Subtree caching implementation in blink-core

Source/core/paint/BlockPainter.cpp

Index: Source/core/paint/BlockPainter.cpp
diff --git a/Source/core/paint/BlockPainter.cpp b/Source/core/paint/BlockPainter.cpp
index fa3bd8fdc40e0f87b703d8c4106853eab75b144f..9c1a5a36e7af498059060876b357e7b91f8a1b04 100644
--- a/Source/core/paint/BlockPainter.cpp
+++ b/Source/core/paint/BlockPainter.cpp
@@ -17,34 +17,19 @@
 #include "core/paint/DeprecatedPaintLayer.h"
 #include "core/paint/InlinePainter.h"
 #include "core/paint/LayoutObjectDrawingRecorder.h"
+#include "core/paint/LayoutObjectSubtreeRecorder.h"
 #include "core/paint/LineBoxListPainter.h"
 #include "core/paint/PaintInfo.h"
 #include "core/paint/ScopeRecorder.h"
 #include "core/paint/ScrollRecorder.h"
 #include "core/paint/ScrollableAreaPainter.h"
 #include "platform/graphics/paint/ClipRecorder.h"
-#include "platform/graphics/paint/SubtreeRecorder.h"
 #include "wtf/Optional.h"
 
 namespace blink {
 
-// We need to balance the benefit of subtree optimization and the cost of subtree display items.
-// Only output subtree information if the block has multiple children or multiple line boxes.
-static bool needsSubtreeRecorder(const LayoutBlock& layoutBlock)
-{
-    return (layoutBlock.firstChild() && layoutBlock.firstChild()->nextSibling())
-        || (layoutBlock.isLayoutBlockFlow() && toLayoutBlockFlow(layoutBlock).firstLineBox() && toLayoutBlockFlow(layoutBlock).firstLineBox()->nextLineBox());
-}
-
 void BlockPainter::paint(const PaintInfo& paintInfo, const LayoutPoint& paintOffset)
 {
-    Optional<SubtreeRecorder> subtreeRecorder;
-    if (needsSubtreeRecorder(m_layoutBlock)) {
-        subtreeRecorder.emplace(*paintInfo.context, m_layoutBlock, paintInfo.phase);
-        if (subtreeRecorder->canUseCache())
-            return;
-    }
-
     PaintInfo localPaintInfo(paintInfo);
 
     LayoutPoint adjustedPaintOffset = paintOffset + m_layoutBlock.location();
@@ -58,6 +43,10 @@ void BlockPainter::paint(const PaintInfo& paintInfo, const LayoutPoint& paintOff
     if (!overflowBox.intersects(LayoutRect(localPaintInfo.rect)))
         return;
 
+    LayoutObjectSubtreeRecorder subtreeRecorder(*paintInfo.context, m_layoutBlock, paintInfo.phase);
+    if (subtreeRecorder.canUseCache())
+        return;
+
     // There are some cases where not all clipped visual overflow is accounted for.
     // FIXME: reduce the number of such cases.
     ContentsClipBehavior contentsClipBehavior = ForceContentsClip;