Re: compatibility of structs/unions/enums in the middle end

[Richard Biener <richard dot guenther at gmail dot com>]

On Fri, Oct 4, 2019 at 1:55 PM Uecker, Martin
<Martin.Uecker@med.uni-goettingen.de> wrote:
>
> Am Freitag, den 04.10.2019, 12:29 +0200 schrieb Richard Biener:
> > On Wed, Oct 2, 2019 at 8:24 PM Uecker, Martin
> > <Martin.Uecker@med.uni-goettingen.de> wrote:
> > >
> > > Am Mittwoch, den 02.10.2019, 17:37 +0200 schrieb Richard Biener:
>
> > >
> > > ...
> > >
> > > > > > Oh, and LTO does _not_ merge types declared inside a function,
> > > > > > so
> > > > > >
> > > > > > void foo () { struct S { int i; }; }
> > > > > > void bar () { struct S { int i; }; }
> > > > > >
> > > > > > the two S are distinct and objects of that type do not conflict.
> > > > >
> > > > > This is surprising as these types are compatible across TUs. So
> > > > > if some pointer is passed between these functions this is
> > > > > supposed to work.
> > > >
> > > > So if they are compatible the frontend needs to mark them so in this case.
> > >
> > > It can't. The front end never sees the other TU.
> >
> > If the type "leaves" the CU via a call the called function has a prototype
> > through which it "sees" the CU.
>
> The prototype could be local to the function or it could be a void*
> (or other pointer type) argument.
>
>
>
> TU1-----------------------
> #include <stdio.h>
>
> extern void f(void *p);
>
> int main()
> {
>         struct foo { int x; } b;
>         b.x = 2;
>         f(&b);
>         printf("%d\n", b.x);
> }
>
> TU2-----------------------------
> extern void f(void *p)
> {
>         struct foo { int x; } *q = p;
>         q->x = 3;
> }

If the frontend puts those structures at local scope
then yes, the above presents a problem to LTO
at the moment.  So, trigger some inlining,
make main() read b.x after f(&b) and assert that
it is 3.  I think that would fail at the moment.

Richard.

> Best,
> Martin