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129 lines
3.9 KiB
C++
129 lines
3.9 KiB
C++
// <latch> -*- C++ -*-
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// Copyright (C) 2020-2026 Free Software Foundation, Inc.
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//
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// This file is part of the GNU ISO C++ Library. This library is free
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// software; you can redistribute it and/or modify it under the
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// terms of the GNU General Public License as published by the
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// Free Software Foundation; either version 3, or (at your option)
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// any later version.
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// This library is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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// Under Section 7 of GPL version 3, you are granted additional
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// permissions described in the GCC Runtime Library Exception, version
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// 3.1, as published by the Free Software Foundation.
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// You should have received a copy of the GNU General Public License and
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// a copy of the GCC Runtime Library Exception along with this program;
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// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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// <http://www.gnu.org/licenses/>.
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/** @file include/latch
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* This is a Standard C++ Library header.
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*/
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#ifndef _GLIBCXX_LATCH
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#define _GLIBCXX_LATCH 1
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#ifdef _GLIBCXX_SYSHDR
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#pragma GCC system_header
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#endif
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#include <bits/requires_hosted.h> // concurrency
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#define __glibcxx_want_latch
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#include <bits/version.h>
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#ifdef __cpp_lib_latch // C++ >= 20 && atomic_wait
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#include <bits/atomic_base.h>
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#include <ext/numeric_traits.h>
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#include <bits/intcmp.h> // cmp_equal, cmp_less_equal, etc.
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namespace std _GLIBCXX_VISIBILITY(default)
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{
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_GLIBCXX_BEGIN_NAMESPACE_VERSION
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class latch
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{
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public:
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static constexpr ptrdiff_t
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max() noexcept
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{
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using __gnu_cxx::__int_traits;
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constexpr auto __max = __int_traits<__detail::__platform_wait_t>::__max;
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if constexpr (std::cmp_less(__max, __PTRDIFF_MAX__))
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return __max;
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return __PTRDIFF_MAX__;
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}
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constexpr explicit
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latch(ptrdiff_t __expected) noexcept
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: _M_counter(__expected)
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{ __glibcxx_assert(__expected >= 0 && __expected <= max()); }
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~latch() = default;
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latch(const latch&) = delete;
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latch& operator=(const latch&) = delete;
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_GLIBCXX_ALWAYS_INLINE void
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count_down(ptrdiff_t __update = 1)
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{
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__glibcxx_assert(__update >= 0 && __update <= max());
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auto const __old = __atomic_impl::fetch_sub(&_M_counter, __update,
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memory_order::release);
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if (std::cmp_equal(__old, __update))
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__atomic_impl::notify_all(&_M_counter);
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else
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__glibcxx_assert(std::cmp_less(__update, __old));
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}
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_GLIBCXX_ALWAYS_INLINE bool
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try_wait() const noexcept
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{ return __atomic_impl::load(&_M_counter, memory_order::acquire) == 0; }
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_GLIBCXX_ALWAYS_INLINE void
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wait() const noexcept
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{
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auto const __vfn = [this] {
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return __atomic_impl::load(&_M_counter, memory_order::acquire);
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};
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auto const __pred = [](__detail::__platform_wait_t __v) {
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return __v == 0;
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};
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std::__atomic_wait_address(&_M_counter, __pred, __vfn);
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}
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_GLIBCXX_ALWAYS_INLINE void
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arrive_and_wait(ptrdiff_t __update = 1) noexcept
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{
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// The standard specifies this functions as count_down(update); wait();
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// but we combine those two calls into one and avoid the wait() if we
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// know the counter reached zero.
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__glibcxx_assert(__update >= 0 && __update <= max());
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// Use acq_rel here because an omitted wait() would have used acquire:
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auto const __old = __atomic_impl::fetch_sub(&_M_counter, __update,
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memory_order::acq_rel);
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if (std::cmp_equal(__old, __update))
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__atomic_impl::notify_all(&_M_counter);
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else
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{
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__glibcxx_assert(std::cmp_less(__update, __old));
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wait();
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}
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}
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private:
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alignas(__detail::__platform_wait_alignment)
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__detail::__platform_wait_t _M_counter;
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};
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_GLIBCXX_END_NAMESPACE_VERSION
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} // namespace
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#endif // __cpp_lib_latch
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#endif // _GLIBCXX_LATCH
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