Runtime CPU detection for rsqrt().

src/core/SkOpts.h

Index: src/core/SkOpts.h
diff --git a/src/core/SkOpts.h b/src/core/SkOpts.h
index 71abae5d7a57c9b8a74631675ef32a3e98e590e7..f02ec97553519c06ec4356a9d2a7095a6e7d379b 100644
--- a/src/core/SkOpts.h
+++ b/src/core/SkOpts.h
@@ -16,7 +16,10 @@ namespace SkOpts {
     // Called by SkGraphics::Init(), and automatically #if SK_ALLOW_STATIC_GLOBAL_INITIALIZERS.
     void Init();
 
-    // (Function pointers go here).
+    // Declare function pointers here...
+
+    // Returns a fast approximation of 1.0f/sqrtf(x).
+    extern float (*rsqrt)(float);
 }
 
 #endif//SkOpts_DEFINED