gcc/libstdc++-v3/include/parallel/par_loop.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
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// version.

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// General Public License for more details.

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/** @file parallel/par_loop.h
 *  @brief Parallelization of embarrassingly parallel execution by
 *  means of equal splitting.
 *  This file is a GNU parallel extension to the Standard C++ Library.
 */

// Written by Felix Putze.

#ifndef _GLIBCXX_PARALLEL_PAR_LOOP_H
#define _GLIBCXX_PARALLEL_PAR_LOOP_H 1

#include <omp.h>
#include <parallel/settings.h>

namespace __gnu_parallel
{

  /** @brief Embarrassingly parallel algorithm for random access
   * iterators, using hand-crafted parallelization by equal splitting
   * the work.
   *
   *  @param begin Begin iterator of element sequence.
   *  @param end End iterator of element sequence.
   *  @param o User-supplied functor (comparator, predicate, adding
   *  functor, ...)
   *  @param f Functor to "process" an element with op (depends on
   *  desired functionality, e. g. for std::for_each(), ...).
   *  @param r Functor to "add" a single result to the already
   *  processed elements (depends on functionality).
   *  @param base Base value for reduction.
   *  @param output Pointer to position where final result is written to
   *  @param bound Maximum number of elements processed (e. g. for
   *  std::count_n()).
   *  @return User-supplied functor (that may contain a part of the result).
   */
  template<typename RandomAccessIterator, typename Op, typename Fu, typename Red, typename Result>
  Op
  for_each_template_random_access_ed(RandomAccessIterator begin,
				     RandomAccessIterator end, Op o, Fu& f,
				     Red r, Result base, Result& output,
				     typename std::iterator_traits<RandomAccessIterator>::difference_type bound)
  {
    typedef std::iterator_traits<RandomAccessIterator> traits_type;
    typedef typename traits_type::difference_type difference_type;

    const difference_type length = end - begin;
    const difference_type settings_threads = static_cast<difference_type>(get_max_threads());
    const difference_type dmin = settings_threads < length ? settings_threads : length;
    const difference_type dmax = dmin > 1 ? dmin : 1;

    thread_index_t num_threads = static_cast<thread_index_t>(dmax);


    Result *thread_results = new Result[num_threads];

#pragma omp parallel num_threads(num_threads)
    {
      // Neutral element.
      Result reduct = Result();

      thread_index_t p = num_threads;
      thread_index_t iam = omp_get_thread_num();
      difference_type start = iam * length / p;
      difference_type limit = (iam == p - 1) ? length : (iam + 1) * length / p;

      if (start < limit)
	{
	  reduct = f(o, begin + start);
	  start++;
	}

      for (; start < limit; start++)
	reduct = r(reduct, f(o, begin + start));

      thread_results[iam] = reduct;
    }

    for (thread_index_t i = 0; i < num_threads; i++)
      output = r(output, thread_results[i]);

    // Points to last element processed (needed as return value for
    // some algorithms like transform).
    f.finish_iterator = begin + length;

    return o;
  }

} // end namespace

#endif