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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/107608] [13 Regression] Failure on
 fold-overflow-1.c and pr95115.c
Date: Mon, 05 Dec 2022 15:14:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107608

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Aldy Hernandez from comment #7)
> (In reply to Richard Biener from comment #6)
> > (In reply to Jakub Jelinek from comment #0)
> > > ... but then
> > > comes dom2 and happily replaces
> > >   _1 =3D 3.4028234663852885981170418348451692544e+38 * 2.0e+0;
> > >   return _1;
> > > with
> > >   _1 =3D 3.4028234663852885981170418348451692544e+38 * 2.0e+0;
> > >   return  Inf;
> > > (I think this is still correct)
> >=20
> > Note this is also a pessimization code-generation wise since if we
> > preserve the multiplication the result is readily available in a
> > register but as optimized we have another constant pool entry and load.
> >=20
> > So we might want to consider not propagating constants generated by
> > operations
> > we cannot eliminate.  If the only consumer is a compare-and-branch we
> > can of course still end up with a seemingly dead stmt, so this would be=
 only
> > for the missed optimization.
>=20
> Up to y'all if this is the way to go, but here are some thoughts...
>=20
> Off the top of my head, we have VRP and DOM propagating constants.=20
> Technically also simplify_using_ranges, but it's only called from VRP/DOM,
> and it currently only works with integers, so we should be ok here.
>=20
> I think we could limit propagation in may_propagate_copy() which both VRP
> and DOM gate on.  VRP uses it through its use of substitute_and_fold_engi=
ne
> and DOM uses it directly.  Would this work?

I don't think may_propagate_copy is the correct vehicle.  Instead the
substitute_and_fold_engine could only substitute from defs with no
side-effects - IIRC it already refrains from propagating _into_ defs
that will be removed after the propagation.=