Index: gc.c
===================================================================
--- gc.c	(revision 22907)
+++ gc.c	(working copy)
@@ -1127,7 +1127,7 @@
     int ret;
     rb_thread_t *th = GET_THREAD();
     SET_STACK_END;
-    ret = STACK_LENGTH > STACK_LEVEL_MAX - GC_WATER_MARK;
+    ret = STACK_LENGTH > (long)(STACK_LEVEL_MAX - GC_WATER_MARK);
 #ifdef __ia64
     if (!ret) {
         ret = (VALUE*)rb_ia64_bsp() - th->machine_register_stack_start >