Fix GradientShaderBase4fContext::Interval fuzzer assert

src/effects/gradients/Sk4fGradientBase.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Sk4fGradientBase.h"
 
 #include <functional>
@@ skipping 107 matching lines @@
 };
 
 } // anonymous namespace
 
 SkGradientShaderBase::GradientShaderBase4fContext::
 Interval::Interval(const Sk4f& c0, SkScalar p0,
                    const Sk4f& c1, SkScalar p1)
     : fP0(p0)
     , fP1(p1)
     , fZeroRamp((c0 == c1).allTrue()) {
-
     SkASSERT(p0 != p1);
     // Either p0 or p1 can be (-)inf for synthetic clamp edge intervals.
     SkASSERT(SkScalarIsFinite(p0) || SkScalarIsFinite(p1));
 
     const auto dp = p1 - p0;
 
     // Clamp edge intervals are always zero-ramp.
     SkASSERT(SkScalarIsFinite(dp) || fZeroRamp);
     const Sk4f dc = SkScalarIsFinite(dp) ? (c1 - c0) / dp : 0;
 
@@ skipping 128 matching lines @@
 void SkGradientShaderBase::
 GradientShaderBase4fContext::addMirrorIntervals(const SkGradientShaderBase& shader,
                                             const Sk4f& componentScale, bool reverse) {
     const IntervalIterator iter(shader.fOrigColors,
                                 shader.fOrigPos,
                                 shader.fColorCount,
                                 reverse);
     iter.iterate([this, &componentScale] (SkColor c0, SkColor c1, SkScalar p0, SkScalar p1) {
         SkASSERT(fIntervals.empty() || fIntervals.back().fP1 == 2 - p0);
 
-        fIntervals.emplace_back(pack_color(c0, fColorsArePremul, componentScale),
-                                2 - p0,
-                                pack_color(c1, fColorsArePremul, componentScale),
-                                2 - p1);
+        const auto mirror_p0 = 2 - p0;
+        const auto mirror_p1 = 2 - p1;
+        // mirror_p1 & mirror_p1 may collapse for very small values - recheck to avoid
+        // triggering Interval asserts.
+        if (mirror_p0 != mirror_p1) {
+            fIntervals.emplace_back(pack_color(c0, fColorsArePremul, componentScale),
+                                    mirror_p0,
+                                    pack_color(c1, fColorsArePremul, componentScale),
+                                    mirror_p1);
+        }
     });
 }
 
 void SkGradientShaderBase::
 GradientShaderBase4fContext::shadeSpan(int x, int y, SkPMColor dst[], int count) {
     if (fColorsArePremul) {
         this->shadePremulSpan<DstType::L32, ApplyPremul::False>(x, y, dst, count);
     } else {
         this->shadePremulSpan<DstType::L32, ApplyPremul::True>(x, y, dst, count);
     }
@@ skipping 144 matching lines @@
         fDc = DstTraits<dstType, premul>::load(i->fDc);
     }
 
     const Interval* fFirstInterval;
     const Interval* fLastInterval;
     const Interval* fInterval;
     SkScalar        fPrevT;
     Sk4f            fCc;
     Sk4f            fDc;
 };