[C PATCH] Fix -Wunused-function (PR debug/66869)

[Jakub Jelinek <jakub@redhat.com>]

Hi!

The early-debug changes moved warnings about unused functions into cgraph.
The problem is that if we have just unused declarations, they aren't
sometimes even registered with cgraph and therefore we no longer warn.

Here is an attempt to register those with cgraph anyway to get the warning,
for C FE only (no idea where to do that in C++ FE).  Or anyone has better
suggestions what to do?

Bootstrapped/regtested on x86_64-linux and i686-linux.

2016-01-25  Jakub Jelinek  <jakub@redhat.com>

	PR debug/66869
	* c-decl.c (c_write_global_declarations_1): For warn_unused_function,
	ensure creation of cgraph node even if there is no definition.

	* gcc.dg/pr66869.c: New test.

--- gcc/c/c-decl.c.jj	2016-01-21 00:41:47.000000000 +0100
+++ gcc/c/c-decl.c	2016-01-25 16:36:31.973504082 +0100
@@ -10741,11 +10741,19 @@ c_write_global_declarations_1 (tree glob
       if (TREE_CODE (decl) == FUNCTION_DECL
 	  && DECL_INITIAL (decl) == 0
 	  && DECL_EXTERNAL (decl)
-	  && !TREE_PUBLIC (decl)
-	  && C_DECL_USED (decl))
+	  && !TREE_PUBLIC (decl))
 	{
-	  pedwarn (input_location, 0, "%q+F used but never defined", decl);
-	  TREE_NO_WARNING (decl) = 1;
+	  if (C_DECL_USED (decl))
+	    {
+	      pedwarn (input_location, 0, "%q+F used but never defined", decl);
+	      TREE_NO_WARNING (decl) = 1;
+	    }
+	  /* For -Wunused-function push the unused statics into cgraph,
+	     so that check_global_declaration emits the warning.  */
+	  else if (warn_unused_function
+		   && ! DECL_ARTIFICIAL (decl)
+		   && ! TREE_NO_WARNING (decl))
+	    cgraph_node::get_create (decl);
 	}
 
       wrapup_global_declaration_1 (decl);
--- gcc/testsuite/gcc.dg/pr66869.c.jj	2016-01-25 16:38:39.037758657 +0100
+++ gcc/testsuite/gcc.dg/pr66869.c	2016-01-25 16:39:42.346888954 +0100
@@ -0,0 +1,6 @@
+/* PR debug/66869 */
+/* { dg-do compile } */
+/* { dg-options "-Wunused-function" } */
+
+static void test (void); /* { dg-warning "'test' declared 'static' but never defined" } */
+int i;

	Jakub