Re: [C++ PATCH] Optimize C++ comdat ctors/dtors in classes without virtual bases (PR c++/3187, take 4)

[Jakub Jelinek <jakub@redhat.com>]

On Tue, Dec 01, 2009 at 04:46:52AM -0500, Jakub Jelinek wrote:
> Unfortunately, when bootstrapping with extra checking patch (second patch)
> where I wanted to verify that for classes with comdat virtual dtors we
> always emit either both D2 (and D1 alias to it) and D0 dtor, or none of
> them, build in libstdc++ revealed that we emit e.g. in complex_io.cc
> (reduced testcase attached last, cc1plus -O2 cplx.ii) only the
> _ZN1DIi1BIiE1CIiEED2Ev (and _ZN1DIi1BIiE1CIiEED1Ev alias) dtor in
> the _ZN1DIi1BIiE1CIiEED5Ev comdat group, but not _ZN1DIi1BIiE1CIiEED0Ev.
> The class is extern template class, so either the g++ is buggy and emits
> something it never should, because the template is extern, or
> putting the *D0* dtors also in the *D5* comdat group is not a good idea.

Here is the additional patch that allowed the 2 patches to bootstrap.
The problem is with implicitly defined methods in extern template class XXX<NNN>
instantiations.  The synthetized methods don't have
DECL_TEMPLATE_INSTANTIATION set, thus don't go through
instantiate_class_member and thus are emitted whenever something needs them
(and the side with explicit non-extern instantiation doesn't necessarily
define all the functions).  In this light the patch below is an ABI change
(not sure what other compilers do though), as with the patch newly compiled
code with extern template class XXX<NNN> would no longer emit routines
it needs it, instead rely on the CU with
extern template class XXX<NNN>; template class XXX<NNN>;
to emit them.  But as the code emitted by older G++ doesn't guarantee all
the implicitly declared methods are emitted there, mixing new code and old
code could result in undefined references.  The methods are usually inlined,
sure, but e.g. if ipa-inline.c considers them called in cold blocks, it
decides not to inline them.

--- gcc/cp/pt.c.jj	2009-12-01 12:09:10.000000000 +0100
+++ gcc/cp/pt.c	2009-12-01 13:33:39.000000000 +0100
@@ -16055,6 +16055,31 @@ do_type_instantiation (tree t, tree stor
 	if (TREE_CODE (tmp) == FUNCTION_DECL
 	    && DECL_TEMPLATE_INSTANTIATION (tmp))
 	  instantiate_class_member (tmp, extern_p);
+	else if (TREE_CODE (tmp) == FUNCTION_DECL
+		 && DECL_ARTIFICIAL (tmp)
+		 && (DECL_CONSTRUCTOR_P (tmp)
+		     || DECL_DESTRUCTOR_P (tmp)
+		     || DECL_ASSIGNMENT_OPERATOR_P (tmp)))
+	  {
+	    /* Ensure the implicitly declared ctors/dtors/assignment operators
+	       will be either inlined, or extern calls for extern_p,
+	       or defined in the current TU otherwise.  */
+	    DECL_EXTERNAL (tmp) = 1;
+	    if (extern_p)
+	      DECL_NOT_REALLY_EXTERN (tmp) = 0;
+	    else
+	      {
+		mark_definable (tmp);
+		/* Always make artificials weak.  */
+		if (flag_weak)
+		  comdat_linkage (tmp);
+		/* For WIN32 we also want to put explicit instantiations in
+		   linkonce sections.  */
+		else if (TREE_PUBLIC (tmp))
+		  maybe_make_one_only (tmp);
+	      }
+	    DECL_INTERFACE_KNOWN (tmp) = 1;
+	  }
 
     for (tmp = TYPE_FIELDS (t); tmp; tmp = TREE_CHAIN (tmp))
       if (TREE_CODE (tmp) == VAR_DECL && DECL_TEMPLATE_INSTANTIATION (tmp))


	Jakub