What are the advantages of tagged memory as opposed to using the unused bits in a regular pointer as a tag?
In the LISP machines for example, you had an add instruction. Which would happily work correctly on pointers, floats, and integers depending on the data type, at the machine code level. Offers safety and also makes the the compiler simpler.
But where this really shines is in things like, well, lists, since the tags can distinguish atoms from pairs and values like nil, fairly complex list-walking operations can be done in hardware, and pretty quickly too. It also makes hardware implementation of garbage collection possible.
This is just my intuition, but I suspect, these days, it all works out to about the same thing in the end. You use some of the cache for code that implements pointer tagging, or you can sacrifice some die area away from cache for hardwired logic doing the same thing. It probably is in the same ballpark of complexity and speed.
An interesting question for a modern ISA design would be to figure out how to make tagged words and memory work well with SIMD.
Not sure what the issue might be. Let’s say you’re doing a multiply-add: you’d call the one for the data type you want and if any operand were of the wrong type you’d get a fault. Am I missing something?
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!
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.