Revert of Fix UB in SkDivBits

src/core/SkMath.cpp

 /*
  * Copyright 2008 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkMathPriv.h"
 #include "SkFloatBits.h"
 #include "SkFloatingPoint.h"
@@ skipping 27 matching lines @@
     }
     if (x & 0x2) {
         sub_shift(zeros, x, 1);
     }
 
     return zeros;
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
+#define DIVBITS_ITER(n)                                 \
+    case n:                                             \
+        if ((numer = (numer << 1) - denom) >= 0)        \
+            result |= 1 << (n - 1); else numer += denom
 
-#define DIVBITS_ITER(k)                    \
-    case k:                                \
-        if (numer*2 >= denom) {            \
-            numer = numer*2 - denom;       \
-            result |= 1 << (k-1);          \
-        } else {                           \
-            numer *= 2;                    \
-        }
-
-// NOTE: if you're thinking of editing this method, consider replacing it with
-// a simple shift and divide.  This legacy method predated reliable hardware division.
-int32_t SkDivBits(int32_t n, int32_t d, int shift_bias) {
-    SkASSERT(d != 0);
-    if (n == 0) {
+int32_t SkDivBits(int32_t numer, int32_t denom, int shift_bias) {
+    SkASSERT(denom != 0);
+    if (numer == 0) {
         return 0;
     }
 
-    // Make numer and denom positive, and sign hold the resulting sign
-    // We'll be left-shifting numer, so it's important it's a uint32_t.
-    // We put denom in a uint32_t just to keep things simple.
-    int32_t sign = SkExtractSign(n ^ d);
-#if defined(SK_SUPPORT_LEGACY_DIVBITS_UB)
-    // Blink layout tests are baselined to Clang optimizing through the UB.
-    int32_t numer = SkAbs32(n);
-    int32_t denom = SkAbs32(d);
-#else
-    uint32_t numer = SkAbs32(n);
-    uint32_t denom = SkAbs32(d);
-#endif
-
-    // It's probably a bug to use n or d below here.
+    // make numer and denom positive, and sign hold the resulting sign
+    int32_t sign = SkExtractSign(numer ^ denom);
+    numer = SkAbs32(numer);
+    denom = SkAbs32(denom);
 
     int nbits = SkCLZ(numer) - 1;
     int dbits = SkCLZ(denom) - 1;
     int bits = shift_bias - nbits + dbits;
 
     if (bits < 0) {  // answer will underflow
         return 0;
     }
     if (bits > 31) {  // answer will overflow
         return SkApplySign(SK_MaxS32, sign);
     }
 
     denom <<= dbits;
     numer <<= nbits;
 
     SkFixed result = 0;
 
     // do the first one
-    if (numer >= denom) {
-        numer -= denom;
+    if ((numer -= denom) >= 0) {
         result = 1;
+    } else {
+        numer += denom;
     }
 
     // Now fall into our switch statement if there are more bits to compute
     if (bits > 0) {
         // make room for the rest of the answer bits
         result <<= bits;
         switch (bits) {
             DIVBITS_ITER(31); DIVBITS_ITER(30); DIVBITS_ITER(29);
             DIVBITS_ITER(28); DIVBITS_ITER(27); DIVBITS_ITER(26);
             DIVBITS_ITER(25); DIVBITS_ITER(24); DIVBITS_ITER(23);
@@ skipping 51 matching lines @@
         if (SkScalarNearlyZero(*cosValue)) {
             *cosValue = 0;
         }
     }
 
     if (SkScalarNearlyZero(sinValue)) {
         sinValue = 0;
     }
     return sinValue;
 }