87 lines
2.5 KiB
C
87 lines
2.5 KiB
C
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/* Parallel for loops
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Copyright (C) 2019-2022 Free Software Foundation, Inc.
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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#ifndef GDBSUPPORT_PARALLEL_FOR_H
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#define GDBSUPPORT_PARALLEL_FOR_H
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#include <algorithm>
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#if CXX_STD_THREAD
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#include <thread>
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#include "gdbsupport/thread-pool.h"
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#endif
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namespace gdb
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{
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/* A very simple "parallel for". This splits the range of iterators
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into subranges, and then passes each subrange to the callback. The
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work may or may not be done in separate threads.
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This approach was chosen over having the callback work on single
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items because it makes it simple for the caller to do
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once-per-subrange initialization and destruction. */
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template<class RandomIt, class RangeFunction>
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void
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parallel_for_each (RandomIt first, RandomIt last, RangeFunction callback)
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{
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#if CXX_STD_THREAD
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/* So we can use a local array below. */
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const size_t local_max = 16;
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size_t n_threads = std::min (thread_pool::g_thread_pool->thread_count (),
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local_max);
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size_t n_actual_threads = 0;
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std::future<void> futures[local_max];
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size_t n_elements = last - first;
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if (n_threads > 1)
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{
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/* Arbitrarily require that there should be at least 10 elements
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in a thread. */
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if (n_elements / n_threads < 10)
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n_threads = std::max (n_elements / 10, (size_t) 1);
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size_t elts_per_thread = n_elements / n_threads;
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n_actual_threads = n_threads - 1;
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for (int i = 0; i < n_actual_threads; ++i)
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{
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RandomIt end = first + elts_per_thread;
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auto task = [=] ()
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{
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callback (first, end);
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};
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futures[i] = gdb::thread_pool::g_thread_pool->post_task (task);
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first = end;
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}
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}
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#endif /* CXX_STD_THREAD */
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/* Process all the remaining elements in the main thread. */
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callback (first, last);
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#if CXX_STD_THREAD
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for (int i = 0; i < n_actual_threads; ++i)
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futures[i].wait ();
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#endif /* CXX_STD_THREAD */
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}
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}
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#endif /* GDBSUPPORT_PARALLEL_FOR_H */
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