Shuffle/swizzle operations

[valadaptive]

I'd like to replace my homegrown SIMD implementation with this crate. My use case requires support for a "shuffle" / "swizzle" / "permute" operation, which this crate currently doesn't appear to have.

One obstacle I encountered when implementing shuffles in my SIMD implementation is that different architectures' intrinsics take different types for shuffle indices (they differ in signedness and size), so we can't use const generics to specify them currently. However, LLVM appears to optimize constant permutes into shuffle instructions.

My thought is to give each SIMD type an associated Indices type (name not final), which is like its Mask or Block types, and is a [usize; N]. A shuffle function would take it as input.

[AndrewJakubowicz]

Thanks for opening this issue!
We briefly spoke about this at the rendering office hours last week (#office hours > Renderer 2025-07-09 @ 💬).

At a high level, this is definitely something that would be a very welcome addition 😄 !
As an additional reference, portable SIMD has a carefully designed swizzling API. The open question is what API enables LLVM to recognize the shuffle/swizzle so it can potentially optimize it.

Would you be interested in drafting a proof-of-concept?

[valadaptive]

I can look into drafting one in the next few days.

I'm not sure how much special stuff needs to be done for LLVM to optimize shuffles. In my testing, it can optimize _mm_permutevar_ps into a constant shuffle. Not sure about _mm_shuffle_epi8 on non-AVX2 targets, or WASM i8x16_swizzle -> i8x16_shuffle. As far as I know, NEON doesn't even have a constant swizzle instruction.