Bug 1540785 - Part 2: Devirtualize nsDisplayItemBase::GetPerFrameKey() r=mattwoodrow

Differential Revision: https://phabricator.services.mozilla.com/D27580

--HG--
extra : moz-landing-system : lando

layout/generic/TextOverflow.cpp

@@ -153,7 +153,7 @@ class nsDisplayTextOverflowMarker final : public nsDisplayItem {
   nsDisplayTextOverflowMarker(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
                               const nsRect& aRect, nscoord aAscent,
                               const nsStyleTextOverflowSide* aStyle,
-                              uint32_t aLineNumber, uint32_t aIndex)
+                              uint32_t aLineNumber, uint16_t aIndex)
       : nsDisplayItem(aBuilder, aFrame),
         mRect(aRect),
         mStyle(*aStyle),
@@ -189,9 +189,8 @@ class nsDisplayTextOverflowMarker final : public nsDisplayItem {
 
   virtual void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
 
-  virtual uint32_t GetPerFrameKey() const override {
+  virtual uint16_t CalculatePerFrameKey() const override { return mIndex; }
-    return (mIndex << TYPE_BITS) | nsDisplayItem::GetPerFrameKey();
+
-  }
   void PaintTextToContext(gfxContext* aCtx, nsPoint aOffsetFromRect);
 
   virtual bool CreateWebRenderCommands(
@@ -206,7 +205,7 @@ class nsDisplayTextOverflowMarker final : public nsDisplayItem {
   nsRect mRect;  // in reference frame coordinates
   const nsStyleTextOverflowSide mStyle;
   nscoord mAscent;  // baseline for the marker text in mRect
-  uint32_t mIndex;
+  uint16_t mIndex;
 };
 
 static void PaintTextShadowCallback(gfxContext* aCtx, nsPoint aShadowOffset,

layout/mathml/nsMathMLChar.cpp

@@ -1717,7 +1717,7 @@ class nsDisplayMathMLCharForeground final : public nsDisplayItem {
  public:
   nsDisplayMathMLCharForeground(nsDisplayListBuilder* aBuilder,
                                 nsIFrame* aFrame, nsMathMLChar* aChar,
-                                uint32_t aIndex, bool aIsSelected)
+                                uint16_t aIndex, bool aIsSelected)
       : nsDisplayItem(aBuilder, aFrame),
         mChar(aChar),
         mIndex(aIndex),
@@ -1758,13 +1758,11 @@ class nsDisplayMathMLCharForeground final : public nsDisplayItem {
     return GetBounds(aBuilder, &snap);
   }
 
-  virtual uint32_t GetPerFrameKey() const override {
+  virtual uint16_t CalculatePerFrameKey() const override { return mIndex; }
-    return (mIndex << TYPE_BITS) | nsDisplayItem::GetPerFrameKey();
-  }
 
  private:
   nsMathMLChar* mChar;
-  uint32_t mIndex;
+  uint16_t mIndex;
   bool mIsSelected;
 };
 

layout/mathml/nsMathMLFrame.cpp

@@ -303,7 +303,7 @@ void nsMathMLFrame::DisplayBoundingMetrics(nsDisplayListBuilder* aBuilder,
 class nsDisplayMathMLBar final : public nsDisplayItem {
  public:
   nsDisplayMathMLBar(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
-                     const nsRect& aRect, uint32_t aIndex)
+                     const nsRect& aRect, uint16_t aIndex)
       : nsDisplayItem(aBuilder, aFrame), mRect(aRect), mIndex(aIndex) {
     MOZ_COUNT_CTOR(nsDisplayMathMLBar);
   }
@@ -311,15 +311,13 @@ class nsDisplayMathMLBar final : public nsDisplayItem {
   virtual ~nsDisplayMathMLBar() { MOZ_COUNT_DTOR(nsDisplayMathMLBar); }
 #endif
 
-  virtual uint32_t GetPerFrameKey() const override {
+  virtual uint16_t CalculatePerFrameKey() const override { return mIndex; }
-    return (mIndex << TYPE_BITS) | nsDisplayItem::GetPerFrameKey();
-  }
 
   virtual void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
   NS_DISPLAY_DECL_NAME("MathMLBar", TYPE_MATHML_BAR)
  private:
   nsRect mRect;
-  uint32_t mIndex;
+  uint16_t mIndex;
 };
 
 void nsDisplayMathMLBar::Paint(nsDisplayListBuilder* aBuilder,

layout/mathml/nsMathMLmencloseFrame.cpp

@@ -716,9 +716,7 @@ class nsDisplayNotation final : public nsDisplayItem {
   virtual ~nsDisplayNotation() { MOZ_COUNT_DTOR(nsDisplayNotation); }
 #endif
 
-  virtual uint32_t GetPerFrameKey() const override {
+  virtual uint16_t CalculatePerFrameKey() const override { return mType; }
-    return (mType << TYPE_BITS) | nsDisplayItem::GetPerFrameKey();
-  }
 
   virtual void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
   NS_DISPLAY_DECL_NAME("MathMLMencloseNotation", TYPE_MATHML_MENCLOSE_NOTATION)

layout/painting/nsDisplayList.cpp

@@ -3533,7 +3533,7 @@ static Maybe<nsRect> GetViewportRectRelativeToReferenceFrame(
 
 /* static */ nsDisplayBackgroundImage::InitData
 nsDisplayBackgroundImage::GetInitData(nsDisplayListBuilder* aBuilder,
-                                      nsIFrame* aFrame, uint32_t aLayer,
+                                      nsIFrame* aFrame, uint16_t aLayer,
                                       const nsRect& aBackgroundRect,
                                       ComputedStyle* aBackgroundStyle) {
   nsPresContext* presContext = aFrame->PresContext();
@@ -5029,7 +5029,7 @@ bool nsDisplayEventReceiver::CreateWebRenderCommands(
 
 nsDisplayCompositorHitTestInfo::nsDisplayCompositorHitTestInfo(
     nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
-    const mozilla::gfx::CompositorHitTestInfo& aHitTestFlags, uint32_t aIndex,
+    const mozilla::gfx::CompositorHitTestInfo& aHitTestFlags, uint16_t aIndex,
     const mozilla::Maybe<nsRect>& aArea)
     : nsDisplayHitTestInfoItem(aBuilder, aFrame),
       mIndex(aIndex),
@@ -5114,8 +5114,8 @@ bool nsDisplayCompositorHitTestInfo::CreateWebRenderCommands(
   return true;
 }
 
-uint32_t nsDisplayCompositorHitTestInfo::GetPerFrameKey() const {
+uint16_t nsDisplayCompositorHitTestInfo::CalculatePerFrameKey() const {
-  return (mIndex << TYPE_BITS) | nsDisplayItem::GetPerFrameKey();
+  return mIndex;
 }
 
 int32_t nsDisplayCompositorHitTestInfo::ZIndex() const {
@@ -5643,7 +5643,7 @@ nsDisplayWrapList::nsDisplayWrapList(nsDisplayListBuilder* aBuilder,
 nsDisplayWrapList::nsDisplayWrapList(
     nsDisplayListBuilder* aBuilder, nsIFrame* aFrame, nsDisplayList* aList,
     const ActiveScrolledRoot* aActiveScrolledRoot, bool aClearClipChain,
-    uint32_t aIndex)
+    uint16_t aIndex)
     : nsDisplayHitTestInfoItem(aBuilder, aFrame, aActiveScrolledRoot),
       mFrameActiveScrolledRoot(aBuilder->CurrentActiveScrolledRoot()),
       mOverrideZIndex(0),
@@ -6260,7 +6260,7 @@ bool nsDisplayOpacity::CreateWebRenderCommands(
 nsDisplayBlendMode::nsDisplayBlendMode(
     nsDisplayListBuilder* aBuilder, nsIFrame* aFrame, nsDisplayList* aList,
     uint8_t aBlendMode, const ActiveScrolledRoot* aActiveScrolledRoot,
-    uint32_t aIndex)
+    uint16_t aIndex)
     : nsDisplayWrapList(aBuilder, aFrame, aList, aActiveScrolledRoot, true),
       mBlendMode(aBlendMode),
       mIndex(aIndex) {
@@ -6885,9 +6885,9 @@ nsDisplayFixedPosition::nsDisplayFixedPosition(
     const ActiveScrolledRoot* aActiveScrolledRoot,
     const ActiveScrolledRoot* aContainerASR)
     : nsDisplayOwnLayer(aBuilder, aFrame, aList, aActiveScrolledRoot),
+      mContainerASR(aContainerASR),
       mIndex(0),
-      mIsFixedBackground(false),
+      mIsFixedBackground(false) {
-      mContainerASR(aContainerASR) {
   MOZ_COUNT_CTOR(nsDisplayFixedPosition);
   Init(aBuilder);
 }
@@ -6895,13 +6895,12 @@ nsDisplayFixedPosition::nsDisplayFixedPosition(
 nsDisplayFixedPosition::nsDisplayFixedPosition(nsDisplayListBuilder* aBuilder,
                                                nsIFrame* aFrame,
                                                nsDisplayList* aList,
-                                               uint32_t aIndex)
+                                               uint16_t aIndex)
     : nsDisplayOwnLayer(aBuilder, aFrame, aList,
                         aBuilder->CurrentActiveScrolledRoot()),
+      mContainerASR(nullptr),  // XXX maybe this should be something?
       mIndex(aIndex),
-      mIsFixedBackground(true),
+      mIsFixedBackground(true) {
-      mContainerASR(nullptr)  // XXX maybe this should be something?
-{
   MOZ_COUNT_CTOR(nsDisplayFixedPosition);
   Init(aBuilder);
 }
@@ -6916,7 +6915,7 @@ void nsDisplayFixedPosition::Init(nsDisplayListBuilder* aBuilder) {
 /* static */
 nsDisplayFixedPosition* nsDisplayFixedPosition::CreateForFixedBackground(
     nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
-    nsDisplayBackgroundImage* aImage, uint32_t aIndex) {
+    nsDisplayBackgroundImage* aImage, uint16_t aIndex) {
   nsDisplayList temp;
   temp.AppendToTop(aImage);
 
@@ -7041,7 +7040,7 @@ TableType GetTableTypeFromFrame(nsIFrame* aFrame) {
 
 nsDisplayTableFixedPosition::nsDisplayTableFixedPosition(
     nsDisplayListBuilder* aBuilder, nsIFrame* aFrame, nsDisplayList* aList,
-    uint32_t aIndex, nsIFrame* aAncestorFrame)
+    uint16_t aIndex, nsIFrame* aAncestorFrame)
     : nsDisplayFixedPosition(aBuilder, aFrame, aList, aIndex),
       mAncestorFrame(aAncestorFrame),
       mTableType(GetTableTypeFromFrame(aAncestorFrame)) {
@@ -7054,7 +7053,7 @@ nsDisplayTableFixedPosition::nsDisplayTableFixedPosition(
 nsDisplayTableFixedPosition*
 nsDisplayTableFixedPosition::CreateForFixedBackground(
     nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
-    nsDisplayBackgroundImage* aImage, uint32_t aIndex,
+    nsDisplayBackgroundImage* aImage, uint16_t aIndex,
     nsIFrame* aAncestorFrame) {
   nsDisplayList temp;
   temp.AppendToTop(aImage);
@@ -7508,7 +7507,7 @@ static_assert(sizeof(nsDisplayTransform) < 512, "nsDisplayTransform has grown");
 nsDisplayTransform::nsDisplayTransform(nsDisplayListBuilder* aBuilder,
                                        nsIFrame* aFrame, nsDisplayList* aList,
                                        const nsRect& aChildrenBuildingRect,
-                                       uint32_t aIndex)
+                                       uint16_t aIndex)
     : nsDisplayHitTestInfoItem(aBuilder, aFrame),
       mTransform(Some(Matrix4x4())),
       mTransformGetter(nullptr),
@@ -7526,7 +7525,7 @@ nsDisplayTransform::nsDisplayTransform(nsDisplayListBuilder* aBuilder,
 nsDisplayTransform::nsDisplayTransform(nsDisplayListBuilder* aBuilder,
                                        nsIFrame* aFrame, nsDisplayList* aList,
                                        const nsRect& aChildrenBuildingRect,
-                                       uint32_t aIndex,
+                                       uint16_t aIndex,
                                        bool aAllowAsyncAnimation)
     : nsDisplayHitTestInfoItem(aBuilder, aFrame),
       mTransformGetter(nullptr),
@@ -7544,7 +7543,7 @@ nsDisplayTransform::nsDisplayTransform(nsDisplayListBuilder* aBuilder,
 
 nsDisplayTransform::nsDisplayTransform(
     nsDisplayListBuilder* aBuilder, nsIFrame* aFrame, nsDisplayList* aList,
-    const nsRect& aChildrenBuildingRect, uint32_t aIndex,
+    const nsRect& aChildrenBuildingRect, uint16_t aIndex,
     ComputeTransformFunction aTransformGetter)
     : nsDisplayHitTestInfoItem(aBuilder, aFrame),
       mTransformGetter(aTransformGetter),

layout/painting/nsDisplayList.h

@@ -2027,6 +2027,8 @@ MOZ_ALWAYS_INLINE T* MakeDisplayItem(nsDisplayListBuilder* aBuilder,
     item->SetType(T::ItemType());
   }
 
+  item->SetPerFrameKey(item->CalculatePerFrameKey());
+
   // TODO: Ideally we'd determine this before constructing the item,
   // but we'd need a template specialization for every type that has
   // children, or make all callers pass the type.
@@ -2166,8 +2168,19 @@ class nsDisplayItemBase : public nsDisplayItemLink {
    * Pairing this with the Frame() pointer gives a key that
    * uniquely identifies this display item in the display item tree.
    */
-  virtual uint32_t GetPerFrameKey() const { return uint32_t(GetType()); }
+  uint32_t GetPerFrameKey() const {
-  void SetPerFrameKey(const uint32_t aKey) { mKey = aKey; }
+    // The top 8 bits are currently unused.
+    // The middle 16 bits of the per frame key uniquely identify the display
+    // item when there are more than one item of the same type for a frame.
+    // The low 8 bits are the display item type.
+    return (static_cast<uint32_t>(mKey) << TYPE_BITS) |
+           static_cast<uint32_t>(mType);
+  }
+
+  /**
+   * Returns the initial per frame key for this display item.
+   */
+  virtual uint16_t CalculatePerFrameKey() const { return 0; }
 
   /**
    * Returns true if this item was reused during display list merging.
@@ -2278,6 +2291,8 @@ class nsDisplayItemBase : public nsDisplayItemLink {
     }
   }
 
+  void SetPerFrameKey(const uint16_t aKey) { mKey = aKey; }
+
   void SetDeletedFrame();
 
   nsIFrame* mFrame;  // 8
@@ -4384,7 +4399,7 @@ class nsDisplayBackgroundImage : public nsDisplayImageContainer {
    * aBackgroundRect is relative to aFrame.
    */
   static InitData GetInitData(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
-                              uint32_t aLayer, const nsRect& aBackgroundRect,
+                              uint16_t aLayer, const nsRect& aBackgroundRect,
                               mozilla::ComputedStyle* aBackgroundStyle);
 
   explicit nsDisplayBackgroundImage(nsDisplayListBuilder* aBuilder,
@@ -4435,9 +4450,7 @@ class nsDisplayBackgroundImage : public nsDisplayImageContainer {
 
   void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
 
-  uint32_t GetPerFrameKey() const override {
+  uint16_t CalculatePerFrameKey() const override { return mLayer; }
-    return (mLayer << TYPE_BITS) | nsDisplayItem::GetPerFrameKey();
-  }
 
   /**
    * Return the background positioning area.
@@ -4532,7 +4545,7 @@ class nsDisplayBackgroundImage : public nsDisplayImageContainer {
   nsRect mDestRect;
   /* Bounds of this display item */
   nsRect mBounds;
-  uint32_t mLayer;
+  uint16_t mLayer;
   bool mIsRasterImage;
   /* Whether the image should be treated as fixed to the viewport. */
   bool mShouldFixToViewport;
@@ -4557,6 +4570,14 @@ static_assert(static_cast<uint8_t>(TableType::TABLE_TYPE_MAX) <
               "TableType cannot fit with TableTypeBits::COUNT");
 TableType GetTableTypeFromFrame(nsIFrame* aFrame);
 
+static uint16_t CalculateTablePerFrameKey(const uint16_t aIndex,
+                                          const TableType aType) {
+  const uint32_t key = (aIndex << static_cast<uint8_t>(TableTypeBits::COUNT)) |
+                       static_cast<uint8_t>(aType);
+
+  return static_cast<uint16_t>(key);
+}
+
 /**
  * A display item to paint background image for table. For table parts, such
  * as row, row group, col, col group, when drawing its background, we'll
@@ -4579,11 +4600,8 @@ class nsDisplayTableBackgroundImage : public nsDisplayBackgroundImage {
 
   NS_DISPLAY_DECL_NAME("TableBackgroundImage", TYPE_TABLE_BACKGROUND_IMAGE)
 
-  uint32_t GetPerFrameKey() const override {
+  uint16_t CalculatePerFrameKey() const override {
-    return (mLayer << (TYPE_BITS +
+    return CalculateTablePerFrameKey(mLayer, mTableType);
-                       static_cast<uint8_t>(TableTypeBits::COUNT))) |
-           (static_cast<uint8_t>(mTableType) << TYPE_BITS) |
-           nsDisplayItem::GetPerFrameKey();
   }
 
   bool IsInvalid(nsRect& aRect) const override;
@@ -4711,9 +4729,8 @@ class nsDisplayTableThemedBackground : public nsDisplayThemedBackground {
   NS_DISPLAY_DECL_NAME("TableThemedBackground",
                        TYPE_TABLE_THEMED_BACKGROUND_IMAGE)
 
-  uint32_t GetPerFrameKey() const override {
+  uint16_t CalculatePerFrameKey() const override {
-    return (static_cast<uint8_t>(mTableType) << TYPE_BITS) |
+    return static_cast<uint8_t>(mTableType);
-           nsDisplayItem::GetPerFrameKey();
   }
 
   nsIFrame* FrameForInvalidation() const override { return mAncestorFrame; }
@@ -4903,9 +4920,8 @@ class nsDisplayTableBackgroundColor : public nsDisplayBackgroundColor {
     nsDisplayBackgroundColor::RemoveFrame(aFrame);
   }
 
-  uint32_t GetPerFrameKey() const override {
+  uint16_t CalculatePerFrameKey() const override {
-    return (static_cast<uint8_t>(mTableType) << TYPE_BITS) |
+    return static_cast<uint8_t>(mTableType);
-           nsDisplayItem::GetPerFrameKey();
   }
 
   bool CanUseAsyncAnimations(nsDisplayListBuilder* aBuilder) override {
@@ -5151,7 +5167,7 @@ class nsDisplayCompositorHitTestInfo : public nsDisplayHitTestInfoItem {
   nsDisplayCompositorHitTestInfo(
       nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
       const mozilla::gfx::CompositorHitTestInfo& aHitTestFlags,
-      uint32_t aIndex = 0,
+      uint16_t aIndex = 0,
       const mozilla::Maybe<nsRect>& aArea = mozilla::Nothing());
 
   nsDisplayCompositorHitTestInfo(
@@ -5174,7 +5190,7 @@ class nsDisplayCompositorHitTestInfo : public nsDisplayHitTestInfoItem {
       const StackingContextHelper& aSc,
       mozilla::layers::RenderRootStateManager* aManager,
       nsDisplayListBuilder* aDisplayListBuilder) override;
-  uint32_t GetPerFrameKey() const override;
+  uint16_t CalculatePerFrameKey() const override;
   int32_t ZIndex() const override;
   void SetOverrideZIndex(int32_t aZIndex);
 
@@ -5196,7 +5212,7 @@ class nsDisplayCompositorHitTestInfo : public nsDisplayHitTestInfoItem {
 
  private:
   mozilla::Maybe<mozilla::layers::ScrollableLayerGuid::ViewID> mScrollTarget;
-  uint32_t mIndex;
+  uint16_t mIndex;
   mozilla::Maybe<int32_t> mOverrideZIndex;
   int32_t mAppUnitsPerDevPixel;
 };
@@ -5229,7 +5245,7 @@ class nsDisplayWrapList : public nsDisplayHitTestInfoItem {
   nsDisplayWrapList(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
                     nsDisplayList* aList,
                     const ActiveScrolledRoot* aActiveScrolledRoot,
-                    bool aClearClipChain = false, uint32_t aIndex = 0);
+                    bool aClearClipChain = false, uint16_t aIndex = 0);
 
   nsDisplayWrapList(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame)
       : nsDisplayHitTestInfoItem(aBuilder, aFrame),
@@ -5333,9 +5349,7 @@ class nsDisplayWrapList : public nsDisplayHitTestInfoItem {
   bool ComputeVisibility(nsDisplayListBuilder* aBuilder,
                          nsRegion* aVisibleRegion) override;
 
-  uint32_t GetPerFrameKey() const override {
+  uint16_t CalculatePerFrameKey() const override { return mIndex; }
-    return (mIndex << TYPE_BITS) | nsDisplayItem::GetPerFrameKey();
-  }
 
   bool CanMerge(const nsDisplayItem* aItem) const override { return false; }
 
@@ -5441,7 +5455,7 @@ class nsDisplayWrapList : public nsDisplayHitTestInfoItem {
   // Our mBuildingRect may include the visible areas of children.
   nsRect mBaseBuildingRect;
   int32_t mOverrideZIndex;
-  uint32_t mIndex;
+  uint16_t mIndex;
   bool mHasZIndexOverride;
   bool mClearingClipChain = false;
 };
@@ -5616,7 +5630,7 @@ class nsDisplayBlendMode : public nsDisplayWrapList {
   nsDisplayBlendMode(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
                      nsDisplayList* aList, uint8_t aBlendMode,
                      const ActiveScrolledRoot* aActiveScrolledRoot,
-                     uint32_t aIndex = 0);
+                     uint16_t aIndex = 0);
   nsDisplayBlendMode(nsDisplayListBuilder* aBuilder,
                      const nsDisplayBlendMode& aOther)
       : nsDisplayWrapList(aBuilder, aOther),
@@ -5646,9 +5660,7 @@ class nsDisplayBlendMode : public nsDisplayWrapList {
     // LayerTreeInvalidation
   }
 
-  uint32_t GetPerFrameKey() const override {
+  uint16_t CalculatePerFrameKey() const override { return mIndex; }
-    return (mIndex << TYPE_BITS) | nsDisplayItem::GetPerFrameKey();
-  }
 
   LayerState GetLayerState(
       nsDisplayListBuilder* aBuilder, LayerManager* aManager,
@@ -5672,7 +5684,7 @@ class nsDisplayBlendMode : public nsDisplayWrapList {
 
  protected:
   uint8_t mBlendMode;
-  uint32_t mIndex;
+  uint16_t mIndex;
 };
 
 class nsDisplayTableBlendMode : public nsDisplayBlendMode {
@@ -5680,7 +5692,7 @@ class nsDisplayTableBlendMode : public nsDisplayBlendMode {
   nsDisplayTableBlendMode(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
                           nsDisplayList* aList, uint8_t aBlendMode,
                           const ActiveScrolledRoot* aActiveScrolledRoot,
-                          uint32_t aIndex, nsIFrame* aAncestorFrame)
+                          uint16_t aIndex, nsIFrame* aAncestorFrame)
       : nsDisplayBlendMode(aBuilder, aFrame, aList, aBlendMode,
                            aActiveScrolledRoot, aIndex),
         mAncestorFrame(aAncestorFrame),
@@ -5722,11 +5734,8 @@ class nsDisplayTableBlendMode : public nsDisplayBlendMode {
     nsDisplayBlendMode::RemoveFrame(aFrame);
   }
 
-  uint32_t GetPerFrameKey() const override {
+  uint16_t CalculatePerFrameKey() const override {
-    return (mIndex << (TYPE_BITS +
+    return CalculateTablePerFrameKey(mIndex, mTableType);
-                       static_cast<uint8_t>(TableTypeBits::COUNT))) |
-           (static_cast<uint8_t>(mTableType) << TYPE_BITS) |
-           nsDisplayItem::GetPerFrameKey();
   }
 
  protected:
@@ -5784,9 +5793,8 @@ class nsDisplayBlendContainer : public nsDisplayWrapList {
     return false;
   }
 
-  uint32_t GetPerFrameKey() const override {
+  uint16_t CalculatePerFrameKey() const override {
-    return (mIsForBackground ? 1 << TYPE_BITS : 0) |
+    return mIsForBackground ? 1 : 0;
-           nsDisplayItem::GetPerFrameKey();
   }
 
  protected:
@@ -5826,9 +5834,8 @@ class nsDisplayTableBlendContainer : public nsDisplayBlendContainer {
     nsDisplayBlendContainer::RemoveFrame(aFrame);
   }
 
-  uint32_t GetPerFrameKey() const override {
+  uint16_t CalculatePerFrameKey() const override {
-    return (static_cast<uint8_t>(mTableType) << TYPE_BITS) |
+    return static_cast<uint8_t>(mTableType);
-           nsDisplayItem::GetPerFrameKey();
   }
 
  protected:
@@ -6148,15 +6155,15 @@ class nsDisplayFixedPosition : public nsDisplayOwnLayer {
       : nsDisplayOwnLayer(aBuilder, aOther),
         mAnimatedGeometryRootForScrollMetadata(
             aOther.mAnimatedGeometryRootForScrollMetadata),
+        mContainerASR(aOther.mContainerASR),
         mIndex(aOther.mIndex),
-        mIsFixedBackground(aOther.mIsFixedBackground),
+        mIsFixedBackground(aOther.mIsFixedBackground) {
-        mContainerASR(aOther.mContainerASR) {
     MOZ_COUNT_CTOR(nsDisplayFixedPosition);
   }
 
   static nsDisplayFixedPosition* CreateForFixedBackground(
       nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
-      nsDisplayBackgroundImage* aImage, uint32_t aIndex);
+      nsDisplayBackgroundImage* aImage, uint16_t aIndex);
 
 #ifdef NS_BUILD_REFCNT_LOGGING
   ~nsDisplayFixedPosition() override { MOZ_COUNT_DTOR(nsDisplayFixedPosition); }
@@ -6182,9 +6189,7 @@ class nsDisplayFixedPosition : public nsDisplayOwnLayer {
     return mIsFixedBackground;
   }
 
-  uint32_t GetPerFrameKey() const override {
+  uint16_t CalculatePerFrameKey() const override { return mIndex; }
-    return (mIndex << TYPE_BITS) | nsDisplayItem::GetPerFrameKey();
-  }
 
   AnimatedGeometryRoot* AnimatedGeometryRootForScrollMetadata() const override {
     return mAnimatedGeometryRootForScrollMetadata;
@@ -6204,21 +6209,21 @@ class nsDisplayFixedPosition : public nsDisplayOwnLayer {
  protected:
   // For background-attachment:fixed
   nsDisplayFixedPosition(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
-                         nsDisplayList* aList, uint32_t aIndex);
+                         nsDisplayList* aList, uint16_t aIndex);
   void Init(nsDisplayListBuilder* aBuilder);
   ViewID GetScrollTargetId();
 
   RefPtr<AnimatedGeometryRoot> mAnimatedGeometryRootForScrollMetadata;
-  uint32_t mIndex;
-  bool mIsFixedBackground;
   RefPtr<const ActiveScrolledRoot> mContainerASR;
+  uint16_t mIndex;
+  bool mIsFixedBackground;
 };
 
 class nsDisplayTableFixedPosition : public nsDisplayFixedPosition {
  public:
   static nsDisplayTableFixedPosition* CreateForFixedBackground(
       nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
-      nsDisplayBackgroundImage* aImage, uint32_t aIndex,
+      nsDisplayBackgroundImage* aImage, uint16_t aIndex,
       nsIFrame* aAncestorFrame);
 
   NS_DISPLAY_DECL_NAME("TableFixedPosition", TYPE_TABLE_FIXED_POSITION)
@@ -6237,16 +6242,13 @@ class nsDisplayTableFixedPosition : public nsDisplayFixedPosition {
     nsDisplayFixedPosition::RemoveFrame(aFrame);
   }
 
-  uint32_t GetPerFrameKey() const override {
+  uint16_t CalculatePerFrameKey() const override {
-    return (mIndex << (TYPE_BITS +
+    return CalculateTablePerFrameKey(mIndex, mTableType);
-                       static_cast<uint8_t>(TableTypeBits::COUNT))) |
-           (static_cast<uint8_t>(mTableType) << TYPE_BITS) |
-           nsDisplayItem::GetPerFrameKey();
   }
 
  protected:
   nsDisplayTableFixedPosition(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
-                              nsDisplayList* aList, uint32_t aIndex,
+                              nsDisplayList* aList, uint16_t aIndex,
                               nsIFrame* aAncestorFrame);
 
   nsDisplayTableFixedPosition(nsDisplayListBuilder* aBuilder,
@@ -6687,15 +6689,15 @@ class nsDisplayTransform : public nsDisplayHitTestInfoItem {
    */
   nsDisplayTransform(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
                      nsDisplayList* aList, const nsRect& aChildrenBuildingRect,
-                     uint32_t aIndex);
+                     uint16_t aIndex);
 
   nsDisplayTransform(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
                      nsDisplayList* aList, const nsRect& aChildrenBuildingRect,
-                     uint32_t aIndex, bool aAllowAsyncAnimation);
+                     uint16_t aIndex, bool aAllowAsyncAnimation);
 
   nsDisplayTransform(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
                      nsDisplayList* aList, const nsRect& aChildrenBuildingRect,
-                     uint32_t aIndex,
+                     uint16_t aIndex,
                      ComputeTransformFunction aTransformGetter);
 
 #ifdef NS_BUILD_REFCNT_LOGGING
@@ -6762,9 +6764,7 @@ class nsDisplayTransform : public nsDisplayHitTestInfoItem {
 
   bool CanMerge(const nsDisplayItem* aItem) const override { return false; }
 
-  uint32_t GetPerFrameKey() const override {
+  uint16_t CalculatePerFrameKey() const override { return mIndex; }
-    return (mIndex << TYPE_BITS) | nsDisplayItem::GetPerFrameKey();
-  }
 
   nsDisplayItemGeometry* AllocateGeometry(
       nsDisplayListBuilder* aBuilder) override {
@@ -7010,8 +7010,9 @@ class nsDisplayTransform : public nsDisplayHitTestInfoItem {
   RefPtr<AnimatedGeometryRoot> mAnimatedGeometryRootForChildren;
   RefPtr<AnimatedGeometryRoot> mAnimatedGeometryRootForScrollMetadata;
   nsRect mChildrenBuildingRect;
-  uint32_t mIndex;
   mutable RetainedDisplayList mChildren;
+  uint16_t mIndex;
+
   // The untransformed bounds of |mChildren|.
   nsRect mChildBounds;
   // The transformed bounds of this display item.