Index: utils/UpdateTestChecks/common.py
===================================================================
--- utils/UpdateTestChecks/common.py
+++ utils/UpdateTestChecks/common.py
@@ -47,6 +47,7 @@
 
 RUN_LINE_RE = re.compile('^\s*[;#]\s*RUN:\s*(.*)$')
 CHECK_PREFIX_RE = re.compile('--?check-prefix(?:es)?[= ](\S+)')
+PREFIX_RE = re.compile('^[a-zA-Z0-9_-]+$')
 CHECK_RE = re.compile(r'^\s*[;#]\s*([^:]+?)(?:-NEXT|-NOT|-DAG|-LABEL)?:')
 
 OPT_FUNCTION_RE = re.compile(
@@ -264,3 +265,24 @@
 def add_analyze_checks(output_lines, comment_marker, prefix_list, func_dict, func_name):
   check_label_format = '{} %s-LABEL: \'%s\''.format(comment_marker)
   add_checks(output_lines, comment_marker, prefix_list, func_dict, func_name, check_label_format, False, True)
+
+
+def check_prefix(prefix):
+  if not PREFIX_RE.match(prefix):
+        print('WARNING: Supplied prefix \'%s\' is invalid. Prefix must contain only alphanumeric characters, hyphens and underscores.' %
+              (prefix,), file=sys.stderr)
+
+
+def verify_filecheck_prefixes(fc_cmd):
+  fc_cmd_parts = fc_cmd.split()
+  for part in fc_cmd_parts:
+    if "check-prefix=" in part:
+      prefix = part.split('=', 1)[1]
+      check_prefix(prefix)
+    elif "check-prefixes=" in part:
+      prefixes = part.split('=', 1)[1].split(',')
+      for prefix in prefixes:
+        check_prefix(prefix)
+        if prefixes.count(prefix) > 1:
+          print('WARNING: Supplied prefix \'%s\' is not unique in the prefix list.' %
+                (prefix,), file=sys.stderr)
Index: utils/update_analyze_test_checks.py
===================================================================
--- utils/update_analyze_test_checks.py
+++ utils/update_analyze_test_checks.py
@@ -92,6 +92,7 @@
     prefix_list = []
     for l in run_lines:
       (tool_cmd, filecheck_cmd) = tuple([cmd.strip() for cmd in l.split('|', 1)])
+      common.verify_filecheck_prefixes(filecheck_cmd)
 
       if not tool_cmd.startswith(opt_basename + ' '):
         print('WARNING: Skipping non-%s RUN line: %s' % (opt_basename, l), file=sys.stderr)
Index: utils/update_cc_test_checks.py
===================================================================
--- utils/update_cc_test_checks.py
+++ utils/update_cc_test_checks.py
@@ -168,6 +168,7 @@
 
       # Extract -check-prefix in FileCheck args
       filecheck_cmd = commands[-1]
+      common.verify_filecheck_prefixes(filecheck_cmd)
       if not filecheck_cmd.startswith('FileCheck '):
         print('WARNING: Skipping non-FileChecked RUN line: ' + l, file=sys.stderr)
         continue
Index: utils/update_llc_test_checks.py
===================================================================
--- utils/update_llc_test_checks.py
+++ utils/update_llc_test_checks.py
@@ -89,6 +89,7 @@
       filecheck_cmd = ''
       if len(commands) > 1:
         filecheck_cmd = commands[1]
+      common.verify_filecheck_prefixes(filecheck_cmd)
       if not llc_cmd.startswith('llc '):
         print('WARNING: Skipping non-llc RUN line: ' + l, file=sys.stderr)
         continue
Index: utils/update_mca_test_checks.py
===================================================================
--- utils/update_mca_test_checks.py
+++ utils/update_mca_test_checks.py
@@ -116,6 +116,7 @@
       _warn('could not split tool and filecheck commands: {}'.format(run_line))
       continue
 
+    common.verify_filecheck_prefixes(filecheck_cmd)
     tool_basename = os.path.splitext(os.path.basename(args.llvm_mca_binary))[0]
 
     if not tool_cmd.startswith(tool_basename + ' '):
Index: utils/update_mir_test_checks.py
===================================================================
--- utils/update_mir_test_checks.py
+++ utils/update_mir_test_checks.py
@@ -122,6 +122,7 @@
         commands = [cmd.strip() for cmd in l.split('|', 1)]
         llc_cmd = commands[0]
         filecheck_cmd = commands[1] if len(commands) > 1 else ''
+        common.verify_filecheck_prefixes(filecheck_cmd)
 
         if not llc_cmd.startswith('llc '):
             warn('Skipping non-llc RUN line: {}'.format(l), test_file=test)
Index: utils/update_test_checks.py
===================================================================
--- utils/update_test_checks.py
+++ utils/update_test_checks.py
@@ -103,7 +103,7 @@
     prefix_list = []
     for l in run_lines:
       (tool_cmd, filecheck_cmd) = tuple([cmd.strip() for cmd in l.split('|', 1)])
-
+      common.verify_filecheck_prefixes(filecheck_cmd)
       if not tool_cmd.startswith(opt_basename + ' '):
         print('WARNING: Skipping non-%s RUN line: %s' % (opt_basename, l), file=sys.stderr)
         continue