diff --git a/clang/test/lit.cfg.py b/clang/test/lit.cfg.py
index 6b508e372267d..dbc63b4f0f97a 100644
--- a/clang/test/lit.cfg.py
+++ b/clang/test/lit.cfg.py
@@ -373,8 +373,19 @@ def calculate_arch_features(arch_string):
 # default configs for the test runs.
 config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"
 
+if lit_config.update_tests:
+    import sys
+    import os
+
+    utilspath = os.path.join(config.llvm_src_root, "utils")
+    sys.path.append(utilspath)
+    from update_any_test_checks import utc_lit_plugin
+
+    lit_config.test_updaters.append(utc_lit_plugin)
+
 # Restrict the size of the on-disk CAS for tests. This allows testing in
 # constrained environments (e.g. small TMPDIR). It also prevents leaving
 # behind large files on file systems that do not support sparse files if a test
 # crashes before resizing the file.
 config.environment["LLVM_CAS_MAX_MAPPING_SIZE"] = "%d" % (100 * 1024 * 1024)
+
diff --git a/llvm/docs/CommandGuide/lit.rst b/llvm/docs/CommandGuide/lit.rst
index c9d5baba3e2f4..adf0075215c46 100644
--- a/llvm/docs/CommandGuide/lit.rst
+++ b/llvm/docs/CommandGuide/lit.rst
@@ -313,6 +313,11 @@ ADDITIONAL OPTIONS
 
  List all of the discovered tests and exit.
 
+.. option:: --update-tests
+
+ Pass failing tests to functions in the ``lit_config.test_updaters`` list to
+ check whether any of them know how to update the test to make it pass.
+
 EXIT STATUS
 -----------
 
diff --git a/llvm/test/lit.cfg.py b/llvm/test/lit.cfg.py
index 3b1e6b72fdde9..588deccfd5a6c 100644
--- a/llvm/test/lit.cfg.py
+++ b/llvm/test/lit.cfg.py
@@ -632,3 +632,13 @@ def have_ld64_plugin_support():
 
 if config.has_logf128:
     config.available_features.add("has_logf128")
+
+if lit_config.update_tests:
+    import sys
+    import os
+
+    utilspath = os.path.join(config.llvm_src_root, "utils")
+    sys.path.append(utilspath)
+    from update_any_test_checks import utc_lit_plugin
+
+    lit_config.test_updaters.append(utc_lit_plugin)
diff --git a/llvm/utils/lit/lit/DiffUpdater.py b/llvm/utils/lit/lit/DiffUpdater.py
new file mode 100644
index 0000000000000..a29c46fb8508f
--- /dev/null
+++ b/llvm/utils/lit/lit/DiffUpdater.py
@@ -0,0 +1,161 @@
+import shutil
+import os
+import shlex
+import pathlib
+
+"""
+This file provides the `diff_test_updater` function, which is invoked on failed RUN lines when lit is executed with --update-tests.
+It checks whether the failed command is `diff` and, if so, uses heuristics to determine which file is the checked-in reference file and which file is output from the test case.
+The heuristics are currently as follows:
+    - if exactly one file originates from the `split-file` command, that file is the reference file and the other is the output file
+    - if exactly one file ends with ".expected" (common pattern in LLVM), that file is the reference file and the other is the output file
+    - if exactly one file path contains ".tmp" (e.g. because it contains the expansion of "%t"), that file is the reference file and the other is the output file
+If the command matches one of these patterns the output file content is copied to the reference file to make the test pass.
+If the reference file originated in `split-file`, the output file content is instead copied to the corresponding slice of the test file.
+Otherwise the test is ignored.
+
+Possible improvements:
+    - Support stdin patterns like "my_binary %s | diff expected.txt"
+    - Scan RUN lines to see if a file is the source of output from a previous command (other than `split-file`).
+      If it is then it is not a reference file that can be copied to, regardless of name, since the test will overwrite it anyways.
+    - Only update the parts that need updating (based on the diff output). Could help avoid noisy updates when e.g. whitespace changes are ignored.
+"""
+
+
+class NormalFileTarget:
+    def __init__(self, target):
+        self.target = target
+
+    def copyFrom(self, source):
+        shutil.copy(source, self.target)
+
+    def __str__(self):
+        return self.target
+
+
+class SplitFileTarget:
+    def __init__(self, slice_start_idx, test_path, lines):
+        self.slice_start_idx = slice_start_idx
+        self.test_path = test_path
+        self.lines = lines
+
+    def copyFrom(self, source):
+        lines_before = self.lines[: self.slice_start_idx + 1]
+        self.lines = self.lines[self.slice_start_idx + 1 :]
+        slice_end_idx = None
+        for i, l in enumerate(self.lines):
+            if SplitFileTarget._get_split_line_path(l) != None:
+                slice_end_idx = i
+                break
+        if slice_end_idx is not None:
+            lines_after = self.lines[slice_end_idx:]
+        else:
+            lines_after = []
+        with open(source, "r") as f:
+            new_lines = lines_before + f.readlines() + lines_after
+        with open(self.test_path, "w") as f:
+            for l in new_lines:
+                f.write(l)
+
+    def __str__(self):
+        return f"slice in {self.test_path}"
+
+    @staticmethod
+    def get_target_dir(commands, test_path):
+        # posix=True breaks Windows paths because \ is treated as an escaping character
+        for cmd in commands:
+            split = shlex.split(cmd, posix=False)
+            if "split-file" not in split:
+                continue
+            start_idx = split.index("split-file")
+            split = split[start_idx:]
+            if len(split) < 3:
+                continue
+            p = unquote(split[1].strip())
+            if not test_path.samefile(p):
+                continue
+            return unquote(split[2].strip())
+        return None
+
+    @staticmethod
+    def create(path, commands, test_path, target_dir):
+        path = pathlib.Path(path)
+        with open(test_path, "r") as f:
+            lines = f.readlines()
+        for i, l in enumerate(lines):
+            p = SplitFileTarget._get_split_line_path(l)
+            if p and path.samefile(os.path.join(target_dir, p)):
+                idx = i
+                break
+        else:
+            return None
+        return SplitFileTarget(idx, test_path, lines)
+
+    @staticmethod
+    def _get_split_line_path(l):
+        if len(l) < 6:
+            return None
+        if l.startswith("//"):
+            l = l[2:]
+        else:
+            l = l[1:]
+        if l.startswith("--- "):
+            l = l[4:]
+        else:
+            return None
+        return l.rstrip()
+
+
+def unquote(s):
+    if len(s) > 1 and s[0] == s[-1] and (s[0] == '"' or s[0] == "'"):
+        return s[1:-1]
+    return s
+
+
+def get_source_and_target(a, b, test_path, commands):
+    """
+    Try to figure out which file is the test output and which is the reference.
+    """
+    split_target_dir = SplitFileTarget.get_target_dir(commands, test_path)
+    if split_target_dir:
+        a_target = SplitFileTarget.create(a, commands, test_path, split_target_dir)
+        b_target = SplitFileTarget.create(b, commands, test_path, split_target_dir)
+        if a_target and b_target:
+            return None
+        if a_target:
+            return b, a_target
+        if b_target:
+            return a, b_target
+
+    expected_suffix = ".expected"
+    if a.endswith(expected_suffix) and not b.endswith(expected_suffix):
+        return b, NormalFileTarget(a)
+    if b.endswith(expected_suffix) and not a.endswith(expected_suffix):
+        return a, NormalFileTarget(b)
+
+    tmp_substr = ".tmp"
+    if tmp_substr in a and not tmp_substr in b:
+        return a, NormalFileTarget(b)
+    if tmp_substr in b and not tmp_substr in a:
+        return b, NormalFileTarget(a)
+
+    return None
+
+
+def filter_flags(args):
+    return [arg for arg in args if not arg.startswith("-")]
+
+
+def diff_test_updater(result, test, commands):
+    args = filter_flags(result.command.args)
+    if len(args) != 3:
+        return None
+    [cmd, a, b] = args
+    if cmd != "diff":
+        return None
+    res = get_source_and_target(a, b, pathlib.Path(test.getFilePath()), commands)
+    if not res:
+        return f"update-diff-test: could not deduce source and target from {a} and {b}"
+    source, target = res
+    target.copyFrom(source)
+    return f"update-diff-test: copied {source} to {target}"
diff --git a/llvm/utils/lit/lit/LitConfig.py b/llvm/utils/lit/lit/LitConfig.py
index 5dc712ae28370..dffafeb8b6081 100644
--- a/llvm/utils/lit/lit/LitConfig.py
+++ b/llvm/utils/lit/lit/LitConfig.py
@@ -8,6 +8,7 @@
 import lit.formats
 import lit.TestingConfig
 import lit.util
+from lit.DiffUpdater import diff_test_updater
 
 # LitConfig must be a new style class for properties to work
 class LitConfig(object):
@@ -38,6 +39,7 @@ def __init__(
         parallelism_groups={},
         per_test_coverage=False,
         gtest_sharding=True,
+        update_tests=False,
     ):
         # The name of the test runner.
         self.progname = progname
@@ -89,6 +91,8 @@ def __init__(
         self.parallelism_groups = parallelism_groups
         self.per_test_coverage = per_test_coverage
         self.gtest_sharding = bool(gtest_sharding)
+        self.update_tests = update_tests
+        self.test_updaters = [diff_test_updater]
 
     @property
     def maxIndividualTestTime(self):
diff --git a/llvm/utils/lit/lit/TestRunner.py b/llvm/utils/lit/lit/TestRunner.py
index da7fa86fd3917..6f4db5b1b134e 100644
--- a/llvm/utils/lit/lit/TestRunner.py
+++ b/llvm/utils/lit/lit/TestRunner.py
@@ -13,6 +13,7 @@
 import tempfile
 import threading
 import typing
+import traceback
 from typing import Optional, Tuple
 
 import io
@@ -47,6 +48,16 @@ def __init__(self, message):
         super().__init__(message)
 
 
+class TestUpdaterException(Exception):
+    """
+    There was an error not during test execution, but while invoking a function
+    in test_updaters on a failing RUN line.
+    """
+
+    def __init__(self, message):
+        super().__init__(message)
+
+
 kIsWindows = platform.system() == "Windows"
 
 # Don't use close_fds on Windows.
@@ -1154,6 +1165,28 @@ def executeScriptInternal(
                 str(result.timeoutReached),
             )
 
+        if (
+            litConfig.update_tests
+            and result.exitCode != 0
+            and not timeoutInfo
+            # In theory tests marked XFAIL can fail in the form of XPASS, but the
+            # test updaters are not expected to be able to fix that, so always skip for XFAIL
+            and not test.isExpectedToFail()
+        ):
+            for test_updater in litConfig.test_updaters:
+                try:
+                    update_output = test_updater(result, test, commands)
+                except Exception as e:
+                    output = out
+                    output += err
+                    output += "Exception occurred in test updater:\n"
+                    output += traceback.format_exc()
+                    raise TestUpdaterException(output)
+                if update_output:
+                    for line in update_output.splitlines():
+                        out += f"# {line}\n"
+                    break
+
     return out, err, exitCode, timeoutInfo
 
 
diff --git a/llvm/utils/lit/lit/cl_arguments.py b/llvm/utils/lit/lit/cl_arguments.py
index ed78256ee414b..dcbe553c6d482 100644
--- a/llvm/utils/lit/lit/cl_arguments.py
+++ b/llvm/utils/lit/lit/cl_arguments.py
@@ -204,6 +204,12 @@ def parse_args():
         action="store_true",
         help="Exit with status zero even if some tests fail",
     )
+    execution_group.add_argument(
+        "--update-tests",
+        dest="update_tests",
+        action="store_true",
+        help="Try to update regression tests to reflect current behavior, if possible",
+    )
     execution_test_time_group = execution_group.add_mutually_exclusive_group()
     execution_test_time_group.add_argument(
         "--skip-test-time-recording",
diff --git a/llvm/utils/lit/lit/llvm/config.py b/llvm/utils/lit/lit/llvm/config.py
index b3dfddba483f5..4ef24ae20f8c1 100644
--- a/llvm/utils/lit/lit/llvm/config.py
+++ b/llvm/utils/lit/lit/llvm/config.py
@@ -64,12 +64,17 @@ def __init__(self, lit_config, config):
             self.with_environment("_TAG_REDIR_ERR", "TXT")
             self.with_environment("_CEE_RUNOPTS", "FILETAG(AUTOCVT,AUTOTAG) POSIX(ON)")
 
+        if lit_config.update_tests:
+            self.use_lit_shell = True
+
         # Choose between lit's internal shell pipeline runner and a real shell.
         # If LIT_USE_INTERNAL_SHELL is in the environment, we use that as an
         # override.
         lit_shell_env = os.environ.get("LIT_USE_INTERNAL_SHELL")
         if lit_shell_env:
             self.use_lit_shell = lit.util.pythonize_bool(lit_shell_env)
+            if not self.use_lit_shell and lit_config.update_tests:
+                print("note: --update-tests is not supported when using external shell")
 
         if not self.use_lit_shell:
             features.add("shell")
diff --git a/llvm/utils/lit/lit/main.py b/llvm/utils/lit/lit/main.py
index 24ba804f0c363..745e376de7d52 100755
--- a/llvm/utils/lit/lit/main.py
+++ b/llvm/utils/lit/lit/main.py
@@ -42,6 +42,7 @@ def main(builtin_params={}):
         config_prefix=opts.configPrefix,
         per_test_coverage=opts.per_test_coverage,
         gtest_sharding=opts.gtest_sharding,
+        update_tests=opts.update_tests,
     )
 
     discovered_tests = lit.discovery.find_tests_for_inputs(
diff --git a/llvm/utils/lit/lit/worker.py b/llvm/utils/lit/lit/worker.py
index 8e78bfd45d38b..0e36f3e9a2424 100644
--- a/llvm/utils/lit/lit/worker.py
+++ b/llvm/utils/lit/lit/worker.py
@@ -13,6 +13,7 @@
 
 import lit.Test
 import lit.util
+from lit.TestRunner import TestUpdaterException
 
 
 _lit_config = None
@@ -75,6 +76,10 @@ def _execute_test_handle_errors(test, lit_config):
     try:
         result = test.config.test_format.execute(test, lit_config)
         return _adapt_result(result)
+    except TestUpdaterException as e:
+        if lit_config.debug:
+            raise
+        return lit.Test.Result(lit.Test.UNRESOLVED, str(e))
     except:
         if lit_config.debug:
             raise
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/.gitignore b/llvm/utils/lit/tests/Inputs/diff-test-update/.gitignore
new file mode 100644
index 0000000000000..aea8ee3be4982
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/.gitignore
@@ -0,0 +1,10 @@
+; diff-tmp-dir.test clobbers this file
+empty.txt
+; these test cases are clobbered when run, so they're recreated each time
+single-split-file.test
+single-split-file-populated.test
+multiple-split-file.test
+multiple-split-file-populated.test
+single-split-file-no-expected.test
+split-c-comments.test
+split whitespace.test
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/1.in b/llvm/utils/lit/tests/Inputs/diff-test-update/1.in
new file mode 100644
index 0000000000000..b7d6715e2df11
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/1.in
@@ -0,0 +1 @@
+FOO
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/2.in b/llvm/utils/lit/tests/Inputs/diff-test-update/2.in
new file mode 100644
index 0000000000000..ba578e48b1836
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/2.in
@@ -0,0 +1 @@
+BAR
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/diff-bail.test b/llvm/utils/lit/tests/Inputs/diff-test-update/diff-bail.test
new file mode 100644
index 0000000000000..ded931384f192
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/diff-bail.test
@@ -0,0 +1,3 @@
+# There is no indication here of which file is the reference file to update
+# RUN: diff %S/1.in %S/2.in
+
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/diff-bail2.test b/llvm/utils/lit/tests/Inputs/diff-test-update/diff-bail2.test
new file mode 100644
index 0000000000000..26e12a3b2b289
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/diff-bail2.test
@@ -0,0 +1,7 @@
+# RUN: mkdir %t
+# RUN: cp %S/1.in %t/1.txt
+# RUN: cp %S/2.in %t/2.txt
+
+# There is no indication here of which file is the reference file to update
+# RUN: diff %t/1.txt %t/2.txt
+
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/diff-expected.test b/llvm/utils/lit/tests/Inputs/diff-test-update/diff-expected.test
new file mode 100644
index 0000000000000..a26c6d338f964
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/diff-expected.test
@@ -0,0 +1,5 @@
+# RUN: mkdir %t
+# RUN: cp %S/1.in %t/my-file.expected
+# RUN: cp %S/2.in %t/my-file.txt
+# RUN: diff %t/my-file.expected %t/my-file.txt
+
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/diff-tmp-dir.test b/llvm/utils/lit/tests/Inputs/diff-test-update/diff-tmp-dir.test
new file mode 100644
index 0000000000000..a4fab7e74c007
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/diff-tmp-dir.test
@@ -0,0 +1,8 @@
+# RUN: mkdir %t
+# Tests that if file A is in the %t directory and file B is not,
+# the diff test updater copies from file A to B.
+# RUN: echo "" > %S/empty.txt
+# RUN: cp %S/1.in %t/1.txt
+
+# RUN: diff %t/1.txt %S/empty.txt
+
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/diff-tmp.test b/llvm/utils/lit/tests/Inputs/diff-test-update/diff-tmp.test
new file mode 100644
index 0000000000000..042bf244ebaa1
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/diff-tmp.test
@@ -0,0 +1,3 @@
+# RUN: cp %S/1.in %t.txt
+# RUN: cp %S/2.in %S/diff-t-out.txt
+# RUN: diff %t.txt %S/diff-t-out.txt
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/lit.cfg b/llvm/utils/lit/tests/Inputs/diff-test-update/lit.cfg
new file mode 100644
index 0000000000000..9bd255276638a
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/lit.cfg
@@ -0,0 +1,8 @@
+import lit.formats
+
+config.name = "diff-test-update"
+config.suffixes = [".test"]
+config.test_format = lit.formats.ShTest()
+config.test_source_root = None
+config.test_exec_root = None
+
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/multiple-split-file-populated.in b/llvm/utils/lit/tests/Inputs/diff-test-update/multiple-split-file-populated.in
new file mode 100644
index 0000000000000..e218ed6a0c6ea
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/multiple-split-file-populated.in
@@ -0,0 +1,17 @@
+# RUN: split-file %s %t
+# RUN: cp %S/1.in %t/out.txt
+# RUN: diff %t/test3.expected %t/out.txt
+
+#--- test1.expected
+unrelated
+#--- test2.expected
+#--- test3.expected
+BAR
+
+BAZ
+
+#--- test4.expected
+filler
+#--- test5.expected
+
+
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/multiple-split-file.in b/llvm/utils/lit/tests/Inputs/diff-test-update/multiple-split-file.in
new file mode 100644
index 0000000000000..c47db99912c24
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/multiple-split-file.in
@@ -0,0 +1,13 @@
+# RUN: split-file %s %t
+# RUN: cp %S/1.in %t/out.txt
+# RUN: diff %t/test3.expected %t/out.txt
+
+#--- test1.expected
+unrelated
+#--- test2.expected
+#--- test3.expected
+#--- test4.expected
+filler
+#--- test5.expected
+
+
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/multiple-split-file.out b/llvm/utils/lit/tests/Inputs/diff-test-update/multiple-split-file.out
new file mode 100644
index 0000000000000..c1d2782d3c2d4
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/multiple-split-file.out
@@ -0,0 +1,14 @@
+# RUN: split-file %s %t
+# RUN: cp %S/1.in %t/out.txt
+# RUN: diff %t/test3.expected %t/out.txt
+
+#--- test1.expected
+unrelated
+#--- test2.expected
+#--- test3.expected
+FOO
+#--- test4.expected
+filler
+#--- test5.expected
+
+
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/single-split-file-no-expected.in b/llvm/utils/lit/tests/Inputs/diff-test-update/single-split-file-no-expected.in
new file mode 100644
index 0000000000000..510dc7afba16b
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/single-split-file-no-expected.in
@@ -0,0 +1,6 @@
+# RUN: split-file %s %t
+# RUN: cp %S/1.in %t/out.txt
+# RUN: diff %t/test.txt %t/out.txt
+
+#--- test.txt
+
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/single-split-file-no-expected.out b/llvm/utils/lit/tests/Inputs/diff-test-update/single-split-file-no-expected.out
new file mode 100644
index 0000000000000..f52e3004aee15
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/single-split-file-no-expected.out
@@ -0,0 +1,6 @@
+# RUN: split-file %s %t
+# RUN: cp %S/1.in %t/out.txt
+# RUN: diff %t/test.txt %t/out.txt
+
+#--- test.txt
+FOO
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/single-split-file-populated.in b/llvm/utils/lit/tests/Inputs/diff-test-update/single-split-file-populated.in
new file mode 100644
index 0000000000000..63042cf9b86bc
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/single-split-file-populated.in
@@ -0,0 +1,7 @@
+# RUN: split-file %s %t
+# RUN: cp %S/1.in %t/out.txt
+# RUN: diff %t/test.expected %t/out.txt
+
+#--- test.expected
+BAR
+
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/single-split-file.in b/llvm/utils/lit/tests/Inputs/diff-test-update/single-split-file.in
new file mode 100644
index 0000000000000..422ccf2ef6813
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/single-split-file.in
@@ -0,0 +1,5 @@
+# RUN: split-file %s %t
+# RUN: cp %S/1.in %t/out.txt
+# RUN: diff %t/test.expected %t/out.txt
+
+#--- test.expected
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/single-split-file.out b/llvm/utils/lit/tests/Inputs/diff-test-update/single-split-file.out
new file mode 100644
index 0000000000000..5552ad328ec5c
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/single-split-file.out
@@ -0,0 +1,6 @@
+# RUN: split-file %s %t
+# RUN: cp %S/1.in %t/out.txt
+# RUN: diff %t/test.expected %t/out.txt
+
+#--- test.expected
+FOO
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/split-both.test b/llvm/utils/lit/tests/Inputs/diff-test-update/split-both.test
new file mode 100644
index 0000000000000..f564f446cc94b
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/split-both.test
@@ -0,0 +1,11 @@
+# RUN: split-file %s %t
+# RUN: diff %t/split-both.expected %t/split-both.out
+
+# ignore the fact that it's called ".expected"
+# when comparing two files originating in split-file
+
+#--- split-both.expected
+FOO
+#--- split-both.out
+BAR
+
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/split-c-comments.in b/llvm/utils/lit/tests/Inputs/diff-test-update/split-c-comments.in
new file mode 100644
index 0000000000000..3cda60118f5ba
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/split-c-comments.in
@@ -0,0 +1,6 @@
+// RUN: split-file %s %t
+// RUN: cp %S/1.in %t/out.txt
+// RUN: diff %t/test.txt %t/out.txt
+//
+//--- test.txt
+
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/split-c-comments.out b/llvm/utils/lit/tests/Inputs/diff-test-update/split-c-comments.out
new file mode 100644
index 0000000000000..5020804f198b1
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/split-c-comments.out
@@ -0,0 +1,6 @@
+// RUN: split-file %s %t
+// RUN: cp %S/1.in %t/out.txt
+// RUN: diff %t/test.txt %t/out.txt
+//
+//--- test.txt
+FOO
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/split-whitespace.in b/llvm/utils/lit/tests/Inputs/diff-test-update/split-whitespace.in
new file mode 100644
index 0000000000000..ad48d2ae4953c
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/split-whitespace.in
@@ -0,0 +1,6 @@
+// RUN: split-file "%s" "%t"
+// RUN: cp %S/1.in "%t/out.txt"
+// RUN: diff "%t/test.txt" "%t/out.txt"
+//
+//--- test.txt
+
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/split-whitespace.out b/llvm/utils/lit/tests/Inputs/diff-test-update/split-whitespace.out
new file mode 100644
index 0000000000000..cb28124101ac6
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/split-whitespace.out
@@ -0,0 +1,6 @@
+// RUN: split-file "%s" "%t"
+// RUN: cp %S/1.in "%t/out.txt"
+// RUN: diff "%t/test.txt" "%t/out.txt"
+//
+//--- test.txt
+FOO
diff --git a/llvm/utils/lit/tests/Inputs/diff-test-update/unrelated-split.test b/llvm/utils/lit/tests/Inputs/diff-test-update/unrelated-split.test
new file mode 100644
index 0000000000000..b04eff36721de
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/diff-test-update/unrelated-split.test
@@ -0,0 +1,11 @@
+# the fact that this test runs split-file is unrelated
+# to the diffed files
+
+# RUN: mkdir %t
+# RUN: split-file %s %t
+# RUN: cp %S/1.in %t/unrelated-split.expected
+# RUN: cp %S/2.in %t/unrelated-split.txt
+# RUN: diff %t/unrelated-split.expected %t/unrelated-split.txt
+
+#--- distraction.txt
+
diff --git a/llvm/utils/lit/tests/Inputs/pass-test-update/fail.test b/llvm/utils/lit/tests/Inputs/pass-test-update/fail.test
new file mode 100644
index 0000000000000..a410ef9293129
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/pass-test-update/fail.test
@@ -0,0 +1 @@
+# RUN: not echo "fail"
diff --git a/llvm/utils/lit/tests/Inputs/pass-test-update/lit.cfg b/llvm/utils/lit/tests/Inputs/pass-test-update/lit.cfg
new file mode 100644
index 0000000000000..e0dbc3220678c
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/pass-test-update/lit.cfg
@@ -0,0 +1,14 @@
+import lit.formats
+
+config.name = "pass-test-update"
+config.suffixes = [".test"]
+config.test_format = lit.formats.ShTest()
+config.test_source_root = None
+config.test_exec_root = None
+
+import sys
+import os
+sys.path.append(os.path.dirname(__file__))
+from should_not_run import should_not_run
+lit_config.test_updaters.append(should_not_run)
+
diff --git a/llvm/utils/lit/tests/Inputs/pass-test-update/pass-silent.test b/llvm/utils/lit/tests/Inputs/pass-test-update/pass-silent.test
new file mode 100644
index 0000000000000..07cafbc66a897
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/pass-test-update/pass-silent.test
@@ -0,0 +1,2 @@
+# RUN: echo ""
+
diff --git a/llvm/utils/lit/tests/Inputs/pass-test-update/pass.test b/llvm/utils/lit/tests/Inputs/pass-test-update/pass.test
new file mode 100644
index 0000000000000..3a12c5fee4bb0
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/pass-test-update/pass.test
@@ -0,0 +1 @@
+# RUN: echo "passed"
diff --git a/llvm/utils/lit/tests/Inputs/pass-test-update/should_not_run.py b/llvm/utils/lit/tests/Inputs/pass-test-update/should_not_run.py
new file mode 100644
index 0000000000000..5b39d208a2ed6
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/pass-test-update/should_not_run.py
@@ -0,0 +1,2 @@
+def should_not_run(foo, bar, baz):
+    raise Exception("this test updater should only run on failure")
diff --git a/llvm/utils/lit/tests/Inputs/pass-test-update/xfail.test b/llvm/utils/lit/tests/Inputs/pass-test-update/xfail.test
new file mode 100644
index 0000000000000..044b6a19b4a19
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/pass-test-update/xfail.test
@@ -0,0 +1,2 @@
+# XFAIL: *
+# RUN: not echo "failed"
diff --git a/llvm/utils/lit/tests/Inputs/pass-test-update/xpass.test b/llvm/utils/lit/tests/Inputs/pass-test-update/xpass.test
new file mode 100644
index 0000000000000..e0c1c39762ee9
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/pass-test-update/xpass.test
@@ -0,0 +1,2 @@
+# XFAIL: *
+# RUN: echo "accidentally passed"
diff --git a/llvm/utils/lit/tests/diff-test-update.py b/llvm/utils/lit/tests/diff-test-update.py
new file mode 100644
index 0000000000000..8b9f4610f7f95
--- /dev/null
+++ b/llvm/utils/lit/tests/diff-test-update.py
@@ -0,0 +1,29 @@
+# RUN: cp %S/Inputs/diff-test-update/single-split-file.in %S/Inputs/diff-test-update/single-split-file.test
+# RUN: cp %S/Inputs/diff-test-update/single-split-file-populated.in %S/Inputs/diff-test-update/single-split-file-populated.test
+# RUN: cp %S/Inputs/diff-test-update/multiple-split-file.in %S/Inputs/diff-test-update/multiple-split-file.test
+# RUN: cp %S/Inputs/diff-test-update/multiple-split-file-populated.in %S/Inputs/diff-test-update/multiple-split-file-populated.test
+# RUN: cp %S/Inputs/diff-test-update/single-split-file-no-expected.in %S/Inputs/diff-test-update/single-split-file-no-expected.test
+# RUN: cp %S/Inputs/diff-test-update/split-c-comments.in %S/Inputs/diff-test-update/split-c-comments.test
+# RUN: cp %S/Inputs/diff-test-update/split-whitespace.in "%S/Inputs/diff-test-update/split whitespace.test"
+
+# RUN: not %{lit} --update-tests -v %S/Inputs/diff-test-update | FileCheck %s
+
+# RUN: diff --strip-trailing-cr %S/Inputs/diff-test-update/single-split-file.out %S/Inputs/diff-test-update/single-split-file.test
+# RUN: diff --strip-trailing-cr %S/Inputs/diff-test-update/single-split-file.out %S/Inputs/diff-test-update/single-split-file-populated.test
+# RUN: diff --strip-trailing-cr %S/Inputs/diff-test-update/multiple-split-file.out %S/Inputs/diff-test-update/multiple-split-file.test
+# RUN: diff --strip-trailing-cr %S/Inputs/diff-test-update/multiple-split-file.out %S/Inputs/diff-test-update/multiple-split-file-populated.test
+# RUN: diff --strip-trailing-cr %S/Inputs/diff-test-update/single-split-file-no-expected.out %S/Inputs/diff-test-update/single-split-file-no-expected.test
+# RUN: diff --strip-trailing-cr %S/Inputs/diff-test-update/split-c-comments.out %S/Inputs/diff-test-update/split-c-comments.test
+# RUN: diff --strip-trailing-cr %S/Inputs/diff-test-update/split-whitespace.out "%S/Inputs/diff-test-update/split whitespace.test"
+
+
+# CHECK: # update-diff-test: could not deduce source and target from {{.*}}1.in and {{.*}}2.in
+# CHECK: # update-diff-test: could not deduce source and target from {{.*}}1.txt and {{.*}}2.txt
+# CHECK: # update-diff-test: copied {{.*}}my-file.txt to {{.*}}my-file.expected
+# CHECK: # update-diff-test: copied {{.*}}1.txt to {{.*}}empty.txt
+# CHECK: # update-diff-test: copied {{.*}}diff-tmp.test.tmp.txt to {{.*}}diff-t-out.txt
+# CHECK: # update-diff-test: could not deduce source and target from {{.*}}split-both.expected and {{.*}}split-both.out
+# CHECK: # update-diff-test: copied {{.*}}unrelated-split.txt to {{.*}}unrelated-split.expected
+
+
+# CHECK: Failed: 14 (100.00%)
diff --git a/llvm/utils/lit/tests/pass-test-update.py b/llvm/utils/lit/tests/pass-test-update.py
new file mode 100644
index 0000000000000..2e9f1be2bccab
--- /dev/null
+++ b/llvm/utils/lit/tests/pass-test-update.py
@@ -0,0 +1,34 @@
+# RUN: %{lit} --update-tests --ignore-fail -v %S/Inputs/pass-test-update | FileCheck %s --implicit-check-not Exception
+
+# CHECK: UNRESOLVED: pass-test-update :: fail.test (1 of 5)
+# CHECK: ******************** TEST 'pass-test-update :: fail.test' FAILED ********************
+# CHECK: # {{R}}UN: at line 1
+# CHECK: not echo "fail"
+# CHECK: # executed command: not echo fail
+# CHECK: # .---command stdout------------
+# CHECK: # | fail
+# CHECK: # `-----------------------------
+# CHECK: # error: command failed with exit status: 1
+# CHECK: Exception occurred in test updater:
+# CHECK: Traceback (most recent call last):
+# CHECK:   File {{.*}}, line {{.*}}, in {{.*}}
+# CHECK:     update_output = test_updater(result, test, commands)
+# CHECK:   File "{{.*}}{{/|\\}}should_not_run.py", line {{.*}}, in should_not_run
+# CHECK:     raise Exception("this test updater should only run on failure")
+# CHECK: Exception: this test updater should only run on failure
+# CHECK: ********************
+# CHECK: PASS: pass-test-update :: pass-silent.test (2 of 5)
+# CHECK: PASS: pass-test-update :: pass.test (3 of 5)
+# CHECK: {{X}}FAIL: pass-test-update :: xfail.test (4 of 5)
+# CHECK: XPASS: pass-test-update :: xpass.test (5 of 5)
+# CHECK: ******************** TEST 'pass-test-update :: xpass.test' FAILED ********************
+# CHECK: Exit Code: 0
+# CHECK: Command Output (stdout):
+# CHECK: --
+# CHECK: # {{R}}UN: at line 2
+# CHECK: echo "accidentally passed"
+# CHECK: # executed command: echo 'accidentally passed'
+# CHECK: # .---command stdout------------
+# CHECK: # | accidentally passed
+# CHECK: # `-----------------------------
+# CHECK: ********************
diff --git a/llvm/utils/update_any_test_checks.py b/llvm/utils/update_any_test_checks.py
index e8eef1a46c504..ec277f140a34f 100755
--- a/llvm/utils/update_any_test_checks.py
+++ b/llvm/utils/update_any_test_checks.py
@@ -34,9 +34,12 @@ def find_utc_tool(search_path, utc_name):
     return None
 
 
-def run_utc_tool(utc_name, utc_tool, testname):
+def run_utc_tool(utc_name, utc_tool, testname, environment):
     result = subprocess.run(
-        [utc_tool, testname], stdout=subprocess.PIPE, stderr=subprocess.PIPE
+        [utc_tool, testname],
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        env=environment,
     )
     return (result.returncode, result.stdout, result.stderr)
 
@@ -60,6 +63,42 @@ def expand_listfile_args(arg_list):
     return exp_arg_list
 
 
+def utc_lit_plugin(result, test, commands):
+    testname = test.getFilePath()
+    if not testname:
+        return None
+
+    script_name = os.path.abspath(__file__)
+    utc_search_path = os.path.join(os.path.dirname(script_name), os.path.pardir)
+
+    with open(testname, "r") as f:
+        header = f.readline().strip()
+
+    m = RE_ASSERTIONS.search(header)
+    if m is None:
+        return None
+
+    utc_name = m.group(1)
+    utc_tool = find_utc_tool([utc_search_path], utc_name)
+    if not utc_tool:
+        return f"update-utc-tests: {utc_name} not found"
+
+    return_code, stdout, stderr = run_utc_tool(
+        utc_name, utc_tool, testname, test.config.environment
+    )
+
+    stderr = stderr.decode(errors="replace")
+    if return_code != 0:
+        if stderr:
+            return f"update-utc-tests: {utc_name} exited with return code {return_code}\n{stderr.rstrip()}"
+        return f"update-utc-tests: {utc_name} exited with return code {return_code}"
+
+    stdout = stdout.decode(errors="replace")
+    if stdout:
+        return f"update-utc-tests: updated {testname}\n{stdout.rstrip()}"
+    return f"update-utc-tests: updated {testname}"
+
+
 def main():
     from argparse import RawTextHelpFormatter
 
@@ -78,6 +117,11 @@ def main():
         nargs="*",
         help="Additional directories to scan for update_*_test_checks scripts",
     )
+    parser.add_argument(
+        "--path",
+        help="""Additional directories to scan for executables invoked by the update_*_test_checks scripts,
+separated by the platform path separator""",
+    )
     parser.add_argument("tests", nargs="+")
     config = parser.parse_args()
 
@@ -88,6 +132,10 @@ def main():
     script_name = os.path.abspath(__file__)
     utc_search_path.append(os.path.join(os.path.dirname(script_name), os.path.pardir))
 
+    local_env = os.environ.copy()
+    if config.path:
+        local_env["PATH"] = config.path + os.pathsep + local_env["PATH"]
+
     not_autogenerated = []
     utc_tools = {}
     have_error = False
@@ -117,7 +165,7 @@ def main():
                         continue
 
                 future = executor.submit(
-                    run_utc_tool, utc_name, utc_tools[utc_name], testname
+                    run_utc_tool, utc_name, utc_tools[utc_name], testname, local_env
                 )
                 jobs.append((testname, future))