diff --git a/build/gen.py b/build/gen.py
index 772d090..ae9c40d 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -5,15 +5,16 @@
 
 """Generates build.ninja that will build GN."""
 
-import contextlib
-import errno
-import optparse
+import argparse
 import os
 import platform
 import re
+import shlex
 import subprocess
 import sys
-import tempfile
+
+# IMPORTANT: This script is also executed as python2 on
+# GN's CI builders.
 
 try:  # py3
     from shlex import quote as shell_quote
@@ -97,33 +98,82 @@
   def is_zos(self):
     return self._platform == 'zos'
 
+class ArgumentsList:
+  """Helper class to accumulate ArgumentParser argument definitions
+  and be able to regenerate a corresponding command-line to be
+  written in the generated Ninja file for the 'regen' rule.
+  """
+  def __init__(self):
+    self._arguments = []
+
+  def add(self, *args, **kwargs):
+    """Add an argument definition, use as argparse.ArgumentParser.add_argument()."""
+    self._arguments.append((args, kwargs))
+
+  def add_to_parser(self, parser):
+    """Add all known arguments to parser."""
+    for args, kwargs in self._arguments:
+      parser.add_argument(*args, **kwargs)
+
+  def gen_command_line_args(self, parser_arguments):
+    """Generate a gen.py argument list to be embedded in a Ninja file."""
+    result = []
+    for args, kwargs in self._arguments:
+      if len(args) == 2:
+        long_option = args[1]
+      else:
+        long_option = args[0]
+      dest = kwargs.get('dest', None)
+      if dest is None:
+        assert long_option.startswith('--')
+        dest = long_option[2:].replace('-', '_')
+
+      if getattr(parser_arguments, dest, None) is None:
+        # This was not set on the command-line so skip it.
+        continue
+
+      action = kwargs.get('action', None)
+      if action == 'store_true':
+        if getattr(parser_arguments, dest):
+          result.append(long_option)
+      elif action == 'store' or action is None:
+        result.append('%s=%s' % (long_option, getattr(parser_arguments, dest)))
+      elif action == 'append':
+        for item in getattr(parser_arguments, dest):
+          result.append('%s=%s' % (long_option, item))
+      else:
+        assert action is None, "Unsupported action " + action
+    return ' '.join(shell_quote(item) for item in result)
+
 
 def main(argv):
-  parser = optparse.OptionParser(description=sys.modules[__name__].__doc__)
-  parser.add_option('-d', '--debug', action='store_true',
+  parser = argparse.ArgumentParser(description=sys.modules[__name__].__doc__)
+  args_list = ArgumentsList()
+
+  args_list.add('-d', '--debug', action='store_true',
                     help='Do a debug build. Defaults to release build.')
-  parser.add_option('--platform',
+  args_list.add('--platform',
                     help='target platform (' +
                          '/'.join(Platform.known_platforms()) + ')',
                     choices=Platform.known_platforms())
-  parser.add_option('--host',
+  args_list.add('--host',
                     help='host platform (' +
                          '/'.join(Platform.known_platforms()) + ')',
                     choices=Platform.known_platforms())
-  parser.add_option('--use-lto', action='store_true',
+  args_list.add('--use-lto', action='store_true',
                     help='Enable the use of LTO')
-  parser.add_option('--use-icf', action='store_true',
+  args_list.add('--use-icf', action='store_true',
                     help='Enable the use of Identical Code Folding')
-  parser.add_option('--no-last-commit-position', action='store_true',
+  args_list.add('--no-last-commit-position', action='store_true',
                     help='Do not generate last_commit_position.h.')
-  parser.add_option('--out-path',
+  args_list.add('--out-path',
                     help='The path to generate the build files in.')
-  parser.add_option('--no-strip', action='store_true',
+  args_list.add('--no-strip', action='store_true',
                     help='Don\'t strip release build. Useful for profiling.')
-  parser.add_option('--no-static-libstdc++', action='store_true',
+  args_list.add('--no-static-libstdc++', action='store_true',
                     default=False, dest='no_static_libstdcpp',
                     help='Don\'t link libstdc++ statically')
-  parser.add_option('--link-lib',
+  args_list.add('--link-lib',
                     action='append',
                     metavar='LINK_LIB',
                     default=[],
@@ -134,7 +184,7 @@
                           'used multiple times. Useful to link custom malloc ' +
                           'or cpu profiling libraries.'))
   if sys.platform == 'zos':
-    parser.add_option('--zoslib-dir',
+    args_list.add('--zoslib-dir',
                       action='store',
                       default='../third_party/zoslib',
                       dest='zoslib_dir',
@@ -143,10 +193,8 @@
                             'add -I<ZOSLIB_DIR>/install/include to the compile ' +
                             'commands. See README.md for details.'))
 
-  options, args = parser.parse_args(argv)
-
-  if args:
-    parser.error('Unrecognized command line arguments: %s.' % ', '.join(args))
+  args_list.add_to_parser(parser)
+  options = parser.parse_args(argv)
 
   platform = Platform(options.platform)
   if options.host:
@@ -160,7 +208,7 @@
   if not options.no_last_commit_position:
     GenerateLastCommitPosition(host,
                                os.path.join(out_dir, 'last_commit_position.h'))
-  WriteGNNinja(os.path.join(out_dir, 'build.ninja'), platform, host, options)
+  WriteGNNinja(os.path.join(out_dir, 'build.ninja'), platform, host, options, args_list)
   return 0
 
 
@@ -198,11 +246,11 @@
 
 def WriteGenericNinja(path, static_libraries, executables,
                       cxx, ar, ld, platform, host, options,
-                      cflags=[], ldflags=[], libflags=[],
-                      include_dirs=[], solibs=[]):
-  args = ' -d' if options.debug else ''
-  for link_lib in options.link_libs:
-    args +=  ' --link-lib=' + shell_quote(link_lib)
+                      args_list, cflags=[], ldflags=[],
+                      libflags=[], include_dirs=[], solibs=[]):
+  args = args_list.gen_command_line_args(options)
+  if args:
+    args = " " + args
 
   ninja_header_lines = [
     'cxx = ' + cxx,
@@ -308,7 +356,7 @@
             os.path.relpath(template_filename, os.path.dirname(path)) + '\n')
 
 
-def WriteGNNinja(path, platform, host, options):
+def WriteGNNinja(path, platform, host, options, args_list):
   if platform.is_msvc():
     cxx = os.environ.get('CXX', 'cl.exe')
     ld = os.environ.get('LD', 'link.exe')
@@ -830,9 +878,8 @@
   executables['gn_unittests']['libs'].extend(static_libraries.keys())
 
   WriteGenericNinja(path, static_libraries, executables, cxx, ar, ld,
-                    platform, host, options, cflags, ldflags,
-                    libflags, include_dirs, libs)
-
+                    platform, host, options, args_list,
+                    cflags, ldflags, libflags, include_dirs, libs)
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv[1:]))