[llvm] x86: fix musttail sibcall miscompilation (PR #168956)

[Folkert de Vries via llvm-commits]

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folkertdev wrote:

I believe that would currently run into this 

https://github.com/llvm/llvm-project/blob/d748c81218bee39dafb9cc0c00ed7831a3ed44c3/llvm/lib/Target/X86/X86ISelLoweringCall.cpp#L2937-L2955

The arm backend does not have such a check. Though perhaps we can remove this logic now that we actually do implement shuffling arguments around on the stack?

https://github.com/llvm/llvm-project/pull/168956