`scatter` and `gather` are not strict provenance compatible

[RalfJung]

The implementations of gather and scatter in portable-simd currently are not compatible with strict provenance. Specifically, it is SimdConstPtr::wrapping_add that transmutes a vector of pointers to a vector of integers, does integer arithmetic, and then transmutes back the result. It also has safety comments that are somewhat in conflict with what I am writing in rust-lang/rust#95547. ;)

[calebzulawski]

I think this is easy to fix, we just need a new intrinsic for vector pointer offset

[RalfJung]

So how would one start fixing this? Add a new simd_offset case around here? Can I just call bx.inbounds_gep with SIMD types?

[calebzulawski]

Yeah, that seems right to me. It might also be possible to implement it in rustc_codegen_ssa?

[RalfJung]

It might also be possible to implement it in rustc_codegen_ssa?

Are there other SIMD intrinsics that do that that could be used as a template?

[calebzulawski]

Hmm, I was thinking offsetting pointers is pretty backend-agnostic but I don't see anything similar. It's probably fine putting it in the LLVM backend with all of the other SIMD intrinsics.