[llvm] [RFC] Extend FASTCC to use up to 22 registers under APX (PR #164638)

[Alex Bezzubikov via llvm-commits]

zuban32 wrote:

> FASTCC is not support in X64 in the C code, so we don't need to worry about compatibility issue, see https://godbolt.org/z/fq99T6h63

I think you're mixing up two orthogonal things here, IIRC fastcall is `x86_fastcall` CC, and fastcall calls still have to adhere to its ABI. While `fastcc` is omitting any ABI at all.

But that doesn't make your change unneeded, just my 2 cents

https://github.com/llvm/llvm-project/pull/164638