Fix blink_perf.paint regression about tables

third_party/WebKit/Source/core/layout/LayoutTableSection.h

 /*
  * Copyright (C) 1997 Martin Jones (mjones@kde.org)
  *           (C) 1997 Torben Weis (weis@kde.org)
  *           (C) 1998 Waldo Bastian (bastian@kde.org)
  *           (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2009, 2013 Apple Inc. All rights
  * reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 200 matching lines @@
   const LayoutTableCell* firstRowCellAdjoiningTableStart() const;
   const LayoutTableCell* firstRowCellAdjoiningTableEnd() const;
 
   CellStruct& cellAt(unsigned row, unsigned effectiveColumn) {
     return m_grid[row].row[effectiveColumn];
   }
   const CellStruct& cellAt(unsigned row, unsigned effectiveColumn) const {
     return m_grid[row].row[effectiveColumn];
   }
   LayoutTableCell* primaryCellAt(unsigned row, unsigned effectiveColumn) {
-    if (effectiveColumn >= numCols(row))

[mstensho (USE GERRIT), 2017/01/20 10:02:04]

I find it strange that this makes a lot of difference. numCols() looks cheap.

Does it help if you rewrite primaryCellAt() as:

    Row& rowVector = m_grid[row].row;
    if (effectiveColumn >= rowVector.size())
      return nullptr;
    return rowVector[effectiveColumn].primaryCell();

[a.suchit, 2017/01/23 06:35:32]

Done.

-      return nullptr;
     CellStruct& c = m_grid[row].row[effectiveColumn];
     return c.primaryCell();
   }
   const LayoutTableCell* primaryCellAt(unsigned row,
                                        unsigned effectiveColumn) const {
     return const_cast<LayoutTableSection*>(this)->primaryCellAt(
         row, effectiveColumn);
   }
 
   unsigned numCols(unsigned row) const { return m_grid[row].row.size(); }
@@ skipping 242 matching lines @@
   // The use is to disable a painting optimization where we just paint the
   // invalidated cells.
   bool m_hasMultipleCellLevels;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutTableSection, isTableSection());
 
 }  // namespace blink
 
 #endif  // LayoutTableSection_h