re PR debug/54177 (Segfault in cselib_lookup due to NULL_RTX passed from vt_add_function_parameter)

PR debug/54177
* var-tracking.c (vt_add_function_parameter): Bail if
var_lowpart fails.

From-SVN: r191999

gcc/ChangeLog

+2012-10-02  Alexandre Oliva <aoliva@redhat.com>
+
+	PR debug/54177
+	* var-tracking.c (vt_add_function_parameter): Bail if
+	var_lowpart fails.
+
 2012-10-02  H.J. Lu  <hongjiu.lu@intel.com>
 
 	PR target/54741

gcc/var-tracking.c

@@ -9428,6 +9428,7 @@ vt_add_function_parameter (tree parm)
       && GET_CODE (incoming) != PARALLEL)
     {
       cselib_val *val;
+      rtx lowpart;
 
       /* ??? We shouldn't ever hit this, but it may happen because
 	 arguments passed by invisible reference aren't dealt with
@@ -9436,7 +9437,11 @@ vt_add_function_parameter (tree parm)
       if (offset)
 	return;
 
-      val = cselib_lookup_from_insn (var_lowpart (mode, incoming), mode, true,
+      lowpart = var_lowpart (mode, incoming);
+      if (!lowpart)
+	return;
+
+      val = cselib_lookup_from_insn (lowpart, mode, true,
 				     VOIDmode, get_insns ());
 
       /* ??? Float-typed values in memory are not handled by