Don't cache subsequence whose layer is not fully contained by repaint rect

Source/platform/graphics/paint/DisplayItemList.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "platform/graphics/paint/DisplayItemList.h"
 
 #include "platform/NotImplemented.h"
 #include "platform/TraceEvent.h"
 #include "platform/graphics/paint/DrawingDisplayItem.h"
@@ skipping 76 matching lines @@
         showDebugData();
         WTFLogAlways("DisplayItem %s has duplicated id with previous %s (index=%d)\n",
             displayItem.asDebugString().utf8().data(), m_newDisplayItems[index].asDebugString().utf8().data(), static_cast<int>(index));
 #endif
         ASSERT_NOT_REACHED();
     }
     addItemToIndexIfNeeded(displayItem, m_newDisplayItems.size() - 1, m_newDisplayItemIndicesByClient);
 #endif // ENABLE(ASSERT)
 
     ASSERT(!displayItem.skippedCache()); // Only DisplayItemList can set the flag.
-    if (skippingCache())
+    if (skippingCache()) {
+        // TODO(wangxianzhu): Distinguish skippingCache and uncacheablility.

[chrishtr, 2015/09/08 22:48:00]

What's the difference exactly?

[Xianzhu, 2015/09/09 16:50:33]

Wanted to distinguish "skip-cache but cacheable" and "uncachable". However, the
former is not used for now so I removed uncacheable flag in the new patch. (I
planned to use the former to optimize full-subtree invalidation to avoid mark
every object for invalidation.)

         displayItem.setSkippedCache();
+        displayItem.setUncacheable();
+    }
 }
 
 void DisplayItemList::beginScope()
 {
     ASSERT_WITH_SECURITY_IMPLICATION(m_nextScope < UINT_MAX);
     m_scopeStack.append(m_nextScope++);
     beginSkippingCache();
 }
 
 void DisplayItemList::endScope()
@@ skipping 511 matching lines @@
 
 void DisplayItemList::replay(GraphicsContext& context)
 {
     TRACE_EVENT0("blink,benchmark", "DisplayItemList::replay");
     ASSERT(m_newDisplayItems.isEmpty());
     for (DisplayItem& displayItem : m_currentDisplayItems)
         displayItem.replay(context);
 }
 
 } // namespace blink