The LLVM x86-64 ABI requires the top bits to be zero. GCC treats them as undefined. Until a recent clarification, the x86-64 psABI made the upper bits undefined by omission only, which is why I think most people followed the GCC interpretation.
In theory. In practice the vast majority of Linux userland programs are compiled with GCC so unless GCC did something particularly braindead they are unlikely to break compatibility with that and so it's the ABI everyone needs to target. Which is also what happened in this case: The standard was updated to mandate the GCC behavior.
https://github.com/llvm/llvm-project/issues/12579 https://groups.google.com/g/x86-64-abi/c/h7FFh30oS3s/m/Gksan... https://gitlab.com/x86-psABIs/x86-64-ABI/-/merge_requests/61