[SPv2] Defer decision of raster invalidation after paint for changes z-index, transform, etc.

third_party/WebKit/Source/platform/graphics/paint/DisplayItem.h

Index: third_party/WebKit/Source/platform/graphics/paint/DisplayItem.h
diff --git a/third_party/WebKit/Source/platform/graphics/paint/DisplayItem.h b/third_party/WebKit/Source/platform/graphics/paint/DisplayItem.h
index 0cc8cf31997441ee3289b0ae76d1314c6df377de..a54f911f81676020ef9acbc3461864f9dd74f6bb 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/DisplayItem.h
+++ b/third_party/WebKit/Source/platform/graphics/paint/DisplayItem.h
@@ -182,6 +182,7 @@ public:
         kSubsequence,
         kEndSubsequence,
 
+        kHasBeenMoved,
         kUninitializedType,
         kTypeLast = kUninitializedType
     };
@@ -224,11 +225,17 @@ public:
         const Type type;
     };
 
-    Id getId() const { return Id(*m_client, m_type); }
+    Id getId() const { return Id(client(), getType()); }
 
     virtual void replay(GraphicsContext&) const { }
 
-    const DisplayItemClient& client() const { ASSERT(m_client); return *m_client; }
+    const DisplayItemClient& client() const
+    {
+        DCHECK(!hasBeenMoved());
+        DCHECK(m_client);
+        return *m_client;
+    }
+
     Type getType() const { return m_type; }
 
     // Size of this object in memory, used to move it with memcpy.
@@ -312,11 +319,10 @@ public:
     }
 #endif
 
-    // True if the client is non-null. Because m_client is const, this should
-    // never be false except when we explicitly create a tombstone/"dead display
-    // item" as part of moving an item from one list to another (see:
-    // DisplayItemList::appendByMoving).
-    bool hasValidClient() const { return m_client; }
+    // True if the item is a placeholder at the place of the original item that
+    // has been moved by DisplayItemList::appendByMoving().
+    bool hasBeenMoved() const { return m_type == kHasBeenMoved; }
+    size_t movedToIndex() const { DCHECK(hasBeenMoved()); return m_movedToIndex; }
 
     virtual bool drawsContent() const { return false; }
 
@@ -332,19 +338,21 @@ public:
 #endif
 
 private:
-    // The default DisplayItem constructor is only used by
-    // ContiguousContainer::appendByMoving where an invalid DisplaItem is
+    // The DisplayItem(size_t) constructor is only used by
+    // ContiguousContainer::appendByMoving() where a placeholder DisplayItem is
     // constructed at the source location.
     template <typename T, unsigned alignment> friend class ContiguousContainer;
-
-    DisplayItem()
-        : m_client(nullptr)
-        , m_type(kUninitializedType)
+    DisplayItem(size_t movedToIndex)
+        : m_movedToIndex(movedToIndex)
+        , m_type(kHasBeenMoved)
         , m_derivedSize(sizeof(*this))
         , m_skippedCache(false)
     { }
 
-    const DisplayItemClient* m_client;
+    union {

[pdr., 2016/09/08 01:36:23]

This approach of using a union is very efficient but also very brittle. I can
imagine a person reading this for the first time would have a difficult time
understanding why a subclass like DrawingDisplayItem has two flavors in base
class. The way we move DisplayItems out of the contiguous container and replace
them with an invalid DisplayItem was already a lot of mental overhead.

What do you think of a simpler approach where we use an auxiliary array and
optimize later if needed? For example, before painting we could allocate a flat
array of size_t's that stores the index where the item has moved to.

[Xianzhu, 2016/09/08 03:01:42]

Done.

+        const DisplayItemClient* m_client; // For m_type != HasBeenMoved.
+        size_t m_movedToIndex; // For m_type == HasBeenMoved.
+    };
     static_assert(kTypeLast < (1 << 16), "DisplayItem::Type should fit in 16 bits");
     const Type m_type : 16;
     const unsigned m_derivedSize : 8; // size of the actual derived class