forked from OSchip/llvm-project
74 lines
2.8 KiB
C++
74 lines
2.8 KiB
C++
// -*- C++ -*-
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//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
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#define _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
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#include <__availability>
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#include <__chrono/duration.h>
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#include <__chrono/high_resolution_clock.h>
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#include <__chrono/steady_clock.h>
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#include <__chrono/time_point.h>
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#include <__config>
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#include <__filesystem/file_time_type.h>
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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# pragma GCC system_header
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#endif
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_LIBCPP_BEGIN_NAMESPACE_STD
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static _LIBCPP_CONSTEXPR const int __libcpp_polling_count = 64;
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// Polls a thread for a condition given by a predicate, and backs off based on a backoff policy
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// before polling again.
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//
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// - __f is the "test function" that should return true if polling succeeded, and false if it failed.
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//
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// - __bf is the "backoff policy", which is called with the duration since we started polling. It should
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// return false in order to resume polling, and true if polling should stop entirely for some reason.
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// In general, backoff policies sleep for some time before returning control to the polling loop.
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//
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// - __max_elapsed is the maximum duration to try polling for. If the maximum duration is exceeded,
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// the polling loop will return false to report a timeout.
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template<class _Fn, class _BFn>
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_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI
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bool __libcpp_thread_poll_with_backoff(_Fn&& __f, _BFn&& __bf, chrono::nanoseconds __max_elapsed = chrono::nanoseconds::zero()) {
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auto const __start = chrono::high_resolution_clock::now();
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for (int __count = 0;;) {
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if (__f())
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return true; // _Fn completion means success
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if (__count < __libcpp_polling_count) {
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__count += 1;
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continue;
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}
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chrono::nanoseconds const __elapsed = chrono::high_resolution_clock::now() - __start;
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if (__max_elapsed != chrono::nanoseconds::zero() && __max_elapsed < __elapsed)
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return false; // timeout failure
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if (__bf(__elapsed))
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return false; // _BFn completion means failure
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}
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}
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// A trivial backoff policy that always immediately returns the control to
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// the polling loop.
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//
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// This is not very well-behaved since it will cause the polling loop to spin,
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// so this should most likely only be used on single-threaded systems where there
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// are no other threads to compete with.
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struct __spinning_backoff_policy {
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
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bool operator()(chrono::nanoseconds const&) const {
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return false;
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}
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};
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
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