diff --git a/tools/gn/bin/gyp_flag_compare.py b/tools/gn/bin/gyp_flag_compare.py
index ddd260a..7bece4d 100755
--- a/tools/gn/bin/gyp_flag_compare.py
+++ b/tools/gn/bin/gyp_flag_compare.py
@@ -5,7 +5,33 @@
 # found in the LICENSE file.
 
 """Given the output of -t commands from a ninja build for a gyp and GN generated
-build, report on differences between the command lines."""
+build, report on differences between the command lines.
+
+This script is best used interactively. Here's a recommended setup:
+  $ PYTHONPATH=tools/gn/bin python
+  >>> import sys
+  >>> import pprint
+  >>> sys.displayhook = pprint.pprint
+  >>> import gyp_flag_compare as fc
+  >>> fc.ConfigureBuild(['gyp_define=1', 'define=2'], ['gn_arg=1', 'arg=2'])
+  >>> chrome = fc.Comparison('chrome')
+
+The above starts interactive Python, sets up the output to be pretty-printed
+(useful for making lists, dicts, and sets readable), configures the build with
+GN arguments and GYP defines, and then generates a comparison for that build
+configuration for the "chrome" target.
+
+After that, the |chrome| object can be used to investigate differences in the
+build.
+
+To configure an official build, use this configuration. Disabling NaCl produces
+a more meaningful comparison, as certain files need to get compiled twice
+for the IRT build, which uses different flags:
+  >>> fc.ConfigureBuild(
+          ['disable_nacl=1', 'buildtype=Official', 'branding=Chrome'],
+          ['enable_nacl=false', 'is_official_build=true',
+           'is_chrome_branded=true'])
+"""
 
 
 import os
@@ -13,15 +39,286 @@
 import subprocess
 import sys
 
-
 # Must be in src/.
 os.chdir(os.path.join(os.path.dirname(__file__), '..', '..', '..'))
 
-
-g_total_differences = 0
+_GN_OUT_DIR = 'out/gn_flags'
+_GYP_OUT_DIR = 'out_gyp_flags/Release'
 
 
-def FindAndRemoveArgWithValue(command_line, argname):
+def ConfigureBuild(gyp_args=[], gn_args=[]):
+  print >> sys.stderr, 'Regenerating in %s...' % _GN_OUT_DIR
+  # Currently only Release, non-component.
+  Run('gn gen %s --args="is_debug=false is_component_build=false %s"' % \
+      (_GN_OUT_DIR, ' '.join(gn_args)))
+
+  os.environ.pop('GYP_DEFINES', None)
+  # Remove environment variables required by gn but conflicting with GYP.
+  # Relevant if Windows toolchain isn't provided by depot_tools.
+  os.environ.pop('GYP_MSVS_OVERRIDE_PATH', None)
+  os.environ.pop('WINDOWSSDKDIR', None)
+
+  gyp_defines = ''
+  if len(gyp_args) > 0:
+    gyp_defines = '-D' + ' -D'.join(gyp_args)
+
+  print >> sys.stderr, 'Regenerating in %s...' % _GYP_OUT_DIR
+  Run('python build/gyp_chromium -Goutput_dir=out_gyp_flags ' + \
+      '-Gconfig=Release %s' % gyp_defines)
+
+
+def Counts(dict_of_list):
+  """Given a dictionary whose value are lists, returns a dictionary whose values
+  are the length of the list. This can be used to summarize a dictionary.
+  """
+  return {k: len(v) for k, v in dict_of_list.iteritems()}
+
+
+def CountsByDirname(dict_of_list):
+  """Given a list of files, returns a dict of dirname to counts in that dir."""
+  r = {}
+  for path in dict_of_list:
+    dirname = os.path.dirname(path)
+    r.setdefault(dirname, 0)
+    r[dirname] += 1
+  return r
+
+
+class Comparison(object):
+  """A comparison of the currently-configured build for a target."""
+
+  def __init__(self, gyp_target, gn_target=None):
+    """Creates a comparison of a GN and GYP target. If the target names differ
+    between the two build systems, then two names may be passed.
+    """
+    if gn_target is None:
+      gn_target = gyp_target
+    self._gyp_target = gyp_target
+    self._gn_target = gn_target
+
+    self._skipped = []
+
+    self._total_diffs = 0
+
+    self._missing_gyp_flags = {}
+    self._missing_gn_flags = {}
+
+    self._missing_gyp_files = {}
+    self._missing_gn_files = {}
+
+    self._CompareFiles()
+
+  @property
+  def gyp_files(self):
+    """Returns the set of files that are in the GYP target."""
+    return set(self._gyp_flags.keys())
+
+  @property
+  def gn_files(self):
+    """Returns the set of files that are in the GN target."""
+    return set(self._gn_flags.keys())
+
+  @property
+  def skipped(self):
+    """Returns the list of compiler commands that were not processed during the
+    comparison.
+    """
+    return self._skipped
+
+  @property
+  def total_differences(self):
+    """Returns the total number of differences detected."""
+    return self._total_diffs
+
+  @property
+  def missing_in_gyp(self):
+    """Differences that are only in GYP build but not in GN, indexed by the
+    difference."""
+    return self._missing_gyp_flags
+
+  @property
+  def missing_in_gn(self):
+    """Differences that are only in the GN build but not in GYP, indexed by
+    the difference."""
+    return self._missing_gn_flags
+
+  @property
+  def missing_in_gyp_by_file(self):
+    """Differences that are only in the GYP build but not in GN, indexed by
+    file.
+    """
+    return self._missing_gyp_files
+
+  @property
+  def missing_in_gn_by_file(self):
+    """Differences that are only in the GYP build but not in GN, indexed by
+    file.
+    """
+    return self._missing_gn_files
+
+  def _CompareFiles(self):
+    """Performs the actual target comparison."""
+    if sys.platform == 'win32':
+      # On Windows flags are stored in .rsp files which are created by building.
+      print >> sys.stderr, 'Building in %s...' % _GN_OUT_DIR
+      Run('ninja -C %s -d keeprsp %s' % (_GN_OUT_DIR, self._gn_target))
+      print >> sys.stderr, 'Building in %s...' % _GYP_OUT_DIR
+      Run('ninja -C %s -d keeprsp %s' % (_GYP_OUT_DIR, self._gn_target))
+
+    gn = Run('ninja -C %s -t commands %s' % (_GN_OUT_DIR, self._gn_target))
+    gyp = Run('ninja -C %s -t commands %s' % (_GYP_OUT_DIR, self._gyp_target))
+
+    self._gn_flags = self._GetFlags(gn.splitlines(),
+                                    os.path.join(os.getcwd(), _GN_OUT_DIR))
+    self._gyp_flags = self._GetFlags(gyp.splitlines(),
+                                     os.path.join(os.getcwd(), _GYP_OUT_DIR))
+
+    all_files = sorted(self.gn_files & self.gyp_files)
+    for filename in all_files:
+      gyp_flags = self._gyp_flags[filename]
+      gn_flags = self._gn_flags[filename]
+      self._CompareLists(filename, gyp_flags, gn_flags, 'dash_f')
+      self._CompareLists(filename, gyp_flags, gn_flags, 'defines')
+      self._CompareLists(filename, gyp_flags, gn_flags, 'include_dirs')
+      self._CompareLists(filename, gyp_flags, gn_flags, 'warnings',
+          # More conservative warnings in GN we consider to be OK.
+          dont_care_gyp=[
+            '/wd4091',  # 'keyword' : ignored on left of 'type' when no variable
+                        # is declared.
+            '/wd4456',  # Declaration hides previous local declaration.
+            '/wd4457',  # Declaration hides function parameter.
+            '/wd4458',  # Declaration hides class member.
+            '/wd4459',  # Declaration hides global declaration.
+            '/wd4702',  # Unreachable code.
+            '/wd4800',  # Forcing value to bool 'true' or 'false'.
+            '/wd4838',  # Conversion from 'type' to 'type' requires a narrowing
+                        # conversion.
+          ] if sys.platform == 'win32' else None,
+          dont_care_gn=[
+            '-Wendif-labels',
+            '-Wextra',
+            '-Wsign-compare',
+          ] if not sys.platform == 'win32' else None)
+      self._CompareLists(filename, gyp_flags, gn_flags, 'other')
+
+  def _CompareLists(self, filename, gyp, gn, name,
+                    dont_care_gyp=None, dont_care_gn=None):
+    """Return a report of any differences between gyp and gn lists, ignoring
+    anything in |dont_care_{gyp|gn}| respectively."""
+    if gyp[name] == gn[name]:
+      return
+    if not dont_care_gyp:
+      dont_care_gyp = []
+    if not dont_care_gn:
+      dont_care_gn = []
+    gyp_set = set(gyp[name])
+    gn_set = set(gn[name])
+    missing_in_gyp = gyp_set - gn_set
+    missing_in_gn = gn_set - gyp_set
+    missing_in_gyp -= set(dont_care_gyp)
+    missing_in_gn -= set(dont_care_gn)
+
+    for m in missing_in_gyp:
+      self._missing_gyp_flags.setdefault(name, {}) \
+                             .setdefault(m, []).append(filename)
+      self._total_diffs += 1
+    self._missing_gyp_files.setdefault(filename, {}) \
+                           .setdefault(name, set()).update(missing_in_gyp)
+
+    for m in missing_in_gn:
+      self._missing_gn_flags.setdefault(name, {}) \
+                            .setdefault(m, []).append(filename)
+      self._total_diffs += 1
+    self._missing_gn_files.setdefault(filename, {}) \
+                          .setdefault(name, set()).update(missing_in_gn)
+
+  def _GetFlags(self, lines, build_dir):
+    """Turn a list of command lines into a semi-structured dict."""
+    is_win = sys.platform == 'win32'
+    flags_by_output = {}
+    for line in lines:
+      command_line = shlex.split(line.strip(), posix=not is_win)[1:]
+
+      output_name = _FindAndRemoveArgWithValue(command_line, '-o')
+      dep_name = _FindAndRemoveArgWithValue(command_line, '-MF')
+
+      command_line = _MergeSpacedArgs(command_line, '-Xclang')
+
+      cc_file = [x for x in command_line if x.endswith('.cc') or
+                                            x.endswith('.c') or
+                                            x.endswith('.cpp') or
+                                            x.endswith('.mm') or
+                                            x.endswith('.m')]
+      if len(cc_file) != 1:
+        self._skipped.append(command_line)
+        continue
+      assert len(cc_file) == 1
+
+      if is_win:
+        rsp_file = [x for x in command_line if x.endswith('.rsp')]
+        assert len(rsp_file) <= 1
+        if rsp_file:
+          rsp_file = os.path.join(build_dir, rsp_file[0][1:])
+          with open(rsp_file, "r") as open_rsp_file:
+            command_line = shlex.split(open_rsp_file, posix=False)
+
+      defines = [x for x in command_line if x.startswith('-D')]
+      include_dirs = [x for x in command_line if x.startswith('-I')]
+      dash_f = [x for x in command_line if x.startswith('-f')]
+      warnings = \
+          [x for x in command_line if x.startswith('/wd' if is_win else '-W')]
+      others = [x for x in command_line if x not in defines and \
+                                           x not in include_dirs and \
+                                           x not in dash_f and \
+                                           x not in warnings and \
+                                           x not in cc_file]
+
+      for index, value in enumerate(include_dirs):
+        if value == '-Igen':
+          continue
+        path = value[2:]
+        if not os.path.isabs(path):
+          path = os.path.join(build_dir, path)
+        include_dirs[index] = '-I' + os.path.normpath(path)
+
+      # GYP supports paths above the source root like <(DEPTH)/../foo while such
+      # paths are unsupported by gn. But gn allows to use system-absolute paths
+      # instead (paths that start with single '/'). Normalize all paths.
+      cc_file = [os.path.normpath(os.path.join(build_dir, cc_file[0]))]
+
+      # Filter for libFindBadConstructs.so having a relative path in one and
+      # absolute path in the other.
+      others_filtered = []
+      for x in others:
+        if x.startswith('-Xclang ') and \
+            (x.endswith('libFindBadConstructs.so') or \
+             x.endswith('libFindBadConstructs.dylib')):
+          others_filtered.append(
+              '-Xclang ' +
+              os.path.join(os.getcwd(), os.path.normpath(
+                  os.path.join('out/gn_flags', x.split(' ', 1)[1]))))
+        elif x.startswith('-B'):
+          others_filtered.append(
+              '-B' +
+              os.path.join(os.getcwd(), os.path.normpath(
+                  os.path.join('out/gn_flags', x[2:]))))
+        else:
+          others_filtered.append(x)
+      others = others_filtered
+
+      flags_by_output[cc_file[0]] = {
+        'output': output_name,
+        'depname': dep_name,
+        'defines': sorted(defines),
+        'include_dirs': sorted(include_dirs),  # TODO(scottmg): This is wrong.
+        'dash_f': sorted(dash_f),
+        'warnings': sorted(warnings),
+        'other': sorted(others),
+      }
+    return flags_by_output
+
+
+def _FindAndRemoveArgWithValue(command_line, argname):
   """Given a command line as a list, remove and return the value of an option
   that takes a value as a separate entry.
 
@@ -35,7 +332,7 @@
   return value
 
 
-def MergeSpacedArgs(command_line, argname):
+def _MergeSpacedArgs(command_line, argname):
   """Combine all arguments |argname| with their values, separated by a space."""
   i = 0
   result = []
@@ -50,141 +347,9 @@
   return result
 
 
-def NormalizeSymbolArguments(command_line):
-  """Normalize -g arguments.
-
-  If there's no -g args, it's equivalent to -g0. -g2 is equivalent to -g.
-  Modifies |command_line| in place.
-  """
-  # Strip -g0 if there's no symbols.
-  have_some_symbols = False
-  for x in command_line:
-    if x.startswith('-g') and x != '-g0':
-      have_some_symbols = True
-  if not have_some_symbols and '-g0' in command_line:
-    command_line.remove('-g0')
-
-  # Rename -g2 to -g.
-  if '-g2' in command_line:
-    command_line[command_line.index('-g2')] = '-g'
-
-
-def GetFlags(lines, build_dir):
-  """Turn a list of command lines into a semi-structured dict."""
-  is_win = sys.platform == 'win32'
-  flags_by_output = {}
-  for line in lines:
-    command_line = shlex.split(line.strip(), posix=not is_win)[1:]
-
-    output_name = FindAndRemoveArgWithValue(command_line, '-o')
-    dep_name = FindAndRemoveArgWithValue(command_line, '-MF')
-
-    NormalizeSymbolArguments(command_line)
-
-    command_line = MergeSpacedArgs(command_line, '-Xclang')
-
-    cc_file = [x for x in command_line if x.endswith('.cc') or
-                                          x.endswith('.c') or
-                                          x.endswith('.cpp')]
-    if len(cc_file) != 1:
-      print 'Skipping %s' % command_line
-      continue
-    assert len(cc_file) == 1
-
-    if is_win:
-      rsp_file = [x for x in command_line if x.endswith('.rsp')]
-      assert len(rsp_file) <= 1
-      if rsp_file:
-        rsp_file = os.path.join(build_dir, rsp_file[0][1:])
-        with open(rsp_file, "r") as open_rsp_file:
-          command_line = shlex.split(open_rsp_file, posix=False)
-
-    defines = [x for x in command_line if x.startswith('-D')]
-    include_dirs = [x for x in command_line if x.startswith('-I')]
-    dash_f = [x for x in command_line if x.startswith('-f')]
-    warnings = \
-        [x for x in command_line if x.startswith('/wd' if is_win else '-W')]
-    others = [x for x in command_line if x not in defines and \
-                                         x not in include_dirs and \
-                                         x not in dash_f and \
-                                         x not in warnings and \
-                                         x not in cc_file]
-
-    for index, value in enumerate(include_dirs):
-      if value == '-Igen':
-        continue
-      path = value[2:]
-      if not os.path.isabs(path):
-        path = os.path.join(build_dir, path)
-      include_dirs[index] = '-I' + os.path.normpath(path)
-
-    # GYP supports paths above the source root like <(DEPTH)/../foo while such
-    # paths are unsupported by gn. But gn allows to use system-absolute paths
-    # instead (paths that start with single '/'). Normalize all paths.
-    cc_file = [os.path.normpath(os.path.join(build_dir, cc_file[0]))]
-
-    # Filter for libFindBadConstructs.so having a relative path in one and
-    # absolute path in the other.
-    others_filtered = []
-    for x in others:
-      if x.startswith('-Xclang ') and x.endswith('libFindBadConstructs.so'):
-        others_filtered.append(
-            '-Xclang ' +
-            os.path.join(os.getcwd(),
-                         os.path.normpath(
-                             os.path.join('out/gn_flags', x.split(' ', 1)[1]))))
-      elif x.startswith('-B'):
-        others_filtered.append(
-            '-B' +
-            os.path.join(os.getcwd(),
-                         os.path.normpath(os.path.join('out/gn_flags', x[2:]))))
-      else:
-        others_filtered.append(x)
-    others = others_filtered
-
-    flags_by_output[cc_file[0]] = {
-      'output': output_name,
-      'depname': dep_name,
-      'defines': sorted(defines),
-      'include_dirs': sorted(include_dirs),  # TODO(scottmg): This is wrong.
-      'dash_f': sorted(dash_f),
-      'warnings': sorted(warnings),
-      'other': sorted(others),
-    }
-  return flags_by_output
-
-
-def CompareLists(gyp, gn, name, dont_care_gyp=None, dont_care_gn=None):
-  """Return a report of any differences between gyp and gn lists, ignoring
-  anything in |dont_care_{gyp|gn}| respectively."""
-  global g_total_differences
-  if not dont_care_gyp:
-    dont_care_gyp = []
-  if not dont_care_gn:
-    dont_care_gn = []
-  output = ''
-  if gyp[name] != gn[name]:
-    gyp_set = set(gyp[name])
-    gn_set = set(gn[name])
-    missing_in_gyp = gyp_set - gn_set
-    missing_in_gn = gn_set - gyp_set
-    missing_in_gyp -= set(dont_care_gyp)
-    missing_in_gn -= set(dont_care_gn)
-    if missing_in_gyp or missing_in_gn:
-      output += '  %s differ:\n' % name
-    if missing_in_gyp:
-      output += '    In gyp, but not in GN:\n      %s' % '\n      '.join(
-          sorted(missing_in_gyp)) + '\n'
-      g_total_differences += len(missing_in_gyp)
-    if missing_in_gn:
-      output += '    In GN, but not in gyp:\n      %s' % '\n      '.join(
-          sorted(missing_in_gn)) + '\n\n'
-      g_total_differences += len(missing_in_gn)
-  return output
-
-
 def Run(command_line):
   """Run |command_line| as a subprocess and return stdout. Raises on error."""
+  print >> sys.stderr, command_line
   return subprocess.check_output(command_line, shell=True)
 
 
@@ -194,41 +359,18 @@
     print '   or: %s target' % __file__
     return 1
 
+  ConfigureBuild()
+
+  gyp_target = sys.argv[1]
   if len(sys.argv) == 2:
-    sys.argv.append(sys.argv[1])
+    gn_target = gyp_target
+  else:
+    gn_target = sys.argv[2]
+  comparison = Comparison(gyp_target, gn_target)
 
-  gn_out_dir = 'out/gn_flags'
-  print >> sys.stderr, 'Regenerating in %s...' % gn_out_dir
-  # Currently only Release, non-component.
-  Run('gn gen %s --args="is_debug=false is_component_build=false"' % gn_out_dir)
-  gn = Run('ninja -C %s -t commands %s' % (gn_out_dir, sys.argv[2]))
-  if sys.platform == 'win32':
-    # On Windows flags are stored in .rsp files which are created during build.
-    print >> sys.stderr, 'Building in %s...' % gn_out_dir
-    Run('ninja -C %s -d keeprsp %s' % (gn_out_dir, sys.argv[2]))
-
-  os.environ.pop('GYP_DEFINES', None)
-  # Remove environment variables required by gn but conflicting with GYP.
-  # Relevant if Windows toolchain isn't provided by depot_tools.
-  os.environ.pop('GYP_MSVS_OVERRIDE_PATH', None)
-  os.environ.pop('WINDOWSSDKDIR', None)
-
-  gyp_out_dir = 'out_gyp_flags/Release'
-  print >> sys.stderr, 'Regenerating in %s...' % gyp_out_dir
-  Run('python build/gyp_chromium -Goutput_dir=out_gyp_flags -Gconfig=Release')
-  gyp = Run('ninja -C %s -t commands %s' % (gyp_out_dir, sys.argv[1]))
-  if sys.platform == 'win32':
-    # On Windows flags are stored in .rsp files which are created during build.
-    print >> sys.stderr, 'Building in %s...' % gyp_out_dir
-    Run('ninja -C %s -d keeprsp %s' % (gyp_out_dir, sys.argv[2]))
-
-  all_gyp_flags = GetFlags(gyp.splitlines(),
-                           os.path.join(os.getcwd(), gyp_out_dir))
-  all_gn_flags = GetFlags(gn.splitlines(),
-                          os.path.join(os.getcwd(), gn_out_dir))
-  gyp_files = set(all_gyp_flags.keys())
-  gn_files = set(all_gn_flags.keys())
-  different_source_list = gyp_files != gn_files
+  gyp_files = comparison.gyp_files
+  gn_files = comparison.gn_files
+  different_source_list = comparison.gyp_files != comparison.gn_files
   if different_source_list:
     print 'Different set of sources files:'
     print '  In gyp, not in GN:\n    %s' % '\n    '.join(
@@ -236,42 +378,31 @@
     print '  In GN, not in gyp:\n    %s' % '\n    '.join(
         sorted(gn_files - gyp_files))
     print '\nNote that flags will only be compared for files in both sets.\n'
-  file_list = gyp_files & gn_files
+
+  differing_files = set(comparison.missing_in_gn_by_file.keys()) & \
+                    set(comparison.missing_in_gyp_by_file.keys())
   files_with_given_differences = {}
-  for filename in sorted(file_list):
-    gyp_flags = all_gyp_flags[filename]
-    gn_flags = all_gn_flags[filename]
-    differences = CompareLists(gyp_flags, gn_flags, 'dash_f')
-    differences += CompareLists(gyp_flags, gn_flags, 'defines')
-    differences += CompareLists(gyp_flags, gn_flags, 'include_dirs')
-    differences += CompareLists(gyp_flags, gn_flags, 'warnings',
-        # More conservative warnings in GN we consider to be OK.
-        dont_care_gyp=[
-          '/wd4091',  # 'keyword' : ignored on left of 'type' when no variable
-                      # is declared.
-          '/wd4456',  # Declaration hides previous local declaration.
-          '/wd4457',  # Declaration hides function parameter.
-          '/wd4458',  # Declaration hides class member.
-          '/wd4459',  # Declaration hides global declaration.
-          '/wd4702',  # Unreachable code.
-          '/wd4800',  # Forcing value to bool 'true' or 'false'.
-          '/wd4838',  # Conversion from 'type' to 'type' requires a narrowing
-                      # conversion.
-        ] if sys.platform == 'win32' else None,
-        dont_care_gn=[
-          '-Wendif-labels',
-          '-Wextra',
-          '-Wsign-compare',
-        ] if not sys.platform == 'win32' else None)
-    differences += CompareLists(gyp_flags, gn_flags, 'other')
-    if differences:
-      files_with_given_differences.setdefault(differences, []).append(filename)
+  for filename in differing_files:
+    output = ''
+    missing_in_gyp = comparison.missing_in_gyp_by_file.get(filename, {})
+    missing_in_gn = comparison.missing_in_gn_by_file.get(filename, {})
+    difference_types = sorted(set(missing_in_gyp.keys() + missing_in_gn.keys()))
+    for difference_type in difference_types:
+      output += '  %s differ:\n' % difference_type
+      if difference_type in missing_in_gyp:
+        output += '    In gyp, but not in GN:\n      %s' % '\n      '.join(
+            sorted(missing_in_gyp[difference_type])) + '\n'
+      if difference_type in missing_in_gn:
+        output += '    In GN, but not in gyp:\n      %s' % '\n      '.join(
+            sorted(missing_in_gn[difference_type])) + '\n'
+    if output:
+      files_with_given_differences.setdefault(output, []).append(filename)
 
   for diff, files in files_with_given_differences.iteritems():
     print '\n'.join(sorted(files))
     print diff
 
-  print 'Total differences:', g_total_differences
+  print 'Total differences:', comparison.total_differences
   # TODO(scottmg): Return failure on difference once we're closer to identical.
   return 0