> It would be very easy to support 512-bit vectors everywhere, and just emulate them on most systems with a small number of smaller vectors. It's easy for a compiler to generate good code for this

Wouldn’t that be suboptimal if/when CPUs that support 1024-bit vectors come along?

> Variable-length vectors, on the other hand, are a very challenging problem for compiler devs. You tend to get worse code out than if you just statically picked a size, even if it's not the native size.

Why would it be challenging? You could statically pick a size on a system with variable-length vectors, too. How would that be worse code?