pin before calling acos

src/core/SkGeometry.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 "SkGeometry.h"
 #include "SkMatrix.h"
 #include "SkNx.h"
@@ skipping 725 matching lines @@
     R = (2*a*a*a - 9*a*b + 27*c) / 54;
 
     SkScalar Q3 = Q * Q * Q;
     SkScalar R2MinusQ3 = R * R - Q3;
     SkScalar adiv3 = a / 3;
 
     SkScalar*   roots = tValues;
     SkScalar    r;
 
     if (R2MinusQ3 < 0) { // we have 3 real roots
-        SkScalar theta = SkScalarACos(R / SkScalarSqrt(Q3));
+        // the divide/root can, due to finite precisions, be slightly outside of -1...1
+        SkScalar theta = SkScalarACos(SkScalarPin(R / SkScalarSqrt(Q3), -1, 1));
         SkScalar neg2RootQ = -2 * SkScalarSqrt(Q);
 
         r = neg2RootQ * SkScalarCos(theta/3) - adiv3;
         if (is_unit_interval(r)) {
             *roots++ = r;
         }
         r = neg2RootQ * SkScalarCos((theta + 2*SK_ScalarPI)/3) - adiv3;
         if (is_unit_interval(r)) {
             *roots++ = r;
         }
@@ skipping 581 matching lines @@
         matrix.preScale(SK_Scalar1, -SK_Scalar1);
     }
     if (userMatrix) {
         matrix.postConcat(*userMatrix);
     }
     for (int i = 0; i < conicCount; ++i) {
         matrix.mapPoints(dst[i].fPts, 3);
     }
     return conicCount;
 }