libstdc++
par_loop.h
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1 // -*- C++ -*-
2 
3 // Copyright (C) 2007, 2008, 2009 Free Software Foundation, Inc.
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24 
25 /** @file parallel/par_loop.h
26  * @brief Parallelization of embarrassingly parallel execution by
27  * means of equal splitting.
28  * This file is a GNU parallel extension to the Standard C++ Library.
29  */
30 
31 // Written by Felix Putze.
32 
33 #ifndef _GLIBCXX_PARALLEL_PAR_LOOP_H
34 #define _GLIBCXX_PARALLEL_PAR_LOOP_H 1
35 
36 #include <omp.h>
37 #include <parallel/settings.h>
38 #include <parallel/base.h>
39 #include <parallel/equally_split.h>
40 
41 namespace __gnu_parallel
42 {
43 
44 /** @brief Embarrassingly parallel algorithm for random access
45  * iterators, using hand-crafted parallelization by equal splitting
46  * the work.
47  *
48  * @param begin Begin iterator of element sequence.
49  * @param end End iterator of element sequence.
50  * @param o User-supplied functor (comparator, predicate, adding
51  * functor, ...)
52  * @param f Functor to "process" an element with op (depends on
53  * desired functionality, e. g. for std::for_each(), ...).
54  * @param r Functor to "add" a single result to the already
55  * processed elements (depends on functionality).
56  * @param base Base value for reduction.
57  * @param output Pointer to position where final result is written to
58  * @param bound Maximum number of elements processed (e. g. for
59  * std::count_n()).
60  * @return User-supplied functor (that may contain a part of the result).
61  */
62 template<typename RandomAccessIterator,
63  typename Op,
64  typename Fu,
65  typename Red,
66  typename Result>
67  Op
68  for_each_template_random_access_ed(RandomAccessIterator begin,
69  RandomAccessIterator end,
70  Op o, Fu& f, Red r, Result base,
71  Result& output,
72  typename std::iterator_traits
73  <RandomAccessIterator>::
74  difference_type bound)
75  {
77  typedef typename traits_type::difference_type difference_type;
78  const difference_type length = end - begin;
79  Result *thread_results;
80  bool* constructed;
81 
82  thread_index_t num_threads =
83  __gnu_parallel::min<difference_type>(get_max_threads(), length);
84 
85 # pragma omp parallel num_threads(num_threads)
86  {
87 # pragma omp single
88  {
89  num_threads = omp_get_num_threads();
90  thread_results = static_cast<Result*>(
91  ::operator new(num_threads * sizeof(Result)));
92  constructed = new bool[num_threads];
93  }
94 
95  thread_index_t iam = omp_get_thread_num();
96 
97  // Neutral element.
98  Result* reduct = static_cast<Result*>(::operator new(sizeof(Result)));
99 
100  difference_type
101  start = equally_split_point(length, num_threads, iam),
102  stop = equally_split_point(length, num_threads, iam + 1);
103 
104  if (start < stop)
105  {
106  new(reduct) Result(f(o, begin + start));
107  ++start;
108  constructed[iam] = true;
109  }
110  else
111  constructed[iam] = false;
112 
113  for (; start < stop; ++start)
114  *reduct = r(*reduct, f(o, begin + start));
115 
116  thread_results[iam] = *reduct;
117  } //parallel
118 
119  for (thread_index_t i = 0; i < num_threads; ++i)
120  if (constructed[i])
121  output = r(output, thread_results[i]);
122 
123  // Points to last element processed (needed as return value for
124  // some algorithms like transform).
125  f.finish_iterator = begin + length;
126 
127  delete[] thread_results;
128  delete[] constructed;
129 
130  return o;
131  }
132 
133 } // end namespace
134 
135 #endif /* _GLIBCXX_PARALLEL_PAR_LOOP_H */