libstdc++-v3/include/parallel/par_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/par_loop.h
  26  *  @brief Parallelization of embarrassingly parallel execution by
  27  *  means of equal splitting.
  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_PAR_LOOP_H
  34 #define _GLIBCXX_PARALLEL_PAR_LOOP_H 1
  35
  36 #include <omp.h>
  37 #include <parallel/settings.h>
  38 #include <parallel/base.h>
  39 #include <parallel/equally_split.h>
  40
  41 namespace __gnu_parallel
  42 {
  43   /** @brief Embarrassingly parallel algorithm for random access
  44    * iterators, using hand-crafted parallelization by equal splitting
  45    * the work.
  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, ...)
  51    *  @param __f Functor to "process" an element with __op (depends on
  52    *  desired functionality, e. g. for std::for_each(), ...).
  53    *  @param __r Functor to "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_ed(_RAIter __begin, _RAIter __end,
  68                                          _Op __o, _Fu& __f, _Red __r,
  69                                          _Result __base, _Result& __output,
  70       typename std::iterator_traits<_RAIter>::difference_type __bound)
  71     {
  72       typedef std::iterator_traits<_RAIter> _TraitsType;
  73       typedef typename _TraitsType::difference_type _DifferenceType;
  74       const _DifferenceType __length = __end - __begin;
  75       _Result *__thread_results;
  76       bool* __constructed;
  77
  78       _ThreadIndex __num_threads = __gnu_parallel::min<_DifferenceType>
  79         (__get_max_threads(), __length);
  80
  81 #     pragma omp parallel num_threads(__num_threads)
  82       {
  83 #       pragma omp single
  84         {
  85           __num_threads = omp_get_num_threads();
  86           __thread_results = static_cast<_Result*>
  87             (::operator new(__num_threads * sizeof(_Result)));
  88           __constructed = new bool[__num_threads];
  89         }
  90
  91         _ThreadIndex __iam = omp_get_thread_num();
  92
  93         // Neutral element.
  94         _Result* __reduct;
  95
  96         _DifferenceType
  97           __start = __equally_split_point(__length, __num_threads, __iam),
  98           __stop = __equally_split_point(__length, __num_threads, __iam + 1);
  99
 100         if (__start < __stop)
 101           {
 102             __reduct = new _Result(__f(__o, __begin + __start));
 103             ++__start;
 104             __constructed[__iam] = true;
 105           }
 106         else
 107           __constructed[__iam] = false;
 108
 109         for (; __start < __stop; ++__start)
 110           *__reduct = __r(*__reduct, __f(__o, __begin + __start));
 111
 112         if (__constructed[__iam])
 113           {
 114             ::new(&__thread_results[__iam]) _Result(*__reduct);
 115             delete __reduct;
 116           }
 117       } //parallel
 118
 119       for (_ThreadIndex __i = 0; __i < __num_threads; ++__i)
 120         if (__constructed[__i])
 121           {
 122             __output = __r(__output, __thread_results[__i]);
 123             __thread_results[__i].~_Result();
 124           }
 125
 126       // Points to last element processed (needed as return value for
 127       // some algorithms like transform).
 128       __f._M_finish_iterator = __begin + __length;
 129
 130       ::operator delete(__thread_results);
 131
 132       delete[] __constructed;
 133
 134       return __o;
 135     }
 136
 137 } // end namespace
 138
 139 #endif /* _GLIBCXX_PARALLEL_PAR_LOOP_H */