Cheaper thread-safe atomic initialization of static references.

Source/wtf/Threading.h

Index: Source/wtf/Threading.h
diff --git a/Source/wtf/Threading.h b/Source/wtf/Threading.h
index 97d4cb390ae0734e9b214a95207e47713bebe7d3..0953a7b40d122a3e004d75a59c41ef3e5db113ef 100644
--- a/Source/wtf/Threading.h
+++ b/Source/wtf/Threading.h
@@ -30,14 +30,22 @@
 #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 {