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[thirdparty/gcc.git] / libstdc++-v3 / include / parallel / partial_sum.h

// -*- C++ -*-

// Copyright (C) 2007 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 2, 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.

// You should have received a copy of the GNU General Public License
// along with this library; see the file COPYING.  If not, write to
// the Free Software Foundation, 59 Temple Place - Suite 330, Boston,
// MA 02111-1307, USA.

// As a special exception, you may use this file as part of a free
// software library without restriction.  Specifically, if other files
// instantiate templates or use macros or inline functions from this
// file, or you compile this file and link it with other files to
// produce an executable, this file does not by itself cause the
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/** @file parallel/partial_sum.h
 *  @brief Parallel implementation of std::partial_sum(), i. e. prefix
 *  sums.
 *  This file is a GNU parallel extension to the Standard C++ Library.
 */

// Written by Johannes Singler.

#ifndef _GLIBCXX_PARALLEL_PARTIAL_SUM_H
#define _GLIBCXX_PARALLEL_PARTIAL_SUM_H 1

#include <omp.h>
#include <bits/stl_algobase.h>
#include <parallel/parallel.h>
#include <parallel/numericfwd.h>

namespace __gnu_parallel
{
  // Problem: there is no 0-element given.

/** @brief Base case prefix sum routine.
  *  @param begin Begin iterator of input sequence.
  *  @param end End iterator of input sequence.
  *  @param result Begin iterator of output sequence.
  *  @param bin_op Associative binary function.
  *  @param value Start value. Must be passed since the neutral
  *  element is unknown in general.
  *  @return End iterator of output sequence. */
template<
    typename InputIterator,
    typename OutputIterator,
    typename BinaryOperation>
  inline OutputIterator
  parallel_partial_sum_basecase(
            InputIterator begin, InputIterator end,
            OutputIterator result, BinaryOperation bin_op,
            typename std::iterator_traits<InputIterator>::value_type value)
  {
    if (begin == end)
      return result;

    while (begin != end)
      {
        value = bin_op(value, *begin);
        *result = value;
        ++result;
        ++begin;
      }
    return result;
  }

/** @brief Parallel partial sum implementation, two-phase approach,
    no recursion.
    *  @param begin Begin iterator of input sequence.
    *  @param end End iterator of input sequence.
    *  @param result Begin iterator of output sequence.
    *  @param bin_op Associative binary function.
    *  @param n Length of sequence.
    *  @param num_threads Number of threads to use.
    *  @return End iterator of output sequence.
    */
template<
    typename InputIterator,
    typename OutputIterator,
    typename BinaryOperation>
  OutputIterator
  parallel_partial_sum_linear(
            InputIterator begin, InputIterator end,
            OutputIterator result, BinaryOperation bin_op,
            typename std::iterator_traits<InputIterator>::difference_type n)
  {
    typedef std::iterator_traits<InputIterator> traits_type;
    typedef typename traits_type::value_type value_type;
    typedef typename traits_type::difference_type difference_type;

    if (begin == end)
      return result;

    thread_index_t num_threads =
        std::min<difference_type>(get_max_threads(), n - 1);

    if (num_threads < 2)
      {
        *result = *begin;
        return parallel_partial_sum_basecase(
            begin + 1, end, result + 1, bin_op, *begin);
      }

    difference_type* borders;
    value_type* sums;

#   pragma omp parallel num_threads(num_threads)
      {
#       pragma omp single
          {
            num_threads = omp_get_num_threads();

            borders = new difference_type[num_threads + 2];

            if (Settings::partial_sum_dilatation == 1.0f)
              equally_split(n, num_threads + 1, borders);
            else
              {
                difference_type chunk_length =
                    ((double)n /
                    ((double)num_threads + Settings::partial_sum_dilatation)),
                    borderstart = n - num_threads * chunk_length;
                borders[0] = 0;
                for (int i = 1; i < (num_threads + 1); ++i)
                  {
                    borders[i] = borderstart;
                    borderstart += chunk_length;
                  }
                borders[num_threads + 1] = n;
              }

            sums = static_cast<value_type*>(
                ::operator new(sizeof(value_type) * num_threads));
            OutputIterator target_end;
          } //single

        thread_index_t iam = omp_get_thread_num();
        if (iam == 0)
          {
            *result = *begin;
            parallel_partial_sum_basecase(begin + 1, begin + borders[1],
                          result + 1, bin_op, *begin);
            new(&(sums[iam])) value_type(*(result + borders[1] - 1));
          }
        else
          {
            new(&(sums[iam])) value_type(
                                std::accumulate(begin + borders[iam] + 1,
                                begin + borders[iam + 1],
                                *(begin + borders[iam]),
                                bin_op, __gnu_parallel::sequential_tag()));
          }

#       pragma omp barrier

#       pragma omp single
          parallel_partial_sum_basecase(
              sums + 1, sums + num_threads, sums + 1, bin_op, sums[0]);

#       pragma omp barrier

        // Still same team.
        parallel_partial_sum_basecase(begin + borders[iam + 1],
                      begin + borders[iam + 2],
                      result + borders[iam + 1], bin_op,
                      sums[iam]);
      } //parallel

    delete[] sums;
    delete[] borders;

    return result + n;
  }

/** @brief Parallel partial sum front-end.
  *  @param begin Begin iterator of input sequence.
  *  @param end End iterator of input sequence.
  *  @param result Begin iterator of output sequence.
  *  @param bin_op Associative binary function.
  *  @return End iterator of output sequence. */
template<
    typename InputIterator,
    typename OutputIterator,
    typename BinaryOperation>
  OutputIterator
  parallel_partial_sum(InputIterator begin, InputIterator end,
                       OutputIterator result, BinaryOperation bin_op)
  {
    _GLIBCXX_CALL(begin - end)

    typedef std::iterator_traits<InputIterator> traits_type;
    typedef typename traits_type::value_type value_type;
    typedef typename traits_type::difference_type difference_type;

    difference_type n = end - begin;

    switch (Settings::partial_sum_algorithm)
      {
      case Settings::LINEAR:
        // Need an initial offset.
        return parallel_partial_sum_linear(begin, end, result, bin_op, n);
      default:
    // Partial_sum algorithm not implemented.
        _GLIBCXX_PARALLEL_ASSERT(0);
        return result + n;
      }
  }
}

#endif