Index: update_llc_test_checks.py
===================================================================
--- update_llc_test_checks.py
+++ update_llc_test_checks.py
@@ -102,6 +102,15 @@
 
       check_prefixes = [item for m in common.CHECK_PREFIX_RE.finditer(filecheck_cmd)
                                for item in m.group(1).split(',')]
+
+      fc_cmd_parts = filecheck_cmd.split()
+      for part in fc_cmd_parts:
+          if "check-prefix=" in part:
+            wrong_prefix = part.split('=')[1]
+            if ',' in wrong_prefix:
+              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']