diff --git a/tools/compare_builds.py b/tools/compare_builds.py
new file mode 100755
index 0000000..838a628
--- /dev/null
+++ b/tools/compare_builds.py
@@ -0,0 +1,661 @@
+#!/usr/bin/env -S python3 -u
+
+"""
+This script helps find various build behaviors that make builds less hermetic
+and repeatable. Depending on the flags, it runs a sequence of builds and looks
+for files that have changed or have been improperly regenerated, updating
+their timestamps incorrectly. It also looks for changes that the build has
+done to the source tree, and for files whose contents are dependent on the
+location of the out directory.
+
+This utility has two major modes, full and incremental. By default, this tool
+runs in full mode. To run in incremental mode, pass the --incremental flag.
+
+
+FULL MODE
+
+In full mode, this tool helps verify BUILD CORRECTNESS by examining its
+REPEATABILITY. In full mode, this tool runs two complete builds in different
+directories and compares the CONTENTS of the two directories. Lists of any
+files that are added, removed or changed are printed, sorted by the timestamp
+of that file, to aid finding which dependencies trigger the rebuilding of
+other files.
+
+
+INCREMENTAL MODE
+
+In incremental mode, this tool helps verfiy the SPEED of the build. It runs two
+builds and looks at the TIMESTAMPS of the generated files, and reports files
+that were changed by the second build. In theory, an incremental build with no
+source files touched should not have any generated targets changed. As in full
+builds, the file list is returned sorted by timestamp.
+
+
+OTHER CHECKS
+
+In both full and incremental mode, this tool looks at the timestamps of all
+source files in the tree, and reports on files that have been touched. In the
+output, these are labeled with the header "Source files touched after start of
+build."
+
+In addition, by default, this tool sets the OUT_DIR environment variable to
+something other than "out" in order to find build rules that are not respecting
+the OUT_DIR. If you see these, you should fix them, but if your build can not
+complete for some reason because of this, you can pass the --no-check-out-dir
+flag to suppress this check.
+
+
+OTHER FLAGS
+
+In full mode, the --detect-embedded-paths flag does the two builds in different
+directories, to help in finding rules that embed the out directory path into
+the targets.
+
+The --hide-build-output flag hides the output of successful bulds, to make
+script output cleaner. The output of builds that fail is still shown.
+
+The --no-build flag is useful if you have already done a build and would
+just like to re-run the analysis.
+
+The --target flag lets you specify a build target other than the default
+full build (droid). You can pass "nothing" as in the example below, or a
+specific target, to reduce the scope of the checks performed.
+
+The --touch flag lets you specify a list of source files to touch between
+the builds, to examine the consequences of editing a particular file.
+
+
+EXAMPLE COMMANDLINES
+
+Please run build/make/tools/compare_builds.py --help for a full listing
+of the commandline flags. Here are a sampling of useful combinations.
+
+  1. Find files changed during an incremental build that doesn't build
+     any targets.
+
+       build/make/tools/compare_builds.py --incremental --target nothing
+
+     Long incremental build times, or consecutive builds that re-run build actions
+     are usually caused by files being touched as part of loading the makefiles.
+
+     The nothing build (m nothing) loads the make and blueprint files, generates
+     the dependency graph, but then doesn't actually build any targets. Checking
+     against this build is the fastest and easiest way to find files that are
+     modified while makefiles are read, for example with $(shell) invocations.
+
+  2. Find packaging targets that are different, ignoring intermediate files.
+
+       build/make/tools/compare_builds.py --subdirs --detect-embedded-paths
+
+     These flags will compare the final staging directories for partitions,
+     as well as the APKs, apexes, testcases, and the like (the full directory
+     list is in the DEFAULT_DIRS variable below). Since these are the files
+     that are ultimately released, it is more important that these files be
+     replicable, even if the intermediates that went into them are not (for
+     example, when debugging symbols are stripped).
+
+  3. Check that all targets are repeatable.
+
+       build/make/tools/compare_builds.py --detect-embedded-paths
+
+     This check will list all of the differences in built targets that it can
+     find. Be aware that the AOSP tree still has quite a few targets that
+     are flagged by this check, so OEM changes might be lost in that list.
+     That said, each file shown here is a potential blocker for a repeatable
+     build.
+
+  4. See what targets are rebuilt when a file is touched between builds.
+
+       build/make/tools/compare_builds.py --incremental \
+            --touch frameworks/base/core/java/android/app/Activity.java
+
+     This check simulates the common engineer workflow of touching a single
+     file and rebuilding the whole system. To see a restricted view, consider
+     also passing a --target option for a common use case. For example:
+
+       build/make/tools/compare_builds.py --incremental --target framework \
+            --touch frameworks/base/core/java/android/app/Activity.java
+"""
+
+import argparse
+import itertools
+import os
+import shutil
+import stat
+import subprocess
+import sys
+
+
+# Soong
+SOONG_UI = "build/soong/soong_ui.bash"
+
+
+# Which directories to use if no --subdirs is supplied without explicit directories.
+DEFAULT_DIRS = (
+    "apex",
+    "data",
+    "product",
+    "ramdisk",
+    "recovery",
+    "root",
+    "system",
+    "system_ext",
+    "system_other",
+    "testcases",
+    "vendor",
+)
+
+
+# Files to skip for incremental timestamp checking
+BUILD_INTERNALS_PREFIX_SKIP = (
+    "soong/.glob/",
+    ".path/",
+)
+
+
+BUILD_INTERNALS_SUFFIX_SKIP = (
+    "/soong/soong_build_metrics.pb",
+    "/.installable_test_files",
+    "/files.db",
+    "/.blueprint.bootstrap",
+    "/build_number.txt",
+    "/build.ninja",
+    "/.out-dir",
+    "/build_fingerprint.txt",
+    "/build_thumbprint.txt",
+    "/.copied_headers_list",
+    "/.installable_files",
+)
+
+
+class DiffType(object):
+  def __init__(self, code, message):
+    self.code = code
+    self.message = message
+
+DIFF_NONE = DiffType("DIFF_NONE", "Files are the same")
+DIFF_MODE = DiffType("DIFF_MODE", "Stat mode bits differ")
+DIFF_SIZE = DiffType("DIFF_SIZE", "File size differs")
+DIFF_SYMLINK = DiffType("DIFF_SYMLINK", "Symlinks point to different locations")
+DIFF_CONTENTS = DiffType("DIFF_CONTENTS", "File contents differ")
+
+
+def main():
+  argparser = argparse.ArgumentParser(description="Diff build outputs from two builds.",
+                                      epilog="Run this command from the root of the tree."
+                                        + " Before running this command, the build environment"
+                                        + " must be set up, including sourcing build/envsetup.sh"
+                                        + " and running lunch.")
+  argparser.add_argument("--detect-embedded-paths", action="store_true",
+      help="Use unique out dirs to detect paths embedded in binaries.")
+  argparser.add_argument("--incremental", action="store_true",
+      help="Compare which files are touched in two consecutive builds without a clean in between.")
+  argparser.add_argument("--hide-build-output", action="store_true",
+      help="Don't print the build output for successful builds")
+  argparser.add_argument("--no-build", dest="run_build", action="store_false",
+      help="Don't build or clean, but do everything else.")
+  argparser.add_argument("--no-check-out-dir", dest="check_out_dir", action="store_false",
+      help="Don't check for rules not honoring movable out directories.")
+  argparser.add_argument("--subdirs", nargs="*",
+      help="Only scan these subdirs of $PRODUCT_OUT instead of the whole out directory."
+           + " The --subdirs argument with no listed directories will give a default list.")
+  argparser.add_argument("--target", default="droid",
+      help="Make target to run. The default is droid")
+  argparser.add_argument("--touch", nargs="+", default=[],
+      help="Files to touch between builds. Must pair with --incremental.")
+  args = argparser.parse_args(sys.argv[1:])
+
+  if args.detect_embedded_paths and args.incremental:
+    sys.stderr.write("Can't pass --detect-embedded-paths and --incremental together.\n")
+    sys.exit(1)
+  if args.detect_embedded_paths and not args.check_out_dir:
+    sys.stderr.write("Can't pass --detect-embedded-paths and --no-check-out-dir together.\n")
+    sys.exit(1)
+  if args.touch and not args.incremental:
+    sys.stderr.write("The --incremental flag is required if the --touch flag is passed.")
+    sys.exit(1)
+
+  AssertAtTop()
+  RequireEnvVar("TARGET_PRODUCT")
+  RequireEnvVar("TARGET_BUILD_VARIANT")
+
+  # Out dir file names:
+  #   - dir_prefix - The directory we'll put everything in (except for maybe the top level
+  #     out/ dir).
+  #   - *work_dir - The directory that we will build directly into. This is in dir_prefix
+  #     unless --no-check-out-dir is set.
+  #   - *out_dir - After building, if work_dir is different from out_dir, we move the out
+  #     directory to here so we can do the comparisions.
+  #   - timestamp_* - Files we touch so we know the various phases between the builds, so we
+  #     can compare timestamps of files.
+  if args.incremental:
+    dir_prefix = "out_incremental"
+    if args.check_out_dir:
+      first_work_dir = first_out_dir = dir_prefix + "/out"
+      second_work_dir = second_out_dir = dir_prefix + "/out"
+    else:
+      first_work_dir = first_out_dir = "out"
+      second_work_dir = second_out_dir = "out"
+  else:
+    dir_prefix = "out_full"
+    first_out_dir = dir_prefix + "/out_1"
+    second_out_dir = dir_prefix + "/out_2"
+    if not args.check_out_dir:
+      first_work_dir = second_work_dir = "out"
+    elif args.detect_embedded_paths:
+      first_work_dir = first_out_dir
+      second_work_dir = second_out_dir
+    else:
+      first_work_dir = dir_prefix + "/work"
+      second_work_dir = dir_prefix + "/work"
+  timestamp_start = dir_prefix + "/timestamp_start"
+  timestamp_between = dir_prefix + "/timestamp_between"
+  timestamp_end = dir_prefix + "/timestamp_end"
+
+  if args.run_build:
+    # Initial clean, if necessary
+    print("Cleaning " + dir_prefix + "/")
+    Clean(dir_prefix)
+    print("Cleaning out/")
+    Clean("out")
+    CreateEmptyFile(timestamp_start)
+    print("Running the first build in " + first_work_dir)
+    RunBuild(first_work_dir, first_out_dir, args.target, args.hide_build_output)
+    for f in args.touch:
+      print("Touching " + f)
+      TouchFile(f)
+    CreateEmptyFile(timestamp_between)
+    print("Running the second build in " + second_work_dir)
+    RunBuild(second_work_dir, second_out_dir, args.target, args.hide_build_output)
+    CreateEmptyFile(timestamp_end)
+    print("Done building")
+    print()
+
+  # Which out directories to scan
+  if args.subdirs is not None:
+    if args.subdirs:
+      subdirs = args.subdirs
+    else:
+      subdirs = DEFAULT_DIRS
+    first_files = ProductFiles(RequireBuildVar(first_out_dir, "PRODUCT_OUT"), subdirs)
+    second_files = ProductFiles(RequireBuildVar(second_out_dir, "PRODUCT_OUT"), subdirs)
+  else:
+    first_files = OutFiles(first_out_dir)
+    second_files = OutFiles(second_out_dir)
+
+  printer = Printer()
+
+  if args.incremental:
+    # Find files that were rebuilt unnecessarily
+    touched_incrementally = FindOutFilesTouchedAfter(first_files,
+                                                     GetFileTimestamp(timestamp_between))
+    printer.PrintList("Touched in incremental build", touched_incrementally)
+  else:
+    # Compare the two out dirs
+    added, removed, changed = DiffFileList(first_files, second_files)
+    printer.PrintList("Added", added)
+    printer.PrintList("Removed", removed)
+    printer.PrintList("Changed", changed, "%s %s")
+
+  # Find files in the source tree that were touched
+  touched_during = FindSourceFilesTouchedAfter(GetFileTimestamp(timestamp_start))
+  printer.PrintList("Source files touched after start of build", touched_during)
+
+  # Find files and dirs that were output to "out" and didn't respect $OUT_DIR
+  if args.check_out_dir:
+    bad_out_dir_contents = FindFilesAndDirectories("out")
+    printer.PrintList("Files and directories created by rules that didn't respect $OUT_DIR",
+                      bad_out_dir_contents)
+
+  # If we didn't find anything, print success message
+  if not printer.printed_anything:
+    print("No bad behaviors found.")
+
+
+def AssertAtTop():
+  """If the current directory is not the top of an android source tree, print an error
+     message and exit."""
+  if not os.access(SOONG_UI, os.X_OK):
+    sys.stderr.write("FAILED: Please run from the root of the tree.\n")
+    sys.exit(1)
+
+
+def RequireEnvVar(name):
+  """Gets an environment variable. If that fails, then print an error message and exit."""
+  result = os.environ.get(name)
+  if not result:
+    sys.stderr.write("error: Can't determine %s. Please run lunch first.\n" % name)
+    sys.exit(1)
+  return result
+
+
+def RunSoong(out_dir, args, capture_output):
+  env = dict(os.environ)
+  env["OUT_DIR"] = out_dir
+  args = [SOONG_UI,] + args
+  if capture_output:
+    proc = subprocess.Popen(args, env=env, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
+    combined_output, none = proc.communicate()
+    return proc.returncode, combined_output
+  else:
+    result = subprocess.run(args, env=env)
+    return result.returncode, None
+
+
+def GetBuildVar(out_dir, name):
+  """Gets a variable from the build system."""
+  returncode, output = RunSoong(out_dir, ["--dumpvar-mode", name], True)
+  if returncode != 0:
+    return None
+  else:
+    return output.decode("utf-8").strip()
+
+
+def RequireBuildVar(out_dir, name):
+  """Gets a variable from the builds system. If that fails, then print an error
+     message and exit."""
+  value = GetBuildVar(out_dir, name)
+  if not value:
+    sys.stderr.write("error: Can't determine %s. Please run lunch first.\n" % name)
+    sys.exit(1)
+  return value
+
+
+def Clean(directory):
+  """"Deletes the supplied directory."""
+  try:
+    shutil.rmtree(directory)
+  except FileNotFoundError:
+    pass
+
+
+def RunBuild(work_dir, out_dir, target, hide_build_output):
+  """Runs a build. If the build fails, prints a message and exits."""
+  returncode, output = RunSoong(work_dir,
+                    ["--build-mode", "--all-modules", "--dir=" + os.getcwd(), target],
+                    hide_build_output)
+  if work_dir != out_dir:
+    os.replace(work_dir, out_dir)
+  if returncode != 0:
+    if hide_build_output:
+      # The build output was hidden, so print it now for debugging
+      sys.stderr.buffer.write(output)
+    sys.stderr.write("FAILED: Build failed. Stopping.\n")
+    sys.exit(1)
+
+
+def DiffFileList(first_files, second_files):
+  """Examines the files.
+
+  Returns:
+    Filenames of files in first_filelist but not second_filelist (added files)
+    Filenames of files in second_filelist but not first_filelist (removed files)
+    2-Tuple of filenames for the files that are in both but are different (changed files)
+  """
+  # List of files, relative to their respective PRODUCT_OUT directories
+  first_filelist = sorted([x for x in first_files], key=lambda x: x[1])
+  second_filelist = sorted([x for x in second_files], key=lambda x: x[1])
+
+  added = []
+  removed = []
+  changed = []
+
+  first_index = 0
+  second_index = 0
+
+  while first_index < len(first_filelist) and second_index < len(second_filelist):
+    # Path relative to source root and path relative to PRODUCT_OUT
+    first_full_filename, first_relative_filename = first_filelist[first_index]
+    second_full_filename, second_relative_filename = second_filelist[second_index]
+
+    if first_relative_filename < second_relative_filename:
+      # Removed
+      removed.append(first_full_filename)
+      first_index += 1
+    elif first_relative_filename > second_relative_filename:
+      # Added
+      added.append(second_full_filename)
+      second_index += 1
+    else:
+      # Both present
+      diff_type = DiffFiles(first_full_filename, second_full_filename)
+      if diff_type != DIFF_NONE:
+        changed.append((first_full_filename, second_full_filename))
+      first_index += 1
+      second_index += 1
+
+  while first_index < len(first_filelist):
+    first_full_filename, first_relative_filename = first_filelist[first_index]
+    removed.append(first_full_filename)
+    first_index += 1
+
+  while second_index < len(second_filelist):
+    second_full_filename, second_relative_filename = second_filelist[second_index]
+    added.append(second_full_filename)
+    second_index += 1
+
+  return (SortByTimestamp(added),
+          SortByTimestamp(removed),
+          SortByTimestamp(changed, key=lambda item: item[1]))
+
+
+def FindOutFilesTouchedAfter(files, timestamp):
+  """Find files in the given file iterator that were touched after timestamp."""
+  result = []
+  for full, relative in files:
+    ts = GetFileTimestamp(full)
+    if ts > timestamp:
+      result.append(TouchedFile(full, ts))
+  return [f.filename for f in sorted(result, key=lambda f: f.timestamp)]
+
+
+def GetFileTimestamp(filename):
+  """Get timestamp for a file (just wraps stat)."""
+  st = os.stat(filename, follow_symlinks=False)
+  return st.st_mtime
+
+
+def SortByTimestamp(items, key=lambda item: item):
+  """Sort the list by timestamp of files.
+  Args:
+    items - the list of items to sort
+    key - a function to extract a filename from each element in items
+  """
+  return [x[0] for x in sorted([(item, GetFileTimestamp(key(item))) for item in items],
+                               key=lambda y: y[1])]
+
+
+def FindSourceFilesTouchedAfter(timestamp):
+  """Find files in the source tree that have changed after timestamp. Ignores
+  the out directory."""
+  result = []
+  for root, dirs, files in os.walk(".", followlinks=False):
+    if root == ".":
+      RemoveItemsFromList(dirs, (".repo", "out", "out_full", "out_incremental"))
+    for f in files:
+      full = os.path.sep.join((root, f))[2:]
+      ts = GetFileTimestamp(full)
+      if ts > timestamp:
+        result.append(TouchedFile(full, ts))
+  return [f.filename for f in sorted(result, key=lambda f: f.timestamp)]
+
+
+def FindFilesAndDirectories(directory):
+  """Finds all files and directories inside a directory."""
+  result = []
+  for root, dirs, files in os.walk(directory, followlinks=False):
+    result += [os.path.sep.join((root, x, "")) for x in dirs]
+    result += [os.path.sep.join((root, x)) for x in files]
+  return result
+
+
+def CreateEmptyFile(filename):
+  """Create an empty file with now as the timestamp at filename."""
+  try:
+    os.makedirs(os.path.dirname(filename))
+  except FileExistsError:
+    pass
+  open(filename, "w").close()
+  os.utime(filename)
+
+
+def TouchFile(filename):
+  os.utime(filename)
+
+
+def DiffFiles(first_filename, second_filename):
+  def AreFileContentsSame(remaining, first_filename, second_filename):
+    """Compare the file contents. They must be known to be the same size."""
+    CHUNK_SIZE = 32*1024
+    with open(first_filename, "rb") as first_file:
+      with open(second_filename, "rb") as second_file:
+        while remaining > 0:
+          size = min(CHUNK_SIZE, remaining)
+          if first_file.read(CHUNK_SIZE) != second_file.read(CHUNK_SIZE):
+            return False
+          remaining -= size
+        return True
+
+  first_stat = os.stat(first_filename, follow_symlinks=False)
+  second_stat = os.stat(first_filename, follow_symlinks=False)
+
+  # Mode bits
+  if first_stat.st_mode != second_stat.st_mode:
+    return DIFF_MODE
+
+  # File size
+  if first_stat.st_size != second_stat.st_size:
+    return DIFF_SIZE
+
+  # Contents
+  if stat.S_ISLNK(first_stat.st_mode):
+    if os.readlink(first_filename) != os.readlink(second_filename):
+      return DIFF_SYMLINK
+  elif stat.S_ISREG(first_stat.st_mode):
+    if not AreFileContentsSame(first_stat.st_size, first_filename, second_filename):
+      return DIFF_CONTENTS
+
+  return DIFF_NONE
+
+
+class FileIterator(object):
+  """Object that produces an iterator containing all files in a given directory.
+
+  Each iteration yields a tuple containing:
+
+  [0] (full) Path to file relative to source tree.
+  [1] (relative) Path to the file relative to the base directory given in the
+      constructor.
+  """
+
+  def __init__(self, base_dir):
+    self._base_dir = base_dir
+
+  def __iter__(self):
+    return self._Iterator(self, self._base_dir)
+
+  def ShouldIncludeFile(self, root, path):
+    return False
+
+  class _Iterator(object):
+    def __init__(self, parent, base_dir):
+      self._parent = parent
+      self._base_dir = base_dir
+      self._walker = os.walk(base_dir, followlinks=False)
+      self._current_index = 0
+      self._current_dir = []
+
+    def __iter__(self):
+      return self
+
+    def __next__(self):
+      # os.walk's iterator will eventually terminate by raising StopIteration
+      while True:
+        if self._current_index >= len(self._current_dir):
+          root, dirs, files = self._walker.__next__()
+          full_paths = [os.path.sep.join((root, f)) for f in files]
+          pairs = [(f, f[len(self._base_dir)+1:]) for f in full_paths]
+          self._current_dir = [(full, relative) for full, relative in pairs
+                               if self._parent.ShouldIncludeFile(root, relative)]
+          self._current_index = 0
+          if not self._current_dir:
+            continue
+        index = self._current_index
+        self._current_index += 1
+        return self._current_dir[index]
+
+
+class OutFiles(FileIterator):
+  """Object that produces an iterator containing all files in a given out directory,
+  except for files which are known to be touched as part of build setup.
+  """
+  def __init__(self, out_dir):
+    super().__init__(out_dir)
+    self._out_dir = out_dir
+
+  def ShouldIncludeFile(self, root, relative):
+    # Skip files in root, although note that this could actually skip
+    # files that are sadly generated directly into that directory.
+    if root == self._out_dir:
+      return False
+    # Skiplist
+    for skip in BUILD_INTERNALS_PREFIX_SKIP:
+      if relative.startswith(skip):
+        return False
+    for skip in BUILD_INTERNALS_SUFFIX_SKIP:
+      if relative.endswith(skip):
+        return False
+    return True
+
+
+class ProductFiles(FileIterator):
+  """Object that produces an iterator containing files in listed subdirectories of $PRODUCT_OUT.
+  """
+  def __init__(self, product_out, subdirs):
+    super().__init__(product_out)
+    self._subdirs = subdirs
+
+  def ShouldIncludeFile(self, root, relative):
+    for subdir in self._subdirs:
+      if relative.startswith(subdir):
+        return True
+    return False
+
+
+class TouchedFile(object):
+  """A file in the out directory with a timestamp."""
+  def __init__(self, filename, timestamp):
+    self.filename = filename
+    self.timestamp = timestamp
+
+
+def RemoveItemsFromList(haystack, needles):
+  for needle in needles:
+    try:
+      haystack.remove(needle)
+    except ValueError:
+      pass
+
+
+class Printer(object):
+  def __init__(self):
+    self.printed_anything = False
+
+  def PrintList(self, title, items, fmt="%s"):
+    if items:
+      if self.printed_anything:
+        sys.stdout.write("\n")
+      sys.stdout.write("%s:\n" % title)
+      for item in items:
+        sys.stdout.write("  %s\n" % fmt % item)
+      self.printed_anything = True
+
+
+if __name__ == "__main__":
+  try:
+    main()
+  except KeyboardInterrupt:
+    pass
+
+
+# vim: ts=2 sw=2 sts=2 nocindent