[Bug lto/112716] LTO optimization with struct with variable size

["uecker at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112716

--- Comment #7 from uecker at gcc dot gnu.org ---
(In reply to rguenther@suse.de from comment #6)
> On Mon, 27 Nov 2023, muecker at gwdg dot de wrote:
> 
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112716
> > 
> > --- Comment #5 from Martin Uecker <muecker at gwdg dot de> ---
> > It works (and is required to work) for other types, e.g.
> > 
> > [[gnu::noinline,gnu::noipa]]
> > int foo(void *p, void *q)
> > {
> >         int n = 5;
> >         int (*p2)[n] = p;
> >         (*p2)[0] = 1;
> >         bar(q);
> >         return (*p2)[0];
> > }
> > 
> > void bar(void* q)
> > {       
> >         int n = 5;
> >         int (*q2)[n] = q;
> >         (*q2)[0] = 2;
> > }
> > 
> > One could argue that there is a weaker requirement for having an object of type
> > int[n] present than for struct { int x[n]; } because we do not access the array
> > directly but it decays to a pointer. (but then, other languages have array
> > assignment, so why does the middle-end care about this C peculiarity?) 
> 
> So in theory we could disregard the VLA-sized components for TBAA
> which would make the access behaved as if it were a int * indirect access.
> I think if you write it as array as above that's already what happens.

What does "disregard the VLA-sized component" mean?

For full interoperability I think one either has to assign 
equivalence classes for structs by ignoring the sizes of all
fields of array type (not just VLA) and also the offsets 
for the following struct members, or, alternately, one has
to give alias set 0 to  structs with VLA fields.  The later
seems preferable and is what I have included in the current
version of my patch for C23 for structs with VLA fields 
(but we could also decide to not support full ISO C rules for
such structs, of course)

> 
> 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.

> 
> As said, for arrays it probably works as-is since these gets the alias
> set of the component.
> 
> > There is also no indication in documentation that structs with variable size
> > follow different rules than conventional structs.   So my preference would be
> > to fix this somehow.  Otherwise we should document this as a limitation.
> 
> Local declared structs in principle follow the same logic (but they
> get promoted "global" due to implementation details I think).