Runtime CPU detection for rsqrt().

src/core/SkOpts.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkOpts_DEFINED
 #define SkOpts_DEFINED
 
 #include "SkTypes.h"
 
 namespace SkOpts {
     // Call to replace pointers to portable functions with pointers to CPU-specific functions.
     // Thread-safe and idempotent.
     // 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