par_loop.h
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// -*- C++ -*-
// Copyright (C) 2007-2026 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
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/** @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 */
equally_split.h
settings.h
Runtime settings and tuning parameters, heuristics to decide whether to use parallelized algorithms.
base.h
Sequential helper functions. This file is a GNU parallel extension to the Standard C++ Library.
__gnu_parallel
GNU parallel code for public use.
Definition balanced_quicksort.h:62
__gnu_parallel::_ThreadIndex
uint16_t _ThreadIndex
Unsigned integer to index a thread number. The maximum thread number (for each processor) must fit in...
Definition types.h:123
__gnu_parallel::__for_each_template_random_access_ed
_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)
Embarrassingly parallel algorithm for random access iterators, using hand-crafted parallelization by ...
Definition par_loop.h:67
__gnu_parallel::min
const _Tp & min(const _Tp &__a, const _Tp &__b)
Equivalent to std::min.
Definition base.h:144
__gnu_parallel::__equally_split_point
_DifferenceType __equally_split_point(_DifferenceType __n, _ThreadIndex __num_threads, _ThreadIndex __thread_no)
function to split a sequence into parts of almost equal size.
Definition equally_split.h:75
std::iterator_traits
Traits class for iterators.
Definition stl_iterator_base_types.h:180