Implement a paint offset cache for slimming paint v2

Source/platform/graphics/paint/DisplayItemList.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 DisplayItemList_h
 #define DisplayItemList_h
 
 #include "platform/PlatformExport.h"
+#include "platform/geometry/LayoutPoint.h"
 #include "platform/graphics/ContiguousContainer.h"
 #include "platform/graphics/paint/DisplayItem.h"
 #include "platform/graphics/paint/Transform3DDisplayItem.h"
 #include "wtf/Alignment.h"
 #include "wtf/HashMap.h"
 #include "wtf/PassOwnPtr.h"
 #include "wtf/Utility.h"
 #include "wtf/Vector.h"
 
 namespace blink {
@@ skipping 20 matching lines @@
 public:
     static PassOwnPtr<DisplayItemList> create()
     {
         return adoptPtr(new DisplayItemList());
     }
 
     // These methods are called during paint invalidation.
     void invalidate(DisplayItemClient);
     void invalidateAll();
 
+    // Record when paint offsets change during paint.
+    void invalidatePaintOffset(DisplayItemClient);
+#if ENABLE(ASSERT)
+    bool paintOffsetWasInvalidated(DisplayItemClient) const;
+#endif
+
+    // Record a new paint offset.
+    // TODO(pdr): Remove these once the paint offset cache is on LayoutObject.
+    void recordPaintOffset(DisplayItemClient, const LayoutPoint&);
+    bool paintOffsetIsUnchanged(DisplayItemClient, const LayoutPoint&) const;
+
     // These methods are called during painting.
     template <typename DisplayItemClass, typename... Args>
     DisplayItemClass& createAndAppend(Args&&... args)
     {
         static_assert(WTF::IsSubclass<DisplayItemClass, DisplayItem>::value,
             "Can only createAndAppend subclasses of DisplayItem.");
         static_assert(sizeof(DisplayItemClass) <= kMaximumDisplayItemSize,
             "DisplayItem subclass is larger than kMaximumDisplayItemSize.");
 
         DisplayItemClass& displayItem = m_newDisplayItems.allocateAndConstruct<DisplayItemClass>(WTF::forward<Args>(args)...);
@@ skipping 58 matching lines @@
         , m_constructionDisabled(false)
         , m_skippingCacheCount(0)
         , m_numCachedItems(0)
         , m_nextScope(1) { }
 
 private:
     friend class DisplayItemListTest;
     friend class DisplayItemListPaintTest;
     friend class DisplayItemListPaintTestForSlimmingPaintV2;
     friend class LayoutObjectDrawingRecorderTest;
+    friend class LayoutObjectDrawingRecorderTestForSlimmingPaintV2;
 
     // Set new item state (scopes, cache skipping, etc) for a new item.
     // TODO(pdr): This only passes a pointer to make the patch easier to review. Change to a reference.
     void processNewItem(DisplayItem*);
 
     void updateValidlyCachedClientsIfNeeded() const;
 
+    // Update the recorded paint offsets to remove any items that no longer have
+    // corresponding cached display items.
+    // TODO(pdr): Remove this once the paint offset cache is on LayoutObject.
+    void removeUnneededPaintOffsetEntries();
+
 #ifndef NDEBUG
     WTF::String displayItemsAsDebugString(const DisplayItems&) const;
 #endif
 
     // Indices into PaintList of all DrawingDisplayItems and BeginSubsequenceDisplayItems of each client.
     // Temporarily used during merge to find out-of-order display items.
     using DisplayItemIndicesByClientMap = HashMap<DisplayItemClient, Vector<size_t>>;
 
     static size_t findMatchingItemFromIndex(const DisplayItem::Id&, const DisplayItemIndicesByClientMap&, const DisplayItems&);
     static void addItemToIndexIfNeeded(const DisplayItem&, size_t index, DisplayItemIndicesByClientMap&);
@@ skipping 15 matching lines @@
     DisplayItems m_currentDisplayItems;
     DisplayItems m_newDisplayItems;
 
     // Contains all clients having valid cached paintings if updated.
     // It's lazily updated in updateValidlyCachedClientsIfNeeded().
     // FIXME: In the future we can replace this with client-side repaint flags
     // to avoid the cost of building and querying the hash table.
     mutable HashSet<DisplayItemClient> m_validlyCachedClients;
     mutable bool m_validlyCachedClientsDirty;
 
+#if ENABLE(ASSERT)
+    // Set of clients which had paint offset changes since the last commit. This is used for
+    // ensuring paint offsets are only updated once and are the same in all phases.
+    HashSet<DisplayItemClient> m_clientsWithPaintOffsetInvalidations;
+#endif
+
     // Allow display item construction to be disabled to isolate the costs of construction
     // in performance metrics.
     bool m_constructionDisabled;
 
     int m_skippingCacheCount;
 
     int m_numCachedItems;
 
     unsigned m_nextScope;
     Vector<unsigned> m_scopeStack;
 
+    // Cache of LayoutObject paint offsets.
+    // TODO(pdr): This should be on LayoutObject itself and is only on the display
+    // item list temporarily until paint invalidation for v2 frees up space on
+    // LayoutObject.
+    using PreviousPaintOffsets = HashMap<DisplayItemClient, LayoutPoint>;
+    PreviousPaintOffsets m_previousPaintOffsets;
+
 #if ENABLE(ASSERT)
     // This is used to check duplicated ids during add(). We could also check during
     // updatePaintList(), but checking during add() helps developer easily find where
     // the duplicated ids are from.
     DisplayItemIndicesByClientMap m_newDisplayItemIndicesByClient;
 #endif
 };
 
 } // namespace blink
 
 #endif // DisplayItemList_h