[Bug target/97194] optimize vector element set/extract at variable position

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97194

--- Comment #9 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #8)
> Note that currently RTL expansion forces a local vector typed variable
> to the stack (instead of allocating a pseudo) when there are
> variable-index accesses to it.  That might be a reason to also handle
> slightly "expensive" extract cases.  But I guess later falling back
> to a stack slot via a splitter or LRA will lead to worse code.

Indeed, but I struggle to see a good reason to bind the entire lifetime of a
variable to memory just because one operation requires that. Cannot GCC instead
create a fresh temporary early at RTL-expand (not split) time for each extract
operation, letting the original variable live in a pseudo, and binding only
that short-lived temporary to memory?

It can result in extra copies if the temporary needs to be loaded from memory
anyway, but I think passes like RTL CSE should be able to propagate them.