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