// -*- C++ -*- // Copyright (C) 2007-2020 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 3, 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. // Under Section 7 of GPL version 3, you are granted additional // permissions described in the GCC Runtime Library Exception, version // 3.1, as published by the Free Software Foundation. // You should have received a copy of the GNU General Public License and // a copy of the GCC Runtime Library Exception along with this program; // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see // <http://www.gnu.org/licenses/>. /** @file parallel/partition.h * @brief Parallel implementation of std::partition(), * std::nth_element(), and std::partial_sort(). * This file is a GNU parallel extension to the Standard C++ Library. */ // Written by Johannes Singler and Felix Putze. #ifndef _GLIBCXX_PARALLEL_PARTITION_H #define _GLIBCXX_PARALLEL_PARTITION_H 1 #include <parallel/basic_iterator.h> #include <parallel/sort.h> #include <parallel/random_number.h> #include <bits/stl_algo.h> #include <parallel/parallel.h> /** @brief Decide whether to declare certain variables volatile. */ #define _GLIBCXX_VOLATILE volatile namespace __gnu_parallel { /** @brief Parallel implementation of std::partition. * @param __begin Begin iterator of input sequence to split. * @param __end End iterator of input sequence to split. * @param __pred Partition predicate, possibly including some kind * of pivot. * @param __num_threads Maximum number of threads to use for this task. * @return Number of elements not fulfilling the predicate. */ template<typename _RAIter, typename _Predicate> typename std::iterator_traits<_RAIter>::difference_type __parallel_partition(_RAIter __begin, _RAIter __end, _Predicate __pred, _ThreadIndex __num_threads) { typedef std::iterator_traits<_RAIter> _TraitsType; typedef typename _TraitsType::value_type _ValueType; typedef typename _TraitsType::difference_type _DifferenceType; _DifferenceType __n = __end - __begin; _GLIBCXX_CALL(__n) const _Settings& __s = _Settings::get(); // shared _GLIBCXX_VOLATILE _DifferenceType __left = 0, __right = __n - 1, __dist = __n, __leftover_left, __leftover_right, __leftnew, __rightnew; // just 0 or 1, but int to allow atomic operations int* __reserved_left = 0, * __reserved_right = 0; _DifferenceType __chunk_size = __s.partition_chunk_size; //at least two chunks per thread if (__dist >= 2 * __num_threads * __chunk_size) # pragma omp parallel num_threads(__num_threads) { # pragma omp single { __num_threads = omp_get_num_threads(); __reserved_left = new int[__num_threads]; __reserved_right = new int[__num_threads]; if (__s.partition_chunk_share > 0.0) __chunk_size = std::max<_DifferenceType> (__s.partition_chunk_size, (double)__n * __s.partition_chunk_share / (double)__num_threads); else __chunk_size = __s.partition_chunk_size; } while (__dist >= 2 * __num_threads * __chunk_size) { # pragma omp single { _DifferenceType __num_chunks = __dist / __chunk_size; for (_ThreadIndex __r = 0; __r < __num_threads; ++__r) { __reserved_left [__r] = 0; // false __reserved_right[__r] = 0; // false } __leftover_left = 0; __leftover_right = 0; } //implicit barrier // Private. _DifferenceType __thread_left, __thread_left_border, __thread_right, __thread_right_border; __thread_left = __left + 1; // Just to satisfy the condition below. __thread_left_border = __thread_left - 1; __thread_right = __n - 1; // Just to satisfy the condition below. __thread_right_border = __thread_right + 1; bool __iam_finished = false; while (!__iam_finished) { if (__thread_left > __thread_left_border) { _DifferenceType __former_dist = __fetch_and_add(&__dist, -__chunk_size); if (__former_dist < __chunk_size) { __fetch_and_add(&__dist, __chunk_size); __iam_finished = true; break; } else { __thread_left = __fetch_and_add(&__left, __chunk_size); __thread_left_border = __thread_left + (__chunk_size - 1); } } if (__thread_right < __thread_right_border) { _DifferenceType __former_dist = __fetch_and_add(&__dist, -__chunk_size); if (__former_dist < __chunk_size) { __fetch_and_add(&__dist, __chunk_size); __iam_finished = true; break; } else { __thread_right = __fetch_and_add(&__right, -__chunk_size); __thread_right_border = __thread_right - (__chunk_size - 1); } } // Swap as usual. while (__thread_left < __thread_right) { while (__pred(__begin[__thread_left]) && __thread_left <= __thread_left_border) ++__thread_left; while (!__pred(__begin[__thread_right]) && __thread_right >= __thread_right_border) --__thread_right; if (__thread_left > __thread_left_border || __thread_right < __thread_right_border) // Fetch new chunk(__s). break; std::iter_swap(__begin + __thread_left, __begin + __thread_right); ++__thread_left; --__thread_right; } } // Now swap the leftover chunks to the right places. if (__thread_left <= __thread_left_border) # pragma omp atomic ++__leftover_left; if (__thread_right >= __thread_right_border) # pragma omp atomic ++__leftover_right; # pragma omp barrier _DifferenceType __leftold = __left, __leftnew = __left - __leftover_left * __chunk_size, __rightold = __right, __rightnew = __right + __leftover_right * __chunk_size; // <=> __thread_left_border + (__chunk_size - 1) >= __leftnew if (__thread_left <= __thread_left_border && __thread_left_border >= __leftnew) { // Chunk already in place, reserve spot. __reserved_left[(__left - (__thread_left_border + 1)) / __chunk_size] = 1; } // <=> __thread_right_border - (__chunk_size - 1) <= __rightnew if (__thread_right >= __thread_right_border && __thread_right_border <= __rightnew) { // Chunk already in place, reserve spot. __reserved_right[((__thread_right_border - 1) - __right) / __chunk_size] = 1; } # pragma omp barrier if (__thread_left <= __thread_left_border && __thread_left_border < __leftnew) { // Find spot and swap. _DifferenceType __swapstart = -1; for (int __r = 0; __r < __leftover_left; ++__r) if (__reserved_left[__r] == 0 && __compare_and_swap(&(__reserved_left[__r]), 0, 1)) { __swapstart = __leftold - (__r + 1) * __chunk_size; break; } #if _GLIBCXX_PARALLEL_ASSERTIONS _GLIBCXX_PARALLEL_ASSERT(__swapstart != -1); #endif std::swap_ranges(__begin + __thread_left_border - (__chunk_size - 1), __begin + __thread_left_border + 1, __begin + __swapstart); } if (__thread_right >= __thread_right_border && __thread_right_border > __rightnew) { // Find spot and swap _DifferenceType __swapstart = -1; for (int __r = 0; __r < __leftover_right; ++__r) if (__reserved_right[__r] == 0 && __compare_and_swap(&(__reserved_right[__r]), 0, 1)) { __swapstart = __rightold + __r * __chunk_size + 1; break; } #if _GLIBCXX_PARALLEL_ASSERTIONS _GLIBCXX_PARALLEL_ASSERT(__swapstart != -1); #endif std::swap_ranges(__begin + __thread_right_border, __begin + __thread_right_border + __chunk_size, __begin + __swapstart); } #if _GLIBCXX_PARALLEL_ASSERTIONS # pragma omp barrier # pragma omp single { for (_DifferenceType __r = 0; __r < __leftover_left; ++__r) _GLIBCXX_PARALLEL_ASSERT(__reserved_left[__r] == 1); for (_DifferenceType __r = 0; __r < __leftover_right; ++__r) _GLIBCXX_PARALLEL_ASSERT(__reserved_right[__r] == 1); } #endif __left = __leftnew; __right = __rightnew; __dist = __right - __left + 1; } # pragma omp flush(__left, __right) } // end "recursion" //parallel _DifferenceType __final_left = __left, __final_right = __right; while (__final_left < __final_right) { // Go right until key is geq than pivot. while (__pred(__begin[__final_left]) && __final_left < __final_right) ++__final_left; // Go left until key is less than pivot. while (!__pred(__begin[__final_right]) && __final_left < __final_right) --__final_right; if (__final_left == __final_right) break; std::iter_swap(__begin + __final_left, __begin + __final_right); ++__final_left; --__final_right; } // All elements on the left side are < piv, all elements on the // right are >= piv delete[] __reserved_left; delete[] __reserved_right; // Element "between" __final_left and __final_right might not have // been regarded yet if (__final_left < __n && !__pred(__begin[__final_left])) // Really swapped. return __final_left; else return __final_left + 1; } /** * @brief Parallel implementation of std::nth_element(). * @param __begin Begin iterator of input sequence. * @param __nth _Iterator of element that must be in position afterwards. * @param __end End iterator of input sequence. * @param __comp Comparator. */ template<typename _RAIter, typename _Compare> void __parallel_nth_element(_RAIter __begin, _RAIter __nth, _RAIter __end, _Compare __comp) { typedef std::iterator_traits<_RAIter> _TraitsType; typedef typename _TraitsType::value_type _ValueType; typedef typename _TraitsType::difference_type _DifferenceType; _GLIBCXX_CALL(__end - __begin) _RAIter __split; _RandomNumber __rng; const _Settings& __s = _Settings::get(); _DifferenceType __minimum_length = std::max<_DifferenceType>(2, std::max(__s.nth_element_minimal_n, __s.partition_minimal_n)); // Break if input range to small. while (static_cast<_SequenceIndex>(__end - __begin) >= __minimum_length) { _DifferenceType __n = __end - __begin; _RAIter __pivot_pos = __begin + __rng(__n); // Swap __pivot_pos value to end. if (__pivot_pos != (__end - 1)) std::iter_swap(__pivot_pos, __end - 1); __pivot_pos = __end - 1; // _Compare must have first_value_type, second_value_type, // result_type // _Compare == // __gnu_parallel::_Lexicographic<S, int, // __gnu_parallel::_Less<S, S> > // __pivot_pos == std::pair<S, int>* __gnu_parallel::__binder2nd<_Compare, _ValueType, _ValueType, bool> __pred(__comp, *__pivot_pos); // Divide, leave pivot unchanged in last place. _RAIter __split_pos1, __split_pos2; __split_pos1 = __begin + __parallel_partition(__begin, __end - 1, __pred, __get_max_threads()); // Left side: < __pivot_pos; __right side: >= __pivot_pos // Swap pivot back to middle. if (__split_pos1 != __pivot_pos) std::iter_swap(__split_pos1, __pivot_pos); __pivot_pos = __split_pos1; // In case all elements are equal, __split_pos1 == 0 if ((__split_pos1 + 1 - __begin) < (__n >> 7) || (__end - __split_pos1) < (__n >> 7)) { // Very unequal split, one part smaller than one 128th // elements not strictly larger than the pivot. __gnu_parallel::__unary_negate<__gnu_parallel:: __binder1st<_Compare, _ValueType, _ValueType, bool>, _ValueType> __pred(__gnu_parallel::__binder1st<_Compare, _ValueType, _ValueType, bool>(__comp, *__pivot_pos)); // Find other end of pivot-equal range. __split_pos2 = __gnu_sequential::partition(__split_pos1 + 1, __end, __pred); } else // Only skip the pivot. __split_pos2 = __split_pos1 + 1; // Compare iterators. if (__split_pos2 <= __nth) __begin = __split_pos2; else if (__nth < __split_pos1) __end = __split_pos1; else break; } // Only at most _Settings::partition_minimal_n __elements __left. __gnu_sequential::nth_element(__begin, __nth, __end, __comp); } /** @brief Parallel implementation of std::partial_sort(). * @param __begin Begin iterator of input sequence. * @param __middle Sort until this position. * @param __end End iterator of input sequence. * @param __comp Comparator. */ template<typename _RAIter, typename _Compare> void __parallel_partial_sort(_RAIter __begin, _RAIter __middle, _RAIter __end, _Compare __comp) { __parallel_nth_element(__begin, __middle, __end, __comp); std::sort(__begin, __middle, __comp); } } //namespace __gnu_parallel #undef _GLIBCXX_VOLATILE #endif /* _GLIBCXX_PARALLEL_PARTITION_H */