2 files changed, 18 insertions, 2 deletions
diff --git a/src/core/SkOpts.cpp b/src/core/SkOpts.cpp
index 4f7c5e9345..7da306c99d 100644
--- a/src/core/SkOpts.cpp
+++ b/src/core/SkOpts.cpp
@@ -20,8 +20,21 @@
     #include <cpu-features.h>
 #endif
 
+static float rsqrt_portable(float x) {
+    // Get initial estimate.
+    int i = *SkTCast<int*>(&x);
+    i = 0x5F1FFFF9 - (i>>1);
+    float estimate = *SkTCast<float*>(&i);
+
+    // One step of Newton's method to refine.
+    const float estimate_sq = estimate*estimate;
+    estimate *= 0.703952253f*(2.38924456f-x*estimate_sq);
+    return estimate;
+}
+
 namespace SkOpts {
-    // (Define default function pointer values here...)
+    // Define default function pointer values here...
+    decltype(rsqrt) rsqrt = rsqrt_portable;
 
     // Each Init_foo() is defined in src/opts/SkOpts_foo.cpp.
     void Init_sse2();
diff --git a/src/core/SkOpts.h b/src/core/SkOpts.h
index 71abae5d7a..f02ec97553 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