Index: clang-tidy/tool/clang-tidy-diff.py
===================================================================
--- clang-tidy/tool/clang-tidy-diff.py
+++ clang-tidy/tool/clang-tidy-diff.py
@@ -24,10 +24,88 @@
 """
 
 import argparse
+import glob
 import json
+import multiprocessing
+import os
 import re
+import shutil
 import subprocess
 import sys
+import tempfile
+import threading
+import traceback
+import yaml
+
+is_py2 = sys.version[0] == '2'
+
+if is_py2:
+    import Queue as queue
+else:
+    import queue as queue
+
+
+def run_tidy(task_queue, lock, timeout):
+  watchdog = None
+  while True:
+    command = task_queue.get()
+    try:
+      proc = subprocess.Popen(command,
+                              stdout=subprocess.PIPE,
+                              stderr=subprocess.PIPE)
+
+      if timeout is not None:
+        watchdog = threading.Timer(timeout, proc.kill)
+        watchdog.start()
+
+      stdout, stderr = proc.communicate()
+
+      with lock:
+        sys.stdout.write(stdout.decode('utf-8') + '\n')
+        if stderr:
+          sys.stderr.write(stderr.decode('utf-8') + '\n')
+    except Exception as e:
+      with lock:
+        sys.stderr.write('Failed: ' + str(e) + ': '.join(command) + '\n')
+    finally:
+      with lock:
+        if (not timeout is None) and (not watchdog is None):
+          if not watchdog.is_alive():
+              sys.stderr.write('Terminated by timeout: ' +
+                               ' '.join(command) + '\n')
+          watchdog.cancel()
+      task_queue.task_done()
+
+
+def start_workers(max_tasks, tidy_caller, task_queue, lock, timeout):
+  for _ in range(max_tasks):
+    t = threading.Thread(target=tidy_caller, args=(task_queue, lock, timeout))
+    t.daemon = True
+    t.start()
+
+def merge_replacement_files(tmpdir, mergefile):
+  """Merge all replacement files in a directory into a single file"""
+  # The fixes suggested by clang-tidy >= 4.0.0 are given under
+  # the top level key 'Diagnostics' in the output yaml files
+  mergekey="Diagnostics"
+  merged=[]
+  for replacefile in glob.iglob(os.path.join(tmpdir, '*.yaml')):
+    content = yaml.safe_load(open(replacefile, 'r'))
+    if not content:
+      continue # Skip empty files.
+    merged.extend(content.get(mergekey, []))
+
+  if merged:
+    # MainSourceFile: The key is required by the definition inside
+    # include/clang/Tooling/ReplacementsYaml.h, but the value
+    # is actually never used inside clang-apply-replacements,
+    # so we set it to '' here.
+    output = { 'MainSourceFile': '', mergekey: merged }
+    with open(mergefile, 'w') as out:
+      yaml.safe_dump(output, out)
+  else:
+    # Empty the file:
+    open(mergefile, 'w').close()
 
 
 def main():
@@ -48,6 +126,10 @@
                       help='custom pattern selecting file paths to check '
                       '(case insensitive, overridden by -regex)')
 
+  parser.add_argument('-j', type=int, default=1,
+                      help='number of tidy instances to be run in parallel.')
+  parser.add_argument('-timeout', type=int, default=None,
+                      help='timeout per each file in seconds.')
   parser.add_argument('-fix', action='store_true', default=False,
                       help='apply suggested fixes')
   parser.add_argument('-checks',
@@ -84,7 +166,7 @@
     match = re.search('^\+\+\+\ \"?(.*?/){%s}([^ \t\n\"]*)' % args.p, line)
     if match:
       filename = match.group(2)
-    if filename == None:
+    if filename is None:
       continue
 
     if args.regex is not None:
@@ -102,44 +184,83 @@
         line_count = int(match.group(3))
       if line_count == 0:
         continue
-      end_line = start_line + line_count - 1;
+      end_line = start_line + line_count - 1
       lines_by_file.setdefault(filename, []).append([start_line, end_line])
 
-  if len(lines_by_file) == 0:
+  if not any(lines_by_file):
     print("No relevant changes found.")
     sys.exit(0)
 
-  line_filter_json = json.dumps(
-    [{"name" : name, "lines" : lines_by_file[name]} for name in lines_by_file],
-    separators = (',', ':'))
+  max_task_count = args.j
+  if max_task_count == 0:
+      max_task_count = multiprocessing.cpu_count()
+  max_task_count = min(len(lines_by_file), max_task_count)
 
-  quote = "";
-  if sys.platform == 'win32':
-    line_filter_json=re.sub(r'"', r'"""', line_filter_json)
-  else:
-    quote = "'";
+  tmpdir = None
+  if args.export_fixes:
+    tmpdir = tempfile.mkdtemp()
+
+  # Tasks for clang-tidy.
+  task_queue = queue.Queue(max_task_count)
+  # A lock for console output.
+  lock = threading.Lock()
+
+  # Run a pool of clang-tidy workers.
+  start_workers(max_task_count, run_tidy, task_queue, lock, args.timeout)
 
-  # Run clang-tidy on files containing changes.
-  command = [args.clang_tidy_binary]
-  command.append('-line-filter=' + quote + line_filter_json + quote)
+  quote = ""
+  if sys.platform != 'win32':
+    quote = "'"
+
+  # Form the common args list.
+  common_clang_tidy_args = []
   if args.fix:
-    command.append('-fix')
-  if args.export_fixes:
-    command.append('-export-fixes=' + args.export_fixes)
+    common_clang_tidy_args.append('-fix')
   if args.checks != '':
-    command.append('-checks=' + quote + args.checks + quote)
+    common_clang_tidy_args.append('-checks=' + quote + args.checks + quote)
   if args.quiet:
-    command.append('-quiet')
+    common_clang_tidy_args.append('-quiet')
   if args.build_path is not None:
-    command.append('-p=%s' % args.build_path)
-  command.extend(lines_by_file.keys())
+    common_clang_tidy_args.append('-p=%s' % args.build_path)
   for arg in args.extra_arg:
-      command.append('-extra-arg=%s' % arg)
+    common_clang_tidy_args.append('-extra-arg=%s' % arg)
   for arg in args.extra_arg_before:
-      command.append('-extra-arg-before=%s' % arg)
-  command.extend(clang_tidy_args)
+    common_clang_tidy_args.append('-extra-arg-before=%s' % arg)
+
+  for name in lines_by_file:
+    line_filter_json = json.dumps(
+      [{"name": name, "lines": lines_by_file[name]}],
+      separators=(',', ':'))
+
+    # Run clang-tidy on files containing changes.
+    command = [args.clang_tidy_binary]
+    command.append('-line-filter=' + quote + line_filter_json + quote)
+    if args.export_fixes:
+      # Get a temporary file. We immediately close the handle so clang-tidy can
+      # overwrite it.
+      (handle, tmp_name) = tempfile.mkstemp(suffix='.yaml', dir=tmpdir)
+      os.close(handle)
+      command.append('-export-fixes=' + tmp_name)
+    command.extend(common_clang_tidy_args)
+    command.append(name)
+    command.extend(clang_tidy_args)
+
+    task_queue.put(command)
+
+  # Wait for all threads to be done.
+  task_queue.join()
+
+  if args.export_fixes:
+    print('Writing fixes to ' + args.export_fixes + ' ...')
+    try:
+      merge_replacement_files(tmpdir, args.export_fixes)
+    except:
+      sys.stderr.write('Error exporting fixes.\n')
+      traceback.print_exc()
+
+  if tmpdir:
+    shutil.rmtree(tmpdir)
 
-  sys.exit(subprocess.call(' '.join(command), shell=True))
 
 if __name__ == '__main__':
   main()
Index: clang-tidy/tool/run-clang-tidy.py
===================================================================
--- clang-tidy/tool/run-clang-tidy.py
+++ clang-tidy/tool/run-clang-tidy.py
@@ -279,8 +279,8 @@
       if file_name_re.search(name):
         task_queue.put(name)
 
-    # Wait for all threads to be done.
-    task_queue.join()
+      # Wait for all threads to be done.
+      task_queue.join()
     if len(failed_files):
       return_code = 1