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      1 // <latch> -*- C++ -*-
      2 
      3 // Copyright (C) 2020-2024 Free Software Foundation, Inc.
      4 //
      5 // This file is part of the GNU ISO C++ Library.  This library is free
      6 // software; you can redistribute it and/or modify it under the
      7 // terms of the GNU General Public License as published by the
      8 // Free Software Foundation; either version 3, or (at your option)
      9 // any later version.
     10 
     11 // This library is distributed in the hope that it will be useful,
     12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
     13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
     14 // GNU General Public License for more details.
     15 
     16 // Under Section 7 of GPL version 3, you are granted additional
     17 // permissions described in the GCC Runtime Library Exception, version
     18 // 3.1, as published by the Free Software Foundation.
     19 
     20 // You should have received a copy of the GNU General Public License and
     21 // a copy of the GCC Runtime Library Exception along with this program;
     22 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
     23 // <http://www.gnu.org/licenses/>.
     24 
     25 /** @file include/latch
     26  *  This is a Standard C++ Library header.
     27  */
     28 
     29 #ifndef _GLIBCXX_LATCH
     30 #define _GLIBCXX_LATCH 1
     31 
     32 #pragma GCC system_header
     33 
     34 #include <bits/requires_hosted.h> // concurrency
     35 
     36 #define __glibcxx_want_latch
     37 #include <bits/version.h>
     38 
     39 #ifdef __cpp_lib_latch // C++ >= 20 && atomic_wait
     40 #include <bits/atomic_base.h>
     41 #include <ext/numeric_traits.h>
     42 
     43 namespace std _GLIBCXX_VISIBILITY(default)
     44 {
     45 _GLIBCXX_BEGIN_NAMESPACE_VERSION
     46 
     47   class latch
     48   {
     49   public:
     50     static constexpr ptrdiff_t
     51     max() noexcept
     52     { return __gnu_cxx::__int_traits<__detail::__platform_wait_t>::__max; }
     53 
     54     constexpr explicit latch(ptrdiff_t __expected) noexcept
     55       : _M_a(__expected) { }
     56 
     57     ~latch() = default;
     58     latch(const latch&) = delete;
     59     latch& operator=(const latch&) = delete;
     60 
     61     _GLIBCXX_ALWAYS_INLINE void
     62     count_down(ptrdiff_t __update = 1)
     63     {
     64       auto const __old = __atomic_impl::fetch_sub(&_M_a,
     65 				    __update, memory_order::release);
     66       if (__old == __update)
     67 	__atomic_impl::notify_all(&_M_a);
     68     }
     69 
     70     _GLIBCXX_ALWAYS_INLINE bool
     71     try_wait() const noexcept
     72     { return __atomic_impl::load(&_M_a, memory_order::acquire) == 0; }
     73 
     74     _GLIBCXX_ALWAYS_INLINE void
     75     wait() const noexcept
     76     {
     77       auto const __pred = [this] { return this->try_wait(); };
     78       std::__atomic_wait_address(&_M_a, __pred);
     79     }
     80 
     81     _GLIBCXX_ALWAYS_INLINE void
     82     arrive_and_wait(ptrdiff_t __update = 1) noexcept
     83     {
     84       count_down(__update);
     85       wait();
     86     }
     87 
     88   private:
     89     alignas(__alignof__(__detail::__platform_wait_t)) __detail::__platform_wait_t _M_a;
     90   };
     91 _GLIBCXX_END_NAMESPACE_VERSION
     92 } // namespace
     93 #endif // __cpp_lib_latch
     94 #endif // _GLIBCXX_LATCH
     95