Explore implementing SIMD types as arrays in the compiler

[workingjubilee]

Instantiating SIMD types as tuple literals becomes absolutely hand-cramping when those structs become wider than 2~8 elements, and even 8 is getting a bit tiresome. CTFE is now powerful enough to significantly sugar some fairly nuanced array creation, e.g.:

const ARRAY: [i8; 32] = {
  let mut arr = [-128; 32];
  arr[27] = -62
  arr[30] = -31;
  arr[31] = -15;
  arr
};

By providing, at the very least, conversions from the obvious arrays into SIMD types, we can make a lot of things easier on ourselves, so that this can just be used with some kind of method like i8x32::from_array(ARRAY).

Because arrays are a much more natural fit, I would honestly like to explore "what compiler changes would be required to let #[repr(simd)] accept arrays straight-out?" but that may quickly become too complicated, so for now it is just a (very) nice-to-have and std::simd conversions can still exist. On the other hand, it might be pretty easy, and that might simplify the overall API for us immensely, so I should at least check.

[calebzulawski]

The vectors should already implement From for conversions to and from arrays. Do we want named methods as well?

[workingjubilee]

Doh.
I got confused because I saw you talking about as_array methods.

[workingjubilee]

OK I mean... should we? 👀

Leaving this open actually since I still want to go check up on this in the compiler proper, but it makes sense to have the issue here since really it's an API concern.

[Lokathor]

so, from_array can be a temporary method, with the advantage that it can be const fn while a From impl cannot yet. However, i think that just a From impl might be cleaner in the long term, if traits will be allowed to have const impls any time at all soon.

[bjorn3]

I believe I once saw a PR to accept arrays in #[repr(simd)], but I can't find it anymore.

[workingjubilee]

That's really cool to hear!
I think this was rust-lang/rust#63531 perhaps?

[Lokathor]

See also, https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Why.20is.20repr(simd).20only.20allowed.20on.20tuple.20structs.3F

[workingjubilee]

Inlining this relevant link from that conversation: rust-lang/rust#5841 👀

[workingjubilee]

rust-lang/rust#78863 implemented this! Nice!