整理

include/boost/atomic/detail/fence_arch_ops_gcc_x86.hpp

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+/*
+ * 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) 2020-2025 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) 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) noexcept
+    {
+        if (order != memory_order_relaxed)
+            __asm__ __volatile__ ("" ::: "memory");
+    }
+};
+
+using fence_arch_operations = fence_arch_operations_gcc_x86;
+
+} // namespace detail
+} // namespace atomics
+} // namespace boost
+
+#include <boost/atomic/detail/footer.hpp>
+
+#endif // BOOST_ATOMIC_DETAIL_FENCE_ARCH_OPS_GCC_X86_HPP_INCLUDED_