70 lines
2.5 KiB
C++
70 lines
2.5 KiB
C++
/*
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* Distributed under the Boost Software License, Version 1.0.
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* (See accompanying file LICENSE_1_0.txt or copy at
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* http://www.boost.org/LICENSE_1_0.txt)
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*
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* Copyright (c) 2020 Andrey Semashev
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*/
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/*!
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* \file atomic/detail/fence_arch_ops_gcc_x86.hpp
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*
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* This header contains implementation of the \c fence_arch_operations struct.
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*/
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#ifndef BOOST_ATOMIC_DETAIL_FENCE_ARCH_OPS_GCC_X86_HPP_INCLUDED_
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#define BOOST_ATOMIC_DETAIL_FENCE_ARCH_OPS_GCC_X86_HPP_INCLUDED_
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#include <boost/memory_order.hpp>
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#include <boost/atomic/detail/config.hpp>
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#include <boost/atomic/detail/header.hpp>
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#ifdef BOOST_HAS_PRAGMA_ONCE
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#pragma once
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#endif
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namespace boost {
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namespace atomics {
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namespace detail {
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//! Fence operations for x86
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struct fence_arch_operations_gcc_x86
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{
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static BOOST_FORCEINLINE void thread_fence(memory_order order) BOOST_NOEXCEPT
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{
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if (order == memory_order_seq_cst)
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{
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// We could generate mfence for a seq_cst fence here, but a dummy lock-prefixed instruction is enough
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// and is faster than mfence on most modern x86 CPUs (as of 2020).
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// Note that we want to apply the atomic operation on any location so that:
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// - It is not shared with other threads. A variable on the stack suits this well.
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// - It is likely in cache. Being close to the top of the stack fits this well.
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// - It does not alias existing data on the stack, so that we don't introduce a false data dependency.
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// See some performance data here: https://shipilev.net/blog/2014/on-the-fence-with-dependencies/
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// Unfortunately, to make tools like valgrind happy, we have to initialize the dummy, which is
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// otherwise not needed.
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unsigned char dummy = 0u;
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__asm__ __volatile__ ("lock; notb %0" : "+m" (dummy) : : "memory");
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}
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else if ((static_cast< unsigned int >(order) & (static_cast< unsigned int >(memory_order_acquire) | static_cast< unsigned int >(memory_order_release))) != 0u)
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{
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__asm__ __volatile__ ("" ::: "memory");
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}
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}
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static BOOST_FORCEINLINE void signal_fence(memory_order order) BOOST_NOEXCEPT
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{
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if (order != memory_order_relaxed)
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__asm__ __volatile__ ("" ::: "memory");
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}
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};
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typedef fence_arch_operations_gcc_x86 fence_arch_operations;
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} // namespace detail
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} // namespace atomics
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} // namespace boost
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#include <boost/atomic/detail/footer.hpp>
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#endif // BOOST_ATOMIC_DETAIL_FENCE_ARCH_OPS_GCC_X86_HPP_INCLUDED_
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