Simplify subtree (now subsequence) caching

Source/platform/graphics/paint/DisplayItemList.cpp

Index: Source/platform/graphics/paint/DisplayItemList.cpp
diff --git a/Source/platform/graphics/paint/DisplayItemList.cpp b/Source/platform/graphics/paint/DisplayItemList.cpp
index 9c8fad763e60b0b3cbb4dbcea05602878a2bd54c..bf8d38c868b3007c82b95050953df488091c6f57 100644
--- a/Source/platform/graphics/paint/DisplayItemList.cpp
+++ b/Source/platform/graphics/paint/DisplayItemList.cpp
@@ -178,20 +178,20 @@ DisplayItems::iterator DisplayItemList::findOutOfOrderCachedItem(DisplayItems::i
 {
     ASSERT(clientCacheIsValid(id.client));
 
-    // Skip indexing of copied items.
-    if (currentIt - context.nextItemToIndex > 0)
-        context.nextItemToIndex = currentIt;
-
     size_t foundIndex = findMatchingItemFromIndex(id, context.displayItemIndicesByClient, m_currentDisplayItems);
     if (foundIndex != kNotFound)
         return m_currentDisplayItems.begin() + foundIndex;
 
-    return findOutOfOrderCachedItemForward(id, context);
+    return findOutOfOrderCachedItemForward(currentIt, id, context);
 }
 
 // Find forward for the item and index all skipped indexable items.
-DisplayItems::iterator DisplayItemList::findOutOfOrderCachedItemForward(const DisplayItem::Id& id, OutOfOrderIndexContext& context)
+DisplayItems::iterator DisplayItemList::findOutOfOrderCachedItemForward(DisplayItems::iterator currentIt, const DisplayItem::Id& id, OutOfOrderIndexContext& context)
 {
+    // Items before currentIt should have been copied. Skip indexing of them.
+    if (currentIt - context.nextItemToIndex > 0)
+        context.nextItemToIndex = currentIt;
+
     DisplayItems::iterator currentEnd = m_currentDisplayItems.end();
     for (; context.nextItemToIndex != currentEnd; ++context.nextItemToIndex) {
         const DisplayItem& item = *context.nextItemToIndex;
@@ -209,9 +209,9 @@ DisplayItems::iterator DisplayItemList::findOutOfOrderCachedItemForward(const Di
 void DisplayItemList::copyCachedSubtree(DisplayItems::iterator& currentIt, DisplayItems& updatedList)
 {
     ASSERT(RuntimeEnabledFeatures::slimmingPaintV2Enabled());
-    ASSERT(currentIt->isBeginSubtree());
+    ASSERT(currentIt->type() == DisplayItem::BeginSubtree);
     ASSERT(!currentIt->scope());
-    DisplayItem::Id endSubtreeId(currentIt->client(), DisplayItem::beginSubtreeTypeToEndSubtreeType(currentIt->type()), 0);
+    DisplayItem::Id endSubtreeId(currentIt->client(), DisplayItem::EndSubtree, 0);
     do {
         // We should always find the EndSubtree display item.
         ASSERT(currentIt != m_currentDisplayItems.end());
@@ -295,32 +295,28 @@ void DisplayItemList::commitNewDisplayItems(DisplayListDiff*)
             ASSERT(newDisplayItem.isCached());
             ASSERT(clientCacheIsValid(newDisplayItem.client()));
             if (!isSynchronized) {
-                DisplayItems::iterator foundIt = findOutOfOrderCachedItem(currentIt, newDisplayItemId, outOfOrderIndexContext);
+                currentIt = findOutOfOrderCachedItem(currentIt, newDisplayItemId, outOfOrderIndexContext);
 
-                if (foundIt == currentEnd) {
+                if (currentIt == currentEnd) {
 #ifndef NDEBUG
                     showDebugData();
                     WTFLogAlways("%s not found in m_currentDisplayItems\n", newDisplayItem.asDebugString().utf8().data());
 #endif
                     ASSERT_NOT_REACHED();
-
-                    // If foundIt == currentEnd, it means that we did not find the cached display item. This should be impossible, but may occur
-                    // if there is a bug in the system, such as under-invalidation, incorrect cache checking or duplicate display ids. In this case,
-                    // attempt to recover rather than crashing or bailing on display of the rest of the display list.
+                    // We did not find the cached display item. This should be impossible, but may occur if there is a bug
+                    // in the system, such as under-invalidation, incorrect cache checking or duplicate display ids.
+                    // In this case, attempt to recover rather than crashing or bailing on display of the rest of the display list.
                     continue;
                 }
-
-                ASSERT(foundIt != currentIt); // because we are in 'if (!isSynchronized)'
-                currentIt = foundIt;
             }
 
             if (newDisplayItem.isCachedDrawing()) {
                 updatedList.appendByMoving(*currentIt, currentIt->derivedSize());
                 ++currentIt;
             } else {
-                ASSERT(newDisplayItem.isCachedSubtree());
+                ASSERT(newDisplayItem.type() == DisplayItem::CachedSubtree);
                 copyCachedSubtree(currentIt, updatedList);
-                ASSERT(updatedList.last().isEndSubtree());
+                ASSERT(updatedList.last().type() == DisplayItem::EndSubtree);
             }
         } else {
 #if ENABLE(ASSERT)