Optimize PaintInvalidatorContext::mapLocalRectToVisualRectInBacking()

third_party/WebKit/Source/core/layout/PaintInvalidationState.cpp

Index: third_party/WebKit/Source/core/layout/PaintInvalidationState.cpp
diff --git a/third_party/WebKit/Source/core/layout/PaintInvalidationState.cpp b/third_party/WebKit/Source/core/layout/PaintInvalidationState.cpp
index 086b5b51fe0577a335892660e0d1b486135072ab..47b3f02008f846ef7a28f711d2cb40dd94451e7a 100644
--- a/third_party/WebKit/Source/core/layout/PaintInvalidationState.cpp
+++ b/third_party/WebKit/Source/core/layout/PaintInvalidationState.cpp
@@ -17,6 +17,7 @@
 #include "core/paint/PaintInvalidator.h"
 #include "core/paint/PaintLayer.h"
 #include "core/paint/PaintPropertyTreeBuilder.h"
+#include "platform/graphics/paint/GeometryMapper.h"
 
 namespace blink {
 
@@ -634,9 +635,15 @@ static const PaintPropertyTreeBuilderContext& dummyTreeBuilderContext() {
   return dummyContext;
 }
 
+static GeometryMapper& dummyGeometryMapper() {

[chrishtr, 2017/03/22 17:54:31]

Is it somehow guaranteed that this GeometryMapper is never used?

[Xianzhu, 2017/03/22 18:50:02]

Yes because the adapter's mapLocalRectToVisualRectInBacking() overrides the
method in PaintInvalidatorContext's. The overriding method uses
PaintInvaldiationState to map the rect.

Create a followup patch to enforce it.

[chrishtr, 2017/03/22 18:56:21]

Ok, please add a comment saying that it's guaranteed by code structure that this
dummy object
is never called.

[Xianzhu, 2017/03/22 19:20:43]

Done.

+  DEFINE_STATIC_LOCAL(std::unique_ptr<GeometryMapper>, dummyMapper,
+                      (GeometryMapper::create()));

[wkorman, 2017/03/22 18:33:40]

We saw pdr@ recently make a perf improvement by adding fast-malloc to an
instance, should we consider similar for GeometryMapper?
https://codereview.chromium.org/2744243010

Maybe it's not worth it just b/c we allocate comparatively few and not in hot
code path.

[Xianzhu, 2017/03/22 18:51:38]

I think it's not worth it.

+  return *dummyMapper;
+}
+
 PaintInvalidatorContextAdapter::PaintInvalidatorContextAdapter(
     const PaintInvalidationState& paintInvalidationState)
-    : PaintInvalidatorContext(dummyTreeBuilderContext()),
+    : PaintInvalidatorContext(dummyTreeBuilderContext(), dummyGeometryMapper()),
       m_paintInvalidationState(paintInvalidationState) {
   forcedSubtreeInvalidationFlags =
       paintInvalidationState.m_forcedSubtreeInvalidationFlags;