Add a 'unique' method to SkRefCnt, document the usage, and add support.

include/core/SkThread_platform.h

Index: include/core/SkThread_platform.h
===================================================================
--- include/core/SkThread_platform.h	(revision 10052)
+++ include/core/SkThread_platform.h	(working copy)
@@ -88,6 +88,10 @@
 
 #endif // SK_BUILD_FOR_ANDROID_FRAMEWORK
 
+int32_t sk_atomic_unprotected_read(const volatile int32_t & x) {

[bsalomon, 2013/07/15 13:04:08]

We've got several implementations of this that are all the same. Would it be
different on any platform?

[bungeman-skia, 2013/07/15 16:02:41]

Yes, we need to clean this up in general. Note that the one which would be in
Chromium is different.

+    return x;
+}
+
 #else  // !SK_BUILD_FOR_ANDROID
 
 /** Implemented by the porting layer, this function adds one to the int
@@ -130,6 +134,12 @@
 */
 SK_API void sk_membar_aquire__after_atomic_conditional_inc();
 
+/** Forces an atomic read of 'x' and marks the read as a benign race.
+    No additional memory barrier is required.
+    This does not need to act as a compiler barrier.
+*/
+SK_API int32_t sk_atomic_unprotected_read(const volatile int32_t & x);
+
 #endif // !SK_BUILD_FOR_ANDROID
 
 #ifdef SK_USE_POSIX_THREADS