> twice the native vector width has ~20% better throughput

Yes, this is what I was saying, but twice the vector width of AVX-512 will perform horrible in SSE, which is why portable SIMD abstractions should make writing code relative to the native vector width simple.

> I pass in the vector width as a generic parameter like this:

> fn do_simd_stuff<const N: usize>(x: Simd<f32, N>) { ... }

My problem is that no portable_simd example code I've seen does this, which causes people to choose one specific N and run with that.

The second part of the problem is how you find the native vector length, so you can instantiate the generic function. IIRC this isn't even exposed in portable_simd and you have to use a seperate crate to get it.