Bug 1350770: Cache the most recent nsGenConNode to speed up future insertions. r=xidorn

layout/base/nsGenConList.cpp

@@ -19,6 +19,7 @@ nsGenConList::Clear()
     delete node;
   }
   mSize = 0;
+  mLastInserted = nullptr;
 }
 
 bool
@@ -41,6 +42,9 @@ nsGenConList::DestroyNodesFor(nsIFrame* aFrame)
     node = nextNode;
   }
 
+  // Modification of the list invalidates the cached pointer.
+  mLastInserted = nullptr;
+
   return true;
 }
 
@@ -108,6 +112,11 @@ nsGenConList::Insert(nsGenConNode* aNode)
   // Check for append.
   if (mList.isEmpty() || NodeAfter(aNode, mList.getLast())) {
     mList.insertBack(aNode);
+  } else if (mLastInserted && mLastInserted != mList.getLast() &&
+             NodeAfter(aNode, mLastInserted) &&
+             NodeAfter(Next(mLastInserted), aNode)) {
+    // Fast path for inserting many consecutive nodes in one place
+    mLastInserted->setNext(aNode);
   } else {
     // Binary search.
 
@@ -142,6 +151,8 @@ nsGenConList::Insert(nsGenConNode* aNode)
   }
   ++mSize;
 
+  mLastInserted = aNode;
+
   // Set the mapping only if it is the first node of the frame.
   // The DEBUG blocks below are for ensuring the invariant required by
   // nsGenConList::DestroyNodesFor. See comment there.

layout/base/nsGenConList.h

@@ -86,7 +86,7 @@ protected:
   uint32_t mSize;
 
 public:
-  nsGenConList() : mSize(0) {}
+  nsGenConList() : mLastInserted(nullptr), mSize(0) {}
   ~nsGenConList() { Clear(); }
   void Clear();
   static nsGenConNode* Next(nsGenConNode* aNode) {
@@ -127,6 +127,10 @@ private:
 
   // Map from frame to the first nsGenConNode of it in the list.
   nsDataHashtable<nsPtrHashKey<nsIFrame>, nsGenConNode*> mNodes;
+
+  // A weak pointer to the node most recently inserted, used to avoid repeated
+  // list traversals in Insert().
+  nsGenConNode* mLastInserted;
 };
 
 #endif /* nsGenConList_h___ */