pin before calling acos

src/pathops/SkPathOpsCubic.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #include "SkGeometry.h"
 #include "SkLineParameters.h"
 #include "SkPathOpsConic.h"
 #include "SkPathOpsCubic.h"
@@ skipping 401 matching lines @@
     double a2 = a * a;
     double Q = (a2 - b * 3) / 9;
     double R = (2 * a2 * a - 9 * a * b + 27 * c) / 54;
     double R2 = R * R;
     double Q3 = Q * Q * Q;
     double R2MinusQ3 = R2 - Q3;
     double adiv3 = a / 3;
     double r;
     double* roots = s;
     if (R2MinusQ3 < 0) {   // we have 3 real roots
-        double theta = acos(R / sqrt(Q3));
+        // the divide/root can, due to finite precisions, be slightly outside of -1...1
+        double theta = acos(SkTPin(R / sqrt(Q3), -1., 1.));
         double neg2RootQ = -2 * sqrt(Q);
 
         r = neg2RootQ * cos(theta / 3) - adiv3;
         *roots++ = r;
 
         r = neg2RootQ * cos((theta + 2 * PI) / 3) - adiv3;
         if (!AlmostDequalUlps(s[0], r)) {
             *roots++ = r;
         }
         r = neg2RootQ * cos((theta - 2 * PI) / 3) - adiv3;
@@ skipping 256 matching lines @@
     for (int index = 0; index < roots; ++index) {
         double t = startT + (endT - startT) * extremeTs[index];
         SkDPoint mid = dCurve.ptAtT(t);
         if (topPt->fY > mid.fY || (topPt->fY == mid.fY && topPt->fX > mid.fX)) {
             topT = t;
             *topPt = mid;
         }
     }
     return topT;
 }