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

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  31 /** @file parallel/partial_sum.h
  32  *  @brief Parallel implementation of std::partial_sum(), i. e. prefix
  33  *  sums.
  34  *  This file is a GNU parallel extension to the Standard C++ Library.
  35  */
  36
  37 // Written by Johannes Singler.
  38
  39 #ifndef _GLIBCXX_PARALLEL_PARTIAL_SUM_H
  40 #define _GLIBCXX_PARALLEL_PARTIAL_SUM_H 1
  41
  42 #include <omp.h>
  43 #include <new>
  44 #include <bits/stl_algobase.h>
  45 #include <parallel/parallel.h>
  46 #include <parallel/numericfwd.h>
  47
  48 namespace __gnu_parallel
  49 {
  50   // Problem: there is no 0-element given.
  51
  52 /** @brief Base case prefix sum routine.
  53   *  @param begin Begin iterator of input sequence.
  54   *  @param end End iterator of input sequence.
  55   *  @param result Begin iterator of output sequence.
  56   *  @param bin_op Associative binary function.
  57   *  @param value Start value. Must be passed since the neutral
  58   *  element is unknown in general.
  59   *  @return End iterator of output sequence. */
  60 template<typename InputIterator,
  61          typename OutputIterator,
  62          typename BinaryOperation>
  63   OutputIterator
  64   parallel_partial_sum_basecase(InputIterator begin, InputIterator end,
  65                                 OutputIterator result, BinaryOperation bin_op,
  66                                 typename std::iterator_traits
  67                                 <InputIterator>::value_type value)
  68   {
  69     if (begin == end)
  70       return result;
  71
  72     while (begin != end)
  73       {
  74         value = bin_op(value, *begin);
  75         *result = value;
  76         ++result;
  77         ++begin;
  78       }
  79     return result;
  80   }
  81
  82 /** @brief Parallel partial sum implementation, two-phase approach,
  83     no recursion.
  84     *  @param begin Begin iterator of input sequence.
  85     *  @param end End iterator of input sequence.
  86     *  @param result Begin iterator of output sequence.
  87     *  @param bin_op Associative binary function.
  88     *  @param n Length of sequence.
  89     *  @param num_threads Number of threads to use.
  90     *  @return End iterator of output sequence.
  91     */
  92 template<typename InputIterator,
  93          typename OutputIterator,
  94          typename BinaryOperation>
  95   OutputIterator
  96   parallel_partial_sum_linear(InputIterator begin, InputIterator end,
  97                               OutputIterator result, BinaryOperation bin_op,
  98                               typename std::iterator_traits
  99                               <InputIterator>::difference_type n)
 100   {
 101     typedef std::iterator_traits<InputIterator> traits_type;
 102     typedef typename traits_type::value_type value_type;
 103     typedef typename traits_type::difference_type difference_type;
 104
 105     if (begin == end)
 106       return result;
 107
 108     thread_index_t num_threads =
 109         std::min<difference_type>(get_max_threads(), n - 1);
 110
 111     if (num_threads < 2)
 112       {
 113         *result = *begin;
 114         return parallel_partial_sum_basecase(
 115             begin + 1, end, result + 1, bin_op, *begin);
 116       }
 117
 118     difference_type* borders;
 119     value_type* sums;
 120
 121 #   pragma omp parallel num_threads(num_threads)
 122       {
 123 #       pragma omp single
 124           {
 125             num_threads = omp_get_num_threads();
 126
 127             borders = new difference_type[num_threads + 2];
 128
 129             if (Settings::partial_sum_dilatation == 1.0f)
 130               equally_split(n, num_threads + 1, borders);
 131             else
 132               {
 133                 difference_type chunk_length =
 134                     ((double)n
 135                      / ((double)num_threads
 136                         + Settings::partial_sum_dilatation)),
 137                   borderstart = n - num_threads * chunk_length;
 138                 borders[0] = 0;
 139                 for (int i = 1; i < (num_threads + 1); ++i)
 140                   {
 141                     borders[i] = borderstart;
 142                     borderstart += chunk_length;
 143                   }
 144                 borders[num_threads + 1] = n;
 145               }
 146
 147             sums = static_cast<value_type*>(::operator new(sizeof(value_type)
 148                                                            * num_threads));
 149             OutputIterator target_end;
 150           } //single
 151
 152         thread_index_t iam = omp_get_thread_num();
 153         if (iam == 0)
 154           {
 155             *result = *begin;
 156             parallel_partial_sum_basecase(begin + 1, begin + borders[1],
 157                                           result + 1, bin_op, *begin);
 158             ::new(&(sums[iam])) value_type(*(result + borders[1] - 1));
 159           }
 160         else
 161           {
 162             ::new(&(sums[iam]))
 163               value_type(std::accumulate(begin + borders[iam] + 1,
 164                                          begin + borders[iam + 1],
 165                                          *(begin + borders[iam]),
 166                                          bin_op,
 167                                          __gnu_parallel::sequential_tag()));
 168           }
 169
 170 #       pragma omp barrier
 171
 172 #       pragma omp single
 173           parallel_partial_sum_basecase(
 174               sums + 1, sums + num_threads, sums + 1, bin_op, sums[0]);
 175
 176 #       pragma omp barrier
 177
 178         // Still same team.
 179         parallel_partial_sum_basecase(begin + borders[iam + 1],
 180                                       begin + borders[iam + 2],
 181                                       result + borders[iam + 1], bin_op,
 182                                       sums[iam]);
 183       } //parallel
 184
 185     ::operator delete(sums);
 186     delete[] borders;
 187
 188     return result + n;
 189   }
 190
 191 /** @brief Parallel partial sum front-end.
 192   *  @param begin Begin iterator of input sequence.
 193   *  @param end End iterator of input sequence.
 194   *  @param result Begin iterator of output sequence.
 195   *  @param bin_op Associative binary function.
 196   *  @return End iterator of output sequence. */
 197 template<typename InputIterator,
 198          typename OutputIterator,
 199          typename BinaryOperation>
 200   OutputIterator
 201   parallel_partial_sum(InputIterator begin, InputIterator end,
 202                        OutputIterator result, BinaryOperation bin_op)
 203   {
 204     _GLIBCXX_CALL(begin - end)
 205
 206     typedef std::iterator_traits<InputIterator> traits_type;
 207     typedef typename traits_type::value_type value_type;
 208     typedef typename traits_type::difference_type difference_type;
 209
 210     difference_type n = end - begin;
 211
 212     switch (Settings::partial_sum_algorithm)
 213       {
 214       case Settings::LINEAR:
 215         // Need an initial offset.
 216         return parallel_partial_sum_linear(begin, end, result, bin_op, n);
 217       default:
 218     // Partial_sum algorithm not implemented.
 219         _GLIBCXX_PARALLEL_ASSERT(0);
 220         return result + n;
 221       }
 222   }
 223 }
 224
 225 #endif