omp_loop.h

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00001 // -*- C++ -*-
00002 
00003 // Copyright (C) 2007, 2008 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 2, 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 // You should have received a copy of the GNU General Public License
00017 // along with this library; see the file COPYING.  If not, write to
00018 // the Free Software Foundation, 59 Temple Place - Suite 330, Boston,
00019 // MA 02111-1307, USA.
00020 
00021 // As a special exception, you may use this file as part of a free
00022 // software library without restriction.  Specifically, if other files
00023 // instantiate templates or use macros or inline functions from this
00024 // file, or you compile this file and link it with other files to
00025 // produce an executable, this file does not by itself cause the
00026 // resulting executable to be covered by the GNU General Public
00027 // License.  This exception does not however invalidate any other
00028 // reasons why the executable file might be covered by the GNU General
00029 // Public License.
00030 
00031 /** @file parallel/omp_loop.h
00032  *  @brief Parallelization of embarrassingly parallel execution by
00033  *  means of an OpenMP for loop.
00034  *  This file is a GNU parallel extension to the Standard C++ Library.
00035  */
00036 
00037 // Written by Felix Putze.
00038 
00039 #ifndef _GLIBCXX_PARALLEL_OMP_LOOP_H
00040 #define _GLIBCXX_PARALLEL_OMP_LOOP_H 1
00041 
00042 #include <omp.h>
00043 
00044 #include <parallel/settings.h>
00045 #include <parallel/basic_iterator.h>
00046 #include <parallel/base.h>
00047 
00048 namespace __gnu_parallel
00049 {
00050 /** @brief Embarrassingly parallel algorithm for random access
00051   * iterators, using an OpenMP for loop.
00052   *
00053   *  @param begin Begin iterator of element sequence.
00054   *  @param end End iterator of element sequence.
00055   *  @param o User-supplied functor (comparator, predicate, adding
00056   *  functor, etc.).
00057   *  @param f Functor to "process" an element with op (depends on
00058   *  desired functionality, e. g. for std::for_each(), ...).
00059   *  @param r Functor to "add" a single result to the already
00060   *  processed elements (depends on functionality).
00061   *  @param base Base value for reduction.
00062   *  @param output Pointer to position where final result is written to
00063   *  @param bound Maximum number of elements processed (e. g. for
00064   *  std::count_n()).
00065   *  @return User-supplied functor (that may contain a part of the result).
00066   */
00067 template<typename RandomAccessIterator,
00068      typename Op,
00069      typename Fu,
00070      typename Red,
00071      typename Result>
00072   Op
00073   for_each_template_random_access_omp_loop(RandomAccessIterator begin,
00074                        RandomAccessIterator end,
00075                        Op o, Fu& f, Red r, Result base,
00076                        Result& output,
00077                        typename std::iterator_traits
00078                        <RandomAccessIterator>::
00079                        difference_type bound)
00080   {
00081     typedef typename
00082         std::iterator_traits<RandomAccessIterator>::difference_type
00083         difference_type;
00084 
00085     difference_type length = end - begin;
00086     thread_index_t num_threads =
00087       __gnu_parallel::min<difference_type>(get_max_threads(), length);
00088 
00089     Result *thread_results;
00090 
00091 #   pragma omp parallel num_threads(num_threads)
00092       {
00093 #       pragma omp single
00094           {
00095             num_threads = omp_get_num_threads();
00096             thread_results = new Result[num_threads];
00097 
00098             for (thread_index_t i = 0; i < num_threads; ++i)
00099               thread_results[i] = Result();
00100           }
00101 
00102         thread_index_t iam = omp_get_thread_num();
00103 
00104 #      pragma omp for schedule(dynamic, _Settings::get().workstealing_chunk_size)
00105         for (difference_type pos = 0; pos < length; ++pos)
00106           thread_results[iam] =
00107               r(thread_results[iam], f(o, begin+pos));
00108       } //parallel
00109 
00110     for (thread_index_t i = 0; i < num_threads; ++i)
00111         output = r(output, thread_results[i]);
00112 
00113     delete [] thread_results;
00114 
00115     // Points to last element processed (needed as return value for
00116     // some algorithms like transform).
00117     f.finish_iterator = begin + length;
00118 
00119     return o;
00120   }
00121 
00122 } // end namespace
00123 
00124 #endif

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