Optimize collapsed border painting

third_party/WebKit/Source/core/layout/LayoutTableCell.cpp

Index: third_party/WebKit/Source/core/layout/LayoutTableCell.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutTableCell.cpp b/third_party/WebKit/Source/core/layout/LayoutTableCell.cpp
index cccc8fbb14f13db05f9aad24967cc044c0a3d93b..c0277f6ace51ff79410d36bc9e23a51df41ad1f3 100644
--- a/third_party/WebKit/Source/core/layout/LayoutTableCell.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutTableCell.cpp
@@ -43,6 +43,7 @@ using namespace HTMLNames;
 struct SameSizeAsLayoutTableCell : public LayoutBlockFlow {
     unsigned bitfields;
     int paddings[2];
+    void* pointer;
 };
 
 static_assert(sizeof(LayoutTableCell) == sizeof(SameSizeAsLayoutTableCell), "LayoutTableCell should stay small");
@@ -65,7 +66,6 @@ void LayoutTableCell::willBeRemovedFromTree()
     LayoutBlockFlow::willBeRemovedFromTree();
 
     section()->setNeedsCellRecalc();
-    section()->removeCachedCollapsedBorders(this);
 }
 
 unsigned LayoutTableCell::parseColSpanFromDOM() const
@@ -908,7 +908,7 @@ void LayoutTableCell::paint(const PaintInfo& paintInfo, const LayoutPoint& paint
 static void addBorderStyle(LayoutTable::CollapsedBorderValues& borderValues,
     CollapsedBorderValue borderValue)
 {
-    if (!borderValue.exists())
+    if (!borderValue.isVisible())
         return;
     size_t count = borderValues.size();
     for (size_t i = 0; i < count; ++i) {
@@ -920,25 +920,36 @@ static void addBorderStyle(LayoutTable::CollapsedBorderValues& borderValues,
 
 void LayoutTableCell::collectBorderValues(LayoutTable::CollapsedBorderValues& borderValues)
 {
-    CollapsedBorderValue startBorder = computeCollapsedStartBorder();
-    CollapsedBorderValue endBorder = computeCollapsedEndBorder();
-    CollapsedBorderValue beforeBorder = computeCollapsedBeforeBorder();
-    CollapsedBorderValue afterBorder = computeCollapsedAfterBorder();
-    LayoutTableSection* section = this->section();
-    bool changed = section->setCachedCollapsedBorder(this, CBSStart, startBorder);
-    changed |= section->setCachedCollapsedBorder(this, CBSEnd, endBorder);
-    changed |= section->setCachedCollapsedBorder(this, CBSBefore, beforeBorder);
-    changed |= section->setCachedCollapsedBorder(this, CBSAfter, afterBorder);
-
-    // In slimming paint mode, we need to invalidate all cells with collapsed border changed.
-    // FIXME: Need a way to invalidate/repaint the borders only. crbug.com/451090#c5.
+    LayoutTableCell::CollapsedBorderValues newValues = {
+        computeCollapsedStartBorder(),
+        computeCollapsedEndBorder(),
+        computeCollapsedBeforeBorder(),
+        computeCollapsedAfterBorder()
+    };
+
+    bool changed;
+    if (!newValues.startBorder.isVisible() && !newValues.endBorder.isVisible() && !newValues.beforeBorder.isVisible() && !newValues.afterBorder.isVisible()) {

[Julien - ping for review, 2016/01/08 14:18:15]

How frequent is this case? It seems like a corner-case optimization that adds
more complexity than it saves memory.

[Xianzhu, 2016/01/08 17:50:45]

This optimization does improve performance of
PerformanceTest/Paint/large-table-background-change-with-invisible-collapsed-borders.html
(previously
PerformanceTest/Layout/large-table-background-change-with-collapsed-borders.html)
from 91ms to 42ms :)
(http://storage.googleapis.com/chromium-telemetry/html-results/results-2015-12...)
by eliminating the whole loop and treewalks for collapsed borders in
TablePainter::paintObject().

I think the case is reasonable in pages that set all tables to collapse borders
but don't specify borders for all tables.

I can stat with top 10k pages.

[Julien - ping for review, 2016/01/11 18:11:36]

This really should have a comment about what it's targeting and include the
great statistics:

"- 11.72% LayoutTableCells have invisible collapsed borders: optimization of
invisible collapsed borders is necessary;"

Also having a helper function for the 4 checks would be nice as it would clarify
what the code is testing.

+        changed = m_collapsedBorderValues;
+        m_collapsedBorderValues = nullptr;
+    } else if (!m_collapsedBorderValues) {
+        changed = true;

[Julien - ping for review, 2016/01/08 14:18:15]

Do we really need that for the first this code runs? If it is needed, we should
put a comment as to *why*.

[Xianzhu, 2016/01/08 17:50:45]

Sorry I'm not sure, do you mean I should comment on the purpose of the 'changed'
variable?

[Julien - ping for review, 2016/01/11 18:11:36]

m_collapsedBorderValue will be nullptr the first time we run this code as this
is run for every paint. I was assuming this wouldn't be needed in this case but
it should be required in a subsequent paint as we could get out of the
optimization above.

+        m_collapsedBorderValues = adoptPtr(new CollapsedBorderValues(newValues));
+    } else {
+        changed = !m_collapsedBorderValues->startBorder.isEquivalentForPainting(newValues.startBorder)
+            || !m_collapsedBorderValues->endBorder.isEquivalentForPainting(newValues.endBorder)
+            || !m_collapsedBorderValues->beforeBorder.isEquivalentForPainting(newValues.beforeBorder)
+            || !m_collapsedBorderValues->afterBorder.isEquivalentForPainting(newValues.afterBorder);
+        if (changed)
+            *m_collapsedBorderValues = newValues;
+    }
+
     if (changed)
         table()->invalidateDisplayItemClient(*this);
 
-    addBorderStyle(borderValues, startBorder);
-    addBorderStyle(borderValues, endBorder);
-    addBorderStyle(borderValues, beforeBorder);
-    addBorderStyle(borderValues, afterBorder);
+    addBorderStyle(borderValues, newValues.startBorder);
+    addBorderStyle(borderValues, newValues.endBorder);
+    addBorderStyle(borderValues, newValues.beforeBorder);
+    addBorderStyle(borderValues, newValues.afterBorder);
 }
 
 static int compareBorderValuesForQSort(const void* pa, const void* pb)