Re: [PATCH] c++, v3: Retry the aliasing of base/complete cdtor optimization at import_export_decl time [PR113208]

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Apr 24, 2024 at 06:39:33PM -0400, Jason Merrill wrote:
> > --- gcc/cp/decl2.cc.jj	2024-04-23 14:49:41.933186265 +0200
> > +++ gcc/cp/decl2.cc	2024-04-24 15:17:09.043625729 +0200
> > @@ -3314,7 +3314,16 @@ tentative_decl_linkage (tree decl)
> >   	     to mark the functions at this point.  */
> >   	  if (DECL_DECLARED_INLINE_P (decl)
> >   	      && (!DECL_IMPLICIT_INSTANTIATION (decl)
> > -		  || DECL_DEFAULTED_FN (decl)))
> > +		  || DECL_DEFAULTED_FN (decl)
> > +		  /* For implicit instantiations of cdtors,
> > +		     if import_export_decl would use comdat linkage,
> > +		     make sure to use it right away, so that maybe_clone_body
> > +		     can use aliases.  See PR113208.  */
> > +		  || (DECL_MAYBE_IN_CHARGE_CDTOR_P (decl)
> > +		      && (flag_implicit_templates
> > +			  || flag_implicit_inline_templates)
> > +		      && flag_weak
> > +		      && TARGET_SUPPORTS_ALIASES)))
> 
> It seems wrong to set DECL_INTERFACE_KNOWN for cdtors that might get an
> explicit instantiation later, and likewise for comdat_linkage when
> !flag_weak; instead of adding this condition to the if, how about adding an
> else like
> 
> >           else if (DECL_MAYBE_IN_CHARGE_CDTOR_P (decl))
> >             /* For implicit instantiations of cdtors,
> > if import_export_decl would use comdat linkage,
> > make sure to use it right away, so that maybe_clone_body
> > can use aliases.  See PR113208.  */
> >             maybe_make_one_only (decl);
> 
> ?

Then can_alias_cdtor would return false, because it ends with:
  /* Don't use aliases for weak/linkonce definitions unless we can put both
     symbols in the same COMDAT group.  */
  return (DECL_INTERFACE_KNOWN (fn)
          && (SUPPORTS_ONE_ONLY || !DECL_WEAK (fn))
          && (!DECL_ONE_ONLY (fn)
              || (HAVE_COMDAT_GROUP && DECL_WEAK (fn))));
Should we change that DECL_INTERFACE_KNOWN (fn) in there to
(DECL_INTERFACE_KNOWN (fn) || something) then and what that
something should be?  HAVE_COMDAT_GROUP && DECL_ONE_ONLY (fn)?

	Jakub