> Note that compact/compress are not actually the key enablers: also with AVX-512 we use table lookups for u64 keys, because this allows us to actually partition a vector and write it both to the left and write sides, as opposed to compressing twice and writing those individually.

That is interesting! So do I understand you correctly that the 512b vectors allow you to implement the algorithm more efficiently? That would indeed be a nice argument for longer SIMD

> Isn't the L1d bandwidth tied to the SIMD width, i.e., it would be unachievable on Skylake if also only using 128-bit vectors there?

It's a hardware detail. Intel does tie it to SIMD width, but it doesn't have to be the case. For example, Apple has 4x128b units but can only load up to 48 bytes (I am not sure about the granularity of the loads) per cycle.