Index: llvm/include/llvm/Support/VirtualFileSystem.h
===================================================================
--- llvm/include/llvm/Support/VirtualFileSystem.h
+++ llvm/include/llvm/Support/VirtualFileSystem.h
@@ -658,6 +658,10 @@
   /// when reading from YAML files.
   std::string ExternalContentsPrefixDir;
 
+  /// If CurrentWorkingDirectory is set, non-absolute paths are considered
+  /// relative to this path.
+  std::string CurrentWorkingDirectory;
+
   /// @name Configuration
   /// @{
 
@@ -752,6 +756,7 @@
   Optional<bool> IsOverlayRelative;
   Optional<bool> UseExternalNames;
   std::string OverlayDir;
+  std::string CurrentWorkingDirectory;
 
 public:
   YAMLVFSWriter() = default;
@@ -764,6 +769,10 @@
 
   void setUseExternalNames(bool UseExtNames) { UseExternalNames = UseExtNames; }
 
+  void setCurrentWorkingDirectory(std::string CWD) {
+    CurrentWorkingDirectory = std::move(CWD);
+  }
+
   void setOverlayDir(StringRef OverlayDirectory) {
     IsOverlayRelative = true;
     OverlayDir.assign(OverlayDirectory.str());
Index: llvm/lib/Support/VirtualFileSystem.cpp
===================================================================
--- llvm/lib/Support/VirtualFileSystem.cpp
+++ llvm/lib/Support/VirtualFileSystem.cpp
@@ -1035,11 +1035,17 @@
 
 llvm::ErrorOr<std::string>
 RedirectingFileSystem::getCurrentWorkingDirectory() const {
+  if (!CurrentWorkingDirectory.empty())
+    return CurrentWorkingDirectory;
   return ExternalFS->getCurrentWorkingDirectory();
 }
 
 std::error_code
 RedirectingFileSystem::setCurrentWorkingDirectory(const Twine &Path) {
+  // Changing the current working directory might have side effects for the
+  // external file system. Unset the special current working directory and have
+  // the external FS deal with them from now on.
+  CurrentWorkingDirectory.clear();
   return ExternalFS->setCurrentWorkingDirectory(Path);
 }
 
@@ -1473,6 +1479,7 @@
         KeyStatusPair("use-external-names", false),
         KeyStatusPair("overlay-relative", false),
         KeyStatusPair("fallthrough", false),
+        KeyStatusPair("current-working-directory", false),
         KeyStatusPair("roots", true),
     };
 
@@ -1533,6 +1540,12 @@
       } else if (Key == "fallthrough") {
         if (!parseScalarBool(I.getValue(), FS->IsFallthrough))
           return false;
+      } else if (Key == "current-working-directory") {
+        StringRef CurrentWorkingDirectory;
+        SmallString<256> Storage;
+        if (!parseScalarString(I.getValue(), CurrentWorkingDirectory, Storage))
+          return false;
+        FS->setCurrentWorkingDirectory(CurrentWorkingDirectory);
       } else {
         llvm_unreachable("key missing from Keys");
       }
@@ -1874,7 +1887,7 @@
 
   void write(ArrayRef<YAMLVFSEntry> Entries, Optional<bool> UseExternalNames,
              Optional<bool> IsCaseSensitive, Optional<bool> IsOverlayRelative,
-             StringRef OverlayDir);
+             StringRef OverlayDir, StringRef CurrentWorkingDirectory);
 };
 
 } // namespace
@@ -1932,6 +1945,7 @@
                        Optional<bool> UseExternalNames,
                        Optional<bool> IsCaseSensitive,
                        Optional<bool> IsOverlayRelative,
+                       StringRef CurrentWorkingDirectory,
                        StringRef OverlayDir) {
   using namespace llvm::sys;
 
@@ -1949,6 +1963,9 @@
     OS << "  'overlay-relative': '" << (UseOverlayRelative ? "true" : "false")
        << "',\n";
   }
+  if (!CurrentWorkingDirectory.empty())
+    OS << "  'current-working-directory': \""
+       << llvm::yaml::escape(CurrentWorkingDirectory) << "\",\n";
   OS << "  'roots': [\n";
 
   if (!Entries.empty()) {
@@ -2004,7 +2021,7 @@
   });
 
   JSONWriter(OS).write(Mappings, UseExternalNames, IsCaseSensitive,
-                       IsOverlayRelative, OverlayDir);
+                       IsOverlayRelative, OverlayDir, CurrentWorkingDirectory);
 }
 
 VFSFromYamlDirIterImpl::VFSFromYamlDirIterImpl(
Index: llvm/unittests/Support/VirtualFileSystemTest.cpp
===================================================================
--- llvm/unittests/Support/VirtualFileSystemTest.cpp
+++ llvm/unittests/Support/VirtualFileSystemTest.cpp
@@ -1994,3 +1994,42 @@
   EXPECT_EQ(FS->getRealPath("/non_existing", RealPath),
             errc::no_such_file_or_directory);
 }
+
+TEST_F(VFSFromYAMLTest, CurrentWorkingDirectory) {
+  IntrusiveRefCntPtr<DummyFileSystem> Lower(new DummyFileSystem());
+  Lower->addDirectory("//root/");
+  Lower->addDirectory("//root/foo");
+  Lower->addRegularFile("//root/foo/a");
+  Lower->addRegularFile("//root/foo/b");
+  IntrusiveRefCntPtr<vfs::FileSystem> FS = getFromYAMLString(
+      "{ 'use-external-names': false,\n"
+      "  'current-working-directory': '//root/bar'\n"
+      "  'roots': [\n"
+      "{\n"
+      "  'type': 'directory',\n"
+      "  'name': '//root/',\n"
+      "  'contents': [ {\n"
+      "                  'type': 'file',\n"
+      "                  'name': 'bar/a',\n"
+      "                  'external-contents': '//root/foo/a'\n"
+      "                }\n"
+      "              ]\n"
+      "}\n"
+      "]\n"
+      "}",
+      Lower);
+  ASSERT_TRUE(FS.get() != nullptr);
+
+  llvm::ErrorOr<std::string> CWD = FS->getCurrentWorkingDirectory();
+  ASSERT_FALSE(CWD.getError());
+  EXPECT_EQ(*CWD, "//root/bar");
+
+  llvm::ErrorOr<vfs::Status> Status = FS->status("./a");
+  ASSERT_FALSE(Status.getError());
+  EXPECT_TRUE(Status->isStatusKnown());
+  EXPECT_FALSE(Status->isDirectory());
+  EXPECT_TRUE(Status->isRegularFile());
+  EXPECT_FALSE(Status->isSymlink());
+  EXPECT_FALSE(Status->isOther());
+  EXPECT_TRUE(Status->exists());
+}