cgraphunit: Avoid code generation differences based on -w/TREE_NO_WARNING [PR94277]

The following testcase FAILs with -fcompare-debug, but not because -g vs.
-g0 would make a difference, but because the second compilation is done with
-w in order not to emit warnings twice and -w seems to affect the *.gkd dump
content.
This is because TREE_NO_WARNING flag, or warn_unused_function does affect
not just whether a warning/pedwarn is printed, but also whether we set
TREE_PUBLIC on such decls.
The following patch makes sure we set it regardless of anything warning
related (TREE_NO_WARNING or warn_unused_function).

2020-03-24  Jakub Jelinek  <jakub@redhat.com>

	PR debug/94277
	* cgraphunit.c (check_global_declaration): For DECL_EXTERNAL and
	non-TREE_PUBLIC non-DECL_ARTIFICIAL FUNCTION_DECLs, set TREE_PUBLIC
	regardless of whether TREE_NO_WARNING is set on it or whether
	warn_unused_function is true or not.

	* gcc.dg/pr94277.c: New test.

gcc/ChangeLog

+2020-03-24  Jakub Jelinek  <jakub@redhat.com>
+
+	PR debug/94277
+	* cgraphunit.c (check_global_declaration): For DECL_EXTERNAL and
+	non-TREE_PUBLIC non-DECL_ARTIFICIAL FUNCTION_DECLs, set TREE_PUBLIC
+	regardless of whether TREE_NO_WARNING is set on it or whether
+	warn_unused_function is true or not.
+
 2020-03-23  Jeff Law  <law@redhat.com>
 
 	PR rtl-optimization/90275

gcc/cgraphunit.c

@@ -1068,15 +1068,15 @@ check_global_declaration (symtab_node *snode)
       && DECL_INITIAL (decl) == 0
       && DECL_EXTERNAL (decl)
       && ! DECL_ARTIFICIAL (decl)
-      && ! TREE_NO_WARNING (decl)
-      && ! TREE_PUBLIC (decl)
-      && (warn_unused_function
-	  || snode->referred_to_p (/*include_self=*/false)))
+      && ! TREE_PUBLIC (decl))
     {
-      if (snode->referred_to_p (/*include_self=*/false))
+      if (TREE_NO_WARNING (decl))
+	;
+      else if (snode->referred_to_p (/*include_self=*/false))
 	pedwarn (input_location, 0, "%q+F used but never defined", decl);
       else
-	warning (OPT_Wunused_function, "%q+F declared %<static%> but never defined", decl);
+	warning (OPT_Wunused_function, "%q+F declared %<static%> but never "
+				       "defined", decl);
       /* This symbol is effectively an "extern" declaration now.  */
       TREE_PUBLIC (decl) = 1;
     }

gcc/testsuite/ChangeLog

+2020-03-24  Jakub Jelinek  <jakub@redhat.com>
+
+	PR debug/94277
+	* gcc.dg/pr94277.c: New test.
+
 2020-03-23  Jeff Law  <law@redhat.com>
 
 	PR target/94144

gcc/testsuite/gcc.dg/pr94277.c

+/* PR debug/94277 */
+/* { dg-do compile } */
+/* { dg-options "-fcompare-debug" } */
+
+static void foo (void);	/* { dg-warning "used but never defined" } */
+
+void
+bar (void)
+{
+  foo ();
+}