Add classification functions

[calebzulawski]

Closes #36.

[calebzulawski]

Looks like aarch64 segfaulted during the travis build (not the test). No clue how that happened...

[workingjubilee]

That's... odd. This requires rebasing anyways, can you do that and see if it repros?

[workingjubilee]

Looks like we do indeed have a particularly gribbly bug!

[calebzulawski]

I read somewhere that LLVM segfaults when an assert fails. When I run this in docker (I don't have access to an actual aarch64 machine) it succeeds, so I thought maybe we're running out of memory and a malloc assert is failing.

[Lokathor]

I'm unclear why codegen-units=1 is in the build flags on some targets but not all targets, and I'm worried about that not being picked up by downstream, when this crate is built into core/std, or is inlined into a user's program.

If we need to reduce codegen-units for this to build without crashing LLVM, and a user of the crate leads the codegen-units at the default quantity of 256 of 16, will their build explode when they use this crate?

[calebzulawski]

I read in an issue elsewhere that codegen-units=1 could fix aarch64 issues, but it didn't here. I just didn't remove it yet. I'm still not completely certain what is causing the issue. I can't replicate it locally with cross, so I think it's particular to the build agent in some way?

[calebzulawski]

I think I've figured out the issue, but I'm not sure what's really happening. The problem is the simd_ne intrinsic which is only used in is_nan, is_normal, and is_subnormal. If I replace it with a naive implementation everything compiles fine on aarch64. I think this is probably a bug in LLVM?

[workingjubilee]

Following up from discussion: these two examples offer repro of our bug, but only when targeting aarch64, but seemingly only when generating tests, and only in release mode.

#[test]
fn is_normal() {
    assert!(core_simd::SimdF32::<64>::splat(0.).is_normal().to_array()[0]);
}

#[test]
fn bug() {
    use core_simd::{SimdF32, SimdU32};
    let v = SimdF32::<64>::splat(0.);
    let m = v.lanes_eq(SimdF32::splat(0.0)) & v.lanes_eq(SimdF32::splat(0.0));
    assert!(m.to_array()[0]);
}

[calebzulawski]

I reduced the maximum lane count from 64 to 32, since the error only occurred with 64-length vectors.

This prevents AVX-512 vectors of u8, but this is really only temporary until we can get an LLVM fix (and get that fixed version into rustc)

[workingjubilee]

Alright, looks like this is good now with that. Have you filed the LLVM bug somewhere? I think with that posted somewhere (anywhere) this can be merged.

[calebzulawski]

There's an LLVM bug filed (though I'm not 100% sure it's the same bug). I still need to file the rust and stdsimd bugs to track it.

[calebzulawski]

Filed #90 and rust-lang/rust#84020.