fix linear gradient assert, by explicitly clamping

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1556993003

Review URL: https://codereview.chromium.org/1556993003

1 files changed, 7 insertions, 4 deletions

diff --git a/src/effects/gradients/SkLinearGradient.cpp b/src/effects/gradients/SkLinearGradient.cpp
index 298466c8a6..4f90c27c8c 100644
--- a/src/effects/gradients/SkLinearGradient.cpp
+++ b/src/effects/gradients/SkLinearGradient.cpp
@@ -668,12 +668,12 @@ void SkLinearGradient::LinearGradientContext::shade4_dx_clamp(SkPMColor dstC[],
         }
 
         fx += n * dx;
-        count -= n;
-        SkASSERT(count >= 0);
+        // fx should now outside of the p0..p1 interval. However, due to float precision loss,
+        // its possible that fx is slightly too small/large, so we clamp it.
         if (dx_is_pos) {
-            SkASSERT(0 == count || fx >= p1);
+            fx = SkTMax(fx, p1);
         } else {
-            SkASSERT(0 == count || fx <= p0);
+            fx = SkTMin(fx, p0);
         }
 
         ramp<apply_alpha>(dstC, n, c, dc, dither0, dither1);
@@ -683,6 +683,9 @@ void SkLinearGradient::LinearGradientContext::shade4_dx_clamp(SkPMColor dstC[],
         if (n & 1) {
             SkTSwap(dither0, dither1);
         }
+
+        count -= n;
+        SkASSERT(count >= 0);
     }
 }