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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/109956] GCC reserves 9 bytes for struct s { int a; char b;
 char t[]; } x = {1, 2, 3};
Date: Thu, 25 May 2023 07:53:36 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109956

--- Comment #11 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Alexander Monakov from comment #8)
> (In reply to joseph@codesourcery.com from comment #6)
> > For the standard, dynamically allocated case, you should only need to=20
> > allocate enough memory to contain the initial part of the struct and th=
e=20
> > array members being accessed - not any padding after that array.  (Ther=
e=20
> > were wording problems before C99 TC2; see DR#282.)
>=20
> I think the following testcase indicates that GCC assumes that tail paddi=
ng
> is accessible:
>=20
> struct S {
> 	int i;
> 	char c;
> 	char fam[];
> };
>=20
> void f(struct S *p, struct S *q)
> {
> 	*p =3D *q;
> }
>=20
> f:
>         movq    (%rsi), %rax
>         movq    %rax, (%rdi)
>         ret
>=20
> Sorry for the tangential remark, but there seems to be a contradiction.

Not only accessible but also not used by sth else.  That is, a following
'char' variable may not be placed into the padding.=