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/* Synchronized work queue.
Copyright (C) 2025 Free Software Foundation, Inc.
This file is part of GDB.
This program 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 of the License, or
(at your option) any later version.
This program 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.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */
#ifndef GDBSUPPORT_WORK_QUEUE_H
#define GDBSUPPORT_WORK_QUEUE_H
#include "gdbsupport/iterator-range.h"
namespace gdb
{
/* Implementation of a thread-safe work queue.
The work items are specified by two iterators of type RandomIt.
BATCH_SIZE is the number of work items to pop in a batch. */
template<typename RandomIt, std::size_t batch_size>
class work_queue
{
public:
/* The work items are specified by the range `[first, last[`. */
work_queue (const RandomIt first, const RandomIt last) noexcept
: m_next (first),
m_last (last)
{
gdb_assert (first <= last);
}
DISABLE_COPY_AND_ASSIGN (work_queue);
/* Pop a batch of work items.
The return value is an iterator range delimiting the work items. */
iterator_range<RandomIt> pop_batch () noexcept
{
for (;;)
{
/* Grab a snapshot of M_NEXT. */
auto next = m_next.load ();
gdb_assert (next <= m_last);
/* The number of items remaining in the queue. */
const auto n_remaining = static_cast<std::size_t> (m_last - next);
/* Are we done? */
if (n_remaining == 0)
return { m_last, m_last };
/* The batch size is proportional to the number of items remaining in
the queue. We do this to try to stike a balance, avoiding
synchronization overhead when there are many items to process at the
start, and avoiding workload imbalance when there are few items to
process at the end. */
const auto this_batch_size = std::min (batch_size, n_remaining);
/* The range of items in this batch. */
const auto this_batch_first = next;
const auto this_batch_last = next + this_batch_size;
/* Update M_NEXT. If the current value of M_NEXT doesn't match NEXT, it
means another thread updated it concurrently, restart. */
if (!m_next.compare_exchange_weak (next, this_batch_last))
continue;
return { this_batch_first, this_batch_last };
}
}
private:
/* The next work item to hand out. */
std::atomic<RandomIt> m_next;
/* The end of the work item range. */
RandomIt m_last;
};
} /* namespace gdb */
#endif /* GDBSUPPORT_WORK_QUEUE_H */
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