Skip PaintPhaseFloat if no floats in a layer

third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
index ff9223db493e4a00a404bf82c6193d0f4663dc12..7adb9fd6ad39f25b118bbba693ceaad4cde0bf5b 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp
@@ -671,7 +671,13 @@ void PaintLayerPainter::paintForegroundForFragments(const PaintLayerFragments& l
         layerFragments, context, localPaintingInfo, paintFlags, clipState);
 
     if (!selectionOnly) {
-        paintForegroundForFragmentsWithPhase(PaintPhaseFloat, layerFragments, context, localPaintingInfo, paintFlags, clipState);
+        if (m_paintLayer.needsPaintPhaseFloat()) {
+            size_t displayItemListSizeBefore = context.paintController().newDisplayItemList().size();
+            paintForegroundForFragmentsWithPhase(PaintPhaseFloat, layerFragments, context, localPaintingInfo, paintFlags, clipState);
+            if (displayItemListSizeBefore == context.paintController().newDisplayItemList().size())

[chrishtr, 2016/01/21 22:09:08]

This looks like a fragile way to try to clear the bit when floats leave this
layer.

[Xianzhu, 2016/01/21 23:58:35]

I think at first we can leave the flag never cleared, so the optimization
applies for layers never have floats. This may not affect how much this CL will
improve performance of Cluster Telemetry. Not sure how much this affects dynamic
change of pages because we lack such benchmarks.

+                m_paintLayer.clearNeedsPaintPhaseFloat();
+        }
+
         paintForegroundForFragmentsWithPhase(PaintPhaseForeground, layerFragments, context, localPaintingInfo, paintFlags, clipState);
 
         if (m_paintLayer.needsPaintPhaseDescendantOutlines()) {