[PE] Add a fast path in GeometryMapper::LocalToAncestorClipRectInternal()

For
  descendant->Parent() == ancestor_clip &&
  descendant->LocalTransformSpace() == ancestor_transform
we can return the clip rect of |descendant| directly to avoid the more
complex logic.

This also reduces the number of entries in the clip cache, which can
speed-up cache lookup.

Premilinary stats show that the fast path is used for about half of all
calls to the function.

Bug: 803867
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I8c390e17cbc7c5e225716e6e90ac8be4dfa615d8
Reviewed-on: https://chromium-review.googlesource.com/963012Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543201}

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

@@ -258,18 +258,32 @@ FloatClipRect GeometryMapper::LocalToAncestorClipRect(
   return result;
 }
 
+static const FloatRoundedRect& GetClipRect(
+    const ClipPaintPropertyNode* clip_node,
+    OverlayScrollbarClipBehavior clip_behavior) {
+  return UNLIKELY(clip_behavior == kExcludeOverlayScrollbarSizeForHitTesting)
+             ? clip_node->ClipRectExcludingOverlayScrollbars()
+             : clip_node->ClipRect();
+}
+
 FloatClipRect GeometryMapper::LocalToAncestorClipRectInternal(
     const ClipPaintPropertyNode* descendant,
     const ClipPaintPropertyNode* ancestor_clip,
     const TransformPaintPropertyNode* ancestor_transform,
     OverlayScrollbarClipBehavior clip_behavior,
     bool& success) {
-  FloatClipRect clip;
   if (descendant == ancestor_clip) {
     success = true;
     return FloatClipRect();
   }
 
+  if (descendant->Parent() == ancestor_clip &&
+      descendant->LocalTransformSpace() == ancestor_transform) {
+    success = true;
+    return FloatClipRect(GetClipRect(descendant, clip_behavior));
+  }
+
+  FloatClipRect clip;
   const ClipPaintPropertyNode* clip_node = descendant;
   Vector<const ClipPaintPropertyNode*> intermediate_nodes;
 
@@ -317,10 +331,7 @@ FloatClipRect GeometryMapper::LocalToAncestorClipRectInternal(
 
     // This is where we generate the roundedness and tightness of clip rect
     // from clip and transform properties, and propagate them to |clip|.
-    FloatClipRect mapped_rect(clip_behavior ==
-                                      kExcludeOverlayScrollbarSizeForHitTesting
-                                  ? (*it)->ClipRectExcludingOverlayScrollbars()
-                                  : (*it)->ClipRect());
+    FloatClipRect mapped_rect(GetClipRect((*it), clip_behavior));
     mapped_rect.Map(transform_matrix);
     clip.Intersect(mapped_rect);
 

third_party/WebKit/Source/platform/graphics/paint/GeometryMapperTest.cpp

@@ -108,8 +108,6 @@ INSTANTIATE_TEST_CASE_P(All,
 
 #define CHECK_SOURCE_TO_DESTINATION_PROJECTION()                  \
   do {                                                            \
-    if (ancestor_state.Transform() == local_state.Transform())    \
-      break;                                                      \
     SCOPED_TRACE("Check SourceToDestinationProjection");          \
     const auto& actual_transform_to_ancestor =                    \
         GeometryMapper::SourceToDestinationProjection(            \
@@ -117,16 +115,22 @@ INSTANTIATE_TEST_CASE_P(All,
     EXPECT_EQ(expected_transform, actual_transform_to_ancestor);  \
   } while (false)
 
-#define CHECK_CACHED_CLIP()                                \
-  do {                                                     \
-    if (ancestor_state.Clip() == local_state.Clip() ||     \
-        ancestor_state.Effect() != local_state.Effect())   \
-      break;                                               \
-    SCOPED_TRACE("Check cached clip");                     \
-    const auto* cached_clip =                              \
-        GetCachedClip(local_state.Clip(), ancestor_state); \
-    DCHECK(cached_clip);                                   \
-    EXPECT_CLIP_RECT_EQ(expected_clip, *cached_clip);      \
+#define CHECK_CACHED_CLIP()                                       \
+  do {                                                            \
+    if (ancestor_state.Effect() != local_state.Effect())          \
+      break;                                                      \
+    SCOPED_TRACE("Check cached clip");                            \
+    const auto* cached_clip =                                     \
+        GetCachedClip(local_state.Clip(), ancestor_state);        \
+    if (ancestor_state.Clip() == local_state.Clip() ||            \
+        (ancestor_state.Clip() == local_state.Clip()->Parent() && \
+         ancestor_state.Transform() ==                            \
+             local_state.Clip()->LocalTransformSpace())) {        \
+      EXPECT_EQ(nullptr, cached_clip);                            \
+      break;                                                      \
+    }                                                             \
+    ASSERT_NE(nullptr, cached_clip);                              \
+    EXPECT_CLIP_RECT_EQ(expected_clip, *cached_clip);             \
   } while (false)
 
 // See the data fields of GeometryMapperTest for variables that will be used in