Refiling "#[repr(simd)] struct(isize, isize) not allowed"

[Centril]

Refiling rust-lang/rfcs#2513 here.

This probably does not require an RFC (according to me and @nrc).
However, we'd like @BurntSushi and @alexcrichton to sign off on this.

cc @rust-lang/compiler
cc @gnzlbg

Repro (playground: https://play.rust-lang.org/?gist=efa6e5caf752ccca651014ae5009feb8&version=nightly&mode=debug&edition=2015):

#![feature(repr_simd)] 

#[repr(simd)] struct A(isize, isize); // ERROR

#[cfg(target_pointer_width = "64")]
type isize_ = i64;

#[cfg(target_pointer_width = "32")]
type isize_ = i32;

#[repr(simd)] struct B(isize_, isize_); // Q: Is this ok?

produces

error[E0077]: SIMD vector element type should be machine type
 --> src/main.rs:3:15
  |
3 | #[repr(simd)] struct A(isize, isize);
  |               ^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

I am wondering why aren't isize and usize allowed ? I currently work around this to implement simd vectors of pointers, but maybe that's broken for reason I don't understand?

[Centril]

@rfcbot merge

[rfcbot]

Team member @Centril has proposed to merge this. The next step is review by the rest of the tagged teams:

• @Centril
• @aturon
• @cramertj
• @eddyb
• @joshtriplett
• @nikomatsakis
• @nrc
• @pnkfelix
• @scottmcm
• @withoutboats

Concerns:

• signoff-from-alex-and-andrew resolved by Refiling "#[repr(simd)] struct(isize, isize) not allowed" #55078 (comment)

Once a majority of reviewers approve (and none object), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[Centril]

• signoff from @BurntSushi
• signoff from @alexcrichton

[Centril]

@rfcbot concern signoff-from-alex-and-andrew

[scottmcm]

This is just adding another type to a feature that isn't on the path to stabilization, right?

@rfcbot reviewed

[Centril]

@scottmcm yep :) (#27731 tracks repr(simd) in general)

[gnzlbg]

While #[repr(simd)] struct A(isize, isize); currently errors, #[repr(simd)] struct A<T>(T, T); type B = A<isize>; currently accidentally works on nightly.

The packed_simd crate has used this "bug" to export vectors of pointers and vectors of usize/isize/msize for a while and uses them in some of the examples (e.g. the examples/aobench tiled.rs algorithm).

Lifting the restriction makes something that's already possible and useful a bit easier to do. Also, the current plan is to never stabilize repr(simd), but to expose it through a std API. On stable, some of these are already exposed via std::arch (e.g. __m128), but none of them interact with isize/usize - so this does not interact with stable types at all.

Something like RFC2366 with support for packed SIMD vectors of pointers would need to be added to the language for this to land on stable.

[alexcrichton]

The #[repr(simd)] feature currently (and I can't forsee at least) have any path to stabilization. In that sense I would not personally recommend working on this feature as it's basically just a building block for std::arch, which we must expose to the language somehow.

I suspect, however, that fixing this is probably a few lines of a patch. While I wouldn't at all prioritize this I wouldn't stop it if someone were motivated.