Don't use cached subsequence when clip changes

third_party/WebKit/Source/core/paint/PaintLayerClipper.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 133 matching lines @@
     // before we call calculateClipRects so that calculateClipRects will hit
     // the cache.
     ClipRects* parentClipRects = 0;
     if (context.rootLayer != m_layoutObject.layer() && m_layoutObject.layer()->parent())
         parentClipRects = m_layoutObject.layer()->parent()->clipper().getClipRects(context);
     RefPtr<ClipRects> clipRects = ClipRects::create();
     calculateClipRects(context, *clipRects);
     return storeClipRectsInCache(context, parentClipRects, *clipRects);
 }
 
 void PaintLayerClipper::clearClipRectsIncludingDescendants()

[chrishtr, 2015/11/18 00:32:02]

It looks like PaintLayer::updateLayerPositionsAfterLayout() calls this
unconditionally?

[Xianzhu, 2015/11/18 22:38:14]

You are right.

The latest patch checks actual change of painting clip rects.

 {
     m_cache = nullptr;
+    m_layoutObject.layer()->setNeedsRepaint();
 
     for (PaintLayer* layer = m_layoutObject.layer()->firstChild(); layer; layer = layer->nextSibling()) {
         layer->clipper().clearClipRectsIncludingDescendants();
     }
 }
 
 void PaintLayerClipper::clearClipRectsIncludingDescendants(ClipRectsCacheSlot cacheSlot)
 {
     if (m_cache)
         m_cache->clear(cacheSlot);
+    m_layoutObject.layer()->setNeedsRepaint();
 
     for (PaintLayer* layer = m_layoutObject.layer()->firstChild(); layer; layer = layer->nextSibling()) {
         layer->clipper().clearClipRectsIncludingDescendants(cacheSlot);
     }
 }
 
 
 LayoutRect PaintLayerClipper::localClipRect(const PaintLayer* clippingRootLayer) const
 {
     LayoutRect layerBounds;
@@ skipping 146 matching lines @@
     if (context.respectOverflowClip == IgnoreOverflowClip)
         return false;
 
     if (layer->isRootLayer() && context.respectOverflowClipForViewport == IgnoreOverflowClip)
         return false;
 
     return true;
 }
 
 } // namespace blink