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

// -*- C++ -*-

// Copyright (C) 2007 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the terms
// of the GNU General Public License as published by the Free Software
// Foundation; either version 2, or (at your option) any later
// version.

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// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with this library; see the file COPYING.  If not, write to
// the Free Software Foundation, 59 Temple Place - Suite 330, Boston,
// MA 02111-1307, USA.

// As a special exception, you may use this file as part of a free
// software library without restriction.  Specifically, if other files
// instantiate templates or use macros or inline functions from this
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/** @file parallel/queue.h
 *  @brief Lock-free double-ended queue.
 *  This file is a GNU parallel extension to the Standard C++ Library.
 */

// Written by Johannes Singler.

#ifndef _GLIBCXX_PARALLEL_QUEUE_H
#define _GLIBCXX_PARALLEL_QUEUE_H 1

#include <parallel/types.h>
#include <parallel/base.h>
#include <parallel/compatibility.h>

/** @brief Decide whether to declare certain variable volatile in this file. */
#define _GLIBCXX_VOLATILE volatile

namespace __gnu_parallel
{
  /**@brief Double-ended queue of bounded size, allowing lock-free
   *  atomic access.  push_front() and pop_front() must not be called
   *  concurrently to each other, while pop_back() can be called
   *  concurrently at all times.
   *  @c empty(), @c size(), and @c top() are intentionally not provided.
   *  Calling them would not make sense in a concurrent setting.
   *  @param T Contained element type. */
  template<typename T>
  class RestrictedBoundedConcurrentQueue
  {
  private:
    /** @brief Array of elements, seen as cyclic buffer. */
    T* base;

    /** @brief Maximal number of elements contained at the same time. */
    sequence_index_t max_size;

    /** @brief Cyclic begin and end pointers contained in one
	atomically changeable value. */
    _GLIBCXX_VOLATILE lcas_t borders;

  public:
    /** @brief Constructor. Not to be called concurrent, of course.
     *  @param max_size Maximal number of elements to be contained. */
    RestrictedBoundedConcurrentQueue(sequence_index_t max_size)
    {
      this->max_size = max_size;
      base = new T[max_size];
      borders = encode2(0, 0);
#pragma omp flush
    }

    /** @brief Destructor. Not to be called concurrent, of course. */
    ~RestrictedBoundedConcurrentQueue()
    {
      delete[] base;
    }

    /** @brief Pushes one element into the queue at the front end.
     *  Must not be called concurrently with pop_front(). */
    void push_front(const T& t)
    {
      lcas_t former_borders = borders;
      int former_front, former_back;
      decode2(former_borders, former_front, former_back);
      *(base + former_front % max_size) = t;
#if _GLIBCXX_ASSERTIONS
      // Otherwise: front - back > max_size eventually.
      _GLIBCXX_PARALLEL_ASSERT(((former_front + 1) - former_back) <= max_size);
#endif
      fetch_and_add(&borders, encode2(1, 0));
    }

    /** @brief Pops one element from the queue at the front end.
     *  Must not be called concurrently with pop_front(). */
    bool pop_front(T& t)
    {
      int former_front, former_back;
#pragma omp flush
      decode2(borders, former_front, former_back);
      while (former_front > former_back)
	{
	  // Chance.
	  lcas_t former_borders = encode2(former_front, former_back);
	  lcas_t new_borders = encode2(former_front - 1, former_back);
	  if (compare_and_swap(&borders, former_borders, new_borders))
	    {
	      t = *(base + (former_front - 1) % max_size);
	      return true;
	    }
#pragma omp flush
	  decode2(borders, former_front, former_back);
	}
      return false;
    }

    /** @brief Pops one element from the queue at the front end.
     *  Must not be called concurrently with pop_front(). */
    bool pop_back(T& t)	//queue behavior
    {
      int former_front, former_back;
#pragma omp flush
      decode2(borders, former_front, former_back);
      while (former_front > former_back)
	{
	  // Chance.
	  lcas_t former_borders = encode2(former_front, former_back);
	  lcas_t new_borders = encode2(former_front, former_back + 1);
	  if (compare_and_swap(&borders, former_borders, new_borders))
	    {
	      t = *(base + former_back % max_size);
	      return true;
	    }
#pragma omp flush
	  decode2(borders, former_front, former_back);
	}
      return false;
    }
  };
}	//namespace __gnu_parallel

#undef _GLIBCXX_VOLATILE

#endif