The current specification allows some instructions to have two vector
source operands read from the same vector register but with different
EEW.
…
(cue folks furiously trying to construct one...)
haha i tried but can’t think of a use case !!
We do have a choice of:
1) Mandate all implementations raise an illegal exception in this
case. This is my preferred route, as this would be a minor errata for
existing implementations (doesn't affect software), and we would not reuse
this state/encoding for other purposes.
2) Allow either correct execution or illegal exception (as with
misaligned).
3) Consider "reserved", implying implementations that support it are
non-conforming unless we later go with 2).
I don’t have a strong opinion, but I prefer a route that allows us to recover those instruction encodings — they seem to be getting scarce hence represent value. You said there were already requests for extra instructions — would this space be usef for any of them (or other as-yet-unforeeen instructions)?
Does (3) give us the best route to reuse the encodings in the future? I’m a bit confused about the permanence of (1), and I don’t like the possibility software fragmentation that will arise from (2).
