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From: "siddhesh 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: Wed, 25 Jan 2023 12:44:59 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107952

--- Comment #12 from Siddhesh Poyarekar <siddhesh 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?

There's an open bug to document these semantics:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D77650(In reply to
rguenther@suse.de from comment #11)
> On Tue, 24 Jan 2023, qing.zhao at oracle dot com wrote:
>=20
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107952
> >=20
> > --- Comment #10 from Qing Zhao <qing.zhao at oracle dot com> ---
> > > --- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
> > >=20
> > > GCC handles for example
> > >=20
> > > struct A { char data[1]; };
> > > struct B { int n; struct A a; };
> > >=20
> > > as if the a.data[] array is a flex-array.
> >=20
> > Okay. Then the maximum size of __builtin_object_size for it should be -=
1,
> > right?
>=20
> I think so.

Why?  If the a B object is allocated with a visible allocator call, we can
return the correct size here too.=