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

[Phoebe Wang via llvm-commits]

phoebewang 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

I see, thanks! There's nothing to do with the C `fastcall` then.

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