[Bug target/113059] [14 regression] fftw fails tests for -O3 -m32 -march=znver2 since r14-6210-ge44ed92dbbe9d4

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113059

--- Comment #12 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #11)
> IMHO REG_UNUSED notes should stay correct or be removed.  Instead of a full
> solution can we just wipe them in postreload-cse as we know that pass will
> break them?  Any user then should recompute the notes.  Originally pass
> properties were thought of a vehicle indicating these kind of IL features,
> so we
> could add PROP_rtl_unused_notes and if not set treat them as possibly
> invalid when present (instead of actively wiping them), and when a pass
> requires them re-compute them.
> 
> The problem with properties is that they are always set/unset irrespective
> of whether a pass did something destroying it.  That could be
> circumvented by postreload-cse only clearing the flag when it does
> something.
> 
> Not (re-)using pass properties for this but a flag in struct function
> works as well of course (or treat it as part of the DF state).

I was hoping we could have some df checking which would discover invalid
REG_UNUSED notes and so we would know which passes need tweaking, but I'm
afraid my df knowledge is insufficient for that.
Dunno how often postreload-cse actually extends life time of some register,
which should determine whether we want to drop REG_UNUSED notes unconditionally
at the end of postreload-cse, or whether we e.g. want just some flag whether
we've extended lifetime of something during it and only remove REG_UNUSED notes
if that flag is set.