Implement a paint offset cache for slimming paint v2

Source/core/layout/LayoutObject.h

 /*
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2004 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 991 matching lines @@
     LayoutSize previousPaintInvalidationRectSize() const { return previousPaintInvalidationRect().size(); }
     void setPreviousPaintInvalidationRect(const LayoutRect& rect) { m_previousPaintInvalidationRect = rect; }
 
     // Only adjusts if the paint invalidation container is not a composited scroller.
     void adjustPreviousPaintInvalidationForScrollIfNeeded(const DoubleSize& scrollDelta);
 
     // The previous position of the top-left corner of the object in its previous paint backing.
     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);
     void clearShouldDoFullPaintInvalidation() { m_bitfields.setFullPaintInvalidationReason(PaintInvalidationNone); }
 
     bool shouldInvalidateOverflowForPaint() const { return m_bitfields.shouldInvalidateOverflowForPaint(); }
 
     virtual void clearPaintInvalidationState(const PaintInvalidationState&);
 
     bool mayNeedPaintInvalidation() const { return m_bitfields.mayNeedPaintInvalidation(); }
@@ skipping 398 matching lines @@
     static bool s_affectsParentBlock;
 
     // This stores the paint invalidation rect from the previous frame. This rect does *not* account for composited scrolling. See
     // adjustInvalidationRectForCompositedScrolling().
     LayoutRect m_previousPaintInvalidationRect;
 
     // This stores the position in the paint invalidation backing's coordinate.
     // 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.
 class DeprecatedDisableModifyLayoutTreeStructureAsserts {
     WTF_MAKE_NONCOPYABLE(DeprecatedDisableModifyLayoutTreeStructureAsserts);
 public:
     DeprecatedDisableModifyLayoutTreeStructureAsserts();
 
     static bool canModifyLayoutTreeStateInAnyState();
 
@@ skipping 216 matching lines @@
 void showTree(const blink::LayoutObject*);
 void showLineTree(const blink::LayoutObject*);
 void showLayoutTree(const blink::LayoutObject* object1);
 // We don't make object2 an optional parameter so that showLayoutTree
 // can be called from gdb easily.
 void showLayoutTree(const blink::LayoutObject* object1, const blink::LayoutObject* object2);
 
 #endif
 
 #endif // LayoutObject_h