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

   1 // -*- C++ -*-
   2
   3 // Copyright (C) 2007, 2008 Free Software Foundation, Inc.
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  14 // General Public License for more details.
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  21 // As a special exception, you may use this file as part of a free
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  29 // Public License.
  30
  31 /** @file parallel/queue.h
  32  *  @brief Lock-free double-ended queue.
  33  *  This file is a GNU parallel extension to the Standard C++ Library.
  34  */
  35
  36 // Written by Johannes Singler.
  37
  38 #ifndef _GLIBCXX_PARALLEL_QUEUE_H
  39 #define _GLIBCXX_PARALLEL_QUEUE_H 1
  40
  41 #include <parallel/types.h>
  42 #include <parallel/base.h>
  43 #include <parallel/compatibility.h>
  44
  45 /** @brief Decide whether to declare certain variable volatile in this file. */
  46 #define _GLIBCXX_VOLATILE volatile
  47
  48 namespace __gnu_parallel
  49 {
  50   /**@brief Double-ended queue of bounded size, allowing lock-free
  51    *  atomic access.  push_front() and pop_front() must not be called
  52    *  concurrently to each other, while pop_back() can be called
  53    *  concurrently at all times.
  54    *  @c empty(), @c size(), and @c top() are intentionally not provided.
  55    *  Calling them would not make sense in a concurrent setting.
  56    *  @param T Contained element type. */
  57   template<typename T>
  58     class RestrictedBoundedConcurrentQueue
  59     {
  60     private:
  61       /** @brief Array of elements, seen as cyclic buffer. */
  62       T* base;
  63
  64       /** @brief Maximal number of elements contained at the same time. */
  65       sequence_index_t max_size;
  66
  67       /** @brief Cyclic begin and end pointers contained in one
  68           atomically changeable value. */
  69       _GLIBCXX_VOLATILE lcas_t borders;
  70
  71     public:
  72       /** @brief Constructor. Not to be called concurrent, of course.
  73        *  @param max_size Maximal number of elements to be contained. */
  74       RestrictedBoundedConcurrentQueue(sequence_index_t max_size)
  75       {
  76         this->max_size = max_size;
  77         base = new T[max_size];
  78         borders = encode2(0, 0);
  79 #pragma omp flush
  80       }
  81
  82       /** @brief Destructor. Not to be called concurrent, of course. */
  83       ~RestrictedBoundedConcurrentQueue()
  84       { delete[] base; }
  85
  86       /** @brief Pushes one element into the queue at the front end.
  87        *  Must not be called concurrently with pop_front(). */
  88       void
  89       push_front(const T& t)
  90       {
  91         lcas_t former_borders = borders;
  92         int former_front, former_back;
  93         decode2(former_borders, former_front, former_back);
  94         *(base + former_front % max_size) = t;
  95 #if _GLIBCXX_ASSERTIONS
  96         // Otherwise: front - back > max_size eventually.
  97         _GLIBCXX_PARALLEL_ASSERT(((former_front + 1) - former_back)
  98                                  <= max_size);
  99 #endif
 100         fetch_and_add(&borders, encode2(1, 0));
 101       }
 102
 103       /** @brief Pops one element from the queue at the front end.
 104        *  Must not be called concurrently with pop_front(). */
 105       bool
 106       pop_front(T& t)
 107       {
 108         int former_front, former_back;
 109 #pragma omp flush
 110         decode2(borders, former_front, former_back);
 111         while (former_front > former_back)
 112           {
 113             // Chance.
 114             lcas_t former_borders = encode2(former_front, former_back);
 115             lcas_t new_borders = encode2(former_front - 1, former_back);
 116             if (compare_and_swap(&borders, former_borders, new_borders))
 117               {
 118                 t = *(base + (former_front - 1) % max_size);
 119                 return true;
 120               }
 121 #pragma omp flush
 122             decode2(borders, former_front, former_back);
 123           }
 124         return false;
 125       }
 126
 127       /** @brief Pops one element from the queue at the front end.
 128        *  Must not be called concurrently with pop_front(). */
 129       bool
 130       pop_back(T& t)    //queue behavior
 131       {
 132         int former_front, former_back;
 133 #pragma omp flush
 134         decode2(borders, former_front, former_back);
 135         while (former_front > former_back)
 136           {
 137             // Chance.
 138             lcas_t former_borders = encode2(former_front, former_back);
 139             lcas_t new_borders = encode2(former_front, former_back + 1);
 140             if (compare_and_swap(&borders, former_borders, new_borders))
 141               {
 142                 t = *(base + former_back % max_size);
 143                 return true;
 144               }
 145 #pragma omp flush
 146             decode2(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 */