Index: utils/update_llc_test_checks.py
===================================================================
--- utils/update_llc_test_checks.py
+++ utils/update_llc_test_checks.py
@@ -102,6 +102,13 @@
 
       check_prefixes = [item for m in common.CHECK_PREFIX_RE.finditer(filecheck_cmd)
                                for item in m.group(1).split(',')]
+
+      if len(check_prefixes) > 1:
+          wrong_prefix = ",".join(check_prefixes)
+          if ("-check-prefix=" + wrong_prefix) in filecheck_cmd:
+            print('WARNING: Prefix \'%s\' is invalid. Did you mean \'-check-prefixes=%s\'?' %
+                  (wrong_prefix, wrong_prefix), file=sys.stderr)
+
       if not check_prefixes:
         check_prefixes = ['CHECK']