Use __sync intrinsics on g++ 4.1+

[SVN r37527]

include/boost/detail/atomic_count.hpp

@@ -93,6 +93,8 @@ typedef long atomic_count;
 #  include <boost/detail/atomic_count_pthreads.hpp>
 #elif defined(WIN32) || defined(_WIN32) || defined(__WIN32__)
 #  include <boost/detail/atomic_count_win32.hpp>
+#elif defined( __GNUC__ ) && ( __GNUC__ * 100 + __GNUC_MINOR__ >= 401 )
+#  include <boost/detail/atomic_count_sync.hpp>
 #elif defined(__GLIBCPP__) || defined(__GLIBCXX__)
 #  include <boost/detail/atomic_count_gcc.hpp>
 #elif defined(BOOST_HAS_PTHREADS)

include/boost/detail/atomic_count_sync.hpp

@@ -0,0 +1,57 @@
+#ifndef BOOST_DETAIL_ATOMIC_COUNT_SYNC_HPP_INCLUDED
+#define BOOST_DETAIL_ATOMIC_COUNT_SYNC_HPP_INCLUDED
+
+//
+//  boost/detail/atomic_count_sync.hpp
+//
+//  atomic_count for g++ 4.1+
+//
+//  http://gcc.gnu.org/onlinedocs/gcc-4.1.1/gcc/Atomic-Builtins.html
+//
+//  Copyright 2007 Peter Dimov
+//
+//  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)
+//
+
+namespace boost
+{
+
+namespace detail
+{
+
+class atomic_count
+{
+public:
+
+    explicit atomic_count( long v ) : value_( v ) {}
+
+    void operator++()
+    {
+        __sync_add_and_fetch( &value_, 1 );
+    }
+
+    long operator--()
+    {
+        return __sync_add_and_fetch( &value_, -1 );
+    }
+
+    operator long() const
+    {
+        return __sync_fetch_and_add( &value_, 0 );
+    }
+
+private:
+
+    atomic_count(atomic_count const &);
+    atomic_count & operator=(atomic_count const &);
+
+    mutable long value_;
+};
+
+} // namespace detail
+
+} // namespace boost
+
+#endif // #ifndef BOOST_DETAIL_ATOMIC_COUNT_SYNC_HPP_INCLUDED