2009-12-18  Jimmy Guo  <jguo@yahoo-inc.com>

	PR libstdc++/40088
	* src/locale_init.cc (locale::locale()): Optimize the common case
	where _S_global still points to _S_classic.

--- libstdc++-v3/src/locale_init.cc	(revision 155341)
+++ libstdc++-v3/src/locale_init.cc	(revision 155343)
@@ -100,9 +100,25 @@ namespace std 
   locale::locale() throw() : _M_impl(0)
   { 
     _S_initialize();
-    __gnu_cxx::lock sentry(__gnu_internal::locale_mutex);
-    _S_global->_M_add_reference();
+
+    // Checked locking to optimize the common case where _S_global
+    // still points to _S_classic (locale::_S_initialize_once()):
+    // - If they are the same, just increment the reference count and
+    //   we are done.  This effectively constructs a C locale object
+    //   identical to the static c_locale.
+    // - Otherwise, _S_global can and may be destroyed due to
+    //   locale::global() call on another thread, in which case we
+    //   fall back to lock protected access to both _S_global and
+    //   its reference count.
     _M_impl = _S_global;
+    if (_M_impl == _S_classic)
+      _M_impl->_M_add_reference();
+    else
+      {
+        __gnu_cxx::lock sentry(__gnu_internal::locale_mutex);
+        _S_global->_M_add_reference();
+        _M_impl = _S_global;
+      }
   }
 
   locale