Bug 1920673. Avoid calling IsSingular on a matrix before inverting it, just try to invert it and see if it succeeds. r=gfx-reviewers,nical

This repeated work does show up in profiles.

Differential Revision: https://phabricator.services.mozilla.com/D223289

gfx/2d/Matrix.h

@@ -2227,6 +2227,15 @@ class Matrix4x4TypedFlagged
     return clone;
   }
 
+  Maybe<Matrix4x4TypedFlagged<TargetUnits, SourceUnits>> MaybeInverse() const {
+    typedef Matrix4x4TypedFlagged<TargetUnits, SourceUnits> InvertedMatrix;
+    InvertedMatrix clone = Cast<InvertedMatrix>();
+    if (clone.Invert()) {
+      return Some(clone);
+    }
+    return Nothing();
+  }
+
   template <typename NewTargetUnits>
   bool operator==(
       const Matrix4x4TypedFlagged<TargetUnits, NewTargetUnits>& aMatrix) const {

layout/base/nsLayoutUtils.cpp

@@ -2059,11 +2059,10 @@ static bool TransformGfxPointFromAncestor(RelativeTo aFrame,
     auto matrix = nsLayoutUtils::GetTransformToAncestor(
         RelativeTo{text ? text : aFrame.mFrame, aFrame.mViewportType},
         aAncestor);
-    if (matrix.IsSingular()) {
+    aMatrixCache = matrix.MaybeInverse();
+    if (aMatrixCache.isNothing()) {
       return false;
     }
-    matrix.Invert();
-    aMatrixCache.emplace(matrix);
   }
 
   const Matrix4x4Flagged& ctm = *aMatrixCache;
@@ -2211,10 +2210,10 @@ nsLayoutUtils::TransformResult nsLayoutUtils::TransformRect(
   }
   Matrix4x4Flagged downToDest = GetTransformToAncestor(
       RelativeTo{aToFrame}, RelativeTo{nearestCommonAncestor});
-  if (downToDest.IsSingular()) {
+  // invert downToDest in place
+  if (!downToDest.Invert()) {
     return NONINVERTIBLE_TRANSFORM;
   }
-  downToDest.Invert();
   aRect = TransformFrameRectToAncestor(aFromFrame, aRect,
                                        RelativeTo{nearestCommonAncestor});
 

layout/painting/nsDisplayList.cpp

@@ -7239,7 +7239,8 @@ bool nsDisplayTransform::UntransformRect(const nsRect& aTransformedBounds,
                                          const Matrix4x4& aMatrix,
                                          float aAppUnitsPerPixel,
                                          nsRect* aOutRect) {
-  if (aMatrix.IsSingular()) {
+  Maybe<Matrix4x4> inverse = aMatrix.MaybeInverse();
+  if (inverse.isNothing()) {
     return false;
   }
 
@@ -7255,7 +7256,7 @@ bool nsDisplayTransform::UntransformRect(const nsRect& aTransformedBounds,
       NSAppUnitsToFloatPixels(aChildBounds.width, aAppUnitsPerPixel),
       NSAppUnitsToFloatPixels(aChildBounds.height, aAppUnitsPerPixel));
 
-  result = aMatrix.Inverse().ProjectRectBounds(result, childGfxBounds);
+  result = inverse->ProjectRectBounds(result, childGfxBounds);
   *aOutRect = nsLayoutUtils::RoundGfxRectToAppRect(ThebesRect(result),
                                                    aAppUnitsPerPixel);
   return true;