Index: llvm/utils/gn/README.rst
===================================================================
--- llvm/utils/gn/README.rst
+++ llvm/utils/gn/README.rst
@@ -82,6 +82,24 @@
 `utils/gn/secondary`.  For example, the build file for `llvm/lib/Support` is in
 `utils/gn/secondary/llvm/lib/Support`.
 
+.. _Syncing GN files from CMake files:
+
+Syncing GN files from CMake files
+=================================
+
+Sometimes after pulling in the latest changes, the GN build doesn't work.
+Most of the time this is due to someone adding a file to CMakeLists.txt file.
+Run `llvm/utils/gn/build/sync_source_lists_from_cmake.py` to print a report
+of which files need to be added to or removed from `BUILD.gn` files to
+match the corresponding `CMakeLists.txt`. You have to manually read the output
+of the script and implement its suggestions.
+
+If new `CMakeLists.txt` files have been added, you have to manually create
+a new corresponding `BUILD.gn` file below `llvm/utils/gn/secondary/`.
+
+If the dependencies in a `CMakeLists.txt` file have been changed, you have to
+manually analyze and fix.
+
 .. _Philosophy:
 
 Philosophy
Index: llvm/utils/gn/build/sync_source_lists_from_cmake.py
===================================================================
--- /dev/null
+++ llvm/utils/gn/build/sync_source_lists_from_cmake.py
@@ -0,0 +1,56 @@
+#!/usr/bin/env python
+
+"""Helps to keepin BUILD.gn files in sync with the corresponding CMakeLists.txt.
+
+For each BUILD.gn file in the tree, checks if the list of cpp files in
+it is identical to the list of cpp files in the corresponding CMakeLists.txt
+file, and prints the difference if not."""
+
+from __future__ import print_function
+
+import re
+import os
+import subprocess
+
+def main():
+    gn_files = subprocess.check_output(
+            ['git', 'ls-files', '*BUILD.gn']).splitlines()
+
+    # Matches e.g. |   "foo.cpp",|.
+    gn_cpp_re = re.compile(r'^\s*"([^"]+\.(?:cpp|h))",$', re.MULTILINE)
+    # Matches e.g. |   "foo.cpp"|.
+    cmake_cpp_re = re.compile(r'^\s*([A-Za-z_0-9/-]+\.(?:cpp|h))$',
+                              re.MULTILINE)
+
+    for gn_file in gn_files:
+        # The CMakeLists.txt for llvm/utils/gn/secondary/foo/BUILD.gn is
+        # directly at foo/CMakeLists.txt.
+        strip_prefix = 'llvm/utils/gn/secondary/'
+        if not gn_file.startswith(strip_prefix):
+            continue
+        cmake_file = os.path.join(
+                os.path.dirname(gn_file[len(strip_prefix):]), 'CMakeLists.txt')
+        if not os.path.exists(cmake_file):
+            continue
+
+        def get_sources(source_re, text):
+            return set([m.group(1) for m in source_re.finditer(text)])
+        gn_cpp = get_sources(gn_cpp_re, open(gn_file).read())
+        cmake_cpp = get_sources(cmake_cpp_re, open(cmake_file).read())
+
+        if gn_cpp == cmake_cpp:
+            continue
+
+        print(gn_file)
+        print('add:')
+        if cmake_cpp - gn_cpp:
+            for s in cmake_cpp - gn_cpp:
+                print('    "%s",' % s)
+        if gn_cpp - cmake_cpp:
+            print('remove:')
+            for s in gn_cpp - cmake_cpp:
+                print(s)
+        print()
+
+if __name__ == '__main__':
+    main()
Index: llvm/utils/gn/secondary/llvm/lib/Support/BUILD.gn
===================================================================
--- llvm/utils/gn/secondary/llvm/lib/Support/BUILD.gn
+++ llvm/utils/gn/secondary/llvm/lib/Support/BUILD.gn
@@ -39,6 +39,7 @@
     "BinaryStreamWriter.cpp",
     "BlockFrequency.cpp",
     "BranchProbability.cpp",
+    "BuryPointer.cpp",
     "COM.cpp",
     "CachePruning.cpp",
     "Chrono.cpp",