Re: [PATCH] c++: Retry the aliasing of base/complete cdtor optimization at import_export_decl time [PR113208]

[Jan Hubicka <hubicka@ucw.cz>]

> 
> I've tried to see what actually happens during linking without LTO, so compiled
> pr113208_0.C with -O1 -fkeep-inline-functions -std=c++20 with vanilla trunk
> (so it has those 2 separate comdats, one for C2 and one for C1), though I've
> changed the
> void m(k);
> line to
> __attribute__((noipa)) void m(k) {}
> in the testcase, then compiled
> pr113208_1.C with -O2 -fkeep-inline-functions -std=c++20 -fno-omit-frame-pointer
> so that one can clearly differentiate from where the implementation was
> picked and finally added
> template <typename _Tp> struct _Vector_base {
>   int g() const;
>   _Vector_base(int, int);
> };
> 
> struct QualityValue;
> template <>
> _Vector_base<QualityValue>::_Vector_base(int, int) {}
> template <>
> int _Vector_base<QualityValue>::g() const { return 0; }
> int main () {}
> If I link this, I see _ZN6vectorI12QualityValueEC2ERKS1_ and
> _ZN6vectorI12QualityValueEC1ERKS1_ as separate functions with the
> omitted frame pointer bodies, so clearly the pr113208_0.C versions prevailed
> in both cases.  It is unclear why that isn't the case for LTO.

I think it is because of -fkeep-inline-functions which makes the first
object file to define both symbols, while with LTO we optimize out one
of them.  

So to reproduce same behaviour with non-LTO we would probably need use
-O1 and arrange the contructor to be unilinable instead of using
-fkeep-inline-functions.

Honza