replace SkFixedDiv impl with native 64bit math

BUG=skia:
TBR=

Review URL: https://codereview.chromium.org/1022543003

bench/MathBench.cpp

@@ -545,6 +545,50 @@ private:
 
 ///////////////////////////////////////////////////////////////////////////////
 
+class DivBitsBench : public Benchmark {
+protected:
+    enum {
+        N = 1000
+    };
+    volatile int32_t fSrc[N], fDst[N];
+public:
+    DivBitsBench() {
+        SkRandom rand;
+        for (int i = 0; i < N; ++i) {
+            fSrc[i] = rand.nextU();
+        }
+    }
+protected:
+    virtual void onDraw(const int loops, SkCanvas*) {
+        for (int j = 0; j < loops; ++j) {
+            for (int i = 0; i < N - 4; ++i) {
+                fDst[i] = SkDivBits(fSrc[i], fSrc[i] >> 3, 16);
+            }
+        }
+    }
+    virtual const char* onGetName() {
+        return "divbits";
+    }
+};
+DEF_BENCH( return new DivBitsBench; )
+
+class FixedDivBench : public DivBitsBench {
+protected:
+    virtual void onDraw(const int loops, SkCanvas*) {
+        for (int j = 0; j < loops; ++j) {
+            for (int i = 0; i < N - 4; ++i) {
+                fDst[i] = SkFixedDiv(fSrc[i], fSrc[i] >> 3);
+            }
+        }
+    }
+    virtual const char* onGetName() {
+        return "fixeddiv";
+    }
+};
+DEF_BENCH( return new FixedDivBench; )
+
+///////////////////////////////////////////////////////////////////////////////
+
 template <typename T>
 class DivModBench : public Benchmark {
     SkString fName;

include/core/SkFixed.h

@@ -78,7 +78,10 @@ typedef int32_t             SkFixed;
 #define SkFixedAbs(x)       SkAbs32(x)
 #define SkFixedAve(a, b)    (((a) + (b)) >> 1)
 
-#define SkFixedDiv(numer, denom)    SkDivBits(numer, denom, 16)
+static inline int32_t SkFixedDiv(int32_t numer, int32_t denom) {
+    int64_t tmp = ((int64_t)numer << 16) / denom;
+    return (int32_t)tmp;
+}
 
 //////////////////////////////////////////////////////////////////////////////////////////////////////
 // Now look for ASM overrides for our portable versions (should consider putting this in its own file)