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From: "aldyh 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 16:30:34 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107608

--- Comment #9 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #8)
> (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 loa=
d.
> > >=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/D=
OM,
> > 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 V=
RP
> > and DOM gate on.  VRP uses it through its use of substitute_and_fold_en=
gine
> > and DOM uses it directly.  Would this work?
>=20
> 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.

So where do you suggest we clamp this?  The uses I can think of are VRP
(various places in tree-ssa-propagate.cc) and DOM (cprop_operand).=