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This update also happens to include a bug fix for an actual instruction selection bug in the compiler that miscompiled the Swift runtime libraries for certain devices. So if the security benefits didn’t convince you, consider making some mobile release engineers happy :)

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Could provide a link with some info? I tried to look around to confirm but didn’t find anything. This sounds like it could be really useful.

Someone's reported this as https://bugs.swift.org/browse/SR-16047. The bug is that A11 and older processors don't have a CAS instruction, and LLVM's lowering accidentally added xzr to the set of "valid registers" for the replacement instruction sequence. So if you're seeing crashes in MetadataAllocator in iOS 15.4, this is probably going to fix it for you.

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