Bug 1377993 - Make node slots less memory hungry in common cases.

Tag UXP Issue #1344

9 files changed, 203 insertions, 178 deletions

diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index 7d926f6bb..81cc41210 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -1595,7 +1595,7 @@ Element::BindToTree(nsIDocument* aDocument, nsIContent* aParent,
 #endif
   {
     if (aBindingParent) {
-      nsDOMSlots *slots = DOMSlots();
+      nsExtendedDOMSlots* slots = ExtendedDOMSlots();
 
       slots->mBindingParent = aBindingParent; // Weak, so no addref happens.
     }
@@ -1618,7 +1618,7 @@ Element::BindToTree(nsIDocument* aDocument, nsIContent* aParent,
     }
     ShadowRoot* parentContainingShadow = aParent->GetContainingShadow();
     if (parentContainingShadow) {
-      DOMSlots()->mContainingShadow = parentContainingShadow;
+      ExtendedDOMSlots()->mContainingShadow = parentContainingShadow;
     }
   }
 
@@ -2007,7 +2007,7 @@ Element::UnbindFromTree(bool aDeep, bool aNullParent)
   }
 #endif
 
-  nsDOMSlots* slots = GetExistingDOMSlots();
+  nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
   if (slots) {
     if (clearBindingParent) {
       slots->mBindingParent = nullptr;
@@ -2055,7 +2055,7 @@ Element::UnbindFromTree(bool aDeep, bool aNullParent)
 nsICSSDeclaration*
 Element::GetSMILOverrideStyle()
 {
-  Element::nsDOMSlots *slots = DOMSlots();
+  Element::nsExtendedDOMSlots* slots = ExtendedDOMSlots();
 
   if (!slots->mSMILOverrideStyle) {
     slots->mSMILOverrideStyle = new nsDOMCSSAttributeDeclaration(this, true);
@@ -2067,7 +2067,7 @@ Element::GetSMILOverrideStyle()
 DeclarationBlock*
 Element::GetSMILOverrideStyleDeclaration()
 {
-  Element::nsDOMSlots *slots = GetExistingDOMSlots();
+  Element::nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
   return slots ? slots->mSMILOverrideStyleDeclaration.get() : nullptr;
 }
 
@@ -2075,7 +2075,7 @@ nsresult
 Element::SetSMILOverrideStyleDeclaration(DeclarationBlock* aDeclaration,
                                          bool aNotify)
 {
-  Element::nsDOMSlots *slots = DOMSlots();
+  Element::nsExtendedDOMSlots* slots = ExtendedDOMSlots();
 
   slots->mSMILOverrideStyleDeclaration = aDeclaration;
 
@@ -3988,7 +3988,7 @@ Element::ClearDataset()
 nsDataHashtable<nsRefPtrHashKey<DOMIntersectionObserver>, int32_t>*
 Element::RegisteredIntersectionObservers()
 {
-  nsDOMSlots* slots = DOMSlots();
+  nsExtendedDOMSlots* slots = ExtendedDOMSlots();
   return &slots->mRegisteredIntersectionObservers;
 }
 
@@ -4041,7 +4041,7 @@ Element::UpdateIntersectionObservation(DOMIntersectionObserver* aObserver, int32
 void
 Element::SetCustomElementData(CustomElementData* aData)
 {
-  nsDOMSlots *slots = DOMSlots();
+  nsExtendedDOMSlots *slots = ExtendedDOMSlots();
   MOZ_ASSERT(!slots->mCustomElementData, "Custom element data may not be changed once set.");
   slots->mCustomElementData = aData;
 }
diff --git a/dom/base/Element.h b/dom/base/Element.h
index 6d8bc823e..e7218ee93 100644
--- a/dom/base/Element.h
+++ b/dom/base/Element.h
@@ -398,7 +398,7 @@ public:
    */
   inline CustomElementData* GetCustomElementData() const
   {
-    nsDOMSlots *slots = GetExistingDOMSlots();
+    nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
     if (slots) {
       return slots->mCustomElementData;
     }
@@ -837,7 +837,7 @@ public:
 
   ShadowRoot *FastGetShadowRoot() const
   {
-    nsDOMSlots* slots = GetExistingDOMSlots();
+    nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
     return slots ? slots->mShadowRoot.get() : nullptr;
   }
 
diff --git a/dom/base/FragmentOrElement.cpp b/dom/base/FragmentOrElement.cpp
index bf0679d9f..5755fb817 100644
--- a/dom/base/FragmentOrElement.cpp
+++ b/dom/base/FragmentOrElement.cpp
@@ -530,8 +530,7 @@ nsNodeSupportsWeakRefTearoff::GetWeakReference(nsIWeakReference** aInstancePtr)
 //----------------------------------------------------------------------
 FragmentOrElement::nsDOMSlots::nsDOMSlots()
   : nsINode::nsSlots(),
-    mDataset(nullptr),
-    mBindingParent(nullptr)
+    mDataset(nullptr)
 {
 }
 
@@ -543,84 +542,104 @@ FragmentOrElement::nsDOMSlots::~nsDOMSlots()
 }
 
 void
-FragmentOrElement::nsDOMSlots::Traverse(nsCycleCollectionTraversalCallback &cb, bool aIsXUL)
+FragmentOrElement::nsDOMSlots::Traverse(nsCycleCollectionTraversalCallback &cb)
 {
   NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mSlots->mStyle");
   cb.NoteXPCOMChild(mStyle.get());
 
-  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mSlots->mSMILOverrideStyle");
-  cb.NoteXPCOMChild(mSMILOverrideStyle.get());
-
   NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mSlots->mAttributeMap");
   cb.NoteXPCOMChild(mAttributeMap.get());
 
-  if (aIsXUL) {
-    NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mSlots->mControllers");
-    cb.NoteXPCOMChild(mControllers);
+  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mSlots->mChildrenList");
+  cb.NoteXPCOMChild(NS_ISUPPORTS_CAST(nsIDOMNodeList*, mChildrenList));
+
+  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mSlots->mClassList");
+  cb.NoteXPCOMChild(mClassList.get());
+
+  if (!mExtendedSlots) {
+    return;
   }
 
-  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mSlots->mXBLBinding");
-  cb.NoteNativeChild(mXBLBinding, NS_CYCLE_COLLECTION_PARTICIPANT(nsXBLBinding));
+  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mExtendedSlots->mSMILOverrideStyle");
+  cb.NoteXPCOMChild(mExtendedSlots->mSMILOverrideStyle.get());
 
-  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mSlots->mXBLInsertionParent");
-  cb.NoteXPCOMChild(mXBLInsertionParent.get());
+  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mExtendedSlots->mControllers");
+  cb.NoteXPCOMChild(mExtendedSlots->mControllers);
 
-  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mSlots->mShadowRoot");
-  cb.NoteXPCOMChild(NS_ISUPPORTS_CAST(nsIContent*, mShadowRoot));
+  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mExtendedSlots->mLabelsList");
+  cb.NoteXPCOMChild(NS_ISUPPORTS_CAST(nsIDOMNodeList*,mExtendedSlots-> mLabelsList));
 
-  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mSlots->mContainingShadow");
-  cb.NoteXPCOMChild(NS_ISUPPORTS_CAST(nsIContent*, mContainingShadow));
+  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mExtendedSlots->mShadowRoot");
+  cb.NoteXPCOMChild(NS_ISUPPORTS_CAST(nsIContent*, mExtendedSlots->mShadowRoot));
 
-  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mSlots->mChildrenList");
-  cb.NoteXPCOMChild(NS_ISUPPORTS_CAST(nsIDOMNodeList*, mChildrenList));
+  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mExtendedSlots->mContainingShadow");
+  cb.NoteXPCOMChild(NS_ISUPPORTS_CAST(nsIContent*, mExtendedSlots->mContainingShadow));
 
-  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mSlots->mLabelsList");
-  cb.NoteXPCOMChild(NS_ISUPPORTS_CAST(nsIDOMNodeList*, mLabelsList));
+  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mExtendedSlots->mXBLBinding");
+  cb.NoteNativeChild(mExtendedSlots->mXBLBinding,
+                     NS_CYCLE_COLLECTION_PARTICIPANT(nsXBLBinding));
 
-  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mSlots->mClassList");
-  cb.NoteXPCOMChild(mClassList.get());
+  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mExtendedSlots->mXBLInsertionParent");
+  cb.NoteXPCOMChild(mExtendedSlots->mXBLInsertionParent.get());
 
-  if (mCustomElementData) {
-    for (uint32_t i = 0; i < mCustomElementData->mCallbackQueue.Length(); i++) {
-      mCustomElementData->mCallbackQueue[i]->Traverse(cb);
+  if (mExtendedSlots->mCustomElementData) {
+    for (uint32_t i = 0;
+         i < mExtendedSlots->mCustomElementData->mCallbackQueue.Length(); i++) {
+      mExtendedSlots->mCustomElementData->mCallbackQueue[i]->Traverse(cb);
     }
   }
 
-  for (auto iter = mRegisteredIntersectionObservers.Iter(); !iter.Done(); iter.Next()) {
+  for (auto iter = mExtendedSlots->mRegisteredIntersectionObservers.Iter();
+       !iter.Done(); iter.Next()) {
     DOMIntersectionObserver* observer = iter.Key();
-    NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mSlots->mRegisteredIntersectionObservers[i]");
+    NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb,
+                                       "mExtendedSlots->mRegisteredIntersectionObservers[i]");
     cb.NoteXPCOMChild(observer);
   }
+
+  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mExtendedSlots->mFrameLoaderOrOpener");
+  cb.NoteXPCOMChild(mExtendedSlots->mFrameLoaderOrOpener);
 }
 
 void
-FragmentOrElement::nsDOMSlots::Unlink(bool aIsXUL)
+FragmentOrElement::nsDOMSlots::Unlink()
 {
   mStyle = nullptr;
-  mSMILOverrideStyle = nullptr;
   if (mAttributeMap) {
     mAttributeMap->DropReference();
     mAttributeMap = nullptr;
   }
-  if (aIsXUL)
-    NS_IF_RELEASE(mControllers);
-
-  MOZ_ASSERT(!mXBLBinding);
-
-  mXBLInsertionParent = nullptr;
-  mShadowRoot = nullptr;
-  mContainingShadow = nullptr;
   mChildrenList = nullptr;
-  mLabelsList = nullptr;
-  mCustomElementData = nullptr;
   mClassList = nullptr;
-  mRegisteredIntersectionObservers.Clear();
+
+  if (!mExtendedSlots) {
+    return;
+  }
+
+  mExtendedSlots->mSMILOverrideStyle = nullptr;
+  mExtendedSlots->mControllers = nullptr;
+  mExtendedSlots->mLabelsList = nullptr;
+  mExtendedSlots->mShadowRoot = nullptr;
+  mExtendedSlots->mContainingShadow = nullptr;
+  MOZ_ASSERT(!(mExtendedSlots->mXBLBinding));
+  mExtendedSlots->mXBLInsertionParent = nullptr;
+  mExtendedSlots->mCustomElementData = nullptr;
+  mExtendedSlots->mRegisteredIntersectionObservers.Clear();
+  nsCOMPtr<nsIFrameLoader> frameLoader =
+    do_QueryInterface(mExtendedSlots->mFrameLoaderOrOpener);
+  if (frameLoader) {
+    static_cast<nsFrameLoader*>(frameLoader.get())->Destroy();
+  }
+  mExtendedSlots->mFrameLoaderOrOpener = nullptr;
 }
 
 size_t
 FragmentOrElement::nsDOMSlots::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
 {
   size_t n = aMallocSizeOf(this);
+  if (mExtendedSlots) {
+    n += aMallocSizeOf(mExtendedSlots.get());
+  }
 
   if (mAttributeMap) {
     n += mAttributeMap->SizeOfIncludingThis(aMallocSizeOf);
@@ -641,6 +660,19 @@ FragmentOrElement::nsDOMSlots::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) c
   return n;
 }
 
+FragmentOrElement::nsExtendedDOMSlots::nsExtendedDOMSlots()
+  : mBindingParent(nullptr)
+{
+}
+
+FragmentOrElement::nsExtendedDOMSlots::~nsExtendedDOMSlots()
+{
+  nsCOMPtr<nsIFrameLoader> frameLoader = do_QueryInterface(mFrameLoaderOrOpener);
+  if (frameLoader) {
+    static_cast<nsFrameLoader*>(frameLoader.get())->Destroy();
+  }
+}
+
 FragmentOrElement::FragmentOrElement(already_AddRefed<mozilla::dom::NodeInfo>& aNodeInfo)
   : nsIContent(aNodeInfo)
 {
@@ -962,7 +994,7 @@ FragmentOrElement::IsLink(nsIURI** aURI) const
 nsIContent*
 FragmentOrElement::GetBindingParent() const
 {
-  nsDOMSlots *slots = GetExistingDOMSlots();
+  nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
 
   if (slots) {
     return slots->mBindingParent;
@@ -974,7 +1006,7 @@ nsXBLBinding*
 FragmentOrElement::GetXBLBinding() const
 {
   if (HasFlag(NODE_MAY_BE_IN_BINDING_MNGR)) {
-    nsDOMSlots *slots = GetExistingDOMSlots();
+    nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
     if (slots) {
       return slots->mXBLBinding;
     }
@@ -1009,11 +1041,11 @@ FragmentOrElement::SetXBLBinding(nsXBLBinding* aBinding,
 
   if (aBinding) {
     SetFlags(NODE_MAY_BE_IN_BINDING_MNGR);
-    nsDOMSlots *slots = DOMSlots();
+    nsExtendedDOMSlots* slots = ExtendedDOMSlots();
     slots->mXBLBinding = aBinding;
     bindingManager->AddBoundContent(this);
   } else {
-    nsDOMSlots *slots = GetExistingDOMSlots();
+    nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
     if (slots) {
       slots->mXBLBinding = nullptr;
     }
@@ -1028,7 +1060,7 @@ nsIContent*
 FragmentOrElement::GetXBLInsertionParent() const
 {
   if (HasFlag(NODE_MAY_BE_IN_BINDING_MNGR)) {
-    nsDOMSlots *slots = GetExistingDOMSlots();
+    nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
     if (slots) {
       return slots->mXBLInsertionParent;
     }
@@ -1040,7 +1072,7 @@ FragmentOrElement::GetXBLInsertionParent() const
 ShadowRoot*
 FragmentOrElement::GetContainingShadow() const
 {
-  nsDOMSlots *slots = GetExistingDOMSlots();
+  nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
   if (slots) {
     return slots->mContainingShadow;
   }
@@ -1050,21 +1082,21 @@ FragmentOrElement::GetContainingShadow() const
 void
 FragmentOrElement::SetShadowRoot(ShadowRoot* aShadowRoot)
 {
-  nsDOMSlots *slots = DOMSlots();
+  nsExtendedDOMSlots* slots = ExtendedDOMSlots();
   slots->mShadowRoot = aShadowRoot;
 }
 
 nsTArray<nsIContent*>&
 FragmentOrElement::DestInsertionPoints()
 {
-  nsDOMSlots *slots = DOMSlots();
+  nsExtendedDOMSlots* slots = ExtendedDOMSlots();
   return slots->mDestInsertionPoints;
 }
 
 nsTArray<nsIContent*>*
 FragmentOrElement::GetExistingDestInsertionPoints() const
 {
-  nsDOMSlots *slots = GetExistingDOMSlots();
+  nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
   if (slots) {
     return &slots->mDestInsertionPoints;
   }
@@ -1075,11 +1107,11 @@ void
 FragmentOrElement::SetXBLInsertionParent(nsIContent* aContent)
 {
   if (aContent) {
-    nsDOMSlots *slots = DOMSlots();
+    nsExtendedDOMSlots* slots = ExtendedDOMSlots();
     SetFlags(NODE_MAY_BE_IN_BINDING_MNGR);
     slots->mXBLInsertionParent = aContent;
   } else {
-    nsDOMSlots *slots = GetExistingDOMSlots();
+    nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
     if (slots) {
       slots->mXBLInsertionParent = nullptr;
     }
@@ -1348,14 +1380,15 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(FragmentOrElement)
   {
     nsDOMSlots *slots = tmp->GetExistingDOMSlots();
     if (slots) {
-      if (tmp->IsElement()) {
+      if (slots->mExtendedSlots && tmp->IsElement()) {
         Element* elem = tmp->AsElement();
-        for (auto iter = slots->mRegisteredIntersectionObservers.Iter(); !iter.Done(); iter.Next()) {
+        for (auto iter = slots->mExtendedSlots->mRegisteredIntersectionObservers.Iter();
+             !iter.Done(); iter.Next()) {
           DOMIntersectionObserver* observer = iter.Key();
           observer->UnlinkTarget(*elem);
         }
       }
-      slots->Unlink(tmp->IsXULElement());
+      slots->Unlink();
     }
   }
 
@@ -1920,7 +1953,7 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INTERNAL(FragmentOrElement)
   {
     nsDOMSlots *slots = tmp->GetExistingDOMSlots();
     if (slots) {
-      slots->Traverse(cb, tmp->IsXULElement());
+      slots->Traverse(cb);
     }
   }
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
diff --git a/dom/base/FragmentOrElement.h b/dom/base/FragmentOrElement.h
index a3a59ee43..4edd88908 100644
--- a/dom/base/FragmentOrElement.h
+++ b/dom/base/FragmentOrElement.h
@@ -15,6 +15,7 @@
 
 #include "mozilla/Attributes.h"
 #include "mozilla/MemoryReporting.h"
+#include "mozilla/UniquePtr.h"
 #include "nsAttrAndChildArray.h"          // member
 #include "nsCycleCollectionParticipant.h" // NS_DECL_CYCLE_*
 #include "nsIContent.h"                   // base class
@@ -239,8 +240,6 @@ protected:
   nsresult CopyInnerTo(FragmentOrElement* aDest);
 
 public:
-  // Because of a bug in MS C++ compiler nsDOMSlots must be declared public,
-  // otherwise nsXULElement::nsXULSlots doesn't compile.
   /**
    * There are a set of DOM- and scripting-specific instance variables
    * that may only be instantiated when a content object is accessed
@@ -249,29 +248,13 @@ public:
    * in a side structure that's only allocated when the content is
    * accessed through the DOM.
    */
-  class nsDOMSlots : public nsINode::nsSlots
+
+  class nsExtendedDOMSlots
   {
   public:
-    nsDOMSlots();
-    virtual ~nsDOMSlots();
-
-    void Traverse(nsCycleCollectionTraversalCallback &cb, bool aIsXUL);
-    void Unlink(bool aIsXUL);
-
-    size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
-
-    /**
-     * The .style attribute (an interface that forwards to the actual
-     * style rules)
-     * @see nsGenericHTMLElement::GetStyle
-     */
-    nsCOMPtr<nsICSSDeclaration> mStyle;
+    nsExtendedDOMSlots();
 
-    /**
-     * The .dataset attribute.
-     * @see nsGenericHTMLElement::GetDataset
-     */
-    nsDOMStringMap* mDataset; // [Weak]
+    ~nsExtendedDOMSlots();
 
     /**
      * SMIL Overridde style rules (for SMIL animation of CSS properties)
@@ -285,35 +268,17 @@ public:
     RefPtr<mozilla::DeclarationBlock> mSMILOverrideStyleDeclaration;
 
     /**
-     * An object implementing nsIDOMMozNamedAttrMap for this content (attributes)
-     * @see FragmentOrElement::GetAttributes
-     */
-    RefPtr<nsDOMAttributeMap> mAttributeMap;
-
-    union {
-      /**
-      * The nearest enclosing content node with a binding that created us.
-      * @see FragmentOrElement::GetBindingParent
-      */
-      nsIContent* mBindingParent;  // [Weak]
-
-      /**
-      * The controllers of the XUL Element.
-      */
-      nsIControllers* mControllers; // [OWNER]
-    };
+    * The nearest enclosing content node with a binding that created us.
+    * @see FragmentOrElement::GetBindingParent
+    */
+    nsIContent* mBindingParent;  // [Weak]
 
     /**
-     * An object implementing the .children property for this element.
-     */
-    RefPtr<nsContentList> mChildrenList;
+    * The controllers of the XUL Element.
+    */
+    nsCOMPtr<nsIControllers> mControllers;
 
     /**
-     * An object implementing the .classList property for this element.
-     */
-    RefPtr<nsDOMTokenList> mClassList;
-
-    /*
      * An object implementing the .labels property for this element.
      */
     RefPtr<nsLabelsNodeList> mLabelsList;
@@ -354,6 +319,55 @@ public:
      */
     nsDataHashtable<nsRefPtrHashKey<DOMIntersectionObserver>, int32_t> 
       mRegisteredIntersectionObservers;
+
+    /**
+     * For XUL to hold either frameloader or opener.
+     */
+    nsCOMPtr<nsISupports> mFrameLoaderOrOpener;
+
+  };
+
+  class nsDOMSlots : public nsINode::nsSlots
+  {
+  public:
+    nsDOMSlots();
+    virtual ~nsDOMSlots();
+
+    void Traverse(nsCycleCollectionTraversalCallback &cb);
+    void Unlink();
+
+    size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+
+    /**
+     * The .style attribute (an interface that forwards to the actual
+     * style rules)
+     * @see nsGenericHTMLElement::GetStyle
+     */
+    nsCOMPtr<nsICSSDeclaration> mStyle;
+
+    /**
+     * The .dataset attribute.
+     * @see nsGenericHTMLElement::GetDataset
+     */
+    nsDOMStringMap* mDataset; // [Weak]
+
+    /**
+     * An object implementing nsIDOMMozNamedAttrMap for this content (attributes)
+     * @see FragmentOrElement::GetAttributes
+     */
+    RefPtr<nsDOMAttributeMap> mAttributeMap;
+
+    /**
+     * An object implementing the .children property for this element.
+     */
+    RefPtr<nsContentList> mChildrenList;
+
+    /**
+     * An object implementing the .classList property for this element.
+     */
+    RefPtr<nsDOMTokenList> mClassList;
+
+    mozilla::UniquePtr<nsExtendedDOMSlots> mExtendedSlots;
   };
 
 protected:
@@ -373,6 +387,26 @@ protected:
     return static_cast<nsDOMSlots*>(GetExistingSlots());
   }
 
+  nsExtendedDOMSlots* ExtendedDOMSlots()
+  {
+    nsDOMSlots* slots = DOMSlots();
+    if (!slots->mExtendedSlots) {
+      slots->mExtendedSlots = MakeUnique<nsExtendedDOMSlots>();
+    }
+
+    return slots->mExtendedSlots.get();
+  }
+
+  nsExtendedDOMSlots* GetExistingExtendedDOMSlots() const
+  {
+    nsDOMSlots* slots = GetExistingDOMSlots();
+    if (slots) {
+      return slots->mExtendedSlots.get();
+    }
+
+    return nullptr;
+  }
+
   /**
    * Calls SetIsElementInStyleScopeFlagOnSubtree for each shadow tree attached
    * to this node, which is assumed to be an Element.
diff --git a/dom/base/ShadowRoot.cpp b/dom/base/ShadowRoot.cpp
index 9540754f7..831987a96 100644
--- a/dom/base/ShadowRoot.cpp
+++ b/dom/base/ShadowRoot.cpp
@@ -75,8 +75,8 @@ ShadowRoot::ShadowRoot(nsIContent* aContent,
 
   SetFlags(NODE_IS_IN_SHADOW_TREE);
 
-  DOMSlots()->mBindingParent = aContent;
-  DOMSlots()->mContainingShadow = this;
+  ExtendedDOMSlots()->mBindingParent = aContent;
+  ExtendedDOMSlots()->mContainingShadow = this;
 
   // Add the ShadowRoot as a mutation observer on the host to watch
   // for mutations because the insertion points in this ShadowRoot
diff --git a/dom/base/nsNodeUtils.cpp b/dom/base/nsNodeUtils.cpp
index 75d408151..c38f08a3d 100644
--- a/dom/base/nsNodeUtils.cpp
+++ b/dom/base/nsNodeUtils.cpp
@@ -301,9 +301,12 @@ nsNodeUtils::LastRelease(nsINode* aNode)
       Element* elem = aNode->AsElement();
       FragmentOrElement::nsDOMSlots* domSlots =
         static_cast<FragmentOrElement::nsDOMSlots*>(slots);
-      for (auto iter = domSlots->mRegisteredIntersectionObservers.Iter(); !iter.Done(); iter.Next()) {
-        DOMIntersectionObserver* observer = iter.Key();
-        observer->UnlinkTarget(*elem);
+      if (domSlots->mExtendedSlots) {
+        for (auto iter = domSlots->mExtendedSlots->mRegisteredIntersectionObservers.Iter();
+             !iter.Done(); iter.Next()) {
+          DOMIntersectionObserver* observer = iter.Key();
+          observer->UnlinkTarget(*elem);
+        }
       }
     }
 
diff --git a/dom/html/nsGenericHTMLElement.cpp b/dom/html/nsGenericHTMLElement.cpp
index 2f890325a..cbf97f1ea 100644
--- a/dom/html/nsGenericHTMLElement.cpp
+++ b/dom/html/nsGenericHTMLElement.cpp
@@ -497,7 +497,7 @@ nsGenericHTMLElement::BindToTree(nsIDocument* aDocument, nsIContent* aParent,
   // We need to consider a labels element is moved to another subtree
   // with different root, it needs to update labels list and its root
   // as well.
-  nsDOMSlots* slots = GetExistingDOMSlots();
+  nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
   if (slots && slots->mLabelsList) {
     slots->mLabelsList->MaybeResetRoot(SubtreeRoot());
   }
@@ -524,7 +524,7 @@ nsGenericHTMLElement::UnbindFromTree(bool aDeep, bool aNullParent)
 
   // We need to consider a labels element is removed from tree,
   // it needs to update labels list and its root as well.
-  nsDOMSlots* slots = GetExistingDOMSlots();
+  nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
   if (slots && slots->mLabelsList) {
     slots->mLabelsList->MaybeResetRoot(SubtreeRoot());
   }
@@ -1730,7 +1730,7 @@ nsGenericHTMLElement::Labels()
 {
   MOZ_ASSERT(IsLabelable(),
              "Labels() only allow labelable elements to use it.");
-  nsDOMSlots* slots = DOMSlots();
+  nsExtendedDOMSlots* slots = ExtendedDOMSlots();
 
   if (!slots->mLabelsList) {
     slots->mLabelsList = new nsLabelsNodeList(SubtreeRoot(), MatchLabelsElement,
diff --git a/dom/xul/nsXULElement.cpp b/dom/xul/nsXULElement.cpp
index e351a46eb..a854f53ec 100644
--- a/dom/xul/nsXULElement.cpp
+++ b/dom/xul/nsXULElement.cpp
@@ -174,33 +174,6 @@ nsXULElement::~nsXULElement()
 {
 }
 
-nsXULElement::nsXULSlots::nsXULSlots()
-    : nsXULElement::nsDOMSlots()
-{
-}
-
-nsXULElement::nsXULSlots::~nsXULSlots()
-{
-    NS_IF_RELEASE(mControllers); // Forces release
-    nsCOMPtr<nsIFrameLoader> frameLoader = do_QueryInterface(mFrameLoaderOrOpener);
-    if (frameLoader) {
-        static_cast<nsFrameLoader*>(frameLoader.get())->Destroy();
-    }
-}
-
-void
-nsXULElement::nsXULSlots::Traverse(nsCycleCollectionTraversalCallback &cb)
-{
-    NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "mSlots->mFrameLoaderOrOpener");
-    cb.NoteXPCOMChild(mFrameLoaderOrOpener);
-}
-
-nsINode::nsSlots*
-nsXULElement::CreateSlots()
-{
-    return new nsXULSlots();
-}
-
 void
 nsXULElement::MaybeUpdatePrivateLifetime()
 {
@@ -326,12 +299,7 @@ NS_IMPL_CYCLE_COLLECTION_CLASS(nsXULElement)
 
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(nsXULElement,
                                                   nsStyledElement)
-    {
-        nsXULSlots* slots = static_cast<nsXULSlots*>(tmp->GetExistingSlots());
-        if (slots) {
-            slots->Traverse(cb);
-        }
-    }
+
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
 
 NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(nsXULElement,
@@ -895,9 +863,9 @@ nsXULElement::UnbindFromTree(bool aDeep, bool aNullParent)
     // mDocument in nsGlobalWindow::SetDocShell, but I'm not
     // sure whether that would fix all possible cycles through
     // mControllers.)
-    nsXULSlots* slots = static_cast<nsXULSlots*>(GetExistingDOMSlots());
+    nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
     if (slots) {
-        NS_IF_RELEASE(slots->mControllers);
+        slots->mControllers = nullptr;
         RefPtr<nsFrameLoader> frameLoader = GetFrameLoader();
         if (frameLoader) {
             frameLoader->Destroy();
@@ -1241,9 +1209,9 @@ nsXULElement::RemoveBroadcaster(const nsAString & broadcasterId)
 void
 nsXULElement::DestroyContent()
 {
-    nsXULSlots* slots = static_cast<nsXULSlots*>(GetExistingDOMSlots());
+    nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
     if (slots) {
-        NS_IF_RELEASE(slots->mControllers);
+        slots->mControllers = nullptr;
         RefPtr<nsFrameLoader> frameLoader = GetFrameLoader();
         if (frameLoader) {
             frameLoader->Destroy();
@@ -1473,7 +1441,7 @@ nsIControllers*
 nsXULElement::GetControllers(ErrorResult& rv)
 {
     if (! Controllers()) {
-        nsDOMSlots* slots = DOMSlots();
+        nsExtendedDOMSlots* slots = ExtendedDOMSlots();
 
         rv = NS_NewXULControllers(nullptr, NS_GET_IID(nsIControllers),
                                   reinterpret_cast<void**>(&slots->mControllers));
@@ -1578,7 +1546,7 @@ nsXULElement::LoadSrc()
     RefPtr<nsFrameLoader> frameLoader = GetFrameLoader();
     if (!frameLoader) {
         // Check if we have an opener we need to be setting
-        nsXULSlots* slots = static_cast<nsXULSlots*>(Slots());
+        nsExtendedDOMSlots* slots = ExtendedDOMSlots();
         nsCOMPtr<nsPIDOMWindowOuter> opener = do_QueryInterface(slots->mFrameLoaderOrOpener);
         if (!opener) {
             // If we are a content-primary xul-browser, we want to take the opener property!
@@ -1624,7 +1592,7 @@ nsXULElement::GetFrameLoaderXPCOM(nsIFrameLoader **aFrameLoader)
 already_AddRefed<nsFrameLoader>
 nsXULElement::GetFrameLoader()
 {
-    nsXULSlots* slots = static_cast<nsXULSlots*>(GetExistingSlots());
+    nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
     if (!slots)
         return nullptr;
 
@@ -1646,7 +1614,7 @@ nsXULElement::GetParentApplication(mozIApplication** aApplication)
 void
 nsXULElement::PresetOpenerWindow(mozIDOMWindowProxy* aWindow, ErrorResult& aRv)
 {
-    nsXULSlots* slots = static_cast<nsXULSlots*>(Slots());
+    nsExtendedDOMSlots* slots = ExtendedDOMSlots();
     MOZ_ASSERT(!slots->mFrameLoaderOrOpener, "A frameLoader or opener is present when calling PresetOpenerWindow");
 
     slots->mFrameLoaderOrOpener = aWindow;
@@ -1662,7 +1630,7 @@ nsXULElement::SetIsPrerendered()
 void
 nsXULElement::InternalSetFrameLoader(nsIFrameLoader* aNewFrameLoader)
 {
-    nsXULSlots* slots = static_cast<nsXULSlots*>(GetExistingDOMSlots());
+    nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
     MOZ_ASSERT(slots);
 
     slots->mFrameLoaderOrOpener = aNewFrameLoader;
diff --git a/dom/xul/nsXULElement.h b/dom/xul/nsXULElement.h
index 164afacd3..80424412f 100644
--- a/dom/xul/nsXULElement.h
+++ b/dom/xul/nsXULElement.h
@@ -609,19 +609,6 @@ protected:
 
     nsresult AddPopupListener(nsIAtom* aName);
 
-    class nsXULSlots : public mozilla::dom::Element::nsDOMSlots
-    {
-    public:
-        nsXULSlots();
-        virtual ~nsXULSlots();
-
-        void Traverse(nsCycleCollectionTraversalCallback &cb);
-
-        nsCOMPtr<nsISupports> mFrameLoaderOrOpener;
-    };
-
-    virtual nsINode::nsSlots* CreateSlots() override;
-
     nsresult LoadSrc();
 
     /**
@@ -677,8 +664,8 @@ protected:
     // Internal accessor. This shadows the 'Slots', and returns
     // appropriate value.
     nsIControllers *Controllers() {
-      nsDOMSlots* slots = GetExistingDOMSlots();
-      return slots ? slots->mControllers : nullptr;
+      nsExtendedDOMSlots* slots = GetExistingExtendedDOMSlots();
+      return slots ? slots->mControllers.get() : nullptr;
     }
 
     void UnregisterAccessKey(const nsAString& aOldValue);