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+// -*- 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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+
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+#ifndef _LIBCPP_SEMAPHORE
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+#define _LIBCPP_SEMAPHORE
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+
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+/*
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+ semaphore synopsis
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+
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+namespace std {
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+
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+template<ptrdiff_t least_max_value = implementation-defined>
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+class counting_semaphore
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+{
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+public:
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+static constexpr ptrdiff_t max() noexcept;
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+
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+constexpr explicit counting_semaphore(ptrdiff_t desired);
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+~counting_semaphore();
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+
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+counting_semaphore(const counting_semaphore&) = delete;
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+counting_semaphore& operator=(const counting_semaphore&) = delete;
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+
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+void release(ptrdiff_t update = 1);
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+void acquire();
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+bool try_acquire() noexcept;
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+template<class Rep, class Period>
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+ bool try_acquire_for(const chrono::duration<Rep, Period>& rel_time);
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+template<class Clock, class Duration>
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+ bool try_acquire_until(const chrono::time_point<Clock, Duration>& abs_time);
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+
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+private:
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+ptrdiff_t counter; // exposition only
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+};
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+
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+using binary_semaphore = counting_semaphore<1>;
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+
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+}
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+
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+*/
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+
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+#include <__availability>
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+#include <__config>
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+#include <__thread/timed_backoff_policy.h>
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+#include <__threading_support>
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+#include <atomic>
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+#include <version>
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+
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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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+
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+#ifdef _LIBCPP_HAS_NO_THREADS
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+# error <semaphore> is not supported on this single threaded system
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+#endif
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+
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+_LIBCPP_PUSH_MACROS
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+#include <__undef_macros>
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+
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+#if _LIBCPP_STD_VER >= 14
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+
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+_LIBCPP_BEGIN_NAMESPACE_STD
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+
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+/*
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+
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+__atomic_semaphore_base is the general-case implementation.
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+It is a typical Dijkstra semaphore algorithm over atomics, wait and notify
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+functions. It avoids contention against users' own use of those facilities.
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+
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+*/
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+
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+class __atomic_semaphore_base
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+{
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+ __atomic_base<ptrdiff_t> __a;
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+
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+public:
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+ _LIBCPP_INLINE_VISIBILITY
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+ constexpr explicit __atomic_semaphore_base(ptrdiff_t __count) : __a(__count)
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+ {
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+ }
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+ _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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+ void release(ptrdiff_t __update = 1)
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+ {
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+ if(0 < __a.fetch_add(__update, memory_order_release))
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+ ;
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+ else if(__update > 1)
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+ __a.notify_all();
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+ else
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+ __a.notify_one();
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+ }
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+ _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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+ void acquire()
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+ {
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+ auto const __test_fn = [this]() -> bool {
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+ auto __old = __a.load(memory_order_relaxed);
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+ return (__old != 0) && __a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed);
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+ };
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+ __cxx_atomic_wait(&__a.__a_, __test_fn);
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+ }
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+ template <class Rep, class Period>
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+ _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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+ bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
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+ {
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+ if (__rel_time == chrono::duration<Rep, Period>::zero())
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+ return try_acquire();
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+ auto const __test_fn = [this]() { return try_acquire(); };
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+ return __libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy(), __rel_time);
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+ }
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+ _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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+ bool try_acquire()
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+ {
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+ auto __old = __a.load(memory_order_acquire);
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+ while (true) {
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+ if (__old == 0)
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+ return false;
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+ if (__a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed))
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+ return true;
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+ }
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+ }
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+};
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+
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+#define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
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+
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+template<ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
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+class counting_semaphore
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+{
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+ __atomic_semaphore_base __semaphore;
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+
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+public:
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+ static constexpr ptrdiff_t max() noexcept {
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+ return __least_max_value;
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+ }
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+
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+ _LIBCPP_INLINE_VISIBILITY
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+ constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore(__count) { }
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+ ~counting_semaphore() = default;
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+
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+ counting_semaphore(const counting_semaphore&) = delete;
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+ counting_semaphore& operator=(const counting_semaphore&) = delete;
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+
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+ _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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+ void release(ptrdiff_t __update = 1)
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+ {
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+ __semaphore.release(__update);
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+ }
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+ _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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+ void acquire()
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+ {
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+ __semaphore.acquire();
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+ }
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+ template<class Rep, class Period>
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+ _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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+ bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
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+ {
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+ return __semaphore.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
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+ }
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+ _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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+ bool try_acquire()
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+ {
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+ return __semaphore.try_acquire();
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+ }
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+ template <class Clock, class Duration>
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+ _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
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+ bool try_acquire_until(chrono::time_point<Clock, Duration> const& __abs_time)
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+ {
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+ auto const current = Clock::now();
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+ if (current >= __abs_time)
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+ return try_acquire();
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+ else
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+ return try_acquire_for(__abs_time - current);
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+ }
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+};
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+
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+using binary_semaphore = counting_semaphore<1>;
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+
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+_LIBCPP_END_NAMESPACE_STD
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+
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+#endif // _LIBCPP_STD_VER >= 14
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+
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+_LIBCPP_POP_MACROS
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+
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+#endif //_LIBCPP_SEMAPHORE
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