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

   1 // -*- C++ -*-
   2
   3 // Copyright (C) 2007-2018 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/quicksort.h
  26  *  @brief Implementation of a unbalanced parallel quicksort (in-place).
  27  *  This file is a GNU parallel extension to the Standard C++ Library.
  28  */
  29
  30 // Written by Johannes Singler.
  31
  32 #ifndef _GLIBCXX_PARALLEL_QUICKSORT_H
  33 #define _GLIBCXX_PARALLEL_QUICKSORT_H 1
  34
  35 #include <parallel/parallel.h>
  36 #include <parallel/partition.h>
  37
  38 namespace __gnu_parallel
  39 {
  40   /** @brief Unbalanced quicksort divide step.
  41    *  @param __begin Begin iterator of subsequence.
  42    *  @param __end End iterator of subsequence.
  43    *  @param __comp Comparator.
  44    *  @param __pivot_rank Desired __rank of the pivot.
  45    *  @param __num_samples Choose pivot from that many samples.
  46    *  @param __num_threads Number of threads that are allowed to work on
  47    *  this part.
  48    */
  49   template<typename _RAIter, typename _Compare>
  50     typename std::iterator_traits<_RAIter>::difference_type
  51     __parallel_sort_qs_divide(_RAIter __begin, _RAIter __end,
  52                               _Compare __comp, typename std::iterator_traits
  53                               <_RAIter>::difference_type __pivot_rank,
  54                               typename std::iterator_traits
  55                               <_RAIter>::difference_type
  56                               __num_samples, _ThreadIndex __num_threads)
  57     {
  58       typedef std::iterator_traits<_RAIter> _TraitsType;
  59       typedef typename _TraitsType::value_type _ValueType;
  60       typedef typename _TraitsType::difference_type _DifferenceType;
  61
  62       _DifferenceType __n = __end - __begin;
  63       __num_samples = std::min(__num_samples, __n);
  64
  65       // Allocate uninitialized, to avoid default constructor.
  66       _ValueType* __samples = static_cast<_ValueType*>
  67         (::operator new(__num_samples * sizeof(_ValueType)));
  68
  69       for (_DifferenceType __s = 0; __s < __num_samples; ++__s)
  70         {
  71           const unsigned long long __index = static_cast<unsigned long long>
  72             (__s) * __n / __num_samples;
  73           ::new(&(__samples[__s])) _ValueType(__begin[__index]);
  74         }
  75
  76       __gnu_sequential::sort(__samples, __samples + __num_samples, __comp);
  77
  78       _ValueType& __pivot = __samples[__pivot_rank * __num_samples / __n];
  79
  80       __gnu_parallel::__binder2nd<_Compare, _ValueType, _ValueType, bool>
  81         __pred(__comp, __pivot);
  82       _DifferenceType __split = __parallel_partition(__begin, __end,
  83                                                      __pred, __num_threads);
  84
  85       for (_DifferenceType __s = 0; __s < __num_samples; ++__s)
  86         __samples[__s].~_ValueType();
  87       ::operator delete(__samples);
  88
  89       return __split;
  90     }
  91
  92   /** @brief Unbalanced quicksort conquer step.
  93    *  @param __begin Begin iterator of subsequence.
  94    *  @param __end End iterator of subsequence.
  95    *  @param __comp Comparator.
  96    *  @param __num_threads Number of threads that are allowed to work on
  97    *  this part.
  98    */
  99   template<typename _RAIter, typename _Compare>
 100     void
 101     __parallel_sort_qs_conquer(_RAIter __begin, _RAIter __end,
 102                                _Compare __comp,
 103                                _ThreadIndex __num_threads)
 104     {
 105       typedef std::iterator_traits<_RAIter> _TraitsType;
 106       typedef typename _TraitsType::value_type _ValueType;
 107       typedef typename _TraitsType::difference_type _DifferenceType;
 108
 109       if (__num_threads <= 1)
 110         {
 111           __gnu_sequential::sort(__begin, __end, __comp);
 112           return;
 113         }
 114
 115       _DifferenceType __n = __end - __begin, __pivot_rank;
 116
 117       if (__n <= 1)
 118         return;
 119
 120       _ThreadIndex __num_threads_left;
 121
 122       if ((__num_threads % 2) == 1)
 123         __num_threads_left = __num_threads / 2 + 1;
 124       else
 125         __num_threads_left = __num_threads / 2;
 126
 127       __pivot_rank = __n * __num_threads_left / __num_threads;
 128
 129       _DifferenceType __split = __parallel_sort_qs_divide
 130         (__begin, __end, __comp, __pivot_rank,
 131          _Settings::get().sort_qs_num_samples_preset, __num_threads);
 132
 133 #pragma omp parallel sections num_threads(2)
 134       {
 135 #pragma omp section
 136         __parallel_sort_qs_conquer(__begin, __begin + __split,
 137                                    __comp, __num_threads_left);
 138 #pragma omp section
 139         __parallel_sort_qs_conquer(__begin + __split, __end,
 140                                    __comp, __num_threads - __num_threads_left);
 141       }
 142     }
 143
 144
 145   /** @brief Unbalanced quicksort main call.
 146    *  @param __begin Begin iterator of input sequence.
 147    *  @param __end End iterator input sequence, ignored.
 148    *  @param __comp Comparator.
 149    *  @param __num_threads Number of threads that are allowed to work on
 150    *  this part.
 151    */
 152   template<typename _RAIter, typename _Compare>
 153     void
 154     __parallel_sort_qs(_RAIter __begin, _RAIter __end,
 155                        _Compare __comp,
 156                        _ThreadIndex __num_threads)
 157     {
 158       _GLIBCXX_CALL(__n)
 159
 160       typedef std::iterator_traits<_RAIter> _TraitsType;
 161       typedef typename _TraitsType::value_type _ValueType;
 162       typedef typename _TraitsType::difference_type _DifferenceType;
 163
 164       _DifferenceType __n = __end - __begin;
 165
 166       // At least one element per processor.
 167       if (__num_threads > __n)
 168         __num_threads = static_cast<_ThreadIndex>(__n);
 169
 170       __parallel_sort_qs_conquer(
 171         __begin, __begin + __n, __comp, __num_threads);
 172     }
 173
 174 } //namespace __gnu_parallel
 175
 176 #endif /* _GLIBCXX_PARALLEL_QUICKSORT_H */