libcxx/include/latch

// -*- C++ -*-
//===----------------------------------------------------------------------===//
//
// 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_LATCH
#define _LIBCPP_LATCH

/*
    latch synopsis

namespace std
{

  class latch                                     // since C++20
  {
  public:
    static constexpr ptrdiff_t max() noexcept;

    constexpr explicit latch(ptrdiff_t __expected);
    ~latch();

    latch(const latch&) = delete;
    latch& operator=(const latch&) = delete;

    void count_down(ptrdiff_t __update = 1);
    bool try_wait() const noexcept;
    void wait() const;
    void arrive_and_wait(ptrdiff_t __update = 1);

  private:
    ptrdiff_t __counter; // exposition only
  };

}

*/

#if __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
#  include <__cxx03/__config>
#else
#  include <__config>

#  if _LIBCPP_HAS_THREADS

#    include <__assert>
#    include <__atomic/atomic.h>
#    include <__atomic/atomic_sync.h>
#    include <__atomic/memory_order.h>
#    include <__cstddef/ptrdiff_t.h>
#    include <limits>
#    include <version>

#    if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#      pragma GCC system_header
#    endif

_LIBCPP_PUSH_MACROS
#    include <__undef_macros>

#    if _LIBCPP_STD_VER >= 20

_LIBCPP_BEGIN_NAMESPACE_STD

class latch {
  atomic<ptrdiff_t> __a_;

public:
  [[nodiscard]] static _LIBCPP_HIDE_FROM_ABI constexpr ptrdiff_t max() noexcept {
    return numeric_limits<ptrdiff_t>::max();
  }

  inline _LIBCPP_HIDE_FROM_ABI constexpr explicit latch(ptrdiff_t __expected) : __a_(__expected) {
    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
        __expected >= 0,
        "latch::latch(ptrdiff_t): latch cannot be "
        "initialized with a negative value");
    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
        __expected <= max(),
        "latch::latch(ptrdiff_t): latch cannot be "
        "initialized with a value greater than max()");
  }

  _LIBCPP_HIDE_FROM_ABI ~latch() = default;
  latch(const latch&)            = delete;
  latch& operator=(const latch&) = delete;

  inline _LIBCPP_HIDE_FROM_ABI void count_down(ptrdiff_t __update = 1) {
    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(__update >= 0, "latch::count_down called with a negative value");
    auto const __old = __a_.fetch_sub(__update, memory_order_release);
    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
        __update <= __old,
        "latch::count_down called with a value greater "
        "than the internal counter");
    if (__old == __update)
      __a_.notify_all();
  }
  [[nodiscard]] inline _LIBCPP_HIDE_FROM_ABI bool try_wait() const noexcept {
    auto __value = __a_.load(memory_order_acquire);
    return try_wait_impl(__value);
  }
  inline _LIBCPP_HIDE_FROM_ABI void wait() const {
    std::__atomic_wait_unless(__a_, memory_order_acquire, [this](ptrdiff_t& __value) -> bool {
      return try_wait_impl(__value);
    });
  }
  inline _LIBCPP_HIDE_FROM_ABI void arrive_and_wait(ptrdiff_t __update = 1) {
    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(__update >= 0, "latch::arrive_and_wait called with a negative value");
    // other preconditions on __update are checked in count_down()

    count_down(__update);
    wait();
  }

private:
  _LIBCPP_HIDE_FROM_ABI bool try_wait_impl(ptrdiff_t& __value) const noexcept { return __value == 0; }
};

_LIBCPP_END_NAMESPACE_STD

#    endif // _LIBCPP_STD_VER >= 20

_LIBCPP_POP_MACROS

#  endif // _LIBCPP_HAS_THREADS

#endif // __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)

#endif // _LIBCPP_LATCH