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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/107952] tree-object-size: inconsistent size
 for flexible arrays nested in structs
Date: Tue, 24 Jan 2023 10:04:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107952

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to qinzhao from comment #7)
> (In reply to Richard Biener from comment #1)
> > GCC considered this as a flex-array.=20
>=20
> do you mean for the following example:
>=20
> typedef struct {
>   char pad;
>   char data[];
> } F2;
>=20
> typedef struct {
>   unsigned pad;
>   F2 flex;
> } S2;
>=20
> although C standard disallow the above, GCC extension treats S2.flex.data=
 as
> a flex-array?=20
>=20
> How about:
>=20
> typedef struct {
>   char pad;
>   char data[];
> } F2;
>=20
> typedef struct {
>   F2 flex;
>   unsigned pad;
> } S2;
>=20
> do we have any documentation on this Gcc extension?

GCC handles for example

struct A { char data[1]; };
struct B { int n; struct A a; };

as if the a.data[] array is a flex-array.  It also handles

struct C { int n; struct A a; int x; };

as if a.data[] can be up to 4 elements large (we allow an array to extend
flexibly to padding - but only if it is trailing).  I see that's not
consistently handled though.

I think the [] syntax should follow the C standard as what testcases are
accepted/rejected by the frontend and any extensions there should be
documented (those are separate from what the former array_at_struct_end_p
allowed semantically and where GCC is careful with optimization because
of code out in the wild).=