Save previous paint offset in LayoutObject

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 1120 matching lines @@
     {
         ASSERT(!RuntimeEnabledFeatures::slimmingPaintOffsetCachingEnabled());
         return m_previousPositionFromPaintInvalidationBacking;
     }
     void setPreviousPositionFromPaintInvalidationBacking(const LayoutPoint& positionFromPaintInvalidationBacking)
     {
         ASSERT(!RuntimeEnabledFeatures::slimmingPaintOffsetCachingEnabled());
         m_previousPositionFromPaintInvalidationBacking = positionFromPaintInvalidationBacking;
     }
 
+    const LayoutPoint& previousPaintOffset() const
+    {
+        ASSERT(RuntimeEnabledFeatures::slimmingPaintOffsetCachingEnabled());
+        return m_previousPaintOffset;
+    }
+    void setPreviousPaintOffset(const LayoutPoint& paintOffset) const
+    {
+        ASSERT(RuntimeEnabledFeatures::slimmingPaintOffsetCachingEnabled());
+        m_previousPaintOffset = paintOffset;
+    }
+
     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 480 matching lines @@
     void setChildNeedsOverflowRecalcAfterStyleChange() { m_bitfields.setChildNeedsOverflowRecalcAfterStyleChange(true); }
 
 private:
     // Store state between styleWillChange and styleDidChange
     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;
+    union {
+        // 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().
+        // TODO(wangxianzhu): Remove this when we enable slimmingPaintOffsetCaching.
+        LayoutPoint m_previousPositionFromPaintInvalidationBacking;
+        // For slimmingPaintOffsetCaching.
+        mutable LayoutPoint m_previousPaintOffset;

[pdr., 2015/09/22 22:44:44]

Do we need to track the initial state in order to disambiguate between a
previousPaintOffset of 0,0 and no previous paint offset?

[Xianzhu, 2015/09/23 18:43:18]

Done. Added uninitializedPaintOffset() for the initial value of paint offset.

+    };
 };
 
 // FIXME: remove this once the layout object lifecycle ASSERTS are no longer hit.
 class DeprecatedDisableModifyLayoutTreeStructureAsserts {
     STACK_ALLOCATED();
     WTF_MAKE_NONCOPYABLE(DeprecatedDisableModifyLayoutTreeStructureAsserts);
 public:
     DeprecatedDisableModifyLayoutTreeStructureAsserts();
 
     static bool canModifyLayoutTreeStateInAnyState();
@@ skipping 218 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