diff --git a/llvm/docs/CommandGuide/lit.rst b/llvm/docs/CommandGuide/lit.rst
--- a/llvm/docs/CommandGuide/lit.rst
+++ b/llvm/docs/CommandGuide/lit.rst
@@ -523,6 +523,9 @@
  %S                      source dir (directory of the file currently being run)
  %p                      same as %S
  %{pathsep}              path separator
+ %{fs-src-root}          root component of file system paths pointing to the LLVM checkout
+ %{fs-tmp-root}          root component of file system paths pointing to the test's temporary directory
+ %{fs-sep}               file system path separator
  %t                      temporary file name unique to the test
  %basename_t             The last path component of %t but without the ``.tmp`` extension
  %T                      parent directory of %t (not unique, deprecated, do not use)
diff --git a/llvm/docs/TestingGuide.rst b/llvm/docs/TestingGuide.rst
--- a/llvm/docs/TestingGuide.rst
+++ b/llvm/docs/TestingGuide.rst
@@ -569,6 +569,18 @@
 
    Expands to the path separator, i.e. ``:`` (or ``;`` on Windows).
 
+``${fs-src-root}``
+   Expands to the root component of file system paths for the source directory,
+   i.e. ``/`` on Unix systems or ``C:\`` (or another drive) on Windows.
+
+``${fs-tmp-root}``
+   Expands to the root component of file system paths for the test's temporary
+   directory, i.e. ``/`` on Unix systems or ``C:\`` (or another drive) on
+   Windows.
+
+``${fs-sep}``
+   Expands to the file system separator, i.e. ``/`` or ``\`` on Windows.
+
 ``%/s, %/S, %/t, %/T:``
 
   Act like the corresponding substitution above but replace any ``\``
diff --git a/llvm/utils/lit/lit/TestRunner.py b/llvm/utils/lit/lit/TestRunner.py
--- a/llvm/utils/lit/lit/TestRunner.py
+++ b/llvm/utils/lit/lit/TestRunner.py
@@ -6,6 +6,7 @@
 import os, signal, subprocess, sys
 import re
 import stat
+import pathlib
 import platform
 import shutil
 import tempfile
@@ -1121,6 +1122,12 @@
                           ('%basename_t', baseName),
                           ('%T', tmpDir)])
 
+    substitutions.extend([
+        ('%{fs-src-root}', pathlib.Path(sourcedir).anchor),
+        ('%{fs-tmp-root}', pathlib.Path(tmpBase).anchor),
+        ('%{fs-sep}', os.path.sep),
+    ])
+
     # "%/[STpst]" should be normalized.
     substitutions.extend([
             ('%/s', sourcepath.replace('\\', '/')),