Harden LinearGradient4fContext

src/effects/gradients/Sk4fLinearGradient.cpp

Index: src/effects/gradients/Sk4fLinearGradient.cpp
diff --git a/src/effects/gradients/Sk4fLinearGradient.cpp b/src/effects/gradients/Sk4fLinearGradient.cpp
index dc6e530a0c0896777c185b0ab2a1ff5c5be4cefb..cd1f770ef98306f20ac452b729e2950998b3eed1 100644
--- a/src/effects/gradients/Sk4fLinearGradient.cpp
+++ b/src/effects/gradients/Sk4fLinearGradient.cpp
@@ -100,12 +100,13 @@ SkScalar pinFx<SkShader::kMirror_TileMode>(SkScalar fx) {
     return f < 0 ? f + 2 : f;
 }
 
-// true when x is in [k1,k2)
+// true when x is in [k1,k2), or [k2, k1) when the interval is reversed.
+// TODO(fmalita): hoist the reversed interval check out of this helper.
 bool in_range(SkScalar x, SkScalar k1, SkScalar k2) {
     SkASSERT(k1 != k2);
     return (k1 < k2)
-        ? (x >= k1 && x < k2)
-        : (x >= k2 && x < k1);
+        ? (x >= k1 && x <  k2)
+        : (x >  k2 && x <= k1);
 }
 
 } // anonymous namespace
@@ -116,7 +117,7 @@ LinearGradient4fContext::LinearGradient4fContext(const SkLinearGradient& shader,
     : INHERITED(shader, rec) {
 
     // Our fast path expects interval points to be monotonically increasing in x.
-    const bool reverseIntervals = this->isFast() && fDstToPos.getScaleX() < 0;
+    const bool reverseIntervals = this->isFast() && signbit(fDstToPos.getScaleX());
     this->buildIntervals(shader, rec, reverseIntervals);
 
     SkASSERT(fIntervals.count() > 0);
@@ -290,6 +291,7 @@ public:
         , fDx(dx)
         , fIsVertical(is_vertical)
     {
+        SkASSERT(fAdvX >= 0);
         SkASSERT(firstInterval <= lastInterval);
         SkASSERT(in_range(fx, i->fP0, i->fP1));
         this->compute_interval_props(fx - i->fP0);