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
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>;
43
44}
45
46*/
47
48#include <__assert> // all public C++ headers provide the assertion handler
49#include <__availability>
50#include <__chrono/time_point.h>
51#include <__config>
52#include <__thread/timed_backoff_policy.h>
53#include <__threading_support>
54#include <atomic>
55#include <version>
56
57#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
58#  pragma GCC system_header
59#endif
60
61#ifdef _LIBCPP_HAS_NO_THREADS
62# error <semaphore> is not supported on this single threaded system
63#endif
64
65_LIBCPP_PUSH_MACROS
66#include <__undef_macros>
67
68#if _LIBCPP_STD_VER >= 14
69
70_LIBCPP_BEGIN_NAMESPACE_STD
71
72/*
73
74__atomic_semaphore_base is the general-case implementation.
75It is a typical Dijkstra semaphore algorithm over atomics, wait and notify
76functions. It avoids contention against users' own use of those facilities.
77
78*/
79
80class __atomic_semaphore_base
81{
82    __atomic_base<ptrdiff_t> __a;
83
84public:
85    _LIBCPP_INLINE_VISIBILITY
86    constexpr explicit __atomic_semaphore_base(ptrdiff_t __count) : __a(__count)
87    {
88    }
89    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
90    void release(ptrdiff_t __update = 1)
91    {
92        if(0 < __a.fetch_add(__update, memory_order_release))
93            ;
94        else if(__update > 1)
95            __a.notify_all();
96        else
97            __a.notify_one();
98    }
99    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
100    void acquire()
101    {
102        auto const __test_fn = [this]() -> bool {
103            auto __old = __a.load(memory_order_relaxed);
104            return (__old != 0) && __a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed);
105        };
106        __cxx_atomic_wait(&__a.__a_, __test_fn);
107    }
108    template <class Rep, class Period>
109    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
110    bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
111    {
112        if (__rel_time == chrono::duration<Rep, Period>::zero())
113            return try_acquire();
114        auto const __test_fn = [this]() { return try_acquire(); };
115        return __libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy(), __rel_time);
116    }
117    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
118    bool try_acquire()
119    {
120        auto __old = __a.load(memory_order_acquire);
121        while (true) {
122            if (__old == 0)
123                return false;
124            if (__a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed))
125                return true;
126        }
127    }
128};
129
130#define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
131
132template<ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
133class counting_semaphore
134{
135    __atomic_semaphore_base __semaphore;
136
137public:
138    static constexpr ptrdiff_t max() noexcept {
139        return __least_max_value;
140    }
141
142    _LIBCPP_INLINE_VISIBILITY
143    constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore(__count) { }
144    ~counting_semaphore() = default;
145
146    counting_semaphore(const counting_semaphore&) = delete;
147    counting_semaphore& operator=(const counting_semaphore&) = delete;
148
149    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
150    void release(ptrdiff_t __update = 1)
151    {
152        __semaphore.release(__update);
153    }
154    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
155    void acquire()
156    {
157        __semaphore.acquire();
158    }
159    template<class Rep, class Period>
160    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
161    bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
162    {
163        return __semaphore.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
164    }
165    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
166    bool try_acquire()
167    {
168        return __semaphore.try_acquire();
169    }
170    template <class Clock, class Duration>
171    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
172    bool try_acquire_until(chrono::time_point<Clock, Duration> const& __abs_time)
173    {
174        auto const current = Clock::now();
175        if (current >= __abs_time)
176            return try_acquire();
177        else
178            return try_acquire_for(__abs_time - current);
179    }
180};
181
182using binary_semaphore = counting_semaphore<1>;
183
184_LIBCPP_END_NAMESPACE_STD
185
186#endif // _LIBCPP_STD_VER >= 14
187
188_LIBCPP_POP_MACROS
189
190#endif //_LIBCPP_SEMAPHORE
191