Bug 1419661, if ExtendedDOMSlots are used before slots, use FatSlots to have fewer allocations, r=mrbkap

dom/base/FragmentOrElement.cpp

@@ -645,7 +645,7 @@ static_assert(sizeof(FragmentOrElement::nsDOMSlots) <= MaxDOMSlotSizeAllowed,
               "DOM slots cannot be grown without consideration");
 
 void
-nsIContent::nsExtendedContentSlots::Unlink()
+nsIContent::nsExtendedContentSlots::UnlinkExtendedSlots()
 {
   mBindingParent = nullptr;
   mXBLInsertionPoint = nullptr;
@@ -654,7 +654,7 @@ nsIContent::nsExtendedContentSlots::Unlink()
 }
 
 void
-nsIContent::nsExtendedContentSlots::Traverse(nsCycleCollectionTraversalCallback& aCb)
+nsIContent::nsExtendedContentSlots::TraverseExtendedSlots(nsCycleCollectionTraversalCallback& aCb)
 {
   NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(aCb, "mExtendedSlots->mBindingParent");
   aCb.NoteXPCOMChild(NS_ISUPPORTS_CAST(nsIContent*, mBindingParent));
@@ -679,10 +679,13 @@ FragmentOrElement::nsDOMSlots::nsDOMSlots()
   : nsIContent::nsContentSlots(),
     mDataset(nullptr)
 {
+  MOZ_COUNT_CTOR(nsDOMSlots);
 }
 
 FragmentOrElement::nsDOMSlots::~nsDOMSlots()
 {
+  MOZ_COUNT_DTOR(nsDOMSlots);
+
   if (mAttributeMap) {
     mAttributeMap->DropReference();
   }
@@ -723,8 +726,8 @@ size_t
 FragmentOrElement::nsDOMSlots::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
 {
   size_t n = aMallocSizeOf(this);
-  if (mExtendedSlots) {
-    n += aMallocSizeOf(mExtendedSlots.get());
+  if (OwnsExtendedSlots()) {
+    n += aMallocSizeOf(GetExtendedContentSlots());
   }
 
   if (mAttributeMap) {
@@ -758,9 +761,9 @@ FragmentOrElement::nsExtendedDOMSlots::~nsExtendedDOMSlots()
 }
 
 void
-FragmentOrElement::nsExtendedDOMSlots::Unlink()
+FragmentOrElement::nsExtendedDOMSlots::UnlinkExtendedSlots()
 {
-  nsIContent::nsExtendedContentSlots::Unlink();
+  nsIContent::nsExtendedContentSlots::UnlinkExtendedSlots();
 
   // Don't clear mXBLBinding, it'll be done in
   // BindingManager::RemovedFromDocument from FragmentOrElement::Unlink.
@@ -780,9 +783,9 @@ FragmentOrElement::nsExtendedDOMSlots::Unlink()
 }
 
 void
-FragmentOrElement::nsExtendedDOMSlots::Traverse(nsCycleCollectionTraversalCallback& aCb)
+FragmentOrElement::nsExtendedDOMSlots::TraverseExtendedSlots(nsCycleCollectionTraversalCallback& aCb)
 {
-  nsIContent::nsExtendedContentSlots::Traverse(aCb);
+  nsIContent::nsExtendedContentSlots::TraverseExtendedSlots(aCb);
 
   NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(aCb, "mExtendedSlots->mSMILOverrideStyle");
   aCb.NoteXPCOMChild(mSMILOverrideStyle.get());

dom/base/FragmentOrElement.h

@@ -171,14 +171,14 @@ public:
    * accessed through the DOM.
    */
 
-  class nsExtendedDOMSlots final : public nsIContent::nsExtendedContentSlots
+  class nsExtendedDOMSlots : public nsIContent::nsExtendedContentSlots
   {
   public:
     nsExtendedDOMSlots();
-    ~nsExtendedDOMSlots() final;
+    ~nsExtendedDOMSlots();
 
-    void Traverse(nsCycleCollectionTraversalCallback&) final;
-    void Unlink() final;
+    void TraverseExtendedSlots(nsCycleCollectionTraversalCallback&) final;
+    void UnlinkExtendedSlots() final;
 
     /**
      * SMIL Overridde style rules (for SMIL animation of CSS properties)
@@ -223,11 +223,11 @@ public:
 
   };
 
-  class nsDOMSlots final : public nsIContent::nsContentSlots
+  class nsDOMSlots : public nsIContent::nsContentSlots
   {
   public:
     nsDOMSlots();
-    ~nsDOMSlots() final;
+    ~nsDOMSlots();
 
     void Traverse(nsCycleCollectionTraversalCallback&) final;
     void Unlink() final;
@@ -263,6 +263,29 @@ public:
     RefPtr<nsDOMTokenList> mClassList;
   };
 
+  /**
+   * In case ExtendedDOMSlots is needed before normal DOMSlots, an instance of
+   * FatSlots class, which combines those two slot types, is created.
+   * This way we can avoid extra allocation for ExtendedDOMSlots.
+   * FatSlots is useful for example when creating Custom Elements.
+   */
+  class FatSlots final : public nsDOMSlots, public nsExtendedDOMSlots
+  {
+  public:
+    FatSlots()
+      : nsDOMSlots()
+      , nsExtendedDOMSlots()
+    {
+      MOZ_COUNT_CTOR(FatSlots);
+      SetExtendedContentSlots(this, false);
+    }
+
+    ~FatSlots() final
+    {
+      MOZ_COUNT_DTOR(FatSlots);
+    }
+  };
+
 protected:
   void GetMarkup(bool aIncludeSelf, nsAString& aMarkup);
   void SetInnerHTMLInternal(const nsAString& aInnerHTML, ErrorResult& aError);
@@ -290,7 +313,18 @@ protected:
 
   nsExtendedDOMSlots* ExtendedDOMSlots()
   {
-    return static_cast<nsExtendedDOMSlots*>(ExtendedContentSlots());
+    nsContentSlots* slots = GetExistingContentSlots();
+    if (!slots) {
+      FatSlots* fatSlots = new FatSlots();
+      mSlots = fatSlots;
+      return fatSlots;
+    }
+
+    if (!slots->GetExtendedContentSlots()) {
+      slots->SetExtendedContentSlots(CreateExtendedSlots(), true);
+    }
+
+    return static_cast<nsExtendedDOMSlots*>(slots->GetExtendedContentSlots());
   }
 
   const nsExtendedDOMSlots* GetExistingExtendedDOMSlots() const

dom/base/nsIContent.h

@@ -791,8 +791,8 @@ protected:
     nsExtendedContentSlots();
     virtual ~nsExtendedContentSlots();
 
-    virtual void Traverse(nsCycleCollectionTraversalCallback&);
-    virtual void Unlink();
+    virtual void TraverseExtendedSlots(nsCycleCollectionTraversalCallback&);
+    virtual void UnlinkExtendedSlots();
 
     /**
      * The nearest enclosing content node with a binding that created us.
@@ -821,11 +821,24 @@ protected:
   class nsContentSlots : public nsINode::nsSlots
   {
   public:
+    nsContentSlots()
+      : nsINode::nsSlots()
+      , mExtendedSlots(0)
+    {
+    }
+
+    ~nsContentSlots()
+    {
+      if (!(mExtendedSlots & sNonOwningExtendedSlotsFlag)) {
+        delete GetExtendedContentSlots();
+      }
+    }
+
     void Traverse(nsCycleCollectionTraversalCallback& aCb) override
     {
       nsINode::nsSlots::Traverse(aCb);
       if (mExtendedSlots) {
-        mExtendedSlots->Traverse(aCb);
+        GetExtendedContentSlots()->TraverseExtendedSlots(aCb);
       }
     }
 
@@ -833,11 +846,33 @@ protected:
     {
       nsINode::nsSlots::Unlink();
       if (mExtendedSlots) {
-        mExtendedSlots->Unlink();
+        GetExtendedContentSlots()->UnlinkExtendedSlots();
       }
     }
 
-    mozilla::UniquePtr<nsExtendedContentSlots> mExtendedSlots;
+    void SetExtendedContentSlots(nsExtendedContentSlots* aSlots, bool aOwning)
+    {
+      mExtendedSlots = reinterpret_cast<uintptr_t>(aSlots);
+      if (!aOwning) {
+        mExtendedSlots |= sNonOwningExtendedSlotsFlag;
+      }
+    }
+
+    bool OwnsExtendedSlots() const
+    {
+      return !(mExtendedSlots & sNonOwningExtendedSlotsFlag);
+    }
+
+    nsExtendedContentSlots* GetExtendedContentSlots() const
+    {
+      return reinterpret_cast<nsExtendedContentSlots*>(
+        mExtendedSlots & ~sNonOwningExtendedSlotsFlag);
+    }
+
+  private:
+    static const uintptr_t sNonOwningExtendedSlotsFlag = 1u;
+
+    uintptr_t mExtendedSlots;
   };
 
   // Override from nsINode
@@ -869,22 +904,22 @@ protected:
   const nsExtendedContentSlots* GetExistingExtendedContentSlots() const
   {
     const nsContentSlots* slots = GetExistingContentSlots();
-    return slots ? slots->mExtendedSlots.get() : nullptr;
+    return slots ? slots->GetExtendedContentSlots() : nullptr;
   }
 
   nsExtendedContentSlots* GetExistingExtendedContentSlots()
   {
     nsContentSlots* slots = GetExistingContentSlots();
-    return slots ? slots->mExtendedSlots.get() : nullptr;
+    return slots ? slots->GetExtendedContentSlots() : nullptr;
   }
 
   nsExtendedContentSlots* ExtendedContentSlots()
   {
     nsContentSlots* slots = ContentSlots();
-    if (!slots->mExtendedSlots) {
-      slots->mExtendedSlots.reset(CreateExtendedSlots());
+    if (!slots->GetExtendedContentSlots()) {
+      slots->SetExtendedContentSlots(CreateExtendedSlots(), true);
     }
-    return slots->mExtendedSlots.get();
+    return slots->GetExtendedContentSlots();
   }
 
   /**

servo/components/style/gecko/wrapper.rs

@@ -651,7 +651,10 @@ impl<'le> GeckoElement<'le> {
     #[inline]
     fn extended_slots(&self) -> Option<&structs::FragmentOrElement_nsExtendedDOMSlots> {
         self.dom_slots().and_then(|s| unsafe {
-            (s._base.mExtendedSlots.mPtr as *const structs::FragmentOrElement_nsExtendedDOMSlots)
+            // For the bit usage, see nsContentSlots::GetExtendedSlots.
+            let e_slots = s._base.mExtendedSlots &
+                !structs::nsIContent_nsContentSlots_sNonOwningExtendedSlotsFlag;
+            (e_slots as *const structs::FragmentOrElement_nsExtendedDOMSlots)
                 .as_ref()
         })
     }