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

   1 // -*- C++ -*-
   2
   3 // Copyright (C) 2007, 2008 Free Software Foundation, Inc.
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  14 // General Public License for more details.
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  29 // Public License.
  30
  31 /** @file parallel/omp_loop_static.h
  32  *  @brief Parallelization of embarrassingly parallel execution by
  33  *  means of an OpenMP for loop with static scheduling.
  34  *  This file is a GNU parallel extension to the Standard C++ Library.
  35  */
  36
  37 // Written by Felix Putze.
  38
  39 #ifndef _GLIBCXX_PARALLEL_OMP_LOOP_STATIC_H
  40 #define _GLIBCXX_PARALLEL_OMP_LOOP_STATIC_H 1
  41
  42 #include <omp.h>
  43
  44 #include <parallel/settings.h>
  45 #include <parallel/basic_iterator.h>
  46
  47 namespace __gnu_parallel
  48 {
  49
  50   /** @brief Embarrassingly parallel algorithm for random access
  51    * iterators, using an OpenMP for loop with static scheduling.
  52    *
  53    *  @param begin Begin iterator of element sequence.
  54    *  @param end End iterator of element sequence.
  55    *  @param o User-supplied functor (comparator, predicate, adding
  56    *  functor, ...).
  57    *  @param f Functor to "process" an element with op (depends on
  58    *  desired functionality, e. g. for std::for_each(), ...).
  59    *  @param r Functor to "add" a single result to the already processed
  60    *  elements (depends on functionality).
  61    *  @param base Base value for reduction.
  62    *  @param output Pointer to position where final result is written to
  63    *  @param bound Maximum number of elements processed (e. g. for
  64    *  std::count_n()).
  65    *  @return User-supplied functor (that may contain a part of the result).
  66    */
  67 template<typename RandomAccessIterator,
  68          typename Op,
  69          typename Fu,
  70          typename Red,
  71          typename Result>
  72   Op
  73   for_each_template_random_access_omp_loop_static(RandomAccessIterator begin,
  74                                                   RandomAccessIterator end,
  75                                                   Op o, Fu& f, Red r,
  76                                                   Result base, Result& output,
  77                                                   typename std::iterator_traits
  78                                                   <RandomAccessIterator>::
  79                                                   difference_type bound)
  80   {
  81     typedef typename
  82       std::iterator_traits<RandomAccessIterator>::difference_type
  83       difference_type;
  84
  85     difference_type length = end - begin;
  86     thread_index_t num_threads =
  87       std::min<difference_type>(get_max_threads(), length);
  88
  89     Result *thread_results;
  90
  91 #   pragma omp parallel num_threads(num_threads)
  92       {
  93 #       pragma omp single
  94           {
  95             num_threads = omp_get_num_threads();
  96             thread_results = new Result[num_threads];
  97
  98             for (thread_index_t i = 0; i < num_threads; ++i)
  99               thread_results[i] = Result();
 100           }
 101
 102         thread_index_t iam = omp_get_thread_num();
 103
 104 #       pragma omp for schedule(static, _Settings::get().workstealing_chunk_size)
 105         for (difference_type pos = 0; pos < length; ++pos)
 106           thread_results[iam] = r(thread_results[iam], f(o, begin+pos));
 107       } //parallel
 108
 109     for (thread_index_t i = 0; i < num_threads; ++i)
 110       output = r(output, thread_results[i]);
 111
 112     delete [] thread_results;
 113
 114     // Points to last element processed (needed as return value for
 115     // some algorithms like transform).
 116     f.finish_iterator = begin + length;
 117
 118     return o;
 119   }
 120
 121 } // end namespace
 122
 123 #endif