#415: M1376936 M1375331

dom/base/nsContentList.cpp

@@ -92,7 +92,8 @@ NS_INTERFACE_MAP_END
 
 
 NS_IMPL_CYCLE_COLLECTING_ADDREF(nsBaseContentList)
-NS_IMPL_CYCLE_COLLECTING_RELEASE(nsBaseContentList)
+NS_IMPL_CYCLE_COLLECTING_RELEASE_WITH_LAST_RELEASE(nsBaseContentList,
+                                                   LastRelease())
 
 
 NS_IMETHODIMP
@@ -612,6 +613,17 @@ nsContentList::NodeWillBeDestroyed(const nsINode* aNode)
   SetDirty();
 }
 
+void
+nsContentList::LastRelease()
+{
+  RemoveFromCaches();
+  if (mRootNode) {
+    mRootNode->RemoveMutationObserver(this);
+    mRootNode = nullptr;
+  }
+  SetDirty();
+}
+
 NS_IMETHODIMP
 nsContentList::GetLength(uint32_t* aLength)
 {

dom/base/nsContentList.h

@@ -95,6 +95,9 @@ public:
   {
     mElements.SetCapacity(aCapacity);
   }
+
+  virtual void LastRelease() {}
+
 protected:
   virtual ~nsBaseContentList();
 
@@ -348,6 +351,8 @@ public:
     Reset();
   }
 
+  virtual void LastRelease() override;
+
 protected:
   /**
    * Returns whether the element matches our criterion

layout/base/nsPresArena.cpp

@@ -131,9 +131,17 @@ nsPresArena::Allocate(uint32_t aCode, size_t aSize)
 
   void* result;
   if (len > 0) {
-    // LIFO behavior for best cache utilization
+    // Remove from the end of the mEntries array to avoid memmoving entries,
+    // and use SetLengthAndRetainStorage to avoid a lot of malloc/free
+    // from ShrinkCapacity on smaller sizes.  500 pointers means the malloc size
+    // for the array is 4096 bytes or more on a 64-bit system.  The next smaller
+    // size is 2048 (with jemalloc), which we consider not worth compacting.
     result = list->mEntries.ElementAt(len - 1);
-    list->mEntries.RemoveElementAt(len - 1);
+    if (list->mEntries.Capacity() > 500) {
+      list->mEntries.RemoveElementAt(len - 1);
+    } else {
+      list->mEntries.SetLengthAndRetainStorage(len - 1);
+    }
 #if defined(DEBUG)
     {
       MOZ_MAKE_MEM_DEFINED(result, list->mEntrySize);