Use DrawingRecorder::canUseCachedDrawing

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 809f8ba106cf29a26706314efc9060c4a52080ff..68ae45ec6128cb035d44eb22ee84a9305c7a9a24 100644
--- a/Source/core/paint/BlockPainter.cpp
+++ b/Source/core/paint/BlockPainter.cpp
@@ -87,8 +87,9 @@ void BlockPainter::paintOverflowControlsIfNeeded(const PaintInfo& paintInfo, con
 {
     PaintPhase phase = paintInfo.phase;
     if (m_renderBlock.hasOverflowClip() && m_renderBlock.style()->visibility() == VISIBLE && (phase == PaintPhaseBlockBackground || phase == PaintPhaseChildBlockBackground) && paintInfo.shouldPaintWithinRoot(&m_renderBlock) && !paintInfo.paintRootBackgroundOnly()) {
-        RenderDrawingRecorder recorder(paintInfo.context, &m_renderBlock, paintInfo.phase, pixelSnappedIntRect(paintOffset, m_renderBlock.visualOverflowRect().size()));
-        ScrollableAreaPainter(*m_renderBlock.layer()->scrollableArea()).paintOverflowControls(paintInfo.context, roundedIntPoint(paintOffset), paintInfo.rect, false /* paintingOverlayControls */);
+        RenderDrawingRecorder recorder(paintInfo.context, m_renderBlock, paintInfo.phase, pixelSnappedIntRect(paintOffset, m_renderBlock.visualOverflowRect().size()));
+        if (!recorder.canUseCachedDrawing())
+            ScrollableAreaPainter(*m_renderBlock.layer()->scrollableArea()).paintOverflowControls(paintInfo.context, roundedIntPoint(paintOffset), paintInfo.rect, false /* paintingOverlayControls */);

[chrishtr, 2014/12/11 20:01:48]

Why are these safe? Unless we are very careful, we have to emit cached display
items in the correct order, or paint order might be messed up.

[Xianzhu, 2014/12/11 20:22:16]

I think we'll emit DrawingDisplayItems/CachedDisplayItems just like before in
the same order, while avoiding unnecessary paint operations on the context.

We have ensured that DrawingRecorders' life-times don't overlap, so we should
never emit DrawingDisplayItems/CachedDisplayItems within the block controlled by
'if (!recorder.canUseCachedDrawing())'.

[chrishtr, 2014/12/12 00:18:08]

Oh right, it's within a block that has a DrawingRecorder. Good point. Nice.

     }
 }
 
@@ -178,14 +179,16 @@ void BlockPainter::paintObject(const PaintInfo& paintInfo, const LayoutPoint& pa
     }
 
     if (paintPhase == PaintPhaseMask && m_renderBlock.style()->visibility() == VISIBLE) {
-        RenderDrawingRecorder recorder(paintInfo.context, &m_renderBlock, paintPhase, bounds);
-        m_renderBlock.paintMask(paintInfo, paintOffset);
+        RenderDrawingRecorder recorder(paintInfo.context, m_renderBlock, paintPhase, bounds);
+        if (!recorder.canUseCachedDrawing())
+            m_renderBlock.paintMask(paintInfo, paintOffset);
         return;
     }
 
     if (paintPhase == PaintPhaseClippingMask && m_renderBlock.style()->visibility() == VISIBLE) {
-        RenderDrawingRecorder recorder(paintInfo.context, &m_renderBlock, paintPhase, bounds);
-        m_renderBlock.paintClippingMask(paintInfo, paintOffset);
+        RenderDrawingRecorder recorder(paintInfo.context, m_renderBlock, paintPhase, bounds);
+        if (!recorder.canUseCachedDrawing())
+            m_renderBlock.paintClippingMask(paintInfo, paintOffset);
         return;
     }
 
@@ -225,8 +228,9 @@ void BlockPainter::paintObject(const PaintInfo& paintInfo, const LayoutPoint& pa
     // If the caret's node's render object's containing block is this block, and the paint action is PaintPhaseForeground,
     // then paint the caret.
     if (paintPhase == PaintPhaseForeground) {
-        RenderDrawingRecorder recorder(paintInfo.context, &m_renderBlock, paintPhase, bounds);
-        paintCarets(paintInfo, paintOffset);
+        RenderDrawingRecorder recorder(paintInfo.context, m_renderBlock, paintPhase, bounds);
+        if (!recorder.canUseCachedDrawing())
+            paintCarets(paintInfo, paintOffset);
     }
 }