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

   1 // -*- C++ -*-
   2
   3 // Copyright (C) 2007-2021 Free Software Foundation, Inc.
   4 //
   5 // This file is part of the GNU ISO C++ Library.  This library is free
   6 // software; you can redistribute it and/or modify it under the terms
   7 // of the GNU General Public License as published by the Free Software
   8 // Foundation; either version 3, or (at your option) any later
   9 // version.
  10
  11 // This library is distributed in the hope that it will be useful, but
  12 // WITHOUT ANY WARRANTY; without even the implied warranty of
  13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14 // General Public License for more details.
  15
  16 // Under Section 7 of GPL version 3, you are granted additional
  17 // permissions described in the GCC Runtime Library Exception, version
  18 // 3.1, as published by the Free Software Foundation.
  19
  20 // You should have received a copy of the GNU General Public License and
  21 // a copy of the GCC Runtime Library Exception along with this program;
  22 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
  23 // <http://www.gnu.org/licenses/>.
  24
  25 /** @file parallel/omp_loop.h
  26  *  @brief Parallelization of embarrassingly parallel execution by
  27  *  means of an OpenMP for loop.
  28  *  This file is a GNU parallel extension to the Standard C++ Library.
  29  */
  30
  31 // Written by Felix Putze.
  32
  33 #ifndef _GLIBCXX_PARALLEL_OMP_LOOP_H
  34 #define _GLIBCXX_PARALLEL_OMP_LOOP_H 1
  35
  36 #include <omp.h>
  37
  38 #include <parallel/settings.h>
  39 #include <parallel/basic_iterator.h>
  40 #include <parallel/base.h>
  41
  42 namespace __gnu_parallel
  43 {
  44   /** @brief Embarrassingly parallel algorithm for random access
  45    * iterators, using an OpenMP for loop.
  46    *
  47    *  @param __begin Begin iterator of element sequence.
  48    *  @param __end End iterator of element sequence.
  49    *  @param __o User-supplied functor (comparator, predicate, adding
  50    *  functor, etc.).
  51    *  @param __f Functor to @a process an element with __op (depends on
  52    *  desired functionality, e. g. for std::for_each(), ...).
  53    *  @param __r Functor to @a add a single __result to the already
  54    *  processed elements (depends on functionality).
  55    *  @param __base Base value for reduction.
  56    *  @param __output Pointer to position where final result is written to
  57    *  @param __bound Maximum number of elements processed (e. g. for
  58    *  std::count_n()).
  59    *  @return User-supplied functor (that may contain a part of the result).
  60    */
  61   template<typename _RAIter,
  62            typename _Op,
  63            typename _Fu,
  64            typename _Red,
  65            typename _Result>
  66     _Op
  67     __for_each_template_random_access_omp_loop(_RAIter __begin, _RAIter __end,
  68                                                _Op __o, _Fu& __f, _Red __r,
  69                                                _Result __base,
  70                                                _Result& __output,
  71       typename std::iterator_traits<_RAIter>::difference_type __bound)
  72     {
  73       typedef typename std::iterator_traits<_RAIter>::difference_type
  74         _DifferenceType;
  75
  76       _DifferenceType __length = __end - __begin;
  77       _ThreadIndex __num_threads = __gnu_parallel::min<_DifferenceType>
  78         (__get_max_threads(), __length);
  79
  80       _Result *__thread_results;
  81
  82 #     pragma omp parallel num_threads(__num_threads)
  83       {
  84 #       pragma omp single
  85         {
  86           __num_threads = omp_get_num_threads();
  87           __thread_results = new _Result[__num_threads];
  88
  89           for (_ThreadIndex __i = 0; __i < __num_threads; ++__i)
  90             __thread_results[__i] = _Result();
  91         }
  92
  93         _ThreadIndex __iam = omp_get_thread_num();
  94
  95 #pragma omp for schedule(dynamic, _Settings::get().workstealing_chunk_size)
  96         for (_DifferenceType __pos = 0; __pos < __length; ++__pos)
  97           __thread_results[__iam] = __r(__thread_results[__iam],
  98                                         __f(__o, __begin+__pos));
  99       } //parallel
 100
 101       for (_ThreadIndex __i = 0; __i < __num_threads; ++__i)
 102         __output = __r(__output, __thread_results[__i]);
 103
 104       delete [] __thread_results;
 105
 106       // Points to last element processed (needed as return value for
 107       // some algorithms like transform).
 108       __f._M_finish_iterator = __begin + __length;
 109
 110       return __o;
 111     }
 112
 113 } // end namespace
 114
 115 #endif /* _GLIBCXX_PARALLEL_OMP_LOOP_H */