diff --git a/clang/test/AST/gen_ast_dump_json_test.py b/clang/test/AST/gen_ast_dump_json_test.py
index 4f4f37cd26d7..b03e3cb23771 100644
--- a/clang/test/AST/gen_ast_dump_json_test.py
+++ b/clang/test/AST/gen_ast_dump_json_test.py
@@ -1,4 +1,6 @@
 #!/usr/bin/env python
+
+from __future__ import print_function
 from collections import OrderedDict
 from shutil import copyfile
 import argparse
@@ -7,8 +9,10 @@ import os
 import pprint
 import re
 import subprocess
+import sys
 import tempfile
 
+
 def normalize(dict_var):
     for k, v in dict_var.items():
         if isinstance(v, OrderedDict):
@@ -56,13 +60,14 @@ def main():
     parser = argparse.ArgumentParser()
     parser.add_argument("--clang", help="The clang binary (could be a relative or absolute path)",
                         action="store", required=True)
-    parser.add_argument("--opts", help="other options",
-                        action="store", default='', type=str)
     parser.add_argument("--source", help="the source file. Command used to generate the json will be of the format <clang> -cc1 -ast-dump=json <opts> <source>",
                         action="store", required=True)
     parser.add_argument("--filters", help="comma separated list of AST filters. Ex: --filters=TypedefDecl,BuiltinType",
                         action="store", default='')
-    parser.add_argument("--update", help="Update the file in-place", action="store_true")
+    update_or_generate_group = parser.add_mutually_exclusive_group()
+    update_or_generate_group.add_argument("--update", help="Update the file in-place", action="store_true")
+    update_or_generate_group.add_argument("--opts", help="other options",
+                                          action="store", default='', type=str)
 
     args = parser.parse_args()
 
@@ -75,7 +80,6 @@ def main():
         print("clang binary specified not present.")
         return -1
 
-    options = args.opts.split(' ')
     filters = set(args.filters.split(',')) if args.filters else set()
     
     note = "// NOTE: CHECK lines have been autogenerated by " \
@@ -85,11 +89,32 @@ def main():
         note += "\n// using --filters=" + args.filters
     
     cmd = [clang_binary, "-cc1"]
+    if args.update:
+        # When updating the first line of the test must be a RUN: line
+        with open(args.source, "r") as srcf:
+            first_line = srcf.readline()
+        if "RUN: %clang_cc1 " not in first_line:
+            sys.exit("When using --update the first line of the input file must contain RUN: %clang_cc1")
+        clang_start = first_line.find("%clang_cc1") + len("%clang_cc1")
+        file_check_idx = first_line.rfind("| FileCheck")
+        if file_check_idx:
+            dump_cmd = first_line[clang_start:file_check_idx]
+        else:
+            dump_cmd = first_line[clang_start:]
+        print("Inferred run arguments as '", dump_cmd, "'", sep="")
+        options = dump_cmd.split()
+        if "-ast-dump=json" not in options:
+            sys.exit("ERROR: RUN: line does not contain -ast-dump=json")
+        if "%s" not in options:
+            sys.exit("ERROR: RUN: line does not contain %s")
+        options.remove("%s")
+    else:
+        options = args.opts.split()
+        options.append("-ast-dump=json")
     cmd.extend(options)
-
-    using_ast_dump_filter = 'ast-dump-filter' in args.opts
-        
-    cmd.extend(["-ast-dump=json", args.source])
+    using_ast_dump_filter = any('ast-dump-filter' in arg for arg in cmd)
+    cmd.append(args.source)
+    print("Will run", cmd)
 
     try:
         json_str = subprocess.check_output(cmd)