Runtime CPU detection for rsqrt().

src/core/SkOpts.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkOnce.h"
 #include "SkOpts.h"
 
 #if defined(SK_CPU_X86)
     #if defined(SK_BUILD_FOR_WIN32)
         #include <intrin.h>
         static void cpuid(uint32_t abcd[4]) { __cpuid((int*)abcd, 1); }
     #else
         #include <cpuid.h>
         static void cpuid(uint32_t abcd[4]) { __get_cpuid(1, abcd+0, abcd+1, abcd+2, abcd+3); }
     #endif
 #elif !defined(SK_ARM_HAS_NEON) && defined(SK_CPU_ARM32) && defined(SK_BUILD_FOR_ANDROID)
     #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();
     void Init_ssse3();
     void Init_sse41();
     void Init_neon();
     //TODO: _dsp2, _armv7, _armv8, _x86, _x86_64, _sse42, _avx, avx2, ... ?
 
     static void init() {
     #if defined(SK_CPU_X86)
@@ skipping 11 matching lines @@
 
     SK_DECLARE_STATIC_ONCE(gInitOnce);
     void Init() { SkOnce(&gInitOnce, init); }
 
 #if SK_ALLOW_STATIC_GLOBAL_INITIALIZERS
     static struct AutoInit {
         AutoInit() { Init(); }
     } gAutoInit;
 #endif
 }