From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
	id 908E63858C54; Thu,  2 Mar 2023 17:34:37 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 908E63858C54
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1677778477;
	bh=d6M2ft+WV1HQDyg/buzVSV2P4SByUMYJ6b1MR7ES1IY=;
	h=From:To:Subject:Date:In-Reply-To:References:From;
	b=FN7qeFBXJ+NEvxnAl98kL43ZEz5Apuf6glLjHnF8LpJjcVbb1RlgkT0FOnk//2SV9
	 +g934uho6uB6T9cTph6OymRXyeEKJ2rlcjyoOH8LniEWlTGeYS1bzPE/QddIOmkUmE
	 gSL8o6XCUDMM4wb5j60toyiF/3b9YRAvXOevERvs=
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: Thu, 02 Mar 2023 17:34:37 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: unknown
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: enhancement
X-Bugzilla-Who: qinzhao at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: qinzhao at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: 
Message-ID: <bug-108896-4-ttciH4vpuJ@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-108896-4@http.gcc.gnu.org/bugzilla/>
References: <bug-108896-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108896

--- Comment #8 from qinzhao at gcc dot gnu.org ---
(In reply to Martin Uecker from comment #7)
> An attribute is certainly simpler and should be easy to add.
yes.
>=20
> I proposed similar extension for C23 and there was some interest,
> but I did not have time to follow up.
>=20
> https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2660.pdf
very interesting proposal!
are there any discussions on this proposal? if so, can you point me to them?
>=20
>=20
> Sizeof is not a constant expression in ISO C for a VLA and it is not a
> constant expression if the struct contains a VLA  (GNU extension).  So th=
is
> is already the case and nothing would need to change. It would also poten=
tially=20=20
> avoid mistakes when computing the size of such a struct.
agreed.
However, my understanding is: VLA is only valid inside a function scope. GCC
use a special SAVE_EXPR to record its size expression. and evaluated during
runtime only once.=20
when this variable length concept is extended to global scope, not sure how=
 to
implement the size expression? need some study here.

>  But the
> rules for initialization are not so clear.
shall we make this clear?
>=20
> I do not think it is a good idea to differentiate between file scope stru=
cts
> and others. This would be confusing.
Yes. agreed.

this proposal basically is to extend the VLA concept from function scope to
global scope. is my understanding correct?

>=20
> Considering that the GNU extensions is rarely used, one could consider
> redefining the meaning of
>=20
> int n =3D 1;
> struct {
>   int n;
>   char buf[n];
> };
>=20
> so that the 'n' refers to the member. Or we add a new syntax similar to
> designators (which intuitively makes sense to me).
designator might be better IMO.

a question here is:

for the following nested structure:=20

struct object {
        ...
        char items;
        ...
        struct inner {
                ...
                int flex[];
        };
} *ptr;

what kind of syntax is good to represent the upper bound of "flex" in the i=
nner
struct with "items" in the outer structure? any suggestion?=