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/usr/include/c++/11/parallel/par_loop.h


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     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 */