[clang] [llvm] [X86] Extend kCFI with a 3-bit arity indicator (PR #121070)

[Scott Constable via llvm-commits]

scottconstable wrote:

> If hashing is changed, consider replacing xxhash64 with xxh3+_64bits

@MaskRay This PR does not change the hashing scheme at all.

https://github.com/llvm/llvm-project/pull/121070