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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: Tue, 04 Apr 2023 15:07:14 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108896

--- Comment #37 from qinzhao at gcc dot gnu.org ---
(In reply to Martin Uecker from comment #36)

>=20
> I considered pointers to arrays:
>=20
> struct P {
>   int n;
>   char (*buf)[.n];
> };
>=20

Okay.=20
Then for the field "buf", it has a pointer type, the size of this field is a
compile-time constant. where in the IR do you put the ".n" (the size of the
array this pointer points to). That's the place I am trying to understand f=
rom
your patch or from the IR dump of a working small testing case.=20


> the FAM case needs more work and I guess there are
> still many other problems with the patch.
>=20
For the FAM case, since the field itself is an ARRAY type, then the ".n" ca=
n be
naturally put to the SIZE of the type of the field.=20

Another thing I'd like to point out, for the original intention of this PR,=
 FAM
case is more important than the pointer to array case, I think. So, should =
we
focus on FAM first?

>=20
> The comments in
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104970
>=20
> imply that the size information does not survive long enough
need to study this a little bit more.

> But the size expression is evaluated each time when the member is
> accessed.=20
How to represent this in IR?

> Maybe the size expressions should be limited to very simple=20
expressions without side effects.
agreed. but I think we might want to focus on FAM first.=