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

[Jason Merrill]

On 4/24/24 15:47, Jakub Jelinek wrote:

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)?

Yes, I think reorganize to

((DECL_INTERFACE_KNOWN (fn) && !DECL_WEAK (fn) && !DECL_ONE_ONLY (fn))
 || (HAVE_COMDAT_GROUP && DECL_ONE_ONLY (fn))

Jason