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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/113059] [14 regression] fftw fails tests for -O3 -m32
 -march=znver2 since r14-6210-ge44ed92dbbe9d4
Date: Wed, 10 Jan 2024 09:53:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113059

--- Comment #13 from rguenther at suse dot de <rguenther at suse dot de> ---
On Wed, 10 Jan 2024, jakub at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113059
>=20
> --- 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 w=
ill
> > break them?  Any user then should recompute the notes.  Originally pass
> > properties were thought of a vehicle indicating these kind of IL featur=
es,
> > 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.
> >=20
> > The problem with properties is that they are always set/unset irrespect=
ive
> > of whether a pass did something destroying it.  That could be
> > circumvented by postreload-cse only clearing the flag when it does
> > something.
> >=20
> > 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).
>=20
> 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 registe=
r,
> which should determine whether we want to drop REG_UNUSED notes unconditi=
onally
> at the end of postreload-cse, or whether we e.g. want just some flag whet=
her
> we've extended lifetime of something during it and only remove REG_UNUSED=
 notes
> if that flag is set.

In theory the insn analysis could look at REG_UNUSED notes and mark
them as "to be extended" (eventually even pointing to the note).  When
eliminating to a such marked REG we could either remove the note or
scrap all notes.

That said - not sure how exactly postreload-cse keeps its state.=