On Wed, Oct 2, 2019 at 3:10 PM Richard Biener
<richard.guent...@gmail.com> wrote:
>
> On Wed, Oct 2, 2019 at 2:35 PM Uecker, Martin
> <martin.uec...@med.uni-goettingen.de> wrote:
> >
> > Am Mittwoch, den 02.10.2019, 14:18 +0200 schrieb Richard Biener:
> > > On Wed, Oct 2, 2019 at 1:57 PM Uecker, Martin
> > > <martin.uec...@med.uni-goettingen.de> wrote:
> > > >
> >
> > Thank you for your answers.
> >
> > > > Finally, how does LTO does it? It somehow also needs to unify
> > > > different tagged types? Could we reuse this mechanism somehow?
> > >
> > > LTO structurally merges types via TYPE_CANONICAL. But rules
> > > for merging depend on language semantics, too much merging
> > > hinders optimization.
> >
> > LTO would need to merge types with identical tag and structure
> > across TUs anyway as this is needed for C programs to work.
> > But this implies that it also must merge such types inside a TU
> > (because merging enforces an equivalence relationship).
> > So if I am not missing anything important, LTO would already
> > implement the exact semantics which I propose for C2X.
>
> Sure LTO handles C2X fine. The issue is that it creates way
> larger equivalence classes than necessary for C2X (it has to
> work across language boundaries where compatibility is much
> less specified).
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.
Richard.
> Richard.
>
> > Best,
> > Martin
