Index: test/CodeGen/AArch64/test-update_mir_test_checks.py-itself.mir
===================================================================
--- test/CodeGen/AArch64/test-update_mir_test_checks.py-itself.mir
+++ test/CodeGen/AArch64/test-update_mir_test_checks.py-itself.mir
@@ -13,6 +13,9 @@
 # STDERR-SAME:     sed -e {{.*}} RUN-NESTED{{.*}}SOURCE{{.*}}STDERR{{.*}}SOURCE
 # STDERR-NOT:  WARNING
 
+# SOURCE-LABEL: # NOTE: Assertions have been autogenerated by
+# SOURCE-SAME:  utils/update_mir_test_checks.py
+#
 # SOURCE:      WARNING
 # SOURCE-NEXT: WARNING
 # SOURCE-NEXT:     sed
Index: utils/update_mir_test_checks.py
===================================================================
--- utils/update_mir_test_checks.py
+++ utils/update_mir_test_checks.py
@@ -29,6 +29,7 @@
 RUN_LINE_RE = re.compile('^\s*[;#]\s*RUN:\s*(.*)$')
 TRIPLE_ARG_RE = re.compile(r'-mtriple[= ]([^ ]+)')
 MARCH_ARG_RE = re.compile(r'-march[= ]([^ ]+)')
+LANG_ARG_RE = re.compile(r'-x[= ]([^ ]+)')
 TRIPLE_IR_RE = re.compile(r'^\s*target\s+triple\s*=\s*"([^"]+)"$')
 CHECK_PREFIX_RE = re.compile('--?check-prefix(?:es)?[= ](\S+)')
 CHECK_RE = re.compile(r'^\s*[;#]\s*([^:]+?)(?:-NEXT|-NOT|-DAG|-LABEL)?:')
@@ -74,10 +75,11 @@
 
 
 class Run:
-    def __init__(self, prefixes, cmd_args, triple):
+    def __init__(self, prefixes, cmd_args, triple, lang):
         self.prefixes = prefixes
         self.cmd_args = cmd_args
         self.triple = triple
+        self.lang = lang
 
     def __getitem__(self, index):
         return [self.prefixes, self.cmd_args, self.triple][index]
@@ -143,6 +145,9 @@
         if m and not triple:
             triple = '{}--'.format(m.group(1))
 
+        m = LANG_ARG_RE.search(llc_cmd)
+        lang = (m.group(1) if m else test.split('.')[-1]).lower()
+
         cmd_args = llc_cmd[len('llc'):].strip()
         cmd_args = cmd_args.replace('< %s', '').replace('%s', '').strip()
 
@@ -152,7 +157,7 @@
             check_prefixes = ['CHECK']
         all_prefixes += check_prefixes
 
-        run_list.append(Run(check_prefixes, cmd_args, triple))
+        run_list.append(Run(check_prefixes, cmd_args, triple, lang))
 
     # Remove any common prefixes. We'll just leave those entirely alone.
     common_prefixes = set([prefix for prefix in all_prefixes
@@ -344,7 +349,7 @@
         warn('Ignoring common prefixes: {}'.format(common_prefixes),
              test_file=test)
 
-    comment_char = '#' if test.endswith('.mir') else ';'
+    comment_char = '#' if any(run.lang == 'mir' for run in run_list) else ';'
     autogenerated_note = ('{} NOTE: Assertions have been autogenerated by '
                           'utils/{}'.format(comment_char,
                                             os.path.basename(__file__)))