re PR middle-end/68878 (471.omnetpp in SPEC CPU 2006 is miscompiled with LTO)

	PR lto/68878
	* lto-symtab.c (lto_symtab_prevailing_virtual_decl): New function.
	* lto-symtab.h (lto_symtab_prevailing_virtual_decl): Declare.
	(lto_symtab_prevailing_decl): Use it.

From-SVN: r231671

gcc/lto/ChangeLog

+2015-12-10  Jan Hubicka  <hubicka@ucw.cz>
+
+	PR lto/68878
+	* lto-symtab.c (lto_symtab_prevailing_virtual_decl): New function.
+	* lto-symtab.h (lto_symtab_prevailing_virtual_decl): Declare.
+	(lto_symtab_prevailing_decl): Use it.
+
 2015-12-15  Ilya Verbin  <ilya.verbin@intel.com>
 
 	* lto.c: Include stringpool.h and fold-const.h.

gcc/lto/lto-symtab.c

@@ -517,6 +517,8 @@ lto_symtab_merge_p (tree prevailing, tree decl)
 		 "TREE_CODE mismatch\n");
       return false;
     }
+  gcc_checking_assert (TREE_CHAIN (prevailing) == TREE_CHAIN (decl));
+  
   if (TREE_CODE (prevailing) == FUNCTION_DECL)
     {
       if (DECL_BUILT_IN (prevailing) != DECL_BUILT_IN (decl))
@@ -883,6 +885,11 @@ lto_symtab_merge_symbols_1 (symtab_node *prevailing)
 	  else
 	    {
 	      DECL_INITIAL (e->decl) = error_mark_node;
+	      if (e->lto_file_data)
+		{
+		  lto_free_function_in_decl_state_for_node (e);
+		  e->lto_file_data = NULL;
+		}
 	      symtab->call_varpool_removal_hooks (dyn_cast<varpool_node *> (e));
 	    }
 	  e->remove_all_references ();
@@ -968,3 +975,33 @@ lto_symtab_merge_symbols (void)
 	}
     }
 }
+
+/* Virtual tables may matter for code generation even if they are not
+   directly refernced by the code because they may be used for devirtualizaiton.
+   For this reason it is important to merge even virtual tables that have no
+   associated symbol table entries.  Without doing so we lose optimization
+   oppurtunities by losing track of the vtable constructor.
+   FIXME: we probably ought to introduce explicit symbol table entries for
+   those before streaming.  */
+
+tree
+lto_symtab_prevailing_virtual_decl (tree decl)
+{
+  gcc_checking_assert (!type_in_anonymous_namespace_p (DECL_CONTEXT (decl))
+		       && DECL_ASSEMBLER_NAME_SET_P (decl));
+
+  symtab_node *n = symtab_node::get_for_asmname
+		     (DECL_ASSEMBLER_NAME (decl));
+  while (n && ((!DECL_EXTERNAL (n->decl) && !TREE_PUBLIC (n->decl))
+	       || !DECL_VIRTUAL_P (n->decl)))
+    n = n->next_sharing_asm_name;
+  if (n)
+    {
+      lto_symtab_prevail_decl (n->decl, decl);
+      decl = n->decl;
+    }
+  else
+    symtab_node::get_create (decl);
+
+  return decl;
+}

gcc/lto/lto-symtab.h

@@ -20,6 +20,7 @@ along with GCC; see the file COPYING3.  If not see
 extern void lto_symtab_merge_decls (void);
 extern void lto_symtab_merge_symbols (void);
 extern tree lto_symtab_prevailing_decl (tree decl);
+extern tree lto_symtab_prevailing_virtual_decl (tree decl);
 
 /* Mark DECL to be previailed by PREVAILING.
    Use DECL_ABSTRACT_ORIGIN and DECL_CHAIN as special markers; those do not
@@ -31,6 +32,7 @@ inline void
 lto_symtab_prevail_decl (tree prevailing, tree decl)
 {
   gcc_checking_assert (DECL_ABSTRACT_ORIGIN (decl) != error_mark_node);
+  gcc_assert (TREE_PUBLIC (decl) || DECL_EXTERNAL (decl));
   DECL_CHAIN (decl) = prevailing;
   DECL_ABSTRACT_ORIGIN (decl) = error_mark_node;
 }
@@ -43,5 +45,12 @@ lto_symtab_prevailing_decl (tree decl)
   if (DECL_ABSTRACT_ORIGIN (decl) == error_mark_node)
     return DECL_CHAIN (decl);
   else
-    return decl;
+    {
+      if ((TREE_CODE (decl) == VAR_DECL || TREE_CODE (decl) == FUNCTION_DECL)
+	  && DECL_VIRTUAL_P (decl)
+	  && (TREE_PUBLIC (decl) || DECL_EXTERNAL (decl))
+	  && !symtab_node::get (decl))
+	return lto_symtab_prevailing_virtual_decl (decl);
+      return decl;
+    }
 }