 // WITHOUT ANY WARRANTY; without even the implied warranty of
00013 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
00014 // General Public License for more details.
00015 
00016 // Under Section 7 of GPL version 3, you are granted additional
00017 // permissions described in the GCC Runtime Library Exception, version
00018 // 3.1, as published by the Free Software Foundation.
00019 
00020 // You should have received a copy of the GNU General Public License and
00021 // a copy of the GCC Runtime Library Exception along with this program;
00022 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
00023 // <http://www.gnu.org/licenses/>.
00024 
00025 /** @file parallel/queue.h
00026  *  @brief Lock-free double-ended queue.
00027  *  This file is a GNU parallel extension to the Standard C++ Library.
00028  */
00029 
00030 // Written by Johannes Singler.
00031 
00032 #ifndef _GLIBCXX_PARALLEL_QUEUE_H
00033 #define _GLIBCXX_PARALLEL_QUEUE_H 1
00034 
00035 #include <parallel/types.h>
00036 #include <parallel/base.h>
00037 #include <parallel/compatibility.h>
00038 
00039 /** @brief Decide whether to declare certain variable volatile in this file. */
00040 #define _GLIBCXX_VOLATILE volatile
00041 
00042 namespace __gnu_parallel
00043 {
00044   /**@brief Double-ended queue of bounded size, allowing lock-free
00045    *  atomic access.  push_front() and pop_front() must not be called
00046    *  concurrently to each other, while pop_back() can be called
00047    *  concurrently at all times.
00048    *  @c empty(), @c size(), and @c top() are intentionally not provided.
00049    *  Calling them would not make sense in a concurrent setting.
00050    *  @param T Contained element type. */
00051   template<typename T>
00052     class RestrictedBoundedConcurrentQueue
00053     {
00054     private:
00055       /** @brief Array of elements, seen as cyclic buffer. */
00056       T* base;
00057 
00058       /** @brief Maximal number of elements contained at the same time. */
00059       sequence_index_t max_size;
00060 
00061       /** @brief Cyclic begin and end pointers contained in one
00062       atomically changeable value. */
00063       _GLIBCXX_VOLATILE lcas_t borders;
00064 
00065     public:
00066       /** @brief Constructor. Not to be called concurrent, of course.
00067        *  @param max_size Maximal number of elements to be contained. */
00068       RestrictedBoundedConcurrentQueue(sequence_index_t max_size)
00069       {
00070     this->max_size = max_size;
00071     base = new T[max_size];
00072     borders = encode2(0, 0);
00073 #pragma omp flush
00074       }
00075 
00076       /** @brief Destructor. Not to be called concurrent, of course. */
00077       ~RestrictedBoundedConcurrentQueue()
00078       { delete[] base; }
00079 
00080       /** @brief Pushes one element into the queue at the front end.
00081        *  Must not be called concurrently with pop_front(). */
00082       void
00083       push_front(const T& t)
00084       {
00085     lcas_t former_borders = borders;
00086     int former_front, former_back;
00087     decode2(former_borders, former_front, former_back);
00088     *(base + former_front % max_size) = t;
00089 #if _GLIBCXX_ASSERTIONS
00090     // Otherwise: front - back > max_size eventually.
00091     _GLIBCXX_PARALLEL_ASSERT(((former_front + 1) - former_back)
00092                  <= max_size);
00093 #endif
00094     fetch_and_add(&borders, encode2(1, 0));
00095       }
00096 
00097       /** @brief Pops one element from the queue at the front end.
00098        *  Must not be called concurrently with pop_front(). */
00099       bool
00100       pop_front(T& t)
00101       {
00102     int former_front, former_back;
00103 #pragma omp flush
00104     decode2(borders, former_front, former_back);
00105     while (former_front > former_back)
00106       {
00107         // Chance.
00108         lcas_t former_borders = encode2(former_front, former_back);
00109         lcas_t new_borders = encode2(former_front - 1, former_back);
00110         if (compare_and_swap(&borders, former_borders, new_borders))
00111           {
00112         t = *(base + (former_front - 1) % max_size);
00113         return true;
00114           }
00115 #pragma omp flush
00116         decode2(borders, former_front, former_back);
00117       }
00118     return false;
00119       }
00120 
00121       /** @brief Pops one element from the queue at the front end.
00122        *  Must not be called concurrently with pop_front(). */
00123       bool
00124       pop_back(T& t)    //queue behavior
00125       {
00126     int former_front, former_back;
00127 #pragma omp flush
00128     decode2(borders, former_front, former_back);
00129     while (former_front > former_back)
00130       {
00131         // Chance.
00132         lcas_t former_borders = encode2(former_front, former_back);
00133         lcas_t new_borders = encode2(former_front, former_back + 1);
00134         if (compare_and_swap(&borders, former_borders, new_borders))
00135           {
00136         t = *(base + former_back % max_size);
00137         return true;
00138           }
00139 #pragma omp flush
00140         decode2(borders, former_front, former_back);
00141       }
00142     return false;
00143       }
00144   };
00145 }   //namespace __gnu_parallel
00146 
00147 #undef _GLIBCXX_VOLATILE
00148 
00149 #endif /* _GLIBCXX_PARALLEL_QUEUE_H */

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