compatibility of structs/unions/enums in the middle end

[Richard Biener]

On October 2, 2019 3:55:43 PM GMT+02:00, "Uecker, Martin" <Martin.Uecker@med.uni-goettingen.de> wrote:
>Am Mittwoch, den 02.10.2019, 15:12 +0200 schrieb Richard Biener:
>> On Wed, Oct 2, 2019 at 3:10 PM Richard Biener
>> <richard.guenther@gmail.com> wrote:
>> > 
>> > On Wed, Oct 2, 2019 at 2:35 PM Uecker, Martin
>> > <Martin.Uecker@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.Uecker@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).
>
>Ok, using this would also for our purposes would pessimize things
>too much.
>
>> 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. 

Richard. 

>Best,
>Martin