[QUESTION] what do the simd cmp intrinsics lower to?

[pro465]

https://github.com/rust-lang/portable-simd/blob/master/crates/core_simd/src/intrinsics.rs#L49-L54

they didn't have any comments explaining what specific LLVM instruction they get lowered to, and i also couldn't find any instruction on the LLVM reference that closely resembled them

[bjorn3]

They get lowered to the icmp or fcmp llvm instruction depending on the lane type: https://github.com/rust-lang/rust/blob/1c0ed0d4c4bd7b39299966e57806d29ce38c95af/compiler/rustc_codegen_ssa/src/base.rs#L98-L124

[calebzulawski]

It's worth noting that there are no guarantees as to what anything lowers to--the only guarantee is that a function will perform the operation defined in its documentation, and do it relatively efficiently.

[Lokathor]

However please note that the entire portable-simd project is "best effort" in terms of giving a specific instruction.

We guarantee that the comparison will happen (and produce correct outputs) at a logical level, but the exact hardware path that happens is not intended to be guaranteed.

edit: sniped :(

[pro465]

@Lokathor @bjorn3 @calebzulawski

thank you all very much for your helpful answers. i however, didnt understand anything about the code that @bjorn3 pointed to. does it just sign-extend <N x i1> to whatever the return type is? if so, calling intinsics::simd_eq::<Simd<_, _>, bool> should not work, right?

[bjorn3]

In LLVM i1 is the type for booleans.

[pro465]

@bjorn3 oh so, is calling instrinsics::<Simd<N, _>, [bool; N]> fine?

[bjorn3]

I think so. You can try it if you need to know for sure.

[pro465]

ok lemme try

[pro465]

@bjorn3 didn't work :(

https://github.com/pro465/portable-simd/runs/4429119312?check_suite_focus=true

[bjorn3]

Looks like cg_llvm explicitly checks that the return type is an integer vector: https://github.com/rust-lang/rust/blob/1c0ed0d4c4bd7b39299966e57806d29ce38c95af/compiler/rustc_codegen_llvm/src/intrinsic.rs#L907 Maybe intrinsics::simd_eq::<Self, [u8; LANES]>(*self, *other) == [255; LANES] works? Or I think even better intrinsics::simd_reduce_all(intrinsics::simd_eq::<Self, [u8; LANES]>(*self, *other)).

[pro465]

@bjorn3 still didnt work :(

[pro465]

@bjorn3 i think the error is coming from https://github.com/rust-lang/rust/blob/1c0ed0d4c4bd7b39299966e57806d29ce38c95af/compiler/rustc_codegen_llvm/src/intrinsic.rs#L894

which types have is_simd() return true?

[bjorn3]

is_simd() is true for any type with #[repr(simd)]. Of course. It needs to be intrinsics::simd_reduce_all(intrinsics::simd_eq::<Self, Simd<u8, LANES>>(*self, *other)). (Simd<u8, LANES> instead of [u8; LANES])

[pro465]

now that makes sense, thank you!

[pro465]

finally compiled! thank you all, just waiting all tests to pass and then i will close this issue 😄