It's a controversial opinion I've not heard anyone else express, but IMO variable-width vector instructions are the wrong approach, since they're optimizing for

It's a controversial opinion I've not heard anyone else express, but IMO variable-width vector instructions are the wrong approach, since they're optimizing for the easy cases and solving problems that don't matter much, like instruction counts. Although x86's SIMD extensions have a bunch of crippling issues, fixed-width instructions are fundamentally more general because they work both for vectorizable length-N loops, and for the many, many cases where you have a fixed units of work that you can perform in parallel. If you design your architecture to depend on loops to extract full performance, you cut yourself off from many productive uses of vector instructions outside of loops, and make it more difficult to juggle cases where you have more than one loop dimension. Upgrading to new architectures with longer supported vector lengths is best left to recompilation and variable-width vector libraries.