Skip PaintPhaseFloat if no floats in a layer

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

 // Copyright 2014 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.
 
 #include "core/paint/PaintLayerPainter.h"
 
 #include "core/frame/Settings.h"
 #include "core/layout/ClipPathOperation.h"
 #include "core/layout/LayoutBlock.h"
 #include "core/layout/LayoutFrame.h"
@@ skipping 653 matching lines @@
         clipRecorder.emplace(context, *m_paintLayer.layoutObject(), DisplayItem::ClipLayerForeground, layerFragments[0].foregroundRect, &localPaintingInfo, layerFragments[0].paginationOffset, paintFlags);
         clipState = HasClipped;
     }
 
     // We have to loop through every fragment multiple times, since we have to issue paint invalidations in each specific phase in order for
     // interleaving of the fragments to work properly.
     paintForegroundForFragmentsWithPhase(selectionOnly ? PaintPhaseSelection : PaintPhaseDescendantBlockBackgroundsOnly,
         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()) {
             size_t displayItemListSizeBefore = context.paintController().newDisplayItemList().size();
             paintForegroundForFragmentsWithPhase(PaintPhaseDescendantOutlinesOnly, layerFragments, context, localPaintingInfo, paintFlags, clipState);
             if (displayItemListSizeBefore == context.paintController().newDisplayItemList().size())
                 m_paintLayer.clearNeedsPaintPhaseDescendantOutlines();
         }
     }
 }
@@ skipping 56 matching lines @@
     if (!m_paintLayer.containsDirtyOverlayScrollbars())
         return;
 
     PaintLayerPaintingInfo paintingInfo(&m_paintLayer, LayoutRect(enclosingIntRect(damageRect)), paintFlags, LayoutSize());
     paintLayer(context, paintingInfo, PaintLayerPaintingOverlayScrollbars);
 
     m_paintLayer.setContainsDirtyOverlayScrollbars(false);
 }
 
 } // namespace blink