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From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/87489] [8/9/10/11 Regression] Spurious -Wnonnull
 warning
Date: Fri, 19 Feb 2021 23:38:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D87489

--- Comment #16 from Martin Sebor <msebor at gcc dot gnu.org> ---
The test case in
https://gcc.gnu.org/pipermail/gcc-patches/2021-February/565564.html shows t=
hat
running the -Wnonnull pass later, after FRE, allows the warning to detect t=
he
bug in the following test case:

class b {
public:
   long c();
};
class B {
public:
   B() : f() {}
   b *f;
};
long d, e;
class g : B {
public:
   void h() {
     long a =3D f->c();   <<< -Wnonnull
     d =3D e =3D a;
   }
};
class j {
   j();
   g i;
};
j::j() { i.h(); }

My response
(https://gcc.gnu.org/pipermail/gcc-patches/2021-February/565590.html) is co=
pied
below:

> Thanks.  Yes, the warning would be useful here since the f pointer
> in the call to f->c() is null when i.h() is called from j's ctor.
> The FRE3 pass clearly exposes this :
>=20
> void j::j (struct j * const this)
> {
>    long int _9;
>=20
>    <bb 2> [local count: 1073741824]:
>    MEM[(struct B *)this_3(D)] =3D{v} {CLOBBER};
>    MEM[(struct B *)this_3(D)].f =3D 0B;
>    _9 =3D b::c (0B);
>    ...
>=20
> Because the warning runs early (after CCP2), the null pointer is
> not detected.  To see it the warning code would have to work quite
> hard to figure out that the two assignments below refer to the same
> location (it would essentially have to do what FRE does):
>=20
>    MEM[(struct B *)this_3(D)].f =3D 0B;
>    _7 =3D MEM[(struct b * *)_1];
>    _9 =3D b::c (_7);
>=20
> It's probably too late to make this change for GCC 11 as Jeff has
> already decided that it should be deferred until GCC 12, and even
> then there's a concern that doing so might cause false positives.
> I think it's worth revisiting the idea in GCC 12 to see if
> the concern is founded.  Let me make a note of it in the bug.=