latch

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// <latch> -*- C++ -*-
 
// Copyright (C) 2020-2025 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
 
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
 
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
 
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.
 
/** @file include/latch
 *  This is a Standard C++ Library header.
 */
 
#ifndef _GLIBCXX_LATCH
#define _GLIBCXX_LATCH 1
 
#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif
 
#include <bits/requires_hosted.h> // concurrency
 
#define __glibcxx_want_latch
#include <bits/version.h>
 
#ifdef __cpp_lib_latch // C++ >= 20 && atomic_wait
#include <bits/atomic_base.h>
#include <ext/numeric_traits.h>
#include <utility> // cmp_equal, cmp_less_equal, etc.
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
  class latch
  {
  public:
    static constexpr ptrdiff_t
    max() noexcept
    {
      using __gnu_cxx::__int_traits;
      constexpr auto __max = __int_traits<__detail::__platform_wait_t>::__max;
      if constexpr (std::cmp_less(__max, __PTRDIFF_MAX__))
        return __max;
      return __PTRDIFF_MAX__;
    }
 
    constexpr explicit
    latch(ptrdiff_t __expected) noexcept
    : _M_counter(__expected)
    { __glibcxx_assert(__expected >= 0 && __expected <= max()); }
 
    ~latch() = default;
 
    latch(const latch&) = delete;
    latch& operator=(const latch&) = delete;
 
    _GLIBCXX_ALWAYS_INLINE void
    count_down(ptrdiff_t __update = 1)
    {
      __glibcxx_assert(__update >= 0 && __update <= max());
      auto const __old = __atomic_impl::fetch_sub(&_M_counter, __update,
                                                  memory_order::release);
      if (std::cmp_equal(__old, __update))
        __atomic_impl::notify_all(&_M_counter);
      else
        __glibcxx_assert(std::cmp_less(__update, __old));
    }
 
    _GLIBCXX_ALWAYS_INLINE bool
    try_wait() const noexcept
    { return __atomic_impl::load(&_M_counter, memory_order::acquire) == 0; }
 
    _GLIBCXX_ALWAYS_INLINE void
    wait() const noexcept
    {
      auto const __vfn = [this] {
        return __atomic_impl::load(&_M_counter, memory_order::acquire);
      };
      auto const __pred = [](__detail::__platform_wait_t __v) {
        return __v == 0;
      };
      std::__atomic_wait_address(&_M_counter, __pred, __vfn);
    }
 
    _GLIBCXX_ALWAYS_INLINE void
    arrive_and_wait(ptrdiff_t __update = 1) noexcept
    {
      // The standard specifies this functions as count_down(update); wait();
      // but we combine those two calls into one and avoid the wait() if we
      // know the counter reached zero.
 
      __glibcxx_assert(__update >= 0 && __update <= max());
      // Use acq_rel here because an omitted wait() would have used acquire:
      auto const __old = __atomic_impl::fetch_sub(&_M_counter, __update,
                                                  memory_order::acq_rel);
      if (std::cmp_equal(__old, __update))
        __atomic_impl::notify_all(&_M_counter);
      else
        {
          __glibcxx_assert(std::cmp_less(__update, __old));
          wait();
        }
    }
 
  private:
    alignas(__detail::__platform_wait_alignment)
      __detail::__platform_wait_t _M_counter;
  };
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
#endif // __cpp_lib_latch
#endif // _GLIBCXX_LATCH