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_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#include <version>
47
48#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
49#pragma GCC system_header
50#endif
51
52#ifdef _LIBCPP_HAS_NO_THREADS
53# error <latch> is not supported on this single threaded system
54#endif
55
56_LIBCPP_PUSH_MACROS
57#include <__undef_macros>
58
59#if _LIBCPP_STD_VER >= 14
60
61_LIBCPP_BEGIN_NAMESPACE_STD
62
63class latch
64{
65    __atomic_base<ptrdiff_t> __a;
66
67public:
68    static constexpr ptrdiff_t max() noexcept {
69        return numeric_limits<ptrdiff_t>::max();
70    }
71
72    inline _LIBCPP_INLINE_VISIBILITY
73    constexpr explicit latch(ptrdiff_t __expected) : __a(__expected) { }
74
75    ~latch() = default;
76    latch(const latch&) = delete;
77    latch& operator=(const latch&) = delete;
78
79    inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
80    void count_down(ptrdiff_t __update = 1)
81    {
82        auto const __old = __a.fetch_sub(__update, memory_order_release);
83        if(__old == __update)
84            __a.notify_all();
85    }
86    inline _LIBCPP_INLINE_VISIBILITY
87    bool try_wait() const noexcept
88    {
89        return 0 == __a.load(memory_order_acquire);
90    }
91    inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
92    void wait() const
93    {
94        auto const __test_fn = [=]() -> bool {
95            return try_wait();
96        };
97        __cxx_atomic_wait(&__a.__a_, __test_fn);
98    }
99    inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
100    void arrive_and_wait(ptrdiff_t __update = 1)
101    {
102        count_down(__update);
103        wait();
104    }
105};
106
107_LIBCPP_END_NAMESPACE_STD
108
109#endif // _LIBCPP_STD_VER >= 14
110
111_LIBCPP_POP_MACROS
112
113#endif //_LIBCPP_LATCH
114