Bug 279227 – watchOS arm64_32 build broken due to ThreadSafeRefCounted layout

• Component: Web Template Framework
• Status: RESOLVED FIXED

[Elliott Williams]

With debug assertions enabled (a config that EWS and post-commit do not test), watchOS for arm64_32 fails a static assertion in GeneratedSerializers.mm:

[Elliott Williams]

	/Volumes/Data/OpenSource/WebKitBuild/Debug-watchos/DerivedSources/WebKit/GeneratedSerializers.mm:31638:19: error: static assertion failed due to requirement 'sizeof(ShouldBeSameSizeAsThreadSafeDataBufferImpl) == sizeof(WebC
	ore::ThreadSafeDataBufferImpl)'                                                                                                                                                                                                
	 31638 |     static_assert(sizeof(ShouldBeSameSizeAsThreadSafeDataBufferImpl) == sizeof(WebCore::ThreadSafeDataBufferImpl));                                                                                                   
	/Volumes/Data/OpenSource/WebKitBuild/Debug-watchos/DerivedSources/WebKit/GeneratedSerializers.mm:31638:70: note: expression evaluates to '28 == 24'                                                                            
	 31638 |     static_assert(sizeof(ShouldBeSameSizeAsThreadSafeDataBufferImpl) == sizeof(WebCore::ThreadSafeDataBufferImpl));                                                                                                   
	/Volumes/Data/OpenSource/WebKitBuild/Debug-watchos/DerivedSources/WebKit/GeneratedSerializers.mm:47320:19: error: static assertion failed due to requirement 'sizeof(ShouldBeSameSizeAsSerializedScriptValue) == sizeof(WebCore
	::SerializedScriptValue)'                                                                                                                                                                                                      
	 47320 |     static_assert(sizeof(ShouldBeSameSizeAsSerializedScriptValue) == sizeof(WebCore::SerializedScriptValue));                                                                                                         
	/Volumes/Data/OpenSource/WebKitBuild/Debug-watchos/DerivedSources/WebKit/GeneratedSerializers.mm:47320:67: note: expression evaluates to '100 == 96'                                                                           
	 47320 |     static_assert(sizeof(ShouldBeSameSizeAsSerializedScriptValue) == sizeof(WebCore::SerializedScriptValue));                                                                                                         
	2 errors generated.   

I believe that the root problem is that WebCore::ThreadSafeDataBufferImpl is a WTF::ThreadSafeRefCounted, whereas the generated ShouldBeSameSizeAsThreadSafeDataBufferImpl is a WTF::RefCounted. The latter class has m_isOwnedByMainThread, m_areThreadingChecksEnabled, and m_adoptionIsRequired boolean fields that are implement runtime assertion checks. On platforms with 8-byte pointers, these bits seem to get packed in without affecting the overall size of the class, but on arm64_32 they make it a byte larger.

[Radar WebKit Bug Importer]

<rdar://problem/135371304>

[Elliott Williams]

Pull request: https://github.com/WebKit/WebKit/pull/33209

[EWS]

Committed 283247@main (02f9d93edf29): <https://commits.webkit.org/283247@main>

Reviewed commits have been landed. Closing PR #33209 and removing active labels.