https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112716
--- Comment #9 from Martin Uecker <muecker at gwdg dot de> ---
(In reply to Richard Biener from comment #8)
> (In reply to uecker from comment #7)
>
> > >
> > > Note that even without LTO when you enable inlining you'd expose two
> > > different structures with two different alias-sets, possibly leading
> > > to wrong-code (look at the RTL expansion dump and check alias-sets).
> >
> > Yes, for pre C23 this is true for all structs even without VLA.
> > But for C23 this changes.
> >
> > The main case where the GNU extension is interesting and useful is
> > when the VLA field is at the end. So at least for this case it would
> > be nice to have a solution.
>
> So what's the GNU extension here? VLA inside structs are not a C thing?
ISO C does not currently allow VLAs or variably-modified types
inside structs. So all these are GNU extensions.
WG14 is thinking about allowing pointers to VLAs inside structs.
struct foo {
int n;
char (*buf)[.n];
};
But this is a bit different because it would not depend on an
external value.
> I suppose if they were then C23 would make only the "abstract" types
> compatible but the concrete types (actual 'n') would only be compatible
> when 'n' is the same?
Yes, this is how it works for other variably-modified types
in C (since C99) where it is then run-time undefined behavior
if 'n' turns out not to be the same.
>
> I think the GNU extension is incomplete, IIRC you can't have
>
> foo (int n, struct bar { int x; int a[n]; } b) -> struct bar
> {
> return bar;
> }
>
> and there's no way to 'declare' bar in a way that it's usable across
> functions.
You could declare it again in another function
void xyz(int n)
{
struct bar { int x; int a[n]; } y;
foo(n, y);
}
and with C23 compatibility rules this would work.
>
> So I'm not sure assigning C23 "semantics" to this extension is very
> useful. Your examples are awkward workarounds for an incomplete
> language extension.
In any case, we already have the extension and we should
either we make it more useful, or document its limitations,
or deprecate it completely.
