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