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From: "muecker at gwdg dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/108896] provide "element_count" attribute to give more
 context to __builtin_dynamic_object_size() and -fsanitize=bounds
Date: Wed, 08 Mar 2023 20:20:43 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108896

--- Comment #32 from Martin Uecker <muecker at gwdg dot de> ---
Am Mittwoch, dem 08.03.2023 um 19:20 +0000 schrieb qinzhao at gcc dot gnu.o=
rg:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108896
>=20
> --- Comment #30 from qinzhao at gcc dot gnu.org ---
> (In reply to Martin Uecker from comment #28)
>=20
> > The problems with VLA are in my opinion caused by poor
> > implementation (e.g. no stack probing etc) and bad
> > code generation (Linus was not happy about this) and
> > not because anything is fundamentally bad about them
> > from the point of language semantics.
>=20
> you mean gcc's implementation? how about other compilers?

I mostly mean compilers without stack probing (which GCC
has but is not activated by default). Code quality is
probably still bad for most compilers.

>=20
> > VM =3D variably modified.  In C it is a type which is derived from
> > a VLA which is not necessarily itself a VLA, e.g. a pointer to
> > a VLA. But a VLA is also a VM type.
>=20
> Okay.
> >=20
> > > struct foo {
> > > =C2=A0=C2=A0int len;
> > > =C2=A0=C2=A0char (*buf)[.len];
> > > };
> > >=20
> > >=20
> > > This has less issues because the size of the struct then does not dep=
end
> > > on the length.
>=20
> but I am still not clear on why "the size of the above struct 'foo' does =
not
> depend on the .len?"  in my opinion, it should depend on .len. do I miss
> anything here?

Here the last element is not a flexible array member but
a pointer to an array of size len. The size of the pointer is
fixed.

Martin



>=