diff --git a/clang/include/clang/Basic/FileManager.h b/clang/include/clang/Basic/FileManager.h
index 8eca0fd64f0b5..b192dc9fb37b9 100644
--- a/clang/include/clang/Basic/FileManager.h
+++ b/clang/include/clang/Basic/FileManager.h
@@ -339,6 +339,13 @@ class FileManager : public RefCountedBase<FileManager> {
   /// required, which is (almost) never.
   StringRef getCanonicalName(const FileEntry *File);
 
+private:
+  /// Retrieve the canonical name for a given file or directory.
+  ///
+  /// The first param is a key in the CanonicalNames array.
+  StringRef getCanonicalName(const void *Entry, StringRef Name);
+
+public:
   void PrintStats() const;
 };
 
diff --git a/clang/lib/Basic/FileManager.cpp b/clang/lib/Basic/FileManager.cpp
index f9b2afffa5913..1c32aab2fb562 100644
--- a/clang/lib/Basic/FileManager.cpp
+++ b/clang/lib/Basic/FileManager.cpp
@@ -651,34 +651,48 @@ void FileManager::GetUniqueIDMapping(
 }
 
 StringRef FileManager::getCanonicalName(const DirectoryEntry *Dir) {
-  llvm::DenseMap<const void *, llvm::StringRef>::iterator Known
-    = CanonicalNames.find(Dir);
-  if (Known != CanonicalNames.end())
-    return Known->second;
-
-  StringRef CanonicalName(Dir->getName());
-
-  SmallString<4096> CanonicalNameBuf;
-  if (!FS->getRealPath(Dir->getName(), CanonicalNameBuf))
-    CanonicalName = CanonicalNameBuf.str().copy(CanonicalNameStorage);
-
-  CanonicalNames.insert({Dir, CanonicalName});
-  return CanonicalName;
+  return getCanonicalName(Dir, Dir->getName());
 }
 
 StringRef FileManager::getCanonicalName(const FileEntry *File) {
-  llvm::DenseMap<const void *, llvm::StringRef>::iterator Known
-    = CanonicalNames.find(File);
+  return getCanonicalName(File, File->getName());
+}
+
+StringRef FileManager::getCanonicalName(const void *Entry, StringRef Name) {
+  llvm::DenseMap<const void *, llvm::StringRef>::iterator Known =
+      CanonicalNames.find(Entry);
   if (Known != CanonicalNames.end())
     return Known->second;
 
-  StringRef CanonicalName(File->getName());
-
-  SmallString<4096> CanonicalNameBuf;
-  if (!FS->getRealPath(File->getName(), CanonicalNameBuf))
-    CanonicalName = CanonicalNameBuf.str().copy(CanonicalNameStorage);
+  // Name comes from FileEntry/DirectoryEntry::getName(), so it is safe to
+  // store it in the DenseMap below.
+  StringRef CanonicalName(Name);
+
+  SmallString<256> AbsPathBuf;
+  SmallString<256> RealPathBuf;
+  if (!FS->getRealPath(Name, RealPathBuf)) {
+    if (is_style_windows(llvm::sys::path::Style::native)) {
+      // For Windows paths, only use the real path if it doesn't resolve
+      // a substitute drive, as those are used to avoid MAX_PATH issues.
+      AbsPathBuf = Name;
+      if (!FS->makeAbsolute(AbsPathBuf)) {
+        if (llvm::sys::path::root_name(RealPathBuf) ==
+            llvm::sys::path::root_name(AbsPathBuf)) {
+          CanonicalName = RealPathBuf.str().copy(CanonicalNameStorage);
+        } else {
+          // Fallback to using the absolute path.
+          // Simplifying /../ is semantically valid on Windows even in the
+          // presence of symbolic links.
+          llvm::sys::path::remove_dots(AbsPathBuf, /*remove_dot_dot=*/true);
+          CanonicalName = AbsPathBuf.str().copy(CanonicalNameStorage);
+        }
+      }
+    } else {
+      CanonicalName = RealPathBuf.str().copy(CanonicalNameStorage);
+    }
+  }
 
-  CanonicalNames.insert({File, CanonicalName});
+  CanonicalNames.insert({Entry, CanonicalName});
   return CanonicalName;
 }
 
diff --git a/clang/lib/ExtractAPI/ExtractAPIConsumer.cpp b/clang/lib/ExtractAPI/ExtractAPIConsumer.cpp
index 5a490080f38c6..35748ecedd4b0 100644
--- a/clang/lib/ExtractAPI/ExtractAPIConsumer.cpp
+++ b/clang/lib/ExtractAPI/ExtractAPIConsumer.cpp
@@ -184,9 +184,7 @@ struct LocationFileChecker {
     if (ExternalFileEntries.count(File))
       return false;
 
-    StringRef FileName = File->tryGetRealPathName().empty()
-                             ? File->getName()
-                             : File->tryGetRealPathName();
+    StringRef FileName = SM.getFileManager().getCanonicalName(File);
 
     // Try to reduce the include name the same way we tried to include it.
     bool IsQuoted = false;
diff --git a/clang/test/Lexer/case-insensitive-include-win.c b/clang/test/Lexer/case-insensitive-include-win.c
index 6a17d0b552448..ed722feb3d994 100644
--- a/clang/test/Lexer/case-insensitive-include-win.c
+++ b/clang/test/Lexer/case-insensitive-include-win.c
@@ -2,9 +2,15 @@
 // This file should only include code that really needs a Windows host OS to
 // run.
 
+// Note: We must use the real path here, because the logic to detect case
+// mismatches must resolve the real path to figure out the original casing.
+// If we use %t and we are on a substitute drive S: mapping to C:\subst,
+// then we will compare "S:\test.dir\FOO.h" to "C:\subst\test.dir\foo.h"
+// and avoid emitting the diagnostic because the structure is different.
+
 // REQUIRES: system-windows
 // RUN: mkdir -p %t.dir
 // RUN: touch %t.dir/foo.h
-// RUN: not %clang_cl /FI\\?\%t.dir\FOO.h /WX -fsyntax-only %s 2>&1 | FileCheck %s
+// RUN: not %clang_cl /FI \\?\%{t:real}.dir\FOO.h /WX -fsyntax-only %s 2>&1 | FileCheck %s
 
 // CHECK: non-portable path to file '"\\?\
diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
index a8a52a45e4973..b3cee26cab051 100644
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -1688,10 +1688,15 @@ endfunction()
 # use it and can't be in a lit module. Use with make_paths_relative().
 string(CONCAT LLVM_LIT_PATH_FUNCTION
   "# Allow generated file to be relocatable.\n"
-  "from pathlib import Path\n"
+  "import os\n"
+  "import platform\n"
   "def path(p):\n"
   "    if not p: return ''\n"
-  "    return str((Path(__file__).parent / p).resolve())\n"
+  "    # Follows lit.util.abs_path_preserve_drive, which cannot be imported here.\n"
+  "    if platform.system() == 'Windows':\n"
+  "        return os.path.abspath(os.path.join(os.path.dirname(__file__), p))\n"
+  "    else:\n"
+  "        return os.path.realpath(os.path.join(os.path.dirname(__file__), p))\n"
   )
 
 # This function provides an automatic way to 'configure'-like generate a file
diff --git a/llvm/docs/CommandGuide/lit.rst b/llvm/docs/CommandGuide/lit.rst
index e9bd46e84769a..580ba9e33b7ca 100644
--- a/llvm/docs/CommandGuide/lit.rst
+++ b/llvm/docs/CommandGuide/lit.rst
@@ -541,6 +541,16 @@ TestRunner.py:
  %/p                     %p but ``\`` is replaced by ``/``
  %/t                     %t but ``\`` is replaced by ``/``
  %/T                     %T but ``\`` is replaced by ``/``
+ %{s:real}               %s after expanding all symbolic links and substitute drives
+ %{S:real}               %S after expanding all symbolic links and substitute drives
+ %{p:real}               %p after expanding all symbolic links and substitute drives
+ %{t:real}               %t after expanding all symbolic links and substitute drives
+ %{T:real}               %T after expanding all symbolic links and substitute drives
+ %{/s:real}              %/s after expanding all symbolic links and substitute drives
+ %{/S:real}              %/S after expanding all symbolic links and substitute drives
+ %{/p:real}              %/p after expanding all symbolic links and substitute drives
+ %{/t:real}              %/t after expanding all symbolic links and substitute drives
+ %{/T:real}              %/T after expanding all symbolic links and substitute drives
  %{/s:regex_replacement} %/s but escaped for use in the replacement of a ``s@@@`` command in sed
  %{/S:regex_replacement} %/S but escaped for use in the replacement of a ``s@@@`` command in sed
  %{/p:regex_replacement} %/p but escaped for use in the replacement of a ``s@@@`` command in sed
diff --git a/llvm/docs/TestingGuide.rst b/llvm/docs/TestingGuide.rst
index cd18cb4a4a5a1..b62dc280074f4 100644
--- a/llvm/docs/TestingGuide.rst
+++ b/llvm/docs/TestingGuide.rst
@@ -581,7 +581,7 @@ RUN lines:
 ``${fs-sep}``
    Expands to the file system separator, i.e. ``/`` or ``\`` on Windows.
 
-``%/s, %/S, %/t, %/T:``
+``%/s, %/S, %/t, %/T``
 
   Act like the corresponding substitution above but replace any ``\``
   character with a ``/``. This is useful to normalize path separators.
@@ -590,7 +590,17 @@ RUN lines:
 
    Example: ``%/s: C:/Desktop Files/foo_test.s.tmp``
 
-``%:s, %:S, %:t, %:T:``
+``%{s:real}, %{S:real}, %{t:real}, %{T:real}``
+``%{/s:real}, %{/S:real}, %{/t:real}, %{/T:real}``
+
+  Act like the corresponding substitution, including with ``/``, but use
+  the real path by expanding all symbolic links and substitute drives.
+
+   Example: ``%s:  S:\foo_test.s.tmp``
+
+   Example: ``%{/s:real}: C:/SDrive/foo_test.s.tmp``
+
+``%:s, %:S, %:t, %:T``
 
   Act like the corresponding substitution above but remove colons at
   the beginning of Windows paths. This is useful to allow concatenation
diff --git a/llvm/utils/lit/lit/TestRunner.py b/llvm/utils/lit/lit/TestRunner.py
index 664235beaae6b..eb595e5194825 100644
--- a/llvm/utils/lit/lit/TestRunner.py
+++ b/llvm/utils/lit/lit/TestRunner.py
@@ -70,7 +70,7 @@ def change_dir(self, newdir):
         if os.path.isabs(newdir):
             self.cwd = newdir
         else:
-            self.cwd = os.path.realpath(os.path.join(self.cwd, newdir))
+            self.cwd = lit.util.abs_path_preserve_drive(os.path.join(self.cwd, newdir))
 
 class TimeoutHelper(object):
     """
@@ -401,7 +401,7 @@ def executeBuiltinMkdir(cmd, cmd_shenv):
         dir = to_unicode(dir) if kIsWindows else to_bytes(dir)
         cwd = to_unicode(cwd) if kIsWindows else to_bytes(cwd)
         if not os.path.isabs(dir):
-            dir = os.path.realpath(os.path.join(cwd, dir))
+            dir = lit.util.abs_path_preserve_drive(os.path.join(cwd, dir))
         if parent:
             lit.util.mkdir_p(dir)
         else:
@@ -446,7 +446,7 @@ def on_rm_error(func, path, exc_info):
         path = to_unicode(path) if kIsWindows else to_bytes(path)
         cwd = to_unicode(cwd) if kIsWindows else to_bytes(cwd)
         if not os.path.isabs(path):
-            path = os.path.realpath(os.path.join(cwd, path))
+            path = lit.util.abs_path_preserve_drive(os.path.join(cwd, path))
         if force and not os.path.exists(path):
             continue
         try:
@@ -1153,58 +1153,59 @@ def getDefaultSubstitutions(test, tmpDir, tmpBase, normalize_slashes=False):
     substitutions.extend(test.config.substitutions)
     tmpName = tmpBase + '.tmp'
     baseName = os.path.basename(tmpBase)
-    substitutions.extend([('%s', sourcepath),
-                          ('%S', sourcedir),
-                          ('%p', sourcedir),
-                          ('%{pathsep}', os.pathsep),
-                          ('%t', tmpName),
-                          ('%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('\\', '/')),
-            ('%/S', sourcedir.replace('\\', '/')),
-            ('%/p', sourcedir.replace('\\', '/')),
-            ('%/t', tmpBase.replace('\\', '/') + '.tmp'),
-            ('%/T', tmpDir.replace('\\', '/')),
-            ('%/et',tmpName.replace('\\', '\\\\\\\\\\\\\\\\')),
-            ])
-
-    # "%{/[STpst]:regex_replacement}" should be normalized like "%/[STpst]" but we're
-    # also in a regex replacement context of a s@@@ regex.
+
+    substitutions.append(("%{pathsep}", os.pathsep))
+    substitutions.append(("%basename_t", baseName))
+    
+    substitutions.extend(
+        [
+            ("%{fs-src-root}", pathlib.Path(sourcedir).anchor),
+            ("%{fs-tmp-root}", pathlib.Path(tmpBase).anchor),
+            ("%{fs-sep}", os.path.sep),
+        ]
+    )
+
+    substitutions.append(("%/et", tmpName.replace("\\", "\\\\\\\\\\\\\\\\")))
+
     def regex_escape(s):
         s = s.replace('@', r'\@')
         s = s.replace('&', r'\&')
         return s
-    substitutions.extend([
-            ('%{/s:regex_replacement}',
-             regex_escape(sourcepath.replace('\\', '/'))),
-            ('%{/S:regex_replacement}',
-             regex_escape(sourcedir.replace('\\', '/'))),
-            ('%{/p:regex_replacement}',
-             regex_escape(sourcedir.replace('\\', '/'))),
-            ('%{/t:regex_replacement}',
-             regex_escape(tmpBase.replace('\\', '/')) + '.tmp'),
-            ('%{/T:regex_replacement}',
-             regex_escape(tmpDir.replace('\\', '/'))),
-            ])
-
-    # "%:[STpst]" are normalized paths without colons and without a leading
-    # slash.
-    substitutions.extend([
-            ('%:s', colonNormalizePath(sourcepath)),
-            ('%:S', colonNormalizePath(sourcedir)),
-            ('%:p', colonNormalizePath(sourcedir)),
-            ('%:t', colonNormalizePath(tmpBase + '.tmp')),
-            ('%:T', colonNormalizePath(tmpDir)),
-            ])
+
+    path_substitutions = [
+        ("s", sourcepath), ("S", sourcedir), ("p", sourcedir),
+        ("t", tmpName), ("T", tmpDir)
+    ]
+    for path_substitution in path_substitutions:
+        letter = path_substitution[0]
+        path = path_substitution[1]
+
+        # Original path variant
+        substitutions.append(("%" + letter, path))
+
+        # Normalized path separator variant
+        substitutions.append(("%/" + letter, path.replace("\\", "/")))
+
+        # realpath variants
+        # Windows paths with substitute drives are not expanded by default
+        # as they are used to avoid MAX_PATH issues, but sometimes we do
+        # need the fully expanded path.
+        real_path = os.path.realpath(path)
+        substitutions.append(("%{" + letter + ":real}", real_path))
+        substitutions.append(("%{/" + letter + ":real}",
+            real_path.replace("\\", "/")))
+
+        # "%{/[STpst]:regex_replacement}" should be normalized like
+        # "%/[STpst]" but we're also in a regex replacement context
+        # of a s@@@ regex.
+        substitutions.append(
+            ("%{/" + letter + ":regex_replacement}",
+            regex_escape(path.replace("\\", "/"))))
+
+        # "%:[STpst]" are normalized paths without colons and without
+        # a leading slash.
+        substitutions.append(("%:" + letter, colonNormalizePath(path)))
+
     return substitutions
 
 def _memoize(f):
diff --git a/llvm/utils/lit/lit/builtin_commands/diff.py b/llvm/utils/lit/lit/builtin_commands/diff.py
index a12e5e100b1bd..7233d81e72dc3 100644
--- a/llvm/utils/lit/lit/builtin_commands/diff.py
+++ b/llvm/utils/lit/lit/builtin_commands/diff.py
@@ -229,7 +229,7 @@ def main(argv):
     try:
         for file in args:
             if file != "-" and not os.path.isabs(file):
-                file = os.path.realpath(os.path.join(os.getcwd(), file))
+                file = util.abs_path_preserve_drive(file)
 
             if flags.recursive_diff:
                 if file == "-":
diff --git a/llvm/utils/lit/lit/discovery.py b/llvm/utils/lit/lit/discovery.py
index bb77aa310c713..254b8ae5050ad 100644
--- a/llvm/utils/lit/lit/discovery.py
+++ b/llvm/utils/lit/lit/discovery.py
@@ -7,7 +7,7 @@
 import sys
 
 from lit.TestingConfig import TestingConfig
-from lit import LitConfig, Test
+from lit import LitConfig, Test, util
 
 def chooseConfigFileFromDir(dir, config_names):
     for name in config_names:
@@ -52,7 +52,7 @@ def search1(path):
         # configuration to load instead.
         config_map = litConfig.params.get('config_map')
         if config_map:
-            cfgpath = os.path.realpath(cfgpath)
+            cfgpath = util.abs_path_preserve_drive(cfgpath)
             target = config_map.get(os.path.normcase(cfgpath))
             if target:
                 cfgpath = target
@@ -63,13 +63,13 @@ def search1(path):
 
         cfg = TestingConfig.fromdefaults(litConfig)
         cfg.load_from_path(cfgpath, litConfig)
-        source_root = os.path.realpath(cfg.test_source_root or path)
-        exec_root = os.path.realpath(cfg.test_exec_root or path)
+        source_root = util.abs_path_preserve_drive(cfg.test_source_root or path)
+        exec_root = util.abs_path_preserve_drive(cfg.test_exec_root or path)
         return Test.TestSuite(cfg.name, source_root, exec_root, cfg), ()
 
     def search(path):
         # Check for an already instantiated test suite.
-        real_path = os.path.realpath(path)
+        real_path = util.abs_path_preserve_drive(path)
         res = cache.get(real_path)
         if res is None:
             cache[real_path] = res = search1(path)
diff --git a/llvm/utils/lit/lit/util.py b/llvm/utils/lit/lit/util.py
index 75cface848390..4d3802db31fab 100644
--- a/llvm/utils/lit/lit/util.py
+++ b/llvm/utils/lit/lit/util.py
@@ -127,6 +127,23 @@ def usable_core_count():
 
     return n
 
+def abs_path_preserve_drive(path):
+    """Return the absolute path without resolving drive mappings on Windows.
+
+    """
+    if platform.system() == "Windows":
+        # Windows has limitations on path length (MAX_PATH) that
+        # can be worked around using substitute drives, which map
+        # a drive letter to a longer path on another drive.
+        # Since Python 3.8, os.path.realpath resolves sustitute drives,
+        # so we should not use it. In Python 3.7, os.path.realpath
+        # was implemented as os.path.abspath.
+        return os.path.abspath(path)
+    else:
+        # On UNIX, the current directory always has symbolic links resolved,
+        # so any program accepting relative paths cannot preserve symbolic
+        # links in paths and we should always use os.path.realpath.
+        return os.path.realpath(path)
 
 def mkdir(path):
     try:
diff --git a/llvm/utils/lit/tests/Inputs/config-map-discovery/driver.py b/llvm/utils/lit/tests/Inputs/config-map-discovery/driver.py
index db9141b9b1bf9..19a74faeca73e 100644
--- a/llvm/utils/lit/tests/Inputs/config-map-discovery/driver.py
+++ b/llvm/utils/lit/tests/Inputs/config-map-discovery/driver.py
@@ -2,8 +2,7 @@
 import os
 import sys
 
-main_config = sys.argv[1]
-main_config = os.path.realpath(main_config)
+main_config = lit.util.abs_path_preserve_drive(sys.argv[1])
 main_config = os.path.normcase(main_config)
 
 config_map = {main_config : sys.argv[2]}
diff --git a/llvm/utils/lit/tests/Inputs/config-map-discovery/lit.alt.cfg b/llvm/utils/lit/tests/Inputs/config-map-discovery/lit.alt.cfg
index c7b303f50a05c..27860e192fdc1 100644
--- a/llvm/utils/lit/tests/Inputs/config-map-discovery/lit.alt.cfg
+++ b/llvm/utils/lit/tests/Inputs/config-map-discovery/lit.alt.cfg
@@ -5,5 +5,5 @@ config.suffixes = ['.txt']
 config.test_format = lit.formats.ShTest()
 
 import os
-config.test_exec_root = os.path.realpath(os.path.dirname(__file__))
+config.test_exec_root = os.path.dirname(lit.util.abs_path_preserve_drive(__file__))
 config.test_source_root = os.path.join(config.test_exec_root, "tests")
diff --git a/llvm/utils/lit/tests/Inputs/use-llvm-tool-required/lit.cfg b/llvm/utils/lit/tests/Inputs/use-llvm-tool-required/lit.cfg
index b1c510e05ab74..f5101dbf2b052 100644
--- a/llvm/utils/lit/tests/Inputs/use-llvm-tool-required/lit.cfg
+++ b/llvm/utils/lit/tests/Inputs/use-llvm-tool-required/lit.cfg
@@ -1,11 +1,14 @@
 import lit.formats
+import lit.util
+
 config.name = 'use-llvm-tool-required'
 config.suffixes = ['.txt']
 config.test_format = lit.formats.ShTest()
 config.test_source_root = None
 config.test_exec_root = None
 import os.path
-config.llvm_tools_dir = os.path.realpath(os.path.dirname(__file__))
+
+config.llvm_tools_dir = os.path.dirname(lit.util.abs_path_preserve_drive(__file__))
 import lit.llvm
 lit.llvm.initialize(lit_config, config)
 lit.llvm.llvm_config.use_llvm_tool('found', required=True)
diff --git a/llvm/utils/lit/tests/Inputs/use-llvm-tool/lit.cfg b/llvm/utils/lit/tests/Inputs/use-llvm-tool/lit.cfg
index 2bd401aacde35..f248ec596c78c 100644
--- a/llvm/utils/lit/tests/Inputs/use-llvm-tool/lit.cfg
+++ b/llvm/utils/lit/tests/Inputs/use-llvm-tool/lit.cfg
@@ -1,11 +1,14 @@
 import lit.formats
+import lit.util
+
 config.name = 'use-llvm-tool'
 config.suffixes = ['.txt']
 config.test_format = lit.formats.ShTest()
 config.test_source_root = None
 config.test_exec_root = None
 import os.path
-this_dir = os.path.realpath(os.path.dirname(__file__))
+
+this_dir = os.path.dirname(lit.util.abs_path_preserve_drive(__file__))
 config.llvm_tools_dir = os.path.join(this_dir, 'build')
 import lit.llvm
 lit.llvm.initialize(lit_config, config)
diff --git a/llvm/utils/llvm-lit/llvm-lit.in b/llvm/utils/llvm-lit/llvm-lit.in
index 33ec8017cf05f..0b6683b6b6230 100755
--- a/llvm/utils/llvm-lit/llvm-lit.in
+++ b/llvm/utils/llvm-lit/llvm-lit.in
@@ -8,7 +8,7 @@ config_map = {}
 
 def map_config(source_dir, site_config):
     global config_map
-    source_dir = os.path.realpath(source_dir)
+    source_dir = os.path.abspath(source_dir)
     source_dir = os.path.normcase(source_dir)
     site_config = os.path.normpath(site_config)
     config_map[source_dir] = site_config