Re: [GCC 13][PATCH] PR101836: Add a new option -fstrict-flex-array[=n] and use it in __builtin_object_size

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Jun 30, 2022 at 9:30 PM Qing Zhao <qing.zhao@oracle.com> wrote:
>
>
>
> > On Jun 30, 2022, at 1:03 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> >
> > On Thu, Jun 30, 2022 at 03:31:00PM +0000, Qing Zhao wrote:
> >>> No, that’s not true.  A FIELD_DELC is only shared for cv variants of a structure.
> >>
> >> Sorry for my dump questions:
> >>
> >> 1. What do you mean by “cv variants” of a structure?
> >
> > const/volatile qualified variants.  So
> Okay. I see. thanks.
> >
> >> 2. For the following example:
> >>
> >> struct AX { int n; short ax[];};
> >
> > struct AX, const struct AX, volatile const struct AX etc. types will share
> > the FIELD_DECLs.
>
> Okay.
> >
> >> struct UX {struct AX b; int m;};
> >>
> >> Are there two different FIELD_DECLs in the IR, one for AX.ax, the other one is for UX.b.ax?
> >
> > No, there are just n and ax FIELD_DECLs with DECL_CONTEXT of struct AX and
> > b and m FIELD_DECLs with DECL_CONTEXT of struct UX.
>
> Ah, right.
>
>
> >
> > But, what is important is that when some FIELD_DECL is last in some
> > structure and has array type, it doesn't mean it should have an
> > unconstrained length.
> > In the above case, when struct AX is is followed by some other member, it
> > acts as a strict short ax[0]; field (even when that is an exception), one
> > can tak address of &UX.b.ax[0], but can't dereference that, or &UX.b.ax[1].
>
> So, is this a GNU extension. I see that CLANG gives a warning by default and GCC gives a warning when specify -pedantic:
> [opc@qinzhao-ol8u3-x86 trailing_array]$ cat t3.c
> struct AX
> {
>   int n;
>   short ax[];
> };
>
> struct UX
> {
>   struct AX b;
>   int m;
> };
>
> void warn_ax_local (struct AX *p, struct UX *q)
> {
>   p->ax[2] = 0;
>   q->b.ax[2] = 0;
> }
> [opc@qinzhao-ol8u3-x86 trailing_array]$ clang -O2 -Wall t3.c -S
> t3.c:9:13: warning: field 'b' with variable sized type 'struct AX' not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
>   struct AX b;
>             ^
> [opc@qinzhao-ol8u3-x86 trailing_array]$ gcc -O2 -Wall t3.c -pedantic -S
> t3.c:9:13: warning: invalid use of structure with flexible array member [-Wpedantic]
>     9 |   struct AX b;
>       |             ^
>
> But, Yes, I agree, even though this is only a GNU extension, We still need to handle it and accept it as legal code.
>
> Then, yes, I also agree that encoding the info of is_flexible_array into FIELD_DECL is not good.

Which is why I suggested to encode 'not_flexible_array'.  This way the
FE can mark all a[1] this way in some mode
but leave a[] as possibly flexarray (depending on context).

> How about encoding the info of “has_flexible_array” into the enclosing RECORD_TYPE or UNION_TYPE node?

But that has the same issue.  Consider

struct A { int n; int a[1]; };

where a is considered possibly a flexarray vs.

struct B { struct A a; int b; };

where B.a would be not considered to have a flexarray (again note
'possibly' vs. 'actually does').

Also

struct A a;

has 'a' as _not_ having a flexarray (because it's size is statically
allocated) but

struct A *a;
struct B *b;

a->a[n];

as possibly accessing the flexarray portion of *a while

b->a.a[n]

is not accessing a flexarray because there's a member after a in b.

For your original proposal it's really the field declaration itself
which changes so annotating the FIELD_DECL
seems correct to me.

> For example, in the above example,  the RECORD_TYPE for “struct AX” will be marked as “has_flexible_array”, but that for “struct UX” will not.
>
> >
> > I believe pedantically flexible array members in such cases don't
> > necessarily mean zero length array, could be longer, e.g. for the usual
> > x86_64 alignments
> > struct BX { long long n; short o; short ax[]; };
> > struct VX { struct BX b; int m; };
> > I think it acts as short ax[3]; because the padding at the end of struct BX
> > is so long that 3 short elements fit in there.
> > While if one uses
> > struct BX bx = { 1LL, 2, { 3, 4, 5, 6, 7, 8, 9, 10 } };
> > (a GNU extension), then it acts as short ax[11]; - the initializer is 8
> > elements and after short ax[8]; is padding for another 3 full elemenets.
> > And of course:
> > struct BX *p = malloc (offsetof (struct BX, ax) + n * sizeof (short));
> > means short ax[n].
> > Whether struct WX { struct BX b; };
> > struct WX *p = malloc (offsetof (struct WX, b.ax) + n * sizeof (short));
> > is pedantically acting as short ax[n]; is unclear to me, but we are
> > generally allowing that and people expect it.
>
> Okay, I see now.
> >
> > Though, on the GCC side, I think we are only treating like flexible arrays
> > what is really at the end of structs, not followed by other members.
>
> My understanding is, Permitting flexible array to be followed by other members is a GNU extension.  (Actually, it’s not allowed by standard?).
>
> Thanks a lot for your patience and help.
>
> Qing
> >
> >       Jakub
> >
>