Consider that your SIMD instruction might itself take a tag mask and the ALU need only do equality on the tag field. In fact it could do that in parallel with the ALU op; on mismatch you could simply discard the current state and abort the operation. However realistically you’d want the same set of SIMD instructions as an I tagged architecture anyway.

Also I expect any compiler would assume that the contents of an array subject to SIMD computation would be homogeneous anyway, perhaps trying to enforce it elsewhere.

In any case this doesn’t seem like a big deal to me…but I could be wrong!

Yep, sounds like a good sketch.

I guess to really get into it a good start would be to work with existing SIMD and take a quantitative approach to what is actually the most hot part of it. I wonder if any existing language implementations (eg Common Lisp) attempt to do these kinds of things in the first place.