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From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/106868] [12/13 Regression] Bogus
 -Wdangling-pointer warning with -O1
Date: Mon, 12 Sep 2022 16:18:00 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106868

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #1)
> Confirmed.
>=20
> <bb 2> [local count: 1073741824]:
> alloc (&q);
> q.0_1 =3D q;
> *p_4(D) =3D q.0_1;
> q =3D{v} {CLOBBER(eol)};
> a_8 =3D __builtin_memcpy (q.0_1, "", 1);
> *a_8 =3D 0;
> return;
...
> we somehow confuse q.0_1 =3D q; as assigning the address of the object 'q=
'.

The reason for the false positive is plain to see in the IL: the memcpy cal=
l is
passed a copy of the clobbered q.  It then returns another copy of the same=
 q
which is then used to dereference whatever the pointer points to.  The warn=
ing
is due to the (known) mismatch between how the optimizers and the warning
interpret clobbers: (IIUC) the optimizers treat it as the value of the assi=
gned
variable alone becoming indeterminate, while the warning as all copies of it
becoming so.=