diff --git a/llvm/utils/gn/build/write_vcsrevision.gni b/llvm/utils/gn/build/write_vcsrevision.gni
--- a/llvm/utils/gn/build/write_vcsrevision.gni
+++ b/llvm/utils/gn/build/write_vcsrevision.gni
@@ -9,11 +9,10 @@
 #       Defaults to [ "LLVM" ]
 
 declare_args() {
-  # If this set to false, VCSRevision.h is updated after every git commit.
+  # If this is set to true, VCSRevision.h is updated after every git commit.
   # That's technically correct, but results in rebuilds after every commit.
-  # If it's true (default), VCSRevision.h will usually be somewhat
-  # out-of-date, but builds will be faster.
-  llvm_allow_tardy_revision = true
+  # If it's false (default), VCSRevision.h will not contain a revision.
+  llvm_append_vc_rev = false
 }
 
 template("write_vcsrevision") {
@@ -29,9 +28,10 @@
     }
 
     args = [ rebase_path(header, root_build_dir) ]
-    if (!llvm_allow_tardy_revision) {
+    if (llvm_append_vc_rev) {
       depfile = "$header.d"
       args += [
+        "--write-git-rev",
         "-d",
         rebase_path(depfile, root_build_dir),
       ]
diff --git a/llvm/utils/gn/build/write_vcsrevision.py b/llvm/utils/gn/build/write_vcsrevision.py
--- a/llvm/utils/gn/build/write_vcsrevision.py
+++ b/llvm/utils/gn/build/write_vcsrevision.py
@@ -29,40 +29,39 @@
     parser.add_argument('-d', '--depfile',
                         help='if set, writes a depfile that causes this script '
                              'to re-run each time the current revision changes')
+    parser.add_argument('--write-git-rev', action='store_true',
+                        help='if set, writes git revision, else writes #undef')
     parser.add_argument('--name', action='append',
                         help='if set, writes a depfile that causes this script '
                              'to re-run each time the current revision changes')
     parser.add_argument('vcs_header', help='path to the output file to write')
     args = parser.parse_args()
 
-    if os.path.isdir(os.path.join(LLVM_DIR, '.svn')):
-        print('SVN support not implemented', file=sys.stderr)
-        return 1
-    if os.path.exists(os.path.join(LLVM_DIR, '.git')):
-        print('non-mono-repo git support not implemented', file=sys.stderr)
-        return 1
-
-    git, use_shell = which('git'), False
-    if not git:
-        git = which('git.exe')
-    if not git:
-        git = which('git.bat')
-        use_shell = True
-
-    git_dir = subprocess.check_output([git, 'rev-parse', '--git-dir'],
-                                      cwd=LLVM_DIR, shell=use_shell).decode().strip()
-    if not os.path.isdir(git_dir):
-        print('.git dir not found at "%s"' % git_dir, file=sys.stderr)
-        return 1
-
-    rev = subprocess.check_output([git, 'rev-parse', '--short', 'HEAD'],
-                                  cwd=git_dir, shell=use_shell).decode().strip()
-    url = subprocess.check_output([git, 'remote', 'get-url', 'origin'],
-                                  cwd=git_dir, shell=use_shell).decode().strip()
     vcsrevision_contents = ''
-    for name in args.name:
-        vcsrevision_contents += '#define %s_REVISION "%s"\n' % (name, rev)
-        vcsrevision_contents += '#define %s_REPOSITORY "%s"\n' % (name, url)
+    if args.write_git_rev:
+        git, use_shell = which('git'), False
+        if not git: git = which('git.exe')
+        if not git: git, use_shell = which('git.bat'), True
+        git_dir = subprocess.check_output(
+                [git, 'rev-parse', '--git-dir'],
+                cwd=LLVM_DIR, shell=use_shell).decode().strip()
+        if not os.path.isdir(git_dir):
+            print('.git dir not found at "%s"' % git_dir, file=sys.stderr)
+            return 1
+
+        rev = subprocess.check_output(
+                [git, 'rev-parse', '--short', 'HEAD'],
+                cwd=git_dir, shell=use_shell).decode().strip()
+        url = subprocess.check_output(
+                [git, 'remote', 'get-url', 'origin'],
+                cwd=git_dir, shell=use_shell).decode().strip()
+        for name in args.name:
+            vcsrevision_contents += '#define %s_REVISION "%s"\n' % (name, rev)
+            vcsrevision_contents += '#define %s_REPOSITORY "%s"\n' % (name, url)
+    else:
+        for name in args.name:
+            vcsrevision_contents += '#undef %s_REVISION\n' % name
+            vcsrevision_contents += '#undef %s_REPOSITORY\n' % name
 
     # If the output already exists and is identical to what we'd write,
     # return to not perturb the existing file's timestamp.