par_loop.h

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// -*- C++ -*-

// Copyright (C) 2007-2019 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the terms
// of the GNU General Public License as published by the Free Software
// Foundation; either version 3, or (at your option) any later
// version.

// This library is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file parallel/par_loop.h
 *  @brief Parallelization of embarrassingly parallel execution by
 *  means of equal splitting.
 *  This file is a GNU parallel extension to the Standard C++ Library.
 */

// Written by Felix Putze.

#ifndef _GLIBCXX_PARALLEL_PAR_LOOP_H
#define _GLIBCXX_PARALLEL_PAR_LOOP_H 1

#include <omp.h>
#include <parallel/settings.h>
#include <parallel/base.h>
#include <parallel/equally_split.h>

namespace __gnu_parallel
{
  /** @brief Embarrassingly parallel algorithm for random access
   * iterators, using hand-crafted parallelization by equal splitting
   * the work.
   *
   *  @param __begin Begin iterator of element sequence.
   *  @param __end End iterator of element sequence.
   *  @param __o User-supplied functor (comparator, predicate, adding
   *  functor, ...)
   *  @param __f Functor to "process" an element with __op (depends on
   *  desired functionality, e. g. for std::for_each(), ...).
   *  @param __r Functor to "add" a single __result to the already
   *  processed elements (depends on functionality).
   *  @param __base Base value for reduction.
   *  @param __output Pointer to position where final result is written to
   *  @param __bound Maximum number of elements processed (e. g. for
   *  std::count_n()).
   *  @return User-supplied functor (that may contain a part of the result).
   */
  template<typename _RAIter,
           typename _Op,
           typename _Fu,
           typename _Red,
           typename _Result>
    _Op
    __for_each_template_random_access_ed(_RAIter __begin, _RAIter __end,
                                         _Op __o, _Fu& __f, _Red __r,
                                         _Result __base, _Result& __output,
      typename std::iterator_traits<_RAIter>::difference_type __bound)
    {
      typedef std::iterator_traits<_RAIter> _TraitsType;
      typedef typename _TraitsType::difference_type _DifferenceType;
      const _DifferenceType __length = __end - __begin;
      _Result *__thread_results;
      bool* __constructed;

      _ThreadIndex __num_threads = __gnu_parallel::min<_DifferenceType>
        (__get_max_threads(), __length);

#     pragma omp parallel num_threads(__num_threads)
      {
#       pragma omp single
        {
          __num_threads = omp_get_num_threads();
          __thread_results = static_cast<_Result*>
            (::operator new(__num_threads * sizeof(_Result)));
          __constructed = new bool[__num_threads];
        }

        _ThreadIndex __iam = omp_get_thread_num();

        // Neutral element.
        _Result* __reduct;

        _DifferenceType
          __start = __equally_split_point(__length, __num_threads, __iam),
          __stop = __equally_split_point(__length, __num_threads, __iam + 1);

        if (__start < __stop)
          {
            __reduct = new _Result(__f(__o, __begin + __start));
            ++__start;
            __constructed[__iam] = true;
          }
        else
          __constructed[__iam] = false;

        for (; __start < __stop; ++__start)
          *__reduct = __r(*__reduct, __f(__o, __begin + __start));

        if (__constructed[__iam])
          {
            ::new(&__thread_results[__iam]) _Result(*__reduct);
            delete __reduct;
          }
      } //parallel

      for (_ThreadIndex __i = 0; __i < __num_threads; ++__i)
        if (__constructed[__i])
          {
            __output = __r(__output, __thread_results[__i]);
            __thread_results[__i].~_Result();
          }

      // Points to last element processed (needed as return value for
      // some algorithms like transform).
      __f._M_finish_iterator = __begin + __length;

      ::operator delete(__thread_results);

      delete[] __constructed;

      return __o;
    }

} // end namespace

#endif /* _GLIBCXX_PARALLEL_PAR_LOOP_H */