Bug 287777 – [WebCore] Reduce size of CustomPaintCanvas, HTMLCanvasElement, OffscreenCanvas by 8 bytes each

• Status: RESOLVED
• Resolution: FIXED
• Component: New Bugs

[David Kilzer (:ddkilzer)]

Summary:
Reduce WebCore::CustomPaintCanvas, WebCore::HTMLCanvasElement, and WebCore::OffscreenCanvas by 8 bytes each:

WebCore::CanvasBase   120 bytes (no change)
WebCore::CustomPaintCanvas   168->160 bytes
WebCore::HTMLCanvasElement   304->296 bytes
WebCore::OffscreenCanvas  224 -> 216 bytes

[Radar WebKit Bug Importer]

<rdar://problem/144952951>

[David Kilzer (:ddkilzer)]

Before (note that dump-class-layout miscalculates padding for WebCore::CustomPaintCanvas):

$ xcrun python3 ./Tools/Scripts/dump-class-layout -b WebKitBuild -c Release -a arm64e WebCore WebCore::CanvasBase
  +0 <120> WebCore::CanvasBase
  +0 < 16>     WTF::AbstractRefCountedAndCanMakeWeakPtr<WebCore::CanvasBase> WTF::AbstractRefCountedAndCanMakeWeakPtr<WebCore::CanvasBase>
  +0 <  8>         WTF::AbstractRefCounted WTF::AbstractRefCounted
  +0 <  8>             __vtbl_ptr_type * _vptr
  +8 <  8>         WTF::CanMakeWeakPtr<WebCore::CanvasBase> WTF::CanMakeWeakPtr<WebCore::CanvasBase>
  +8 <  8>             WTF::WeakPtrFactory<WebCore::CanvasBase, WTF::DefaultWeakPtrImpl> m_weakPtrFactory
  +8 <  8>                 WTF::RefPtr<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > m_impl
  +8 <  8>                     WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>::StorageType m_ptr
 +16 <  8>     WebCore::IntSize m_size
 +16 <  4>         int m_width
 +20 <  4>         int m_height
 +24 <  8>     WTF::RefPtr<WebCore::ImageBuffer, WTF::RawPtrTraits<WebCore::ImageBuffer>, WTF::DefaultRefDerefTraits<WebCore::ImageBuffer> > m_imageBuffer
 +24 <  8>         WTF::RawPtrTraits<WebCore::ImageBuffer>::StorageType m_ptr
 +32 <  8>     std::__1::atomic<unsigned long> m_imageBufferMemoryCost
 +32 <  8>         std::__1::__atomic_base<unsigned long, true> std::__1::__atomic_base<unsigned long, true>
 +32 <  8>             std::__1::__atomic_base<unsigned long, false> std::__1::__atomic_base<unsigned long, false>
 +32 <  8>                 std::__1::__cxx_atomic_impl<unsigned long, std::__1::__cxx_atomic_base_impl<unsigned long> > __a_
 +32 <  8>                     std::__1::__cxx_atomic_base_impl<unsigned long> std::__1::__cxx_atomic_base_impl<unsigned long>
 +32 <  8>                         _Atomic(unsigned long) __a_value
 +40 <  8>     std::__1::unique_ptr<WebCore::GraphicsContextStateSaver, std::__1::default_delete<WebCore::GraphicsContextStateSaver> > m_contextStateSaver
 +40 <  8>         std::__1::__compressed_pair<WebCore::GraphicsContextStateSaver *, std::__1::default_delete<WebCore::GraphicsContextStateSaver> > __ptr_
 +40 <  8>             std::__1::__compressed_pair_elem<WebCore::GraphicsContextStateSaver *, 0, false> std::__1::__compressed_pair_elem<WebCore::GraphicsContextStateSaver *, 0, false>
 +40 <  8>                 WebCore::GraphicsContextStateSaver * __value_
 +40 <  1>             std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::GraphicsContextStateSaver>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::GraphicsContextStateSaver>, 1, true>
 +40 <  1>                 std::__1::default_delete<WebCore::GraphicsContextStateSaver> std::__1::default_delete<WebCore::GraphicsContextStateSaver>
 +48 <  8>     WTF::String m_lastFillText
 +48 <  8>         WTF::RefPtr<WTF::StringImpl, WTF::RawPtrTraits<WTF::StringImpl>, WTF::DefaultRefDerefTraits<WTF::StringImpl> > m_impl
 +48 <  8>             WTF::RawPtrTraits<WTF::StringImpl>::StorageType m_ptr
 +56 < 16>     WebCore::CanvasNoiseInjection m_canvasNoiseInjection
 +56 < 16>         WebCore::IntRect m_postProcessDirtyRect
 +56 <  8>             WebCore::IntPoint m_location
 +56 <  4>                 int m_x
 +60 <  4>                 int m_y
 +64 <  8>             WebCore::IntSize m_size
 +64 <  4>                 int m_width
 +68 <  4>                 int m_height
 +72 <  8>     WTF::Markable<unsigned long long, WTF::IntegralMarkableTraits<unsigned long long, 9223372036854775807> > m_canvasNoiseHashSalt
 +72 <  8>         unsigned long long m_value
 +80 <  1>     bool m_originClean
 +81 <  1>     bool m_hasCreatedImageBuffer
 +82 <  6>     <PADDING: 6 bytes>
 +88 < 16>     WTF::WeakHashSet<WebCore::CanvasObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes> m_observers
 +88 <  8>         WTF::WeakHashSet<WebCore::CanvasObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes>::WeakPtrImplSet m_set
 +88 <  8>             WTF::HashSet<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTableTraits, 255>::HashTableType m_impl
 +88 <  8>                 WTF::HashTable<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::IdentityExtractor, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, 255>::(anonymous union) None
 +96 <  4>         unsigned int m_operationCountSinceLastCleanup
+100 <  4>         unsigned int m_maxOperationCountWithoutCleanup
+104 < 16>     WTF::WeakHashSet<WebCore::CanvasDisplayBufferObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes> m_displayBufferObservers
+104 <  8>         WTF::WeakHashSet<WebCore::CanvasDisplayBufferObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes>::WeakPtrImplSet m_set
+104 <  8>             WTF::HashSet<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTableTraits, 255>::HashTableType m_impl
+104 <  8>                 WTF::HashTable<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::IdentityExtractor, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, 255>::(anonymous union) None
+112 <  4>         unsigned int m_operationCountSinceLastCleanup
+116 <  4>         unsigned int m_maxOperationCountWithoutCleanup
Total byte size: 120
Total pad bytes: 6
Padding percentage: 5.00 %

$ xcrun python3 ./Tools/Scripts/dump-class-layout -b WebKitBuild -c Release -a arm64e WebCore WebCore::OffscreenCanvas
  +0 <224> WebCore::OffscreenCanvas
  +0 < 32>     WebCore::ActiveDOMObject WebCore::ActiveDOMObject
  +0 <  8>         WTF::AbstractRefCounted WTF::AbstractRefCounted
  +0 <  8>             __vtbl_ptr_type * _vptr
  +8 < 16>         WebCore::ContextDestructionObserver WebCore::ContextDestructionObserver
  +8 <  8>             __vtbl_ptr_type * _vptr
 +16 <  8>             WTF::WeakPtr<WebCore::ScriptExecutionContext, WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl> > m_scriptExecutionContext
 +16 <  8>                 WTF::RefPtr<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > m_impl
 +16 <  8>                     WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>::StorageType m_ptr
 +24 <  8>         uint64_t m_pendingActivityInstanceCount
 +32 <  4>     WTF::RefCounted<WebCore::OffscreenCanvas> WTF::RefCounted<WebCore::OffscreenCanvas>
 +32 <  4>         WTF::RefCountedBase WTF::RefCountedBase
 +32 <  4>             unsigned int m_refCount
 +36 <  4>     <PADDING: 4 bytes>
 +40 <120>     WebCore::CanvasBase WebCore::CanvasBase
 +40 < 16>         WTF::AbstractRefCountedAndCanMakeWeakPtr<WebCore::CanvasBase> WTF::AbstractRefCountedAndCanMakeWeakPtr<WebCore::CanvasBase>
 +40 <  8>             WTF::AbstractRefCounted WTF::AbstractRefCounted
 +40 <  8>                 __vtbl_ptr_type * _vptr
 +48 <  8>             WTF::CanMakeWeakPtr<WebCore::CanvasBase> WTF::CanMakeWeakPtr<WebCore::CanvasBase>
 +48 <  8>                 WTF::WeakPtrFactory<WebCore::CanvasBase, WTF::DefaultWeakPtrImpl> m_weakPtrFactory
 +48 <  8>                     WTF::RefPtr<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > m_impl
 +48 <  8>                         WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>::StorageType m_ptr
 +56 <  8>         WebCore::IntSize m_size
 +56 <  4>             int m_width
 +60 <  4>             int m_height
 +64 <  8>         WTF::RefPtr<WebCore::ImageBuffer, WTF::RawPtrTraits<WebCore::ImageBuffer>, WTF::DefaultRefDerefTraits<WebCore::ImageBuffer> > m_imageBuffer
 +64 <  8>             WTF::RawPtrTraits<WebCore::ImageBuffer>::StorageType m_ptr
 +72 <  8>         std::__1::atomic<unsigned long> m_imageBufferMemoryCost
 +72 <  8>             std::__1::__atomic_base<unsigned long, true> std::__1::__atomic_base<unsigned long, true>
 +72 <  8>                 std::__1::__atomic_base<unsigned long, false> std::__1::__atomic_base<unsigned long, false>
 +72 <  8>                     std::__1::__cxx_atomic_impl<unsigned long, std::__1::__cxx_atomic_base_impl<unsigned long> > __a_
 +72 <  8>                         std::__1::__cxx_atomic_base_impl<unsigned long> std::__1::__cxx_atomic_base_impl<unsigned long>
 +72 <  8>                             _Atomic(unsigned long) __a_value
 +80 <  8>         std::__1::unique_ptr<WebCore::GraphicsContextStateSaver, std::__1::default_delete<WebCore::GraphicsContextStateSaver> > m_contextStateSaver
 +80 <  8>             std::__1::__compressed_pair<WebCore::GraphicsContextStateSaver *, std::__1::default_delete<WebCore::GraphicsContextStateSaver> > __ptr_
 +80 <  8>                 std::__1::__compressed_pair_elem<WebCore::GraphicsContextStateSaver *, 0, false> std::__1::__compressed_pair_elem<WebCore::GraphicsContextStateSaver *, 0, false>
 +80 <  8>                     WebCore::GraphicsContextStateSaver * __value_
 +80 <  1>                 std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::GraphicsContextStateSaver>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::GraphicsContextStateSaver>, 1, true>
 +80 <  1>                     std::__1::default_delete<WebCore::GraphicsContextStateSaver> std::__1::default_delete<WebCore::GraphicsContextStateSaver>
 +88 <  8>         WTF::String m_lastFillText
 +88 <  8>             WTF::RefPtr<WTF::StringImpl, WTF::RawPtrTraits<WTF::StringImpl>, WTF::DefaultRefDerefTraits<WTF::StringImpl> > m_impl
 +88 <  8>                 WTF::RawPtrTraits<WTF::StringImpl>::StorageType m_ptr
 +96 < 16>         WebCore::CanvasNoiseInjection m_canvasNoiseInjection
 +96 < 16>             WebCore::IntRect m_postProcessDirtyRect
 +96 <  8>                 WebCore::IntPoint m_location
 +96 <  4>                     int m_x
+100 <  4>                     int m_y
+104 <  8>                 WebCore::IntSize m_size
+104 <  4>                     int m_width
+108 <  4>                     int m_height
+112 <  8>         WTF::Markable<unsigned long long, WTF::IntegralMarkableTraits<unsigned long long, 9223372036854775807> > m_canvasNoiseHashSalt
+112 <  8>             unsigned long long m_value
+120 <  1>         bool m_originClean
+121 <  1>         bool m_hasCreatedImageBuffer
+122 <  6>         <PADDING: 6 bytes>
+128 < 16>         WTF::WeakHashSet<WebCore::CanvasObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes> m_observers
+128 <  8>             WTF::WeakHashSet<WebCore::CanvasObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes>::WeakPtrImplSet m_set
+128 <  8>                 WTF::HashSet<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTableTraits, 255>::HashTableType m_impl
+128 <  8>                     WTF::HashTable<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::IdentityExtractor, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, 255>::(anonymous union) None
+136 <  4>             unsigned int m_operationCountSinceLastCleanup
+140 <  4>             unsigned int m_maxOperationCountWithoutCleanup
+144 < 16>         WTF::WeakHashSet<WebCore::CanvasDisplayBufferObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes> m_displayBufferObservers
+144 <  8>             WTF::WeakHashSet<WebCore::CanvasDisplayBufferObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes>::WeakPtrImplSet m_set
+144 <  8>                 WTF::HashSet<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTableTraits, 255>::HashTableType m_impl
+144 <  8>                     WTF::HashTable<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::IdentityExtractor, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, 255>::(anonymous union) None
+152 <  4>             unsigned int m_operationCountSinceLastCleanup
+156 <  4>             unsigned int m_maxOperationCountWithoutCleanup
+160 < 24>     WebCore::EventTarget WebCore::EventTarget
+160 <  8>         __vtbl_ptr_type * _vptr
+168 <  8>         WebCore::ScriptWrappable WebCore::ScriptWrappable
+168 <  8>             JSC::Weak<WebCore::JSDOMObject> m_wrapper
+168 <  8>                 JSC::WeakImpl * m_impl
+176 <  8>         WTF::CanMakeWeakPtrWithBitField<EventTarget, WeakPtrFactoryInitialization::Lazy, WeakPtrImplWithEventTargetData> WTF::CanMakeWeakPtrWithBitField<EventTarget, WeakPtrFactoryInitialization::Lazy, WeakPtrImplWithEventTargetData>
+176 <  8>             WTF::WeakPtrFactoryWithBitField<WebCore::EventTarget, WebCore::WeakPtrImplWithEventTargetData> m_weakPtrFactory
+176 <  8>                 WTF::CompactRefPtrTuple<WebCore::WeakPtrImplWithEventTargetData, unsigned short> m_impl
+176 <  8>                     WTF::CompactPointerTuple<WebCore::WeakPtrImplWithEventTargetData *, unsigned short> m_data
+176 <  8>                         uint64_t m_data
+184 <  8>     std::__1::unique_ptr<WebCore::CanvasRenderingContext, std::__1::default_delete<WebCore::CanvasRenderingContext> > m_context
+184 <  8>         std::__1::__compressed_pair<WebCore::CanvasRenderingContext *, std::__1::default_delete<WebCore::CanvasRenderingContext> > __ptr_
+184 <  8>             std::__1::__compressed_pair_elem<WebCore::CanvasRenderingContext *, 0, false> std::__1::__compressed_pair_elem<WebCore::CanvasRenderingContext *, 0, false>
+184 <  8>                 WebCore::CanvasRenderingContext * __value_
+184 <  1>             std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CanvasRenderingContext>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CanvasRenderingContext>, 1, true>
+184 <  1>                 std::__1::default_delete<WebCore::CanvasRenderingContext> std::__1::default_delete<WebCore::CanvasRenderingContext>
+192 <  8>     WTF::RefPtr<WebCore::PlaceholderRenderingContextSource, WTF::RawPtrTraits<WebCore::PlaceholderRenderingContextSource>, WTF::DefaultRefDerefTraits<WebCore::PlaceholderRenderingContextSource> > m_placeholderSource
+192 <  8>         WTF::RawPtrTraits<WebCore::PlaceholderRenderingContextSource>::StorageType m_ptr
+200 <  8>     WTF::RefPtr<WebCore::Image, WTF::RawPtrTraits<WebCore::Image>, WTF::DefaultRefDerefTraits<WebCore::Image> > m_copiedImage
+200 <  8>         WTF::RawPtrTraits<WebCore::Image>::StorageType m_ptr
+208 <  1>     bool m_detached
+209 <  1>     bool m_hasScheduledCommit
+210 <  6>     <PADDING: 6 bytes>
+216 <  8>     std::__1::unique_ptr<WebCore::CSSParserContext, std::__1::default_delete<WebCore::CSSParserContext> > m_cssParserContext
+216 <  8>         std::__1::__compressed_pair<WebCore::CSSParserContext *, std::__1::default_delete<WebCore::CSSParserContext> > __ptr_
+216 <  8>             std::__1::__compressed_pair_elem<WebCore::CSSParserContext *, 0, false> std::__1::__compressed_pair_elem<WebCore::CSSParserContext *, 0, false>
+216 <  8>                 WebCore::CSSParserContext * __value_
+216 <  1>             std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CSSParserContext>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CSSParserContext>, 1, true>
+216 <  1>                 std::__1::default_delete<WebCore::CSSParserContext> std::__1::default_delete<WebCore::CSSParserContext>
Total byte size: 224
Total pad bytes: 16
Padding percentage: 7.14 %

$ xcrun python3 ./Tools/Scripts/dump-class-layout -b WebKitBuild -c Release -a arm64e WebCore WebCore::CustomPaintCanvas
  +0 <168> WebCore::CustomPaintCanvas
  +0 <120>     <PADDING: 120 bytes>
+120 <  4>     WTF::RefCounted<WebCore::CustomPaintCanvas> WTF::RefCounted<WebCore::CustomPaintCanvas>
+120 <  4>         WTF::RefCountedBase WTF::RefCountedBase
+120 <  4>             unsigned int m_refCount
  +0 <120>     WebCore::CanvasBase WebCore::CanvasBase
  +0 < 16>         WTF::AbstractRefCountedAndCanMakeWeakPtr<WebCore::CanvasBase> WTF::AbstractRefCountedAndCanMakeWeakPtr<WebCore::CanvasBase>
  +0 <  8>             WTF::AbstractRefCounted WTF::AbstractRefCounted
  +0 <  8>                 __vtbl_ptr_type * _vptr
  +8 <  8>             WTF::CanMakeWeakPtr<WebCore::CanvasBase> WTF::CanMakeWeakPtr<WebCore::CanvasBase>
  +8 <  8>                 WTF::WeakPtrFactory<WebCore::CanvasBase, WTF::DefaultWeakPtrImpl> m_weakPtrFactory
  +8 <  8>                     WTF::RefPtr<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > m_impl
  +8 <  8>                         WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>::StorageType m_ptr
 +16 <  8>         WebCore::IntSize m_size
 +16 <  4>             int m_width
 +20 <  4>             int m_height
 +24 <  8>         WTF::RefPtr<WebCore::ImageBuffer, WTF::RawPtrTraits<WebCore::ImageBuffer>, WTF::DefaultRefDerefTraits<WebCore::ImageBuffer> > m_imageBuffer
 +24 <  8>             WTF::RawPtrTraits<WebCore::ImageBuffer>::StorageType m_ptr
 +32 <  8>         std::__1::atomic<unsigned long> m_imageBufferMemoryCost
 +32 <  8>             std::__1::__atomic_base<unsigned long, true> std::__1::__atomic_base<unsigned long, true>
 +32 <  8>                 std::__1::__atomic_base<unsigned long, false> std::__1::__atomic_base<unsigned long, false>
 +32 <  8>                     std::__1::__cxx_atomic_impl<unsigned long, std::__1::__cxx_atomic_base_impl<unsigned long> > __a_
 +32 <  8>                         std::__1::__cxx_atomic_base_impl<unsigned long> std::__1::__cxx_atomic_base_impl<unsigned long>
 +32 <  8>                             _Atomic(unsigned long) __a_value
 +40 <  8>         std::__1::unique_ptr<WebCore::GraphicsContextStateSaver, std::__1::default_delete<WebCore::GraphicsContextStateSaver> > m_contextStateSaver
 +40 <  8>             std::__1::__compressed_pair<WebCore::GraphicsContextStateSaver *, std::__1::default_delete<WebCore::GraphicsContextStateSaver> > __ptr_
 +40 <  8>                 std::__1::__compressed_pair_elem<WebCore::GraphicsContextStateSaver *, 0, false> std::__1::__compressed_pair_elem<WebCore::GraphicsContextStateSaver *, 0, false>
 +40 <  8>                     WebCore::GraphicsContextStateSaver * __value_
 +40 <  1>                 std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::GraphicsContextStateSaver>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::GraphicsContextStateSaver>, 1, true>
 +40 <  1>                     std::__1::default_delete<WebCore::GraphicsContextStateSaver> std::__1::default_delete<WebCore::GraphicsContextStateSaver>
 +48 <  8>         WTF::String m_lastFillText
 +48 <  8>             WTF::RefPtr<WTF::StringImpl, WTF::RawPtrTraits<WTF::StringImpl>, WTF::DefaultRefDerefTraits<WTF::StringImpl> > m_impl
 +48 <  8>                 WTF::RawPtrTraits<WTF::StringImpl>::StorageType m_ptr
 +56 < 16>         WebCore::CanvasNoiseInjection m_canvasNoiseInjection
 +56 < 16>             WebCore::IntRect m_postProcessDirtyRect
 +56 <  8>                 WebCore::IntPoint m_location
 +56 <  4>                     int m_x
 +60 <  4>                     int m_y
 +64 <  8>                 WebCore::IntSize m_size
 +64 <  4>                     int m_width
 +68 <  4>                     int m_height
 +72 <  8>         WTF::Markable<unsigned long long, WTF::IntegralMarkableTraits<unsigned long long, 9223372036854775807> > m_canvasNoiseHashSalt
 +72 <  8>             unsigned long long m_value
 +80 <  1>         bool m_originClean
 +81 <  1>         bool m_hasCreatedImageBuffer
 +82 <  6>         <PADDING: 6 bytes>
 +88 < 16>         WTF::WeakHashSet<WebCore::CanvasObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes> m_observers
 +88 <  8>             WTF::WeakHashSet<WebCore::CanvasObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes>::WeakPtrImplSet m_set
 +88 <  8>                 WTF::HashSet<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTableTraits, 255>::HashTableType m_impl
 +88 <  8>                     WTF::HashTable<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::IdentityExtractor, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, 255>::(anonymous union) None
 +96 <  4>             unsigned int m_operationCountSinceLastCleanup
+100 <  4>             unsigned int m_maxOperationCountWithoutCleanup
+104 < 16>         WTF::WeakHashSet<WebCore::CanvasDisplayBufferObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes> m_displayBufferObservers
+104 <  8>             WTF::WeakHashSet<WebCore::CanvasDisplayBufferObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes>::WeakPtrImplSet m_set
+104 <  8>                 WTF::HashSet<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTableTraits, 255>::HashTableType m_impl
+104 <  8>                     WTF::HashTable<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::IdentityExtractor, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, 255>::(anonymous union) None
+112 <  4>             unsigned int m_operationCountSinceLastCleanup
+116 <  4>             unsigned int m_maxOperationCountWithoutCleanup
+120 <  8>     <PADDING: 8 bytes>
+128 < 16>     WebCore::ContextDestructionObserver WebCore::ContextDestructionObserver
+128 <  8>         __vtbl_ptr_type * _vptr
+136 <  8>         WTF::WeakPtr<WebCore::ScriptExecutionContext, WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl> > m_scriptExecutionContext
+136 <  8>             WTF::RefPtr<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > m_impl
+136 <  8>                 WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>::StorageType m_ptr
+144 <  8>     std::__1::unique_ptr<WebCore::PaintRenderingContext2D, std::__1::default_delete<WebCore::PaintRenderingContext2D> > m_context
+144 <  8>         std::__1::__compressed_pair<WebCore::PaintRenderingContext2D *, std::__1::default_delete<WebCore::PaintRenderingContext2D> > __ptr_
+144 <  8>             std::__1::__compressed_pair_elem<WebCore::PaintRenderingContext2D *, 0, false> std::__1::__compressed_pair_elem<WebCore::PaintRenderingContext2D *, 0, false>
+144 <  8>                 WebCore::PaintRenderingContext2D * __value_
+144 <  1>             std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::PaintRenderingContext2D>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::PaintRenderingContext2D>, 1, true>
+144 <  1>                 std::__1::default_delete<WebCore::PaintRenderingContext2D> std::__1::default_delete<WebCore::PaintRenderingContext2D>
+152 <  8>     WTF::RefPtr<WebCore::Image, WTF::RawPtrTraits<WebCore::Image>, WTF::DefaultRefDerefTraits<WebCore::Image> > m_copiedImage
+152 <  8>         WTF::RawPtrTraits<WebCore::Image>::StorageType m_ptr
+160 <  8>     std::__1::unique_ptr<WebCore::CSSParserContext, std::__1::default_delete<WebCore::CSSParserContext> > m_cssParserContext
+160 <  8>         std::__1::__compressed_pair<WebCore::CSSParserContext *, std::__1::default_delete<WebCore::CSSParserContext> > __ptr_
+160 <  8>             std::__1::__compressed_pair_elem<WebCore::CSSParserContext *, 0, false> std::__1::__compressed_pair_elem<WebCore::CSSParserContext *, 0, false>
+160 <  8>                 WebCore::CSSParserContext * __value_
+160 <  1>             std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CSSParserContext>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CSSParserContext>, 1, true>
+160 <  1>                 std::__1::default_delete<WebCore::CSSParserContext> std::__1::default_delete<WebCore::CSSParserContext>
Total byte size: 168
Total pad bytes: 134
Padding percentage: 79.76 %

$ xcrun python3 ./Tools/Scripts/dump-class-layout -b WebKitBuild -c Release -a arm64e WebCore WebCore::HTMLCanvasElement
  +0 <304> WebCore::HTMLCanvasElement
  +0 <120>     WebCore::HTMLElement WebCore::HTMLElement
  +0 <120>         WebCore::StyledElement WebCore::StyledElement
  +0 <120>             WebCore::Element WebCore::Element
  +0 <104>                 WebCore::ContainerNode WebCore::ContainerNode
  +0 < 88>                     WebCore::Node WebCore::Node
  +0 < 24>                         WebCore::EventTarget WebCore::EventTarget
  +0 <  8>                             __vtbl_ptr_type * _vptr
  +8 <  8>                             WebCore::ScriptWrappable WebCore::ScriptWrappable
  +8 <  8>                                 JSC::Weak<WebCore::JSDOMObject> m_wrapper
  +8 <  8>                                     JSC::WeakImpl * m_impl
 +16 <  8>                             WTF::CanMakeWeakPtrWithBitField<EventTarget, WeakPtrFactoryInitialization::Lazy, WeakPtrImplWithEventTargetData> WTF::CanMakeWeakPtrWithBitField<EventTarget, WeakPtrFactoryInitialization::Lazy, WeakPtrImplWithEventTargetData>
 +16 <  8>                                 WTF::WeakPtrFactoryWithBitField<WebCore::EventTarget, WebCore::WeakPtrImplWithEventTargetData> m_weakPtrFactory
 +16 <  8>                                     WTF::CompactRefPtrTuple<WebCore::WeakPtrImplWithEventTargetData, unsigned short> m_impl
 +16 <  8>                                         WTF::CompactPointerTuple<WebCore::WeakPtrImplWithEventTargetData *, unsigned short> m_data
 +16 <  8>                                             uint64_t m_data
 +24 <  4>                         WTF::CanMakeCheckedPtr<WebCore::Node, WTF::DefaultedOperatorEqual::No> WTF::CanMakeCheckedPtr<WebCore::Node, WTF::DefaultedOperatorEqual::No>
 +24 <  4>                             WTF::CanMakeCheckedPtrBase<WTF::SingleThreadIntegralWrapper<unsigned int>, unsigned int, WTF::DefaultedOperatorEqual::No> WTF::CanMakeCheckedPtrBase<WTF::SingleThreadIntegralWrapper<unsigned int>, unsigned int, WTF::DefaultedOperatorEqual::No>
 +24 <  4>                                 WTF::SingleThreadIntegralWrapper<unsigned int> m_checkedPtrCount
 +24 <  4>                                     unsigned int m_value
 +28 <  4>                         uint32_t m_refCountAndParentBit
 +32 <  2>                         uint16_t m_typeBitFields
 +34 <  2>                         WTF::OptionSet<WebCore::Node::StateFlag> m_stateFlags
 +34 <  2>                             WTF::OptionSet<WebCore::Node::StateFlag>::StorageType m_storage
 +36 <  2>                         WTF::OptionSet<WebCore::Node::ElementStateFlag> m_elementStateFlags
 +36 <  2>                             WTF::OptionSet<WebCore::Node::ElementStateFlag>::StorageType m_storage
 +38 <  2>                         WebCore::Node::StyleBitfields m_styleBitfields
 +38 < :3>                             uint16_t m_styleValidity : 3
 +38 < :13>                             uint16_t m_flags : 13
 +40 <  8>                         WTF::CheckedPtr<WebCore::ContainerNode, WTF::RawPtrTraits<WebCore::ContainerNode> > m_parentNode
 +40 <  8>                             WTF::RawPtrTraits<WebCore::ContainerNode>::StorageType m_ptr
 +48 <  8>                         WebCore::TreeScope * m_treeScope
 +56 <  8>                         WebCore::Node * m_previousSibling
 +64 <  8>                         WTF::CheckedPtr<WebCore::Node, WTF::RawPtrTraits<WebCore::Node> > m_next
 +64 <  8>                             WTF::RawPtrTraits<WebCore::Node>::StorageType m_ptr
 +72 <  8>                         WebCore::RenderObject * m_renderer
 +80 <  8>                         WTF::CompactUniquePtrTuple<WebCore::NodeRareData, unsigned short, std::__1::default_delete<WebCore::NodeRareData> > m_rareDataWithBitfields
 +80 <  8>                             WTF::CompactPointerTuple<WebCore::NodeRareData *, unsigned short> m_data
 +80 <  8>                                 uint64_t m_data
 +88 <  8>                     WTF::CheckedPtr<WebCore::Node, WTF::RawPtrTraits<WebCore::Node> > m_firstChild
 +88 <  8>                         WTF::RawPtrTraits<WebCore::Node>::StorageType m_ptr
 +96 <  8>                     WTF::CheckedPtr<WebCore::Node, WTF::RawPtrTraits<WebCore::Node> > m_lastChild
 +96 <  8>                         WTF::RawPtrTraits<WebCore::Node>::StorageType m_ptr
+104 <  8>                 WebCore::QualifiedName m_tagName
+104 <  8>                     WTF::RefPtr<WebCore::QualifiedName::QualifiedNameImpl, WTF::RawPtrTraits<WebCore::QualifiedName::QualifiedNameImpl>, WTF::DefaultRefDerefTraits<WebCore::QualifiedName::QualifiedNameImpl> > m_impl
+104 <  8>                         WTF::RawPtrTraits<WebCore::QualifiedName::QualifiedNameImpl>::StorageType m_ptr
+112 <  8>                 WTF::RefPtr<WebCore::ElementData, WTF::RawPtrTraits<WebCore::ElementData>, WTF::DefaultRefDerefTraits<WebCore::ElementData> > m_elementData
+112 <  8>                     WTF::RawPtrTraits<WebCore::ElementData>::StorageType m_ptr
+120 <120>     WebCore::CanvasBase WebCore::CanvasBase
+120 < 16>         WTF::AbstractRefCountedAndCanMakeWeakPtr<WebCore::CanvasBase> WTF::AbstractRefCountedAndCanMakeWeakPtr<WebCore::CanvasBase>
+120 <  8>             WTF::AbstractRefCounted WTF::AbstractRefCounted
+120 <  8>                 __vtbl_ptr_type * _vptr
+128 <  8>             WTF::CanMakeWeakPtr<WebCore::CanvasBase> WTF::CanMakeWeakPtr<WebCore::CanvasBase>
+128 <  8>                 WTF::WeakPtrFactory<WebCore::CanvasBase, WTF::DefaultWeakPtrImpl> m_weakPtrFactory
+128 <  8>                     WTF::RefPtr<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > m_impl
+128 <  8>                         WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>::StorageType m_ptr
+136 <  8>         WebCore::IntSize m_size
+136 <  4>             int m_width
+140 <  4>             int m_height
+144 <  8>         WTF::RefPtr<WebCore::ImageBuffer, WTF::RawPtrTraits<WebCore::ImageBuffer>, WTF::DefaultRefDerefTraits<WebCore::ImageBuffer> > m_imageBuffer
+144 <  8>             WTF::RawPtrTraits<WebCore::ImageBuffer>::StorageType m_ptr
+152 <  8>         std::__1::atomic<unsigned long> m_imageBufferMemoryCost
+152 <  8>             std::__1::__atomic_base<unsigned long, true> std::__1::__atomic_base<unsigned long, true>
+152 <  8>                 std::__1::__atomic_base<unsigned long, false> std::__1::__atomic_base<unsigned long, false>
+152 <  8>                     std::__1::__cxx_atomic_impl<unsigned long, std::__1::__cxx_atomic_base_impl<unsigned long> > __a_
+152 <  8>                         std::__1::__cxx_atomic_base_impl<unsigned long> std::__1::__cxx_atomic_base_impl<unsigned long>
+152 <  8>                             _Atomic(unsigned long) __a_value
+160 <  8>         std::__1::unique_ptr<WebCore::GraphicsContextStateSaver, std::__1::default_delete<WebCore::GraphicsContextStateSaver> > m_contextStateSaver
+160 <  8>             std::__1::__compressed_pair<WebCore::GraphicsContextStateSaver *, std::__1::default_delete<WebCore::GraphicsContextStateSaver> > __ptr_
+160 <  8>                 std::__1::__compressed_pair_elem<WebCore::GraphicsContextStateSaver *, 0, false> std::__1::__compressed_pair_elem<WebCore::GraphicsContextStateSaver *, 0, false>
+160 <  8>                     WebCore::GraphicsContextStateSaver * __value_
+160 <  1>                 std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::GraphicsContextStateSaver>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::GraphicsContextStateSaver>, 1, true>
+160 <  1>                     std::__1::default_delete<WebCore::GraphicsContextStateSaver> std::__1::default_delete<WebCore::GraphicsContextStateSaver>
+168 <  8>         WTF::String m_lastFillText
+168 <  8>             WTF::RefPtr<WTF::StringImpl, WTF::RawPtrTraits<WTF::StringImpl>, WTF::DefaultRefDerefTraits<WTF::StringImpl> > m_impl
+168 <  8>                 WTF::RawPtrTraits<WTF::StringImpl>::StorageType m_ptr
+176 < 16>         WebCore::CanvasNoiseInjection m_canvasNoiseInjection
+176 < 16>             WebCore::IntRect m_postProcessDirtyRect
+176 <  8>                 WebCore::IntPoint m_location
+176 <  4>                     int m_x
+180 <  4>                     int m_y
+184 <  8>                 WebCore::IntSize m_size
+184 <  4>                     int m_width
+188 <  4>                     int m_height
+192 <  8>         WTF::Markable<unsigned long long, WTF::IntegralMarkableTraits<unsigned long long, 9223372036854775807> > m_canvasNoiseHashSalt
+192 <  8>             unsigned long long m_value
+200 <  1>         bool m_originClean
+201 <  1>         bool m_hasCreatedImageBuffer
+202 <  6>         <PADDING: 6 bytes>
+208 < 16>         WTF::WeakHashSet<WebCore::CanvasObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes> m_observers
+208 <  8>             WTF::WeakHashSet<WebCore::CanvasObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes>::WeakPtrImplSet m_set
+208 <  8>                 WTF::HashSet<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTableTraits, 255>::HashTableType m_impl
+208 <  8>                     WTF::HashTable<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::IdentityExtractor, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, 255>::(anonymous union) None
+216 <  4>             unsigned int m_operationCountSinceLastCleanup
+220 <  4>             unsigned int m_maxOperationCountWithoutCleanup
+224 < 16>         WTF::WeakHashSet<WebCore::CanvasDisplayBufferObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes> m_displayBufferObservers
+224 <  8>             WTF::WeakHashSet<WebCore::CanvasDisplayBufferObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes>::WeakPtrImplSet m_set
+224 <  8>                 WTF::HashSet<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTableTraits, 255>::HashTableType m_impl
+224 <  8>                     WTF::HashTable<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::IdentityExtractor, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, 255>::(anonymous union) None
+232 <  4>             unsigned int m_operationCountSinceLastCleanup
+236 <  4>             unsigned int m_maxOperationCountWithoutCleanup
+240 < 32>     WebCore::ActiveDOMObject WebCore::ActiveDOMObject
+240 <  8>         WTF::AbstractRefCounted WTF::AbstractRefCounted
+240 <  8>             __vtbl_ptr_type * _vptr
+248 < 16>         WebCore::ContextDestructionObserver WebCore::ContextDestructionObserver
+248 <  8>             __vtbl_ptr_type * _vptr
+256 <  8>             WTF::WeakPtr<WebCore::ScriptExecutionContext, WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl> > m_scriptExecutionContext
+256 <  8>                 WTF::RefPtr<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > m_impl
+256 <  8>                     WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>::StorageType m_ptr
+264 <  8>         uint64_t m_pendingActivityInstanceCount
+272 <  8>     std::__1::unique_ptr<WebCore::CanvasRenderingContext, std::__1::default_delete<WebCore::CanvasRenderingContext> > m_context
+272 <  8>         std::__1::__compressed_pair<WebCore::CanvasRenderingContext *, std::__1::default_delete<WebCore::CanvasRenderingContext> > __ptr_
+272 <  8>             std::__1::__compressed_pair_elem<WebCore::CanvasRenderingContext *, 0, false> std::__1::__compressed_pair_elem<WebCore::CanvasRenderingContext *, 0, false>
+272 <  8>                 WebCore::CanvasRenderingContext * __value_
+272 <  1>             std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CanvasRenderingContext>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CanvasRenderingContext>, 1, true>
+272 <  1>                 std::__1::default_delete<WebCore::CanvasRenderingContext> std::__1::default_delete<WebCore::CanvasRenderingContext>
+280 <  8>     WTF::RefPtr<WebCore::Image, WTF::RawPtrTraits<WebCore::Image>, WTF::DefaultRefDerefTraits<WebCore::Image> > m_copiedImage
+280 <  8>         WTF::RawPtrTraits<WebCore::Image>::StorageType m_ptr
+288 <  8>     std::__1::unique_ptr<WebCore::CSSParserContext, std::__1::default_delete<WebCore::CSSParserContext> > m_cssParserContext
+288 <  8>         std::__1::__compressed_pair<WebCore::CSSParserContext *, std::__1::default_delete<WebCore::CSSParserContext> > __ptr_
+288 <  8>             std::__1::__compressed_pair_elem<WebCore::CSSParserContext *, 0, false> std::__1::__compressed_pair_elem<WebCore::CSSParserContext *, 0, false>
+288 <  8>                 WebCore::CSSParserContext * __value_
+288 <  1>             std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CSSParserContext>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CSSParserContext>, 1, true>
+288 <  1>                 std::__1::default_delete<WebCore::CSSParserContext> std::__1::default_delete<WebCore::CSSParserContext>
+296 <  1>     bool m_ignoreReset
+297 <  1>     bool m_didClearImageBuffer
+298 <  1>     bool m_hasRelevantWebGLEventListener
+299 <  1>     bool m_isSnapshotting
+300 <  4>     <PADDING: 4 bytes>
Total byte size: 304
Total pad bytes: 10
Padding percentage: 3.29 %

[David Kilzer (:ddkilzer)]

After:

$ xcrun python3 ./Tools/Scripts/dump-class-layout -b WebKitBuild -c Release -a arm64e WebCore WebCore::CanvasBase
  +0 <120> WebCore::CanvasBase
  +0 < 16>     WTF::AbstractRefCountedAndCanMakeWeakPtr<WebCore::CanvasBase> WTF::AbstractRefCountedAndCanMakeWeakPtr<WebCore::CanvasBase>
  +0 <  8>         WTF::AbstractRefCounted WTF::AbstractRefCounted
  +0 <  8>             __vtbl_ptr_type * _vptr
  +8 <  8>         WTF::CanMakeWeakPtr<WebCore::CanvasBase> WTF::CanMakeWeakPtr<WebCore::CanvasBase>
  +8 <  8>             WTF::WeakPtrFactory<WebCore::CanvasBase, WTF::DefaultWeakPtrImpl> m_weakPtrFactory
  +8 <  8>                 WTF::RefPtr<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > m_impl
  +8 <  8>                     WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>::StorageType m_ptr
 +16 <  8>     WebCore::IntSize m_size
 +16 <  4>         int m_width
 +20 <  4>         int m_height
 +24 <  8>     WTF::RefPtr<WebCore::ImageBuffer, WTF::RawPtrTraits<WebCore::ImageBuffer>, WTF::DefaultRefDerefTraits<WebCore::ImageBuffer> > m_imageBuffer
 +24 <  8>         WTF::RawPtrTraits<WebCore::ImageBuffer>::StorageType m_ptr
 +32 <  8>     std::__1::atomic<unsigned long> m_imageBufferMemoryCost
 +32 <  8>         std::__1::__atomic_base<unsigned long, true> std::__1::__atomic_base<unsigned long, true>
 +32 <  8>             std::__1::__atomic_base<unsigned long, false> std::__1::__atomic_base<unsigned long, false>
 +32 <  8>                 std::__1::__cxx_atomic_impl<unsigned long, std::__1::__cxx_atomic_base_impl<unsigned long> > __a_
 +32 <  8>                     std::__1::__cxx_atomic_base_impl<unsigned long> std::__1::__cxx_atomic_base_impl<unsigned long>
 +32 <  8>                         _Atomic(unsigned long) __a_value
 +40 <  8>     std::__1::unique_ptr<WebCore::GraphicsContextStateSaver, std::__1::default_delete<WebCore::GraphicsContextStateSaver> > m_contextStateSaver
 +40 <  8>         std::__1::__compressed_pair<WebCore::GraphicsContextStateSaver *, std::__1::default_delete<WebCore::GraphicsContextStateSaver> > __ptr_
 +40 <  8>             std::__1::__compressed_pair_elem<WebCore::GraphicsContextStateSaver *, 0, false> std::__1::__compressed_pair_elem<WebCore::GraphicsContextStateSaver *, 0, false>
 +40 <  8>                 WebCore::GraphicsContextStateSaver * __value_
 +40 <  1>             std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::GraphicsContextStateSaver>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::GraphicsContextStateSaver>, 1, true>
 +40 <  1>                 std::__1::default_delete<WebCore::GraphicsContextStateSaver> std::__1::default_delete<WebCore::GraphicsContextStateSaver>
 +48 <  8>     WTF::String m_lastFillText
 +48 <  8>         WTF::RefPtr<WTF::StringImpl, WTF::RawPtrTraits<WTF::StringImpl>, WTF::DefaultRefDerefTraits<WTF::StringImpl> > m_impl
 +48 <  8>             WTF::RawPtrTraits<WTF::StringImpl>::StorageType m_ptr
 +56 < 16>     WTF::WeakHashSet<WebCore::CanvasObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes> m_observers
 +56 <  8>         WTF::WeakHashSet<WebCore::CanvasObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes>::WeakPtrImplSet m_set
 +56 <  8>             WTF::HashSet<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTableTraits, 255>::HashTableType m_impl
 +56 <  8>                 WTF::HashTable<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::IdentityExtractor, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, 255>::(anonymous union) None
 +64 <  4>         unsigned int m_operationCountSinceLastCleanup
 +68 <  4>         unsigned int m_maxOperationCountWithoutCleanup
 +72 < 16>     WTF::WeakHashSet<WebCore::CanvasDisplayBufferObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes> m_displayBufferObservers
 +72 <  8>         WTF::WeakHashSet<WebCore::CanvasDisplayBufferObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes>::WeakPtrImplSet m_set
 +72 <  8>             WTF::HashSet<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTableTraits, 255>::HashTableType m_impl
 +72 <  8>                 WTF::HashTable<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::IdentityExtractor, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, 255>::(anonymous union) None
 +80 <  4>         unsigned int m_operationCountSinceLastCleanup
 +84 <  4>         unsigned int m_maxOperationCountWithoutCleanup
 +88 < 16>     WebCore::CanvasNoiseInjection m_canvasNoiseInjection
 +88 < 16>         WebCore::IntRect m_postProcessDirtyRect
 +88 <  8>             WebCore::IntPoint m_location
 +88 <  4>                 int m_x
 +92 <  4>                 int m_y
 +96 <  8>             WebCore::IntSize m_size
 +96 <  4>                 int m_width
+100 <  4>                 int m_height
+104 <  8>     WTF::Markable<unsigned long long, WTF::IntegralMarkableTraits<unsigned long long, 9223372036854775807> > m_canvasNoiseHashSalt
+104 <  8>         unsigned long long m_value
+112 <  1>     bool m_originClean
+113 <  1>     bool m_hasCreatedImageBuffer
+114 <  6>     <PADDING: 6 bytes>
Total byte size: 120
Total pad bytes: 6
Padding percentage: 5.00 %

$ xcrun python3 ./Tools/Scripts/dump-class-layout -b WebKitBuild -c Release -a arm64e WebCore WebCore::OffscreenCanvas
  +0 <216> WebCore::OffscreenCanvas
  +0 < 32>     WebCore::ActiveDOMObject WebCore::ActiveDOMObject
  +0 <  8>         WTF::AbstractRefCounted WTF::AbstractRefCounted
  +0 <  8>             __vtbl_ptr_type * _vptr
  +8 < 16>         WebCore::ContextDestructionObserver WebCore::ContextDestructionObserver
  +8 <  8>             __vtbl_ptr_type * _vptr
 +16 <  8>             WTF::WeakPtr<WebCore::ScriptExecutionContext, WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl> > m_scriptExecutionContext
 +16 <  8>                 WTF::RefPtr<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > m_impl
 +16 <  8>                     WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>::StorageType m_ptr
 +24 <  8>         uint64_t m_pendingActivityInstanceCount
 +32 <120>     WebCore::CanvasBase WebCore::CanvasBase
 +32 < 16>         WTF::AbstractRefCountedAndCanMakeWeakPtr<WebCore::CanvasBase> WTF::AbstractRefCountedAndCanMakeWeakPtr<WebCore::CanvasBase>
 +32 <  8>             WTF::AbstractRefCounted WTF::AbstractRefCounted
 +32 <  8>                 __vtbl_ptr_type * _vptr
 +40 <  8>             WTF::CanMakeWeakPtr<WebCore::CanvasBase> WTF::CanMakeWeakPtr<WebCore::CanvasBase>
 +40 <  8>                 WTF::WeakPtrFactory<WebCore::CanvasBase, WTF::DefaultWeakPtrImpl> m_weakPtrFactory
 +40 <  8>                     WTF::RefPtr<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > m_impl
 +40 <  8>                         WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>::StorageType m_ptr
 +48 <  8>         WebCore::IntSize m_size
 +48 <  4>             int m_width
 +52 <  4>             int m_height
 +56 <  8>         WTF::RefPtr<WebCore::ImageBuffer, WTF::RawPtrTraits<WebCore::ImageBuffer>, WTF::DefaultRefDerefTraits<WebCore::ImageBuffer> > m_imageBuffer
 +56 <  8>             WTF::RawPtrTraits<WebCore::ImageBuffer>::StorageType m_ptr
 +64 <  8>         std::__1::atomic<unsigned long> m_imageBufferMemoryCost
 +64 <  8>             std::__1::__atomic_base<unsigned long, true> std::__1::__atomic_base<unsigned long, true>
 +64 <  8>                 std::__1::__atomic_base<unsigned long, false> std::__1::__atomic_base<unsigned long, false>
 +64 <  8>                     std::__1::__cxx_atomic_impl<unsigned long, std::__1::__cxx_atomic_base_impl<unsigned long> > __a_
 +64 <  8>                         std::__1::__cxx_atomic_base_impl<unsigned long> std::__1::__cxx_atomic_base_impl<unsigned long>
 +64 <  8>                             _Atomic(unsigned long) __a_value
 +72 <  8>         std::__1::unique_ptr<WebCore::GraphicsContextStateSaver, std::__1::default_delete<WebCore::GraphicsContextStateSaver> > m_contextStateSaver
 +72 <  8>             std::__1::__compressed_pair<WebCore::GraphicsContextStateSaver *, std::__1::default_delete<WebCore::GraphicsContextStateSaver> > __ptr_
 +72 <  8>                 std::__1::__compressed_pair_elem<WebCore::GraphicsContextStateSaver *, 0, false> std::__1::__compressed_pair_elem<WebCore::GraphicsContextStateSaver *, 0, false>
 +72 <  8>                     WebCore::GraphicsContextStateSaver * __value_
 +72 <  1>                 std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::GraphicsContextStateSaver>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::GraphicsContextStateSaver>, 1, true>
 +72 <  1>                     std::__1::default_delete<WebCore::GraphicsContextStateSaver> std::__1::default_delete<WebCore::GraphicsContextStateSaver>
 +80 <  8>         WTF::String m_lastFillText
 +80 <  8>             WTF::RefPtr<WTF::StringImpl, WTF::RawPtrTraits<WTF::StringImpl>, WTF::DefaultRefDerefTraits<WTF::StringImpl> > m_impl
 +80 <  8>                 WTF::RawPtrTraits<WTF::StringImpl>::StorageType m_ptr
 +88 < 16>         WTF::WeakHashSet<WebCore::CanvasObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes> m_observers
 +88 <  8>             WTF::WeakHashSet<WebCore::CanvasObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes>::WeakPtrImplSet m_set
 +88 <  8>                 WTF::HashSet<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTableTraits, 255>::HashTableType m_impl
 +88 <  8>                     WTF::HashTable<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::IdentityExtractor, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, 255>::(anonymous union) None
 +96 <  4>             unsigned int m_operationCountSinceLastCleanup
+100 <  4>             unsigned int m_maxOperationCountWithoutCleanup
+104 < 16>         WTF::WeakHashSet<WebCore::CanvasDisplayBufferObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes> m_displayBufferObservers
+104 <  8>             WTF::WeakHashSet<WebCore::CanvasDisplayBufferObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes>::WeakPtrImplSet m_set
+104 <  8>                 WTF::HashSet<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTableTraits, 255>::HashTableType m_impl
+104 <  8>                     WTF::HashTable<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::IdentityExtractor, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, 255>::(anonymous union) None
+112 <  4>             unsigned int m_operationCountSinceLastCleanup
+116 <  4>             unsigned int m_maxOperationCountWithoutCleanup
+120 < 16>         WebCore::CanvasNoiseInjection m_canvasNoiseInjection
+120 < 16>             WebCore::IntRect m_postProcessDirtyRect
+120 <  8>                 WebCore::IntPoint m_location
+120 <  4>                     int m_x
+124 <  4>                     int m_y
+128 <  8>                 WebCore::IntSize m_size
+128 <  4>                     int m_width
+132 <  4>                     int m_height
+136 <  8>         WTF::Markable<unsigned long long, WTF::IntegralMarkableTraits<unsigned long long, 9223372036854775807> > m_canvasNoiseHashSalt
+136 <  8>             unsigned long long m_value
+144 <  1>         bool m_originClean
+145 <  1>         bool m_hasCreatedImageBuffer
+146 <  2>     <PADDING: 2 bytes>
+148 <  4>     WTF::RefCounted<WebCore::OffscreenCanvas> WTF::RefCounted<WebCore::OffscreenCanvas>
+148 <  4>         WTF::RefCountedBase WTF::RefCountedBase
+148 <  4>             unsigned int m_refCount
+152 < 24>     WebCore::EventTarget WebCore::EventTarget
+152 <  8>         __vtbl_ptr_type * _vptr
+160 <  8>         WebCore::ScriptWrappable WebCore::ScriptWrappable
+160 <  8>             JSC::Weak<WebCore::JSDOMObject> m_wrapper
+160 <  8>                 JSC::WeakImpl * m_impl
+168 <  8>         WTF::CanMakeWeakPtrWithBitField<EventTarget, WeakPtrFactoryInitialization::Lazy, WeakPtrImplWithEventTargetData> WTF::CanMakeWeakPtrWithBitField<EventTarget, WeakPtrFactoryInitialization::Lazy, WeakPtrImplWithEventTargetData>
+168 <  8>             WTF::WeakPtrFactoryWithBitField<WebCore::EventTarget, WebCore::WeakPtrImplWithEventTargetData> m_weakPtrFactory
+168 <  8>                 WTF::CompactRefPtrTuple<WebCore::WeakPtrImplWithEventTargetData, unsigned short> m_impl
+168 <  8>                     WTF::CompactPointerTuple<WebCore::WeakPtrImplWithEventTargetData *, unsigned short> m_data
+168 <  8>                         uint64_t m_data
+176 <  1>     bool m_detached
+177 <  1>     bool m_hasScheduledCommit
+178 <  6>     <PADDING: 6 bytes>
+184 <  8>     std::__1::unique_ptr<WebCore::CanvasRenderingContext, std::__1::default_delete<WebCore::CanvasRenderingContext> > m_context
+184 <  8>         std::__1::__compressed_pair<WebCore::CanvasRenderingContext *, std::__1::default_delete<WebCore::CanvasRenderingContext> > __ptr_
+184 <  8>             std::__1::__compressed_pair_elem<WebCore::CanvasRenderingContext *, 0, false> std::__1::__compressed_pair_elem<WebCore::CanvasRenderingContext *, 0, false>
+184 <  8>                 WebCore::CanvasRenderingContext * __value_
+184 <  1>             std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CanvasRenderingContext>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CanvasRenderingContext>, 1, true>
+184 <  1>                 std::__1::default_delete<WebCore::CanvasRenderingContext> std::__1::default_delete<WebCore::CanvasRenderingContext>
+192 <  8>     WTF::RefPtr<WebCore::PlaceholderRenderingContextSource, WTF::RawPtrTraits<WebCore::PlaceholderRenderingContextSource>, WTF::DefaultRefDerefTraits<WebCore::PlaceholderRenderingContextSource> > m_placeholderSource
+192 <  8>         WTF::RawPtrTraits<WebCore::PlaceholderRenderingContextSource>::StorageType m_ptr
+200 <  8>     WTF::RefPtr<WebCore::Image, WTF::RawPtrTraits<WebCore::Image>, WTF::DefaultRefDerefTraits<WebCore::Image> > m_copiedImage
+200 <  8>         WTF::RawPtrTraits<WebCore::Image>::StorageType m_ptr
+208 <  8>     std::__1::unique_ptr<WebCore::CSSParserContext, std::__1::default_delete<WebCore::CSSParserContext> > m_cssParserContext
+208 <  8>         std::__1::__compressed_pair<WebCore::CSSParserContext *, std::__1::default_delete<WebCore::CSSParserContext> > __ptr_
+208 <  8>             std::__1::__compressed_pair_elem<WebCore::CSSParserContext *, 0, false> std::__1::__compressed_pair_elem<WebCore::CSSParserContext *, 0, false>
+208 <  8>                 WebCore::CSSParserContext * __value_
+208 <  1>             std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CSSParserContext>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CSSParserContext>, 1, true>
+208 <  1>                 std::__1::default_delete<WebCore::CSSParserContext> std::__1::default_delete<WebCore::CSSParserContext>
Total byte size: 216
Total pad bytes: 8
Padding percentage: 3.70 %

$ xcrun python3 ./Tools/Scripts/dump-class-layout -b WebKitBuild -c Release -a arm64e WebCore WebCore::CustomPaintCanvas
  +0 <160> WebCore::CustomPaintCanvas
  +0 <120>     WebCore::CanvasBase WebCore::CanvasBase
  +0 < 16>         WTF::AbstractRefCountedAndCanMakeWeakPtr<WebCore::CanvasBase> WTF::AbstractRefCountedAndCanMakeWeakPtr<WebCore::CanvasBase>
  +0 <  8>             WTF::AbstractRefCounted WTF::AbstractRefCounted
  +0 <  8>                 __vtbl_ptr_type * _vptr
  +8 <  8>             WTF::CanMakeWeakPtr<WebCore::CanvasBase> WTF::CanMakeWeakPtr<WebCore::CanvasBase>
  +8 <  8>                 WTF::WeakPtrFactory<WebCore::CanvasBase, WTF::DefaultWeakPtrImpl> m_weakPtrFactory
  +8 <  8>                     WTF::RefPtr<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > m_impl
  +8 <  8>                         WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>::StorageType m_ptr
 +16 <  8>         WebCore::IntSize m_size
 +16 <  4>             int m_width
 +20 <  4>             int m_height
 +24 <  8>         WTF::RefPtr<WebCore::ImageBuffer, WTF::RawPtrTraits<WebCore::ImageBuffer>, WTF::DefaultRefDerefTraits<WebCore::ImageBuffer> > m_imageBuffer
 +24 <  8>             WTF::RawPtrTraits<WebCore::ImageBuffer>::StorageType m_ptr
 +32 <  8>         std::__1::atomic<unsigned long> m_imageBufferMemoryCost
 +32 <  8>             std::__1::__atomic_base<unsigned long, true> std::__1::__atomic_base<unsigned long, true>
 +32 <  8>                 std::__1::__atomic_base<unsigned long, false> std::__1::__atomic_base<unsigned long, false>
 +32 <  8>                     std::__1::__cxx_atomic_impl<unsigned long, std::__1::__cxx_atomic_base_impl<unsigned long> > __a_
 +32 <  8>                         std::__1::__cxx_atomic_base_impl<unsigned long> std::__1::__cxx_atomic_base_impl<unsigned long>
 +32 <  8>                             _Atomic(unsigned long) __a_value
 +40 <  8>         std::__1::unique_ptr<WebCore::GraphicsContextStateSaver, std::__1::default_delete<WebCore::GraphicsContextStateSaver> > m_contextStateSaver
 +40 <  8>             std::__1::__compressed_pair<WebCore::GraphicsContextStateSaver *, std::__1::default_delete<WebCore::GraphicsContextStateSaver> > __ptr_
 +40 <  8>                 std::__1::__compressed_pair_elem<WebCore::GraphicsContextStateSaver *, 0, false> std::__1::__compressed_pair_elem<WebCore::GraphicsContextStateSaver *, 0, false>
 +40 <  8>                     WebCore::GraphicsContextStateSaver * __value_
 +40 <  1>                 std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::GraphicsContextStateSaver>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::GraphicsContextStateSaver>, 1, true>
 +40 <  1>                     std::__1::default_delete<WebCore::GraphicsContextStateSaver> std::__1::default_delete<WebCore::GraphicsContextStateSaver>
 +48 <  8>         WTF::String m_lastFillText
 +48 <  8>             WTF::RefPtr<WTF::StringImpl, WTF::RawPtrTraits<WTF::StringImpl>, WTF::DefaultRefDerefTraits<WTF::StringImpl> > m_impl
 +48 <  8>                 WTF::RawPtrTraits<WTF::StringImpl>::StorageType m_ptr
 +56 < 16>         WTF::WeakHashSet<WebCore::CanvasObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes> m_observers
 +56 <  8>             WTF::WeakHashSet<WebCore::CanvasObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes>::WeakPtrImplSet m_set
 +56 <  8>                 WTF::HashSet<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTableTraits, 255>::HashTableType m_impl
 +56 <  8>                     WTF::HashTable<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::IdentityExtractor, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, 255>::(anonymous union) None
 +64 <  4>             unsigned int m_operationCountSinceLastCleanup
 +68 <  4>             unsigned int m_maxOperationCountWithoutCleanup
 +72 < 16>         WTF::WeakHashSet<WebCore::CanvasDisplayBufferObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes> m_displayBufferObservers
 +72 <  8>             WTF::WeakHashSet<WebCore::CanvasDisplayBufferObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes>::WeakPtrImplSet m_set
 +72 <  8>                 WTF::HashSet<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTableTraits, 255>::HashTableType m_impl
 +72 <  8>                     WTF::HashTable<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::IdentityExtractor, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, 255>::(anonymous union) None
 +80 <  4>             unsigned int m_operationCountSinceLastCleanup
 +84 <  4>             unsigned int m_maxOperationCountWithoutCleanup
 +88 < 16>         WebCore::CanvasNoiseInjection m_canvasNoiseInjection
 +88 < 16>             WebCore::IntRect m_postProcessDirtyRect
 +88 <  8>                 WebCore::IntPoint m_location
 +88 <  4>                     int m_x
 +92 <  4>                     int m_y
 +96 <  8>                 WebCore::IntSize m_size
 +96 <  4>                     int m_width
+100 <  4>                     int m_height
+104 <  8>         WTF::Markable<unsigned long long, WTF::IntegralMarkableTraits<unsigned long long, 9223372036854775807> > m_canvasNoiseHashSalt
+104 <  8>             unsigned long long m_value
+112 <  1>         bool m_originClean
+113 <  1>         bool m_hasCreatedImageBuffer
+114 <  2>     <PADDING: 2 bytes>
+116 <  4>     WTF::RefCounted<WebCore::CustomPaintCanvas> WTF::RefCounted<WebCore::CustomPaintCanvas>
+116 <  4>         WTF::RefCountedBase WTF::RefCountedBase
+116 <  4>             unsigned int m_refCount
+120 < 16>     WebCore::ContextDestructionObserver WebCore::ContextDestructionObserver
+120 <  8>         __vtbl_ptr_type * _vptr
+128 <  8>         WTF::WeakPtr<WebCore::ScriptExecutionContext, WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl> > m_scriptExecutionContext
+128 <  8>             WTF::RefPtr<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > m_impl
+128 <  8>                 WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>::StorageType m_ptr
+136 <  8>     std::__1::unique_ptr<WebCore::PaintRenderingContext2D, std::__1::default_delete<WebCore::PaintRenderingContext2D> > m_context
+136 <  8>         std::__1::__compressed_pair<WebCore::PaintRenderingContext2D *, std::__1::default_delete<WebCore::PaintRenderingContext2D> > __ptr_
+136 <  8>             std::__1::__compressed_pair_elem<WebCore::PaintRenderingContext2D *, 0, false> std::__1::__compressed_pair_elem<WebCore::PaintRenderingContext2D *, 0, false>
+136 <  8>                 WebCore::PaintRenderingContext2D * __value_
+136 <  1>             std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::PaintRenderingContext2D>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::PaintRenderingContext2D>, 1, true>
+136 <  1>                 std::__1::default_delete<WebCore::PaintRenderingContext2D> std::__1::default_delete<WebCore::PaintRenderingContext2D>
+144 <  8>     WTF::RefPtr<WebCore::Image, WTF::RawPtrTraits<WebCore::Image>, WTF::DefaultRefDerefTraits<WebCore::Image> > m_copiedImage
+144 <  8>         WTF::RawPtrTraits<WebCore::Image>::StorageType m_ptr
+152 <  8>     std::__1::unique_ptr<WebCore::CSSParserContext, std::__1::default_delete<WebCore::CSSParserContext> > m_cssParserContext
+152 <  8>         std::__1::__compressed_pair<WebCore::CSSParserContext *, std::__1::default_delete<WebCore::CSSParserContext> > __ptr_
+152 <  8>             std::__1::__compressed_pair_elem<WebCore::CSSParserContext *, 0, false> std::__1::__compressed_pair_elem<WebCore::CSSParserContext *, 0, false>
+152 <  8>                 WebCore::CSSParserContext * __value_
+152 <  1>             std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CSSParserContext>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CSSParserContext>, 1, true>
+152 <  1>                 std::__1::default_delete<WebCore::CSSParserContext> std::__1::default_delete<WebCore::CSSParserContext>
Total byte size: 160
Total pad bytes: 2
Padding percentage: 1.25 %

$ xcrun python3 ./Tools/Scripts/dump-class-layout -b WebKitBuild -c Release -a arm64e WebCore WebCore::HTMLCanvasElement
  +0 <296> WebCore::HTMLCanvasElement
  +0 <120>     WebCore::HTMLElement WebCore::HTMLElement
  +0 <120>         WebCore::StyledElement WebCore::StyledElement
  +0 <120>             WebCore::Element WebCore::Element
  +0 <104>                 WebCore::ContainerNode WebCore::ContainerNode
  +0 < 88>                     WebCore::Node WebCore::Node
  +0 < 24>                         WebCore::EventTarget WebCore::EventTarget
  +0 <  8>                             __vtbl_ptr_type * _vptr
  +8 <  8>                             WebCore::ScriptWrappable WebCore::ScriptWrappable
  +8 <  8>                                 JSC::Weak<WebCore::JSDOMObject> m_wrapper
  +8 <  8>                                     JSC::WeakImpl * m_impl
 +16 <  8>                             WTF::CanMakeWeakPtrWithBitField<EventTarget, WeakPtrFactoryInitialization::Lazy, WeakPtrImplWithEventTargetData> WTF::CanMakeWeakPtrWithBitField<EventTarget, WeakPtrFactoryInitialization::Lazy, WeakPtrImplWithEventTargetData>
 +16 <  8>                                 WTF::WeakPtrFactoryWithBitField<WebCore::EventTarget, WebCore::WeakPtrImplWithEventTargetData> m_weakPtrFactory
 +16 <  8>                                     WTF::CompactRefPtrTuple<WebCore::WeakPtrImplWithEventTargetData, unsigned short> m_impl
 +16 <  8>                                         WTF::CompactPointerTuple<WebCore::WeakPtrImplWithEventTargetData *, unsigned short> m_data
 +16 <  8>                                             uint64_t m_data
 +24 <  4>                         WTF::CanMakeCheckedPtr<WebCore::Node, WTF::DefaultedOperatorEqual::No> WTF::CanMakeCheckedPtr<WebCore::Node, WTF::DefaultedOperatorEqual::No>
 +24 <  4>                             WTF::CanMakeCheckedPtrBase<WTF::SingleThreadIntegralWrapper<unsigned int>, unsigned int, WTF::DefaultedOperatorEqual::No> WTF::CanMakeCheckedPtrBase<WTF::SingleThreadIntegralWrapper<unsigned int>, unsigned int, WTF::DefaultedOperatorEqual::No>
 +24 <  4>                                 WTF::SingleThreadIntegralWrapper<unsigned int> m_checkedPtrCount
 +24 <  4>                                     unsigned int m_value
 +28 <  4>                         uint32_t m_refCountAndParentBit
 +32 <  2>                         uint16_t m_typeBitFields
 +34 <  2>                         WTF::OptionSet<WebCore::Node::StateFlag> m_stateFlags
 +34 <  2>                             WTF::OptionSet<WebCore::Node::StateFlag>::StorageType m_storage
 +36 <  2>                         WTF::OptionSet<WebCore::Node::ElementStateFlag> m_elementStateFlags
 +36 <  2>                             WTF::OptionSet<WebCore::Node::ElementStateFlag>::StorageType m_storage
 +38 <  2>                         WebCore::Node::StyleBitfields m_styleBitfields
 +38 < :3>                             uint16_t m_styleValidity : 3
 +38 < :13>                             uint16_t m_flags : 13
 +40 <  8>                         WTF::CheckedPtr<WebCore::ContainerNode, WTF::RawPtrTraits<WebCore::ContainerNode> > m_parentNode
 +40 <  8>                             WTF::RawPtrTraits<WebCore::ContainerNode>::StorageType m_ptr
 +48 <  8>                         WebCore::TreeScope * m_treeScope
 +56 <  8>                         WebCore::Node * m_previousSibling
 +64 <  8>                         WTF::CheckedPtr<WebCore::Node, WTF::RawPtrTraits<WebCore::Node> > m_next
 +64 <  8>                             WTF::RawPtrTraits<WebCore::Node>::StorageType m_ptr
 +72 <  8>                         WebCore::RenderObject * m_renderer
 +80 <  8>                         WTF::CompactUniquePtrTuple<WebCore::NodeRareData, unsigned short, std::__1::default_delete<WebCore::NodeRareData> > m_rareDataWithBitfields
 +80 <  8>                             WTF::CompactPointerTuple<WebCore::NodeRareData *, unsigned short> m_data
 +80 <  8>                                 uint64_t m_data
 +88 <  8>                     WTF::CheckedPtr<WebCore::Node, WTF::RawPtrTraits<WebCore::Node> > m_firstChild
 +88 <  8>                         WTF::RawPtrTraits<WebCore::Node>::StorageType m_ptr
 +96 <  8>                     WTF::CheckedPtr<WebCore::Node, WTF::RawPtrTraits<WebCore::Node> > m_lastChild
 +96 <  8>                         WTF::RawPtrTraits<WebCore::Node>::StorageType m_ptr
+104 <  8>                 WebCore::QualifiedName m_tagName
+104 <  8>                     WTF::RefPtr<WebCore::QualifiedName::QualifiedNameImpl, WTF::RawPtrTraits<WebCore::QualifiedName::QualifiedNameImpl>, WTF::DefaultRefDerefTraits<WebCore::QualifiedName::QualifiedNameImpl> > m_impl
+104 <  8>                         WTF::RawPtrTraits<WebCore::QualifiedName::QualifiedNameImpl>::StorageType m_ptr
+112 <  8>                 WTF::RefPtr<WebCore::ElementData, WTF::RawPtrTraits<WebCore::ElementData>, WTF::DefaultRefDerefTraits<WebCore::ElementData> > m_elementData
+112 <  8>                     WTF::RawPtrTraits<WebCore::ElementData>::StorageType m_ptr
+120 < 32>     WebCore::ActiveDOMObject WebCore::ActiveDOMObject
+120 <  8>         WTF::AbstractRefCounted WTF::AbstractRefCounted
+120 <  8>             __vtbl_ptr_type * _vptr
+128 < 16>         WebCore::ContextDestructionObserver WebCore::ContextDestructionObserver
+128 <  8>             __vtbl_ptr_type * _vptr
+136 <  8>             WTF::WeakPtr<WebCore::ScriptExecutionContext, WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl> > m_scriptExecutionContext
+136 <  8>                 WTF::RefPtr<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > m_impl
+136 <  8>                     WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>::StorageType m_ptr
+144 <  8>         uint64_t m_pendingActivityInstanceCount
+152 <120>     WebCore::CanvasBase WebCore::CanvasBase
+152 < 16>         WTF::AbstractRefCountedAndCanMakeWeakPtr<WebCore::CanvasBase> WTF::AbstractRefCountedAndCanMakeWeakPtr<WebCore::CanvasBase>
+152 <  8>             WTF::AbstractRefCounted WTF::AbstractRefCounted
+152 <  8>                 __vtbl_ptr_type * _vptr
+160 <  8>             WTF::CanMakeWeakPtr<WebCore::CanvasBase> WTF::CanMakeWeakPtr<WebCore::CanvasBase>
+160 <  8>                 WTF::WeakPtrFactory<WebCore::CanvasBase, WTF::DefaultWeakPtrImpl> m_weakPtrFactory
+160 <  8>                     WTF::RefPtr<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > m_impl
+160 <  8>                         WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>::StorageType m_ptr
+168 <  8>         WebCore::IntSize m_size
+168 <  4>             int m_width
+172 <  4>             int m_height
+176 <  8>         WTF::RefPtr<WebCore::ImageBuffer, WTF::RawPtrTraits<WebCore::ImageBuffer>, WTF::DefaultRefDerefTraits<WebCore::ImageBuffer> > m_imageBuffer
+176 <  8>             WTF::RawPtrTraits<WebCore::ImageBuffer>::StorageType m_ptr
+184 <  8>         std::__1::atomic<unsigned long> m_imageBufferMemoryCost
+184 <  8>             std::__1::__atomic_base<unsigned long, true> std::__1::__atomic_base<unsigned long, true>
+184 <  8>                 std::__1::__atomic_base<unsigned long, false> std::__1::__atomic_base<unsigned long, false>
+184 <  8>                     std::__1::__cxx_atomic_impl<unsigned long, std::__1::__cxx_atomic_base_impl<unsigned long> > __a_
+184 <  8>                         std::__1::__cxx_atomic_base_impl<unsigned long> std::__1::__cxx_atomic_base_impl<unsigned long>
+184 <  8>                             _Atomic(unsigned long) __a_value
+192 <  8>         std::__1::unique_ptr<WebCore::GraphicsContextStateSaver, std::__1::default_delete<WebCore::GraphicsContextStateSaver> > m_contextStateSaver
+192 <  8>             std::__1::__compressed_pair<WebCore::GraphicsContextStateSaver *, std::__1::default_delete<WebCore::GraphicsContextStateSaver> > __ptr_
+192 <  8>                 std::__1::__compressed_pair_elem<WebCore::GraphicsContextStateSaver *, 0, false> std::__1::__compressed_pair_elem<WebCore::GraphicsContextStateSaver *, 0, false>
+192 <  8>                     WebCore::GraphicsContextStateSaver * __value_
+192 <  1>                 std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::GraphicsContextStateSaver>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::GraphicsContextStateSaver>, 1, true>
+192 <  1>                     std::__1::default_delete<WebCore::GraphicsContextStateSaver> std::__1::default_delete<WebCore::GraphicsContextStateSaver>
+200 <  8>         WTF::String m_lastFillText
+200 <  8>             WTF::RefPtr<WTF::StringImpl, WTF::RawPtrTraits<WTF::StringImpl>, WTF::DefaultRefDerefTraits<WTF::StringImpl> > m_impl
+200 <  8>                 WTF::RawPtrTraits<WTF::StringImpl>::StorageType m_ptr
+208 < 16>         WTF::WeakHashSet<WebCore::CanvasObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes> m_observers
+208 <  8>             WTF::WeakHashSet<WebCore::CanvasObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes>::WeakPtrImplSet m_set
+208 <  8>                 WTF::HashSet<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTableTraits, 255>::HashTableType m_impl
+208 <  8>                     WTF::HashTable<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::IdentityExtractor, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, 255>::(anonymous union) None
+216 <  4>             unsigned int m_operationCountSinceLastCleanup
+220 <  4>             unsigned int m_maxOperationCountWithoutCleanup
+224 < 16>         WTF::WeakHashSet<WebCore::CanvasDisplayBufferObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes> m_displayBufferObservers
+224 <  8>             WTF::WeakHashSet<WebCore::CanvasDisplayBufferObserver, WTF::DefaultWeakPtrImpl, WTF::EnableWeakPtrThreadingAssertions::Yes>::WeakPtrImplSet m_set
+224 <  8>                 WTF::HashSet<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTableTraits, 255>::HashTableType m_impl
+224 <  8>                     WTF::HashTable<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> >, WTF::IdentityExtractor, WTF::DefaultHash<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, WTF::HashTraits<WTF::Ref<WTF::DefaultWeakPtrImpl, WTF::RawPtrTraits<WTF::DefaultWeakPtrImpl>, WTF::DefaultRefDerefTraits<WTF::DefaultWeakPtrImpl> > >, 255>::(anonymous union) None
+232 <  4>             unsigned int m_operationCountSinceLastCleanup
+236 <  4>             unsigned int m_maxOperationCountWithoutCleanup
+240 < 16>         WebCore::CanvasNoiseInjection m_canvasNoiseInjection
+240 < 16>             WebCore::IntRect m_postProcessDirtyRect
+240 <  8>                 WebCore::IntPoint m_location
+240 <  4>                     int m_x
+244 <  4>                     int m_y
+248 <  8>                 WebCore::IntSize m_size
+248 <  4>                     int m_width
+252 <  4>                     int m_height
+256 <  8>         WTF::Markable<unsigned long long, WTF::IntegralMarkableTraits<unsigned long long, 9223372036854775807> > m_canvasNoiseHashSalt
+256 <  8>             unsigned long long m_value
+264 <  1>         bool m_originClean
+265 <  1>         bool m_hasCreatedImageBuffer
+266 <  1>     bool m_ignoreReset
+267 <  1>     bool m_didClearImageBuffer
+268 <  1>     bool m_hasRelevantWebGLEventListener
+269 <  1>     bool m_isSnapshotting
+270 <  2>     <PADDING: 2 bytes>
+272 <  8>     std::__1::unique_ptr<WebCore::CanvasRenderingContext, std::__1::default_delete<WebCore::CanvasRenderingContext> > m_context
+272 <  8>         std::__1::__compressed_pair<WebCore::CanvasRenderingContext *, std::__1::default_delete<WebCore::CanvasRenderingContext> > __ptr_
+272 <  8>             std::__1::__compressed_pair_elem<WebCore::CanvasRenderingContext *, 0, false> std::__1::__compressed_pair_elem<WebCore::CanvasRenderingContext *, 0, false>
+272 <  8>                 WebCore::CanvasRenderingContext * __value_
+272 <  1>             std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CanvasRenderingContext>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CanvasRenderingContext>, 1, true>
+272 <  1>                 std::__1::default_delete<WebCore::CanvasRenderingContext> std::__1::default_delete<WebCore::CanvasRenderingContext>
+280 <  8>     WTF::RefPtr<WebCore::Image, WTF::RawPtrTraits<WebCore::Image>, WTF::DefaultRefDerefTraits<WebCore::Image> > m_copiedImage
+280 <  8>         WTF::RawPtrTraits<WebCore::Image>::StorageType m_ptr
+288 <  8>     std::__1::unique_ptr<WebCore::CSSParserContext, std::__1::default_delete<WebCore::CSSParserContext> > m_cssParserContext
+288 <  8>         std::__1::__compressed_pair<WebCore::CSSParserContext *, std::__1::default_delete<WebCore::CSSParserContext> > __ptr_
+288 <  8>             std::__1::__compressed_pair_elem<WebCore::CSSParserContext *, 0, false> std::__1::__compressed_pair_elem<WebCore::CSSParserContext *, 0, false>
+288 <  8>                 WebCore::CSSParserContext * __value_
+288 <  1>             std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CSSParserContext>, 1, true> std::__1::__compressed_pair_elem<std::__1::default_delete<WebCore::CSSParserContext>, 1, true>
+288 <  1>                 std::__1::default_delete<WebCore::CSSParserContext> std::__1::default_delete<WebCore::CSSParserContext>
Total byte size: 296
Total pad bytes: 2
Padding percentage: 0.68 %

[David Kilzer (:ddkilzer)]

Pull request: https://github.com/WebKit/WebKit/pull/40692

[EWS]

Committed 290488@main (d1a126e1325e): <https://commits.webkit.org/290488@main>

Reviewed commits have been landed. Closing PR #40692 and removing active labels.