Implement a paint offset cache for slimming paint v2

Source/core/layout/LayoutObject.h

Index: Source/core/layout/LayoutObject.h
diff --git a/Source/core/layout/LayoutObject.h b/Source/core/layout/LayoutObject.h
index 06e8d0270fba1af8f836169364b4e2f30ebf0d3b..059f11ca016054bb40673ea5d065996180ac2b58 100644
--- a/Source/core/layout/LayoutObject.h
+++ b/Source/core/layout/LayoutObject.h
@@ -1009,6 +1009,10 @@ public:
     const LayoutPoint& previousPositionFromPaintInvalidationBacking() const { return m_previousPositionFromPaintInvalidationBacking; }
     void setPreviousPositionFromPaintInvalidationBacking(const LayoutPoint& positionFromPaintInvalidationBacking) { m_previousPositionFromPaintInvalidationBacking = positionFromPaintInvalidationBacking; }
 
+    // Cache of the previous paint offset for slimming paint v2.
+    const LayoutPoint& cachedPaintOffset() const;
+    void updateCachedPaintOffset(const LayoutPoint&) const;
+
     PaintInvalidationReason fullPaintInvalidationReason() const { return m_bitfields.fullPaintInvalidationReason(); }
     bool shouldDoFullPaintInvalidation() const { return m_bitfields.fullPaintInvalidationReason() != PaintInvalidationNone; }
     void setShouldDoFullPaintInvalidation(PaintInvalidationReason = PaintInvalidationFull);
@@ -1427,6 +1431,11 @@ private:
     // It is used to detect layoutObject shifts that forces a full invalidation.
     // This point does *not* account for composited scrolling. See adjustInvalidationRectForCompositedScrolling().
     LayoutPoint m_previousPositionFromPaintInvalidationBacking;
+
+    // Previous paint offset for slimming paint v2.
+    // TODO(pdr): This should be merged with m_previousPositionFromPaintInvalidationBacking (https://codereview.chromium.org/1315213002).
+    // DO NOT COMMIT until https://codereview.chromium.org/1315213002 lands.
+    mutable LayoutPoint m_cachedPaintOffset;

[Xianzhu, 2015/08/27 17:20:10]

Would m_previousPaintOffset be better, as we don't use it as a cache (to speedup
access), but compare it to the current paint offset to do invalidation?

[pdr., 2015/08/28 03:31:41]

In the latest patch I've moved the cache to the display item list because we
don't want to increase the size of LayoutObject just yet. This will be
temporary.

I've switched the name as you suggested (m_previousPaintOffsets, etc).

 };
 
 // FIXME: remove this once the layout object lifecycle ASSERTS are no longer hit.