Upgrade to 2022-era LLVM LIBCXX

third_party/libcxx/__thread/poll_with_backoff.h

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+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
+#define _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
+
+#include <__availability>
+#include <__chrono/duration.h>
+#include <__chrono/high_resolution_clock.h>
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+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>
+_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI
+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;
+        continue;
+      }
+      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 {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
+  bool operator()(chrono::nanoseconds const&) const {
+      return false;
+  }
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___THREAD_POLL_WITH_BACKOFF_H

third_party/libcxx/__thread/timed_backoff_policy.h

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+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___THREAD_TIMED_BACKOFF_POLICY_H
+#define _LIBCPP___THREAD_TIMED_BACKOFF_POLICY_H
+
+#include <__config>
+
+#ifndef _LIBCPP_HAS_NO_THREADS
+
+#  include <__chrono/duration.h>
+#  include <__threading_support>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+struct __libcpp_timed_backoff_policy {
+  _LIBCPP_INLINE_VISIBILITY
+  bool operator()(chrono::nanoseconds __elapsed) const
+  {
+      if(__elapsed > chrono::milliseconds(128))
+          __libcpp_thread_sleep_for(chrono::milliseconds(8));
+      else if(__elapsed > chrono::microseconds(64))
+          __libcpp_thread_sleep_for(__elapsed / 2);
+      else if(__elapsed > chrono::microseconds(4))
+        __libcpp_thread_yield();
+      else
+        {} // poll
+      return false;
+  }
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_HAS_NO_THREADS
+
+#endif // _LIBCPP___THREAD_TIMED_BACKOFF_POLICY_H