Save previous paint offset in LayoutObject

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 5be3d0db2781b113becb8e6ad2e003ad48b1ca3a..771b6b0ea330838fb67585cbe4e1629061030089 100644
--- a/Source/core/layout/LayoutObject.h
+++ b/Source/core/layout/LayoutObject.h
@@ -1138,6 +1138,17 @@ public:
         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);
@@ -1638,10 +1649,15 @@ private:
     // 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.