0981fe1017
thread-pool.h requires CXX_STD_THREAD in order to even be included. However, there's no deep reason for this, and during review we found that one patch in the new DWARF indexer series unconditionally requires the thread pool. Because the thread pool already allows a task to be run in the calling thread (for example if it is configured to have no threads in the pool), it seemed straightforward to make this code ok to use when host threads aren't available at all. This patch implements this idea. I built it on a thread-less host (mingw, before my recent configure patch) and verified that the result builds. After the thread-pool change, parallel-for.h no longer needs any CXX_STD_THREAD checks at all, so this patch removes these as well.
79 lines
2.4 KiB
C++
79 lines
2.4 KiB
C++
/* 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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#include "gdbsupport/thread-pool.h"
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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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/* 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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/* Process all the remaining elements in the main thread. */
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callback (first, last);
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for (int i = 0; i < n_actual_threads; ++i)
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futures[i].wait ();
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}
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}
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#endif /* GDBSUPPORT_PARALLEL_FOR_H */
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