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diff --git a/boost/asio/detail/gcc_x86_fenced_block.hpp b/boost/asio/detail/gcc_x86_fenced_block.hpp
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+++ b/boost/asio/detail/gcc_x86_fenced_block.hpp
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+//
+// detail/gcc_x86_fenced_block.hpp
+// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+//
+// Copyright (c) 2003-2012 Christopher M. Kohlhoff (chris at kohlhoff dot com)
+//
+// 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)
+//
+
+#ifndef BOOST_ASIO_DETAIL_GCC_X86_FENCED_BLOCK_HPP
+#define BOOST_ASIO_DETAIL_GCC_X86_FENCED_BLOCK_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include <boost/asio/detail/config.hpp>
+
+#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
+
+#include <boost/asio/detail/push_options.hpp>
+
+namespace boost {
+namespace asio {
+namespace detail {
+
+class gcc_x86_fenced_block
+  : private noncopyable
+{
+public:
+  enum half_t { half };
+  enum full_t { full };
+
+  // Constructor for a half fenced block.
+  explicit gcc_x86_fenced_block(half_t)
+  {
+  }
+
+  // Constructor for a full fenced block.
+  explicit gcc_x86_fenced_block(full_t)
+  {
+    barrier1();
+  }
+
+  // Destructor.
+  ~gcc_x86_fenced_block()
+  {
+    barrier2();
+  }
+
+private:
+  static int barrier1()
+  {
+    int r = 0, m = 1;
+    __asm__ __volatile__ (
+        "xchgl %0, %1" :
+        "=r"(r), "=m"(m) :
+        "0"(1), "m"(m) :
+        "memory", "cc");
+    return r;
+  }
+
+  static void barrier2()
+  {
+#if defined(__SSE2__)
+    __asm__ __volatile__ ("mfence" ::: "memory");
+#else // defined(__SSE2__)
+    barrier1();
+#endif // defined(__SSE2__)
+  }
+};
+
+} // namespace detail
+} // namespace asio
+} // namespace boost
+
+#include <boost/asio/detail/pop_options.hpp>
+
+#endif // defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
+
+#endif // BOOST_ASIO_DETAIL_GCC_X86_FENCED_BLOCK_HPP