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

   1 // -*- C++ -*-
   2
   3 // Copyright (C) 2007, 2008, 2009 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.
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  17 // permissions described in the GCC Runtime Library Exception, version
  18 // 3.1, as published by the Free Software Foundation.
  19
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  21 // a copy of the GCC Runtime Library Exception along with this program;
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  24
  25 /** @file parallel/partial_sum.h
  26  *  @brief Parallel implementation of std::partial_sum(), i.e. prefix
  27 *  sums.
  28  *  This file is a GNU parallel extension to the Standard C++ Library.
  29  */
  30
  31 // Written by Johannes Singler.
  32
  33 #ifndef _GLIBCXX_PARALLEL_PARTIAL_SUM_H
  34 #define _GLIBCXX_PARALLEL_PARTIAL_SUM_H 1
  35
  36 #include <omp.h>
  37 #include <new>
  38 #include <bits/stl_algobase.h>
  39 #include <parallel/parallel.h>
  40 #include <parallel/numericfwd.h>
  41
  42 namespace __gnu_parallel
  43 {
  44   // Problem: there is no 0-element given.
  45
  46 /** @brief Base case prefix sum routine.
  47   *  @param __begin Begin iterator of input sequence.
  48   *  @param __end End iterator of input sequence.
  49   *  @param __result Begin iterator of output sequence.
  50   *  @param __bin_op Associative binary function.
  51   *  @param __value Start value. Must be passed since the neutral
  52   *  element is unknown in general.
  53   *  @return End iterator of output sequence. */
  54 template<typename _IIter,
  55          typename _OutputIterator,
  56          typename _BinaryOperation>
  57   _OutputIterator
  58   __parallel_partial_sum_basecase(_IIter __begin, _IIter __end,
  59                                 _OutputIterator __result, _BinaryOperation __bin_op,
  60                                 typename std::iterator_traits
  61                                 <_IIter>::value_type __value)
  62   {
  63     if (__begin == __end)
  64       return __result;
  65
  66     while (__begin != __end)
  67       {
  68         __value = __bin_op(__value, *__begin);
  69         *__result = __value;
  70         ++__result;
  71         ++__begin;
  72       }
  73     return __result;
  74   }
  75
  76 /** @brief Parallel partial sum implementation, two-phase approach,
  77     no recursion.
  78     *  @param __begin Begin iterator of input sequence.
  79     *  @param __end End iterator of input sequence.
  80     *  @param __result Begin iterator of output sequence.
  81     *  @param __bin_op Associative binary function.
  82     *  @param __n Length of sequence.
  83     *  @param __num_threads Number of threads to use.
  84     *  @return End iterator of output sequence.
  85     */
  86 template<typename _IIter,
  87          typename _OutputIterator,
  88          typename _BinaryOperation>
  89   _OutputIterator
  90   __parallel_partial_sum_linear(_IIter __begin, _IIter __end,
  91                               _OutputIterator __result, _BinaryOperation __bin_op,
  92                               typename std::iterator_traits
  93                               <_IIter>::difference_type __n)
  94   {
  95     typedef std::iterator_traits<_IIter> _TraitsType;
  96     typedef typename _TraitsType::value_type _ValueType;
  97     typedef typename _TraitsType::difference_type _DifferenceType;
  98
  99     if (__begin == __end)
 100       return __result;
 101
 102     _ThreadIndex __num_threads =
 103         std::min<_DifferenceType>(__get_max_threads(), __n - 1);
 104
 105     if (__num_threads < 2)
 106       {
 107         *__result = *__begin;
 108         return __parallel_partial_sum_basecase(
 109             __begin + 1, __end, __result + 1, __bin_op, *__begin);
 110       }
 111
 112     _DifferenceType* __borders;
 113     _ValueType* __sums;
 114
 115     const _Settings& __s = _Settings::get();
 116
 117 #   pragma omp parallel num_threads(__num_threads)
 118       {
 119 #       pragma omp single
 120           {
 121             __num_threads = omp_get_num_threads();
 122
 123             __borders = new _DifferenceType[__num_threads + 2];
 124
 125             if (__s.partial_sum_dilation == 1.0f)
 126               equally_split(__n, __num_threads + 1, __borders);
 127             else
 128               {
 129                 _DifferenceType __chunk_length =
 130                     ((double)__n
 131                      / ((double)__num_threads + __s.partial_sum_dilation)),
 132                   __borderstart = __n - __num_threads * __chunk_length;
 133                 __borders[0] = 0;
 134                 for (int __i = 1; __i < (__num_threads + 1); ++__i)
 135                   {
 136                     __borders[__i] = __borderstart;
 137                     __borderstart += __chunk_length;
 138                   }
 139                 __borders[__num_threads + 1] = __n;
 140               }
 141
 142             __sums = static_cast<_ValueType*>(::operator new(sizeof(_ValueType)
 143                                                            * __num_threads));
 144             _OutputIterator __target_end;
 145           } //single
 146
 147         _ThreadIndex __iam = omp_get_thread_num();
 148         if (__iam == 0)
 149           {
 150             *__result = *__begin;
 151             __parallel_partial_sum_basecase(__begin + 1, __begin + __borders[1],
 152                                           __result + 1, __bin_op, *__begin);
 153             ::new(&(__sums[__iam])) _ValueType(*(__result + __borders[1] - 1));
 154           }
 155         else
 156           {
 157             ::new(&(__sums[__iam]))
 158               _ValueType(std::accumulate(__begin + __borders[__iam] + 1,
 159                                          __begin + __borders[__iam + 1],
 160                                          *(__begin + __borders[__iam]),
 161                                          __bin_op,
 162                                          __gnu_parallel::sequential_tag()));
 163           }
 164
 165 #       pragma omp barrier
 166
 167 #       pragma omp single
 168           __parallel_partial_sum_basecase(
 169               __sums + 1, __sums + __num_threads, __sums + 1, __bin_op, __sums[0]);
 170
 171 #       pragma omp barrier
 172
 173         // Still same team.
 174         __parallel_partial_sum_basecase(__begin + __borders[__iam + 1],
 175                                       __begin + __borders[__iam + 2],
 176                                       __result + __borders[__iam + 1], __bin_op,
 177                                       __sums[__iam]);
 178       } //parallel
 179
 180     ::operator delete(__sums);
 181     delete[] __borders;
 182
 183     return __result + __n;
 184   }
 185
 186 /** @brief Parallel partial sum front-__end.
 187   *  @param __begin Begin iterator of input sequence.
 188   *  @param __end End iterator of input sequence.
 189   *  @param __result Begin iterator of output sequence.
 190   *  @param __bin_op Associative binary function.
 191   *  @return End iterator of output sequence. */
 192 template<typename _IIter,
 193          typename _OutputIterator,
 194          typename _BinaryOperation>
 195   _OutputIterator
 196   __parallel_partial_sum(_IIter __begin, _IIter __end,
 197                        _OutputIterator __result, _BinaryOperation __bin_op)
 198   {
 199     _GLIBCXX_CALL(__begin - __end)
 200
 201     typedef std::iterator_traits<_IIter> _TraitsType;
 202     typedef typename _TraitsType::value_type _ValueType;
 203     typedef typename _TraitsType::difference_type _DifferenceType;
 204
 205     _DifferenceType __n = __end - __begin;
 206
 207     switch (_Settings::get().partial_sum_algorithm)
 208       {
 209       case LINEAR:
 210         // Need an initial offset.
 211         return __parallel_partial_sum_linear(__begin, __end, __result, __bin_op, __n);
 212       default:
 213     // Partial_sum algorithm not implemented.
 214         _GLIBCXX_PARALLEL_ASSERT(0);
 215         return __result + __n;
 216       }
 217   }
 218 }
 219
 220 #endif /* _GLIBCXX_PARALLEL_PARTIAL_SUM_H */