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