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1// -*- C++ -*-
2//===----------------------------------------------------------------------===//
3//
4// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5// See https://llvm.org/LICENSE.txt for license information.
6// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7//
8//===----------------------------------------------------------------------===//
9
10#ifndef _LIBCPP_SEMAPHORE
11#define _LIBCPP_SEMAPHORE
12
13/*
14    semaphore synopsis
15
16namespace std {
17
18template<ptrdiff_t least_max_value = implementation-defined>
19class counting_semaphore                          // since C++20
20{
21public:
22static constexpr ptrdiff_t max() noexcept;
23
24constexpr explicit counting_semaphore(ptrdiff_t desired);
25~counting_semaphore();
26
27counting_semaphore(const counting_semaphore&) = delete;
28counting_semaphore& operator=(const counting_semaphore&) = delete;
29
30void release(ptrdiff_t update = 1);
31void acquire();
32bool try_acquire() noexcept;
33template<class Rep, class Period>
34    bool try_acquire_for(const chrono::duration<Rep, Period>& rel_time);
35template<class Clock, class Duration>
36    bool try_acquire_until(const chrono::time_point<Clock, Duration>& abs_time);
37
38private:
39ptrdiff_t counter; // exposition only
40};
41
42using binary_semaphore = counting_semaphore<1>; // since C++20
43
44}
45
46*/
47
48#include <__config>
49
50#if _LIBCPP_HAS_THREADS
51
52#  include <__assert>
53#  include <__atomic/atomic.h>
54#  include <__atomic/atomic_sync.h>
55#  include <__atomic/memory_order.h>
56#  include <__chrono/time_point.h>
57#  include <__cstddef/ptrdiff_t.h>
58#  include <__thread/poll_with_backoff.h>
59#  include <__thread/support.h>
60#  include <__thread/timed_backoff_policy.h>
61#  include <limits>
62#  include <version>
63
64#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
65#    pragma GCC system_header
66#  endif
67
68_LIBCPP_PUSH_MACROS
69#  include <__undef_macros>
70
71#  if _LIBCPP_STD_VER >= 20
72
73_LIBCPP_BEGIN_NAMESPACE_STD
74
75/*
76
77__atomic_semaphore_base is the general-case implementation.
78It is a typical Dijkstra semaphore algorithm over atomics, wait and notify
79functions. It avoids contention against users' own use of those facilities.
80
81*/
82
83#    define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
84
85class __atomic_semaphore_base {
86  atomic<ptrdiff_t> __a_;
87
88public:
89  _LIBCPP_HIDE_FROM_ABI constexpr explicit __atomic_semaphore_base(ptrdiff_t __count) : __a_(__count) {}
90  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void release(ptrdiff_t __update = 1) {
91    auto __old = __a_.fetch_add(__update, memory_order_release);
92    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
93        __update <= _LIBCPP_SEMAPHORE_MAX - __old, "update is greater than the expected value");
94    if (__old == 0) {
95      __a_.notify_all();
96    }
97  }
98  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void acquire() {
99    std::__atomic_wait_unless(__a_, memory_order_relaxed, [this](ptrdiff_t& __old) {
100      return __try_acquire_impl(__old);
101    });
102  }
103  template <class _Rep, class _Period>
104  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool
105  try_acquire_for(chrono::duration<_Rep, _Period> const& __rel_time) {
106    if (__rel_time == chrono::duration<_Rep, _Period>::zero())
107      return try_acquire();
108    auto const __poll_fn = [this]() { return try_acquire(); };
109    return std::__libcpp_thread_poll_with_backoff(__poll_fn, __libcpp_timed_backoff_policy(), __rel_time);
110  }
111  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool try_acquire() {
112    auto __old = __a_.load(memory_order_relaxed);
113    return __try_acquire_impl(__old);
114  }
115
116private:
117  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool __try_acquire_impl(ptrdiff_t& __old) {
118    while (true) {
119      if (__old == 0)
120        return false;
121      if (__a_.compare_exchange_weak(__old, __old - 1, memory_order_acquire, memory_order_relaxed))
122        return true;
123    }
124  }
125};
126
127template <ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
128class counting_semaphore {
129  __atomic_semaphore_base __semaphore_;
130
131public:
132  static_assert(__least_max_value >= 0, "The least maximum value must be a positive number");
133
134  static constexpr ptrdiff_t max() noexcept { return __least_max_value; }
135
136  _LIBCPP_HIDE_FROM_ABI constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore_(__count) {
137    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
138        __count >= 0,
139        "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
140        "initialized with a negative value");
141    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
142        __count <= max(),
143        "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
144        "initialized with a value greater than max()");
145  }
146  ~counting_semaphore() = default;
147
148  counting_semaphore(const counting_semaphore&)            = delete;
149  counting_semaphore& operator=(const counting_semaphore&) = delete;
150
151  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void release(ptrdiff_t __update = 1) {
152    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(__update >= 0, "counting_semaphore:release called with a negative value");
153    __semaphore_.release(__update);
154  }
155  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void acquire() { __semaphore_.acquire(); }
156  template <class _Rep, class _Period>
157  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool
158  try_acquire_for(chrono::duration<_Rep, _Period> const& __rel_time) {
159    return __semaphore_.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
160  }
161  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool try_acquire() { return __semaphore_.try_acquire(); }
162  template <class _Clock, class _Duration>
163  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool
164  try_acquire_until(chrono::time_point<_Clock, _Duration> const& __abs_time) {
165    auto const __current = _Clock::now();
166    if (__current >= __abs_time)
167      return try_acquire();
168    else
169      return try_acquire_for(__abs_time - __current);
170  }
171};
172
173using binary_semaphore = counting_semaphore<1>;
174
175_LIBCPP_END_NAMESPACE_STD
176
177#  endif // _LIBCPP_STD_VER >= 20
178
179_LIBCPP_POP_MACROS
180
181#endif // _LIBCPP_HAS_THREADS
182
183#if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
184#  include <atomic>
185#  include <cstddef>
186#endif
187
188#endif // _LIBCPP_SEMAPHORE
189