Avoid re-iterate out-of-order display items

Source/platform/graphics/paint/DisplayItem.h

 // 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.
 
 #ifndef DisplayItem_h
 #define DisplayItem_h
 
 #include "platform/PlatformExport.h"
 #include "platform/graphics/ContiguousContainer.h"
 #include "platform/graphics/paint/DisplayItemClient.h"
@@ skipping 166 matching lines @@
         UninitializedType,
         TypeLast = UninitializedType
     };
 
     DisplayItem(const DisplayItemClientWrapper& client, Type type, size_t derivedSize)
         : m_client(client.displayItemClient())
         , m_scope(0)
         , m_type(type)
         , m_derivedSize(derivedSize)
         , m_skippedCache(false)
-        , m_ignoredFromList(false)
 #ifndef NDEBUG
         , m_clientDebugString(client.debugName())
 #endif
     {
         // derivedSize must fit in m_derivedSize.
         // If it doesn't, enlarge m_derivedSize and fix this assert.
         ASSERT_WITH_SECURITY_IMPLICATION(derivedSize < (1 << 8));
         ASSERT_WITH_SECURITY_IMPLICATION(derivedSize >= sizeof(*this));
     }
 
+    virtual ~DisplayItem() { }
+
     // Ids are for matching new DisplayItems with existing DisplayItems.
     struct Id {
         Id(const DisplayItemClient client, const Type type, const unsigned scope)
             : client(client)
             , type(type)
             , scope(scope) { }
 
         bool matches(const DisplayItem& item) const
         {
             // We should always convert to non-cached types before matching.
@@ skipping 18 matching lines @@
             return cachedSubtreeTypeToBeginSubtreeType(m_type);
         return m_type;
     }
 
     // Return the Id with cached type converted to non-cached type.
     Id nonCachedId() const
     {
         return Id(m_client, nonCachedType(), m_scope);
     }
 
-    virtual ~DisplayItem() { }
-
     virtual void replay(GraphicsContext&) { }
 
     DisplayItemClient client() const { return m_client; }
     Type type() const { return m_type; }
 
     void setScope(unsigned scope) { m_scope = scope; }
     unsigned scope() { return m_scope; }
 
     // Size of this object in memory, used to move it with memcpy.
     // This is not sizeof(*this), because it needs to account for the size of
@@ skipping 74 matching lines @@
 
     virtual bool isBegin() const { return false; }
     virtual bool isEnd() const { return false; }
 
 #if ENABLE(ASSERT)
     virtual bool isEndAndPairedWith(DisplayItem::Type otherType) const { return false; }
 #endif
 
     virtual bool drawsContent() const { return false; }
 
-    bool ignoreFromDisplayList() const { return m_ignoredFromList; }
-    void setIgnoredFromDisplayList() { m_ignoredFromList = true; }
+    bool isValid() const { return m_client; }
+    // Re-construct this display item as a permanently invalid item with no client set.
+    void invalidate()

[pdr., 2015/08/18 06:15:21]

This looks correct now, but it's tricky. Because this codepath is only used for
underinvalidation checking, what do you think about renaming invalidate() to
clearClientForUnderinvalidationChecking(), changing m_client to be mutable, and
directly setting m_client to nullptr?

[Xianzhu, 2015/08/18 16:24:44]

Acknowledged and Done.

+    {
+        this->~DisplayItem();
+        new (this) DisplayItem;
+    }
 
 #ifndef NDEBUG
     static WTF::String typeAsDebugString(DisplayItem::Type);
     const WTF::String& clientDebugString() const { return m_clientDebugString; }
     WTF::String asDebugString() const;
     virtual void dumpPropertiesAsDebugString(WTF::StringBuilder&) const;
 #endif
 
 private:
     // The default DisplayItem constructor is only used by
     // ContiguousContainer::appendByMoving where an invalid DisplaItem is
     // constructed at the source location.
     template <typename T, unsigned alignment> friend class ContiguousContainer;
 
     DisplayItem()
         : m_client(nullptr)
         , m_scope(0)
         , m_type(UninitializedType)
         , m_derivedSize(sizeof(*this))
         , m_skippedCache(false)
-        , m_ignoredFromList(true)
 #ifndef NDEBUG
         , m_clientDebugString("invalid")
 #endif
     { }
 
     const DisplayItemClient m_client;
     unsigned m_scope;
     static_assert(TypeLast < (1 << 16), "DisplayItem::Type should fit in 16 bits");
     const Type m_type : 16;
     unsigned m_derivedSize : 8; // size of the actual derived class
     unsigned m_skippedCache : 1;
-    unsigned m_ignoredFromList : 1;
 
 #ifndef NDEBUG
     WTF::String m_clientDebugString;
 #endif
 };
 
 class PLATFORM_EXPORT PairedBeginDisplayItem : public DisplayItem {
 protected:
     PairedBeginDisplayItem(const DisplayItemClientWrapper& client, Type type, size_t derivedSize) : DisplayItem(client, type, derivedSize) { }
 
 private:
     bool isBegin() const final { return true; }
 };
 
 class PLATFORM_EXPORT PairedEndDisplayItem : public DisplayItem {
 protected:
     PairedEndDisplayItem(const DisplayItemClientWrapper& client, Type type, size_t derivedSize) : DisplayItem(client, type, derivedSize) { }
 
 #if ENABLE(ASSERT)
     bool isEndAndPairedWith(DisplayItem::Type otherType) const override = 0;
 #endif
 
 private:
     bool isEnd() const final { return true; }
 };
 
 } // namespace blink
 
 #endif // DisplayItem_h