work around leftshift for negative values

src/core/SkEdge.cpp

 
 /*
  * Copyright 2006 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 "SkEdge.h"
@@ skipping 308 matching lines @@
 /*  f(1/3) = (8a + 12b + 6c + d) / 27
     f(2/3) = (a + 6b + 12c + 8d) / 27
 
     f(1/3)-b = (8a - 15b + 6c + d) / 27
     f(2/3)-c = (a + 6b - 15c + 8d) / 27
 
     use 16/512 to approximate 1/27
 */
 static SkFDot6 cubic_delta_from_line(SkFDot6 a, SkFDot6 b, SkFDot6 c, SkFDot6 d)
 {
-    SkFDot6 oneThird = ((a << 3) - ((b << 4) - b) + 6*c + d) * 19 >> 9;
-    SkFDot6 twoThird = (a + 6*b - ((c << 4) - c) + (d << 3)) * 19 >> 9;
+    // since our parameters may be negative, we don't use << to avoid ASAN warnings
+    SkFDot6 oneThird = (a*8 - b*15 + 6*c + d) * 19 >> 9;
+    SkFDot6 twoThird = (a + 6*b - c*15 + d*8) * 19 >> 9;

[caryclark, 2016/03/08 14:20:23]

I'm curious how this works. If the bug is that the highest number bit is shifted
into the sign bit by the shift of a, b, c, or d, doesn't this imply that the
number may be so large that it can't be safely multiplied by 19?

[reed1, 2016/03/08 14:26:44]

I don't think that is the bug. I think the bug from ASAN is that one of the
values was negative at all (as that is "undefined"), not that we actually
overflowed.

 
     return SkMax32(SkAbs32(oneThird), SkAbs32(twoThird));
 }
 
 int SkCubicEdge::setCubic(const SkPoint pts[4], int shift) {
     SkFDot6 x0, y0, x1, y1, x2, y2, x3, y3;
 
     {
 #ifdef SK_RASTERIZE_EVEN_ROUNDING
         x0 = SkScalarRoundToFDot6(pts[0].fX, shift);
@@ skipping 129 matching lines @@
         success = this->updateLine(oldx, oldy, newx, newy);
         oldx = newx;
         oldy = newy;
     } while (count < 0 && !success);
 
     fCx         = newx;
     fCy         = newy;
     fCurveCount = SkToS8(count);
     return success;
 }