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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/110228] [13/14 Regression] llvm-16 miscompiled due
 to an maybe uninitialized variable
Date: Tue, 04 Jul 2023 06:33:02 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110228

--- Comment #26 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Sergei Trofimovich from comment #24)
> Trying to understand the failure mode here:
>=20
> In bug.c.033t.early_objsz I still see the explicit stores to LocalFlags:
>=20
>   <bb 4> :
>   LookupFlags_15 =3D 0;
>   goto <bb 7>; [INV]
>=20
>   <bb 5> :
>   if (v_13 =3D=3D 1)
>     goto <bb 6>; [INV]
>   else
>     goto <bb 7>; [INV]
>=20
>   <bb 6> :
>   LookupFlags_14 =3D 1;
>=20
>   <bb 7> :
>   # LookupFlags_4 =3D PHI <LookupFlags_15(4), LookupFlags_5(5),
> LookupFlags_14(6)>
>   *p_16(D) =3D LookupFlags_4;
>=20
> But in bug.c.034t.ccp1 I see no stores at all:
>=20
>   if (v_13 =3D=3D 0)
>     goto <bb 6>; [INV]
>   else
>     goto <bb 4>; [INV]
>=20
>   <bb 4> :
>   if (v_13 =3D=3D 1)
>     goto <bb 5>; [INV]
>   else
>     goto <bb 6>; [INV]
>=20
>   <bb 5> :
>=20
>   <bb 6> :
>   # LookupFlags_4 =3D PHI <0(3), LookupFlags_5(4), 1(5)>
>=20
> Specifically '# LookupFlags_4 =3D PHI <0(3), LookupFlags_5(4), 1(5)>' cla=
ims
> that somehow gets values '0' and '1' into a PHI node.
>=20
> AFAIU PHI is an equivalent of a mutable variable in an otherwise immutable
> SSA form. It has to be a write it needed value is not there yet. Why the
> store of '1' was removed? Is it because variable 'v_13' already happens to
> have 0 or 1 value?
>=20
> I wonder why PHI does not look like something below:
>=20
>     # LookupFlags_4 =3D PHI <v_13(3), LookupFlags_5(4), v_13(5)>

LookupFlags is in SSA form thus we are dealing with registers and no "store=
s".
What happens is just constant propagation, LookupFlags_1{4,5} can be
replaced with their assigned constant.=