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

// -*- C++ -*-

// Copyright (C) 2007, 2008 Free Software Foundation, Inc.
//
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/** @file parallel/omp_loop_static.h
 *  @brief Parallelization of embarrassingly parallel execution by
 *  means of an OpenMP for loop with static scheduling.
 *  This file is a GNU parallel extension to the Standard C++ Library.
 */

// Written by Felix Putze.

#ifndef _GLIBCXX_PARALLEL_OMP_LOOP_STATIC_H
#define _GLIBCXX_PARALLEL_OMP_LOOP_STATIC_H 1

#include <omp.h>

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

namespace __gnu_parallel
{

  /** @brief Embarrassingly parallel algorithm for random access
   * iterators, using an OpenMP for loop with static scheduling.
   *
   *  @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_omp_loop_static(RandomAccessIterator begin,
						  RandomAccessIterator end,
						  Op o, Fu& f, Red r,
						  Result base, Result& output,
						  typename std::iterator_traits
						  <RandomAccessIterator>::
						  difference_type bound)
  {
    typedef typename
      std::iterator_traits<RandomAccessIterator>::difference_type
      difference_type;

    difference_type length = end - begin;
    thread_index_t num_threads =
      std::min<difference_type>(get_max_threads(), length);

    Result *thread_results;

#   pragma omp parallel num_threads(num_threads)
      {
#       pragma omp single
          {
            num_threads = omp_get_num_threads();
            thread_results = new Result[num_threads];

            for (thread_index_t i = 0; i < num_threads; ++i)
              thread_results[i] = Result();
          }

        thread_index_t iam = omp_get_thread_num();

#       pragma omp for schedule(static, _Settings::get().workstealing_chunk_size)
        for (difference_type pos = 0; pos < length; ++pos)
          thread_results[iam] = r(thread_results[iam], f(o, begin+pos));
      } //parallel

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

    delete [] thread_results;

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

    return o;
  }

} // end namespace

#endif