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From: "jason at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/97595] [11 Regression] bogus
 -Wstringop-overflow due to DECL_SIZE_UNIT underreporting field size
Date: Mon, 16 Nov 2020 17:10:03 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97595

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Martin Sebor from comment #3)
> I can confirm the warning but I'm not sure the bug is in the middle end
> code.  Let me CC Jason for his comments.
>=20
> The warning triggers for the MEM_REF below:
>=20
>   MEM[(char &)_10 + 224] =3D _24;
>=20
> in the following GIMPLE ("type" is char).  The destination of the access =
is
> this_3(D)->D.45669 which is the basic_istream subobject.  Its size report=
ed
> by DECL_SIZE_UNIT() in component_ref_size() is 16.  The initial offset
> (i.e., _10) is indeterminate but it's taken to be in the range bounded by
> the size of the object, or [0, 16].  Given that, the constant offset 224 =
is
> determined to be out of bounds.

What you're seeing is that the DECL_SIZE of the base subobject excludes any
virtual bases, because they are laid out in the most derived class, while t=
he
TYPE_SIZE of the base type includes them.  CLASSTYPE_SIZE of the base type
should match the DECL_SIZE.

It's definitely OK for a program to refer to a virtual base through any of =
the
derived classes.

> The change introduced in r11-3827 that triggers the warning is the assump=
tion > that unless determined otherwise, an indeterminate offset into an ob=
ject must > be bounded by the object's size.

So this assumption is invalid for base subobjects of types with virtual bas=
es;
such an offset is only bounded by the most-derived object's size.=