[Bug rtl-optimization/97249] Missing vec_select and subreg optimization

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97249

--- Comment #5 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #3)
> Guess you want to figure what built the (vec_select:V8QI (V16QI)) and if
> it was appropriately simplified (and simplify_rtx would handle this case).
> In any case the vec_select is the same as (subreg:V8QI (V16QI)).

This case for vec_select isn't yet handled in simplify-rtx.  It
looks like it does not yet handle any cases that do not use full
vector length?  (Or, in other words, it only handles cases where
all vectors are the same length.)