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From: "manu at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/109123] Bogus warning: pointer used after 'realloc'
 -Wuse-after-free with -O2
Date: Tue, 14 Mar 2023 10:22:02 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109123

--- Comment #6 from Manuel L=C3=B3pez-Ib=C3=A1=C3=B1ez <manu at gcc dot gnu=
.org> ---
(In reply to Manuel L=C3=B3pez-Ib=C3=A1=C3=B1ez from comment #5)
> Is this code motion valid? Is there any point in the middle-end that chec=
ks
> the validity of the pointer beyond a free/realloc?
>=20
> If there is a point where such check happens, perhaps it would be a good
> place to apply no_warning attribute to the pointer.

Answering to myself: It seems the dispute is over the meaning of "use".
According to the middle-end and for the purposes of realloc/free, "use" mea=
ns
dereference, while for the warning "use" is any read of the value.

By only warning for dereferences, the warning may miss some obvious cases l=
ike:

        tmp =3D realloc(v->_begin, sizeof(double) * n);
        v->_end =3D v->_begin + old_size;
        v->_begin =3D tmp;

However, given that the assumption of the middle-end has worked for decades,
and it will be not possible to fix it, warning only for dereferences (or mo=
ving
warning for value-uses to a level not enabled by -Wall) would seem more
user-friendly.=