Index: google3/third_party/llvm/llvm/tools/clang/tools/clang-format/git-clang-format
===================================================================
--- google3/third_party/llvm/llvm/tools/clang/tools/clang-format/git-clang-format
+++ google3/third_party/llvm/llvm/tools/clang/tools/clang-format/git-clang-format
@@ -33,11 +33,12 @@
 import subprocess
 import sys
 
-usage = 'git clang-format [OPTIONS] [<commit>] [<commit>] [--] [<file>...]'
+usage = ('git clang-format [OPTIONS] [<commit>] [<commit>|--staged] '
+         '[--] [<file>...]')
 
 desc = '''
-Run clang-format on all modified lines in the working directory or in a given
-commit.
+Run clang-format on all modified lines in the working directory, in a given
+commit, or in the stage/index.
 
 Forms:
 
@@ -57,6 +58,10 @@
         Run clang-format on all lines in <commit2> that differ from <commit1>.
         Requires --diff.
 
+    git clang-format [<commit>] --staged --diff
+        Run clang-format on all lines in the git stage/index that differ from
+        <commit>, which defaults to HEAD. Requires --diff.
+
 In all of the forms above, <file>... can be used to limit what files are
 affected, using the same syntax as `git diff`. Use `--` to disambiguate between
 files and commits.
@@ -126,6 +131,8 @@
                  help='select hunks interactively')
   p.add_argument('-q', '--quiet', action='count', default=0,
                  help='print less information')
+  p.add_argument('--staged', '--cached', action='store_true',
+                 help='format lines in the stage instead of the working dir')
   p.add_argument('--style',
                  default=config.get('clangformat.style', None),
                  help='passed to clang-format'),
@@ -144,7 +151,10 @@
   del opts.quiet
 
   source, dest, files = interpret_args(opts.args, dash_dash,
-                                       default_commit=opts.commit)
+                                       default_commit=opts.commit,
+                                       staged=opts.staged)
+  if isinstance(dest, Stage) and not opts.diff:
+    die('--diff is required when --staged is used')
   if isinstance(dest, Revision) and not opts.diff:
     die('--diff is required when two commits are given')
   changed_lines = dest.compute_diff_from(source, files)
@@ -207,7 +217,7 @@
   return out
 
 
-def interpret_args(args, dash_dash, default_commit):
+def interpret_args(args, dash_dash, default_commit, staged):
   """Interpret `args` as "[commits] [--] [files]" and return (src, dest, files).
 
   It is assumed that "--" and everything that follows has been removed from
@@ -252,7 +262,11 @@
   if len(commits) > 2:
     die('at most two commits allowed; %d given' % len(commits))
   elif len(commits) == 2:
+    if staged:
+      die('--staged is not allowed when two commits are given')
     return Revision(commits[0]), Revision(commits[1]), files
+  elif staged:
+    return Revision(commits[0]), Stage(), files
   else:
     return Revision(commits[0]), Workdir(), files
 
@@ -283,7 +297,7 @@
 
 
 class TreeLocation (object):
-  """Represents either a commit or the working directory.
+  """Represents either a commit, the stage, or the working directory.
 
   Do not use this directly. Instead, use one of the subclasses."""
 
@@ -439,6 +453,24 @@
   is_workdir = True
 
 
+class Stage (TreeLocation):
+  """Represents the git stage, a.k.a. the index."""
+
+  def create_tree(self, unused_filenames):
+    return run('git', 'write-tree')
+
+  def _file_mode(self, filename):
+    stdout = run('git', 'ls-files', '--stage', filename)
+    # note: there is an optional <tag> as the first element.
+    return int(stdout.split()[-4], 8)
+
+  def _compute_diff_from_base_command(self, source):
+    return ['git', 'diff-index', '-p', '-U0', '--cached', source.revision]
+
+  def _blob_name(self, filename):
+    return ':%s' % filename
+
+
 class Revision (TreeLocation):
   """Represents a specific revision, a.k.a. a commit."""