Reduce copying of local data structures in GeometryMapper and PaintLayerClipper.

third_party/WebKit/Source/platform/graphics/paint/GeometryMapper.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef GeometryMapper_h
 #define GeometryMapper_h
 
 #include "platform/graphics/paint/FloatClipRect.h"
 #include "platform/graphics/paint/PropertyTreeState.h"
 #include "platform/transforms/TransformationMatrix.h"
 #include "wtf/HashMap.h"
 
 namespace blink {
 
 // GeometryMapper is a helper class for fast computations of transformed and
 // visual rects in different PropertyTreeStates. The design document has a
 // number of details on use cases, algorithmic definitions, and running times.
 //
 // NOTE: A GeometryMapper object is only valid for property trees that do not
 // change. If any mutation occurs, a new GeometryMapper object must be allocated
 // corresponding to the new state.
 //
+// ** WARNING** Callers to the methods below may not assume that any const
+// references returned remain const across multiple calls into GeometryMapper.
+// If needed, callers must store local copies of the return values.
+//
 // Design document: http://bit.ly/28P4FDA
 //
 // TODO(chrishtr): take effect tree into account.
 class PLATFORM_EXPORT GeometryMapper {
  public:
   static std::unique_ptr<GeometryMapper> create() {
     return WTF::wrapUnique(new GeometryMapper());
   }
 
   // The runtime of m calls among localToAncestorVisualRect, localToAncestorRect
   // or ancestorToLocalRect with the same |ancestorState| parameter is
   // guaranteed to be O(n + m), where n is the number of transform and clip
   // nodes in their respective property trees.
 
   // If the clips and transforms of |sourceState| are equal to or descendants of
   // those of |destinationState|, returns the same value as
   // localToAncestorVisualRect. Otherwise, maps the input rect to the
   // transform state which is the lowest common ancestor of
   // |sourceState.transform| and |destinationState.transform|, then multiplies
   // it by the the inverse transform mapping from the lowest common ancestor to
   // |destinationState.transform|.
   //
   // DCHECK fails if the clip of |destinationState| is not an ancestor of the
   // clip of |sourceState|, or the inverse transform is not invertible.
-  FloatClipRect sourceToDestinationVisualRect(
+  const FloatClipRect& sourceToDestinationVisualRect(
       const FloatRect&,
       const PropertyTreeState& sourceState,
       const PropertyTreeState& destinationState);
 
   // Same as sourceToDestinationVisualRect() except that only transforms are
   // applied.
-  FloatRect sourceToDestinationRect(
-      const FloatRect&,
+  //
+  // |mappingRect| is both input and output.
+  void sourceToDestinationRect(
       const TransformPaintPropertyNode* sourceTransformNode,
-      const TransformPaintPropertyNode* destinationTransformNode);
+      const TransformPaintPropertyNode* destinationTransformNode,
+      FloatRect& mappingRect);
 
   // Maps from a rect in |localTransformSpace| to its visual rect in
   // |ancestorState|. This is computed by multiplying the rect by its combined
   // transform between |localTransformSpace| and |ancestorSpace|, then
   // flattening into 2D space, then intersecting by the "clip visual rect" for
   // |localTransformState|'s clips. See above for the definition of "clip visual
   // rect".
   //
   // Note that the clip of |ancestorState| is *not* applied.
   //
   // DCHECK fails if any of the paint property tree nodes in
   // |localTransformState| are not equal to or a descendant of that in
   // |ancestorState|.
-  FloatClipRect localToAncestorVisualRect(
+  const FloatClipRect& localToAncestorVisualRect(
       const FloatRect&,
       const PropertyTreeState& localTransformState,
       const PropertyTreeState& ancestorState);
 
   // Maps from a rect in |localTransformNode| space to its transformed rect in
   // |ancestorTransformNode| space. This is computed by multiplying the rect by
   // the combined transform between |localTransformNode| and
   // |ancestorTransformNode|, then flattening into 2D space.
   //
   // DCHECK fails if |localTransformNode| is not equal to or a descendant of
   // |ancestorTransformNode|.
-  FloatRect localToAncestorRect(
-      const FloatRect&,
+  //
+  //|mappingRect| is both input and output.
+  void localToAncestorRect(
       const TransformPaintPropertyNode* localTransformNode,
-      const TransformPaintPropertyNode* ancestorTransformNode);
+      const TransformPaintPropertyNode* ancestorTransformNode,
+      FloatRect& mappingRect);
 
   // Maps from a rect in |ancestorTransformNode| space to its transformed rect
   // in |localTransformNode| space. This is computed by multiplying the rect by
   // the inverse combined transform between |localTransformNode| and
   // |ancestorTransformNode|, if the transform is invertible.
   //
   // DCHECK fails if the combined transform is not invertible, or
   // |localTransformNode| is not equal to or a descendant of
   // |ancestorTransformNode|.
-  FloatRect ancestorToLocalRect(
-      const FloatRect&,
+  //
+  // |mappingRect| is both input and output.
+  void ancestorToLocalRect(
       const TransformPaintPropertyNode* ancestorTransformNode,
-      const TransformPaintPropertyNode* localTransformNode);
+      const TransformPaintPropertyNode* localTransformNode,
+      FloatRect& mappingRect);
 
   // Returns the matrix used in |LocalToAncestorRect|. DCHECK fails iff
   // |localTransformNode| is not equal to or a descendant of
   // |ancestorTransformNode|.
   const TransformationMatrix& localToAncestorMatrix(
       const TransformPaintPropertyNode* localTransformNode,
       const TransformPaintPropertyNode* ancestorTransformNode);
 
   // Returns the "clip visual rect" between |localTransformState| and
   // |ancestorState|. See above for the definition of "clip visual rect".
   FloatClipRect localToAncestorClipRect(
       const PropertyTreeState& localTransformState,
       const PropertyTreeState& ancestorState);
 
   // Like localToAncestorClipRect, except it can handle destination transform
   // spaces which are not direct ancestors of the source transform space.
-  FloatClipRect sourceToDestinationClipRect(
+  const FloatClipRect& sourceToDestinationClipRect(
       const PropertyTreeState& sourceState,
       const PropertyTreeState& destinationState);
 
   // Returns the lowest common ancestor in the paint property tree.
   template <typename NodeType>
   static PLATFORM_EXPORT const NodeType* lowestCommonAncestor(const NodeType*,
                                                               const NodeType*);
 
   void clearCache();
 
  protected:
   GeometryMapper() {}
 
  private:
   // The internal methods do the same things as their public counterparts, but
   // take an extra |success| parameter which indicates if the function is
   // successful on return. See comments of the public functions for failure
   // conditions.
 
-  FloatClipRect sourceToDestinationVisualRectInternal(
+  const FloatClipRect& sourceToDestinationVisualRectInternal(
       const FloatRect&,
       const PropertyTreeState& sourceState,
       const PropertyTreeState& destinationState,
       bool& success);
 
-  FloatClipRect localToAncestorVisualRectInternal(
+  const FloatClipRect& localToAncestorVisualRectInternal(
       const FloatRect&,
       const PropertyTreeState& localTransformState,
       const PropertyTreeState& ancestorState,
       bool& success);
 
-  FloatRect localToAncestorRectInternal(
-      const FloatRect&,
+  void localToAncestorRectInternal(
       const TransformPaintPropertyNode* localTransformNode,
       const TransformPaintPropertyNode* ancestorTransformNode,
+      FloatRect&,
       bool& success);
 
   const TransformationMatrix& localToAncestorMatrixInternal(
       const TransformPaintPropertyNode* localTransformNode,
       const TransformPaintPropertyNode* ancestorTransformNode,
       bool& success);
 
   const FloatClipRect& localToAncestorClipRectInternal(
       const ClipPaintPropertyNode* descendant,
       const ClipPaintPropertyNode* ancestorClip,
       const TransformPaintPropertyNode* ancestorTransform,
       bool& success);
 
-  FloatClipRect sourceToDestinationClipRectInternal(
+  const FloatClipRect& sourceToDestinationClipRectInternal(
       const PropertyTreeState& sourceState,
       const PropertyTreeState& destinationState,
       bool& success);
 
   FloatClipRect slowLocalToAncestorVisualRectWithEffects(
       const FloatRect&,
       const PropertyTreeState& localState,
       const PropertyTreeState& ancestorState,
       bool& success);
 
   friend class GeometryMapperTest;
   friend class PaintLayerClipperTest;
 
+  // These are used to represent various return values of the above
+  // methods.
   const TransformationMatrix m_identity;
   const FloatClipRect m_infiniteClip;
+  FloatClipRect m_tempRect;
 
   DISALLOW_COPY_AND_ASSIGN(GeometryMapper);
 };
 
 }  // namespace blink
 
 #endif  // GeometryMapper_h