| /* |
| * 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_ |