omp_loop.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/omp_loop.h
00026  *  @brief Parallelization of embarrassingly parallel execution by
00027  *  means of an OpenMP for loop.
00028  *  This file is a GNU parallel extension to the Standard C++ Library.
00029  */
00030 
00031 // Written by Felix Putze.
00032 
00033 #ifndef _GLIBCXX_PARALLEL_OMP_LOOP_H
00034 #define _GLIBCXX_PARALLEL_OMP_LOOP_H 1
00035 
00036 #include <omp.h>
00037 
00038 #include <parallel/settings.h>
00039 #include <parallel/basic_iterator.h>
00040 #include <parallel/base.h>
00041 
00042 namespace __gnu_parallel
00043 {
00044 /** @brief Embarrassingly parallel algorithm for random access
00045   * iterators, using an OpenMP for loop.
00046   *
00047   *  @param begin Begin iterator of element sequence.
00048   *  @param end End iterator of element sequence.
00049   *  @param o User-supplied functor (comparator, predicate, adding
00050   *  functor, etc.).
00051   *  @param f Functor to "process" an element with op (depends on
00052   *  desired functionality, e. g. for std::for_each(), ...).
00053   *  @param r Functor to "add" a single result to the already
00054   *  processed elements (depends on functionality).
00055   *  @param base Base value for reduction.
00056   *  @param output Pointer to position where final result is written to
00057   *  @param bound Maximum number of elements processed (e. g. for
00058   *  std::count_n()).
00059   *  @return User-supplied functor (that may contain a part of the result).
00060   */
00061 template<typename RandomAccessIterator,
00062      typename Op,
00063      typename Fu,
00064      typename Red,
00065      typename Result>
00066   Op
00067   for_each_template_random_access_omp_loop(RandomAccessIterator begin,
00068                        RandomAccessIterator end,
00069                        Op o, Fu& f, Red r, Result base,
00070                        Result& output,
00071                        typename std::iterator_traits
00072                        <RandomAccessIterator>::
00073                        difference_type bound)
00074   {
00075     typedef typename
00076         std::iterator_traits<RandomAccessIterator>::difference_type
00077         difference_type;
00078 
00079     difference_type length = end - begin;
00080     thread_index_t num_threads =
00081       __gnu_parallel::min<difference_type>(get_max_threads(), length);
00082 
00083     Result *thread_results;
00084 
00085 #   pragma omp parallel num_threads(num_threads)
00086       {
00087 #       pragma omp single
00088           {
00089             num_threads = omp_get_num_threads();
00090             thread_results = new Result[num_threads];
00091 
00092             for (thread_index_t i = 0; i < num_threads; ++i)
00093               thread_results[i] = Result();
00094           }
00095 
00096         thread_index_t iam = omp_get_thread_num();
00097 
00098 #      pragma omp for schedule(dynamic, _Settings::get().workstealing_chunk_size)
00099         for (difference_type pos = 0; pos < length; ++pos)
00100           thread_results[iam] =
00101               r(thread_results[iam], f(o, begin+pos));
00102       } //parallel
00103 
00104     for (thread_index_t i = 0; i < num_threads; ++i)
00105         output = r(output, thread_results[i]);
00106 
00107     delete [] thread_results;
00108 
00109     // Points to last element processed (needed as return value for
00110     // some algorithms like transform).
00111     f.finish_iterator = begin + length;
00112 
00113     return o;
00114   }
00115 
00116 } // end namespace
00117 
00118 #endif /* _GLIBCXX_PARALLEL_OMP_LOOP_H */

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