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From: "qinzhao at gcc dot gnu.org" <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 16:56:59 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108896

--- Comment #23 from qinzhao at gcc dot gnu.org ---
(In reply to Martin Uecker from comment #13)
>=20
> VLAs and VM types exist since C99 and were made optional in C11.
> The minimal change we adopted to make support for VM types=C2=A0
> (but not VLAs) mandatory again was:
>=20
> https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2778.pdf

So, this will be official in C23? i.e, VM types will be mandatory, but VLA =
will
be optional (and later might be deprecated?)

>=20
> I think using PLACEHOLDER_EXPR that are insert into the size
> expression and then replaced later by the struct being accessed,=C2=A0
> e.g.
>=20
> struct foo {
>  int len;
>  char buf[PLACEHOLDER_EXPR.len]
> };
>=20
> and then later when we have
>=20
> struct foo x;
>=20
> x->buf
>=20
> we would replace in the size of the type for x->buf the placeholder
> with x itself.

I see. Yes, this will resolve the implementation difficulty for filling the
size of the FAM field when the size is the previous declared field in the s=
ame
structure.=20


> Yes, this was what I wanted to do...  My main use case is not flexible
> array members but VM types in struct:
>=20
> struct foo {
>   int len;
>   char (*buf)[.len];
> };
>=20
>=20
> This has less issues because the size of the struct then does not depend
> on the length.

a little confused here:
what's the definition of VM type? it's size will not depend on the ".len" ?=