1// -*- C++ -*-
2//===--------------------------- latch -----------------------------------===//
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_LATCH
11#define _LIBCPP_LATCH
12
13/*
14    latch synopsis
15
16namespace std
17{
18
19  class latch
20  {
21  public:
22    static constexpr ptrdiff_t max() noexcept;
23
24    constexpr explicit latch(ptrdiff_t __expected);
25    ~latch();
26
27    latch(const latch&) = delete;
28    latch& operator=(const latch&) = delete;
29
30    void count_down(ptrdiff_t __update = 1);
31    bool try_wait() const noexcept;
32    void wait() const;
33    void arrive_and_wait(ptrdiff_t __update = 1);
34
35  private:
36    ptrdiff_t __counter; // exposition only
37  };
38
39}
40
41*/
42
43#include <__availability>
44#include <__config>
45#include <atomic>
46
47#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
48#pragma GCC system_header
49#endif
50
51#ifdef _LIBCPP_HAS_NO_THREADS
52# error <latch> is not supported on this single threaded system
53#endif
54
55_LIBCPP_PUSH_MACROS
56#include <__undef_macros>
57
58#if _LIBCPP_STD_VER >= 14
59
60_LIBCPP_BEGIN_NAMESPACE_STD
61
62class latch
63{
64    __atomic_base<ptrdiff_t> __a;
65
66public:
67    static constexpr ptrdiff_t max() noexcept {
68        return numeric_limits<ptrdiff_t>::max();
69    }
70
71    inline _LIBCPP_INLINE_VISIBILITY
72    constexpr explicit latch(ptrdiff_t __expected) : __a(__expected) { }
73
74    ~latch() = default;
75    latch(const latch&) = delete;
76    latch& operator=(const latch&) = delete;
77
78    inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
79    void count_down(ptrdiff_t __update = 1)
80    {
81        auto const __old = __a.fetch_sub(__update, memory_order_release);
82        if(__old == __update)
83            __a.notify_all();
84    }
85    inline _LIBCPP_INLINE_VISIBILITY
86    bool try_wait() const noexcept
87    {
88        return 0 == __a.load(memory_order_acquire);
89    }
90    inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
91    void wait() const
92    {
93        auto const __test_fn = [=]() -> bool {
94            return try_wait();
95        };
96        __cxx_atomic_wait(&__a.__a_, __test_fn);
97    }
98    inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
99    void arrive_and_wait(ptrdiff_t __update = 1)
100    {
101        count_down(__update);
102        wait();
103    }
104};
105
106_LIBCPP_END_NAMESPACE_STD
107
108#endif // _LIBCPP_STD_VER >= 14
109
110_LIBCPP_POP_MACROS
111
112#endif //_LIBCPP_LATCH
113