Optimize collapsed border painting

third_party/WebKit/Source/core/style/CollapsedBorderValue.h

 /*
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 47 matching lines @@
     bool exists() const { return m_precedence != BorderPrecedenceOff; }
     StyleColor color() const { return m_colorIsCurrentColor ? StyleColor::currentColor() : StyleColor(m_color); }
     bool isTransparent() const { return m_transparent; }
     EBorderPrecedence precedence() const { return static_cast<EBorderPrecedence>(m_precedence); }
 
     bool isSameIgnoringColor(const CollapsedBorderValue& o) const
     {
         return width() == o.width() && style() == o.style() && precedence() == o.precedence();
     }
 
-    bool equals(const CollapsedBorderValue& o) const
+    bool visuallyEquals(const CollapsedBorderValue& o) const
     {
+        if (!isVisible() && !o.isVisible())
+            return true;

[Xianzhu, 2016/05/03 19:03:27]

Previously cell borders were cached in LayoutTableSection, separately for the 4
sides and invisible borders were not cached. If a new border previously
invisible is now still invisible (though with some of other fields changed), we
not cache it, and treat it as not changed.

Now we need the above change to treat invisible borders which is previously
invisible unchanged no matter of other fields. This is because now we cache the
4 borders (including invisible ones) of a cell in the same structure, when any
of the borders is visible.

         return color() == o.color() && isTransparent() == o.isTransparent() && isSameIgnoringColor(o);
     }
 
     bool isVisible() const
     {
         return style() > BorderStyleHidden && !isTransparent() && exists();
     }
 
     bool shouldPaint(const CollapsedBorderValue& tableCurrentBorderValue) const
     {
-        // Invisible borders are not cached so painters see visible borders only.
-        ASSERT(isVisible());
-        return isSameIgnoringColor(tableCurrentBorderValue);
+        return isVisible() && isSameIgnoringColor(tableCurrentBorderValue);

[Xianzhu, 2016/05/03 19:03:27]

The reason of this change is the same as the above one.

     }
 
 private:
     Color m_color;
     unsigned m_colorIsCurrentColor : 1;
     unsigned m_width : 23;
     unsigned m_style : 4; // EBorderStyle
     unsigned m_precedence : 3; // EBorderPrecedence
     unsigned m_transparent : 1;
 };
 
 } // namespace blink
 
 #endif // CollapsedBorderValue_h