Raster display item lists via a visual rect RTree.

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

Index: third_party/WebKit/Source/platform/graphics/paint/PaintController.h
diff --git a/third_party/WebKit/Source/platform/graphics/paint/PaintController.h b/third_party/WebKit/Source/platform/graphics/paint/PaintController.h
index b62d5391553bd8bbad4b835d8e28fe7bf7d89b18..2c388cc4735b6daefd7af30085138b972a3c0fe5 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/PaintController.h
+++ b/third_party/WebKit/Source/platform/graphics/paint/PaintController.h
@@ -123,6 +123,9 @@ public:
     const DisplayItemList& displayItemList() const { return paintArtifact().displayItemList(); }
     const Vector<PaintChunk>& paintChunks() const { return paintArtifact().paintChunks(); }
 
+    typedef WTF::HashMap<DisplayItemClient, IntRect> DisplayItemRectMap;

[esprehn, 2015/12/08 19:58:31]

Doesn't this approach the size of the layout tree if you invalidate paint on
many things? Every layout object is a client, so this map contains the entire
layout tree?

[wkorman, 2015/12/08 20:55:10]

I believe you are correct. We obviously won't enable this code path until/unless
we gain comfort with the memory/performance ramifications. When we submit it
will be behind a flag.

We know that we may need to explore alternate data structure/approach for better
memory efficiency, but expectation is that there is a worthwhile performance
gain for memory increase.

What is your level of concern re: the degenerate case where we store the entire
layout tree, in the big picture of overall Blink/Chrome memory usage and
haraken@'s analysis?

https://docs.google.com/presentation/d/1_LRxXp30j60npShHSRVit0B5tLOS4TLiZ68gQ...

[Xianzhu, 2015/12/09 17:09:27]

FYI I have converted DisplayItemClient to an interface, making it possible to
let PaintController access client data. We can add a method
DisplayItemClient::previousPaintBounds() and LayoutObject can just return its
m_previousPaintInvalidaitonRect.

+    const DisplayItemRectMap& displayItemVisualRects() const { return m_displayItemInvalidationRects; }
+
     bool clientCacheIsValid(DisplayItemClient) const;
     bool cacheIsEmpty() const { return m_currentPaintArtifact.isEmpty(); }
 
@@ -238,6 +241,8 @@ private:
     HashSet<DisplayItemClient> m_clientsWithPaintOffsetInvalidations;
 #endif
 
+    DisplayItemRectMap m_displayItemInvalidationRects;
+
     // Allow display item construction to be disabled to isolate the costs of construction
     // in performance metrics.
     bool m_constructionDisabled;