Cheaper thread-safe atomic initialization of static references.

Source/wtf/Threading.h

 /*
  * Copyright (C) 2007, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Justin Haygood (jhaygood@reaktix.com)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 12 matching lines @@
  * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
  * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef Threading_h
 #define Threading_h
 
+#include "wtf/Atomics.h"
 #include "wtf/WTFExport.h"
 #include <stdint.h>
 
-// For portability, we do not use thread-safe statics natively supported by some compilers (e.g. gcc).
-#define AtomicallyInitializedStatic(T, name) \
-    WTF::lockAtomicallyInitializedStaticMutex(); \
-    static T name; \
-    WTF::unlockAtomicallyInitializedStaticMutex();
+// For portability, we do not make use of C++11 thread-safe statics, as supported
+// by some toolchains. Make use of double-checked locking to reduce overhead.
+#define AtomicallyInitializedStaticReference(T, name, initializer) \
+    /* Init to nullptr is thread-safe on all implementations. */   \
+    static void* name##Pointer = nullptr;                          \
+    if (!WTF::acquireLoad(&name##Pointer)) {                       \
+        WTF::lockAtomicallyInitializedStaticMutex();               \
+        if (!WTF::acquireLoad(&name##Pointer))                     \
+            WTF::releaseStore(&name##Pointer, initializer);        \

[Jeffrey Yasskin, 2015/01/21 19:36:06]

It probably makes sense to use:

if (!WTF::acquireLoad(&name##Pointer)) {
  T* initializerResult = initializer;
  WTF::releaseStore(&name##Pointer, initializerResult);
}

to make sure that the initializer returns the right type.

Otherwise, this version looks great. Thanks for implementing this!

[sof, 2015/01/21 20:42:54]

The extra type check is definitely worth it; added.

+        WTF::unlockAtomicallyInitializedStaticMutex();             \
+    }                                                              \
+    T& name = *static_cast<T*>(name##Pointer)
 
 namespace WTF {
 
 #if OS(WIN)
 typedef uint32_t ThreadIdentifier;
 #else
 typedef intptr_t ThreadIdentifier;
 #endif
 
 WTF_EXPORT ThreadIdentifier currentThread();
 
 WTF_EXPORT void lockAtomicallyInitializedStaticMutex();
 WTF_EXPORT void unlockAtomicallyInitializedStaticMutex();
 
 } // namespace WTF
 
 using WTF::ThreadIdentifier;
 using WTF::currentThread;
 
 #endif // Threading_h