Index: llvm/trunk/lib/Support/VirtualFileSystem.cpp
===================================================================
--- llvm/trunk/lib/Support/VirtualFileSystem.cpp
+++ llvm/trunk/lib/Support/VirtualFileSystem.cpp
@@ -1164,14 +1164,14 @@
   /// Looks up the path <tt>[Start, End)</tt> in \p From, possibly
   /// recursing into the contents of \p From if it is a directory.
   ErrorOr<Entry *> lookupPath(sys::path::const_iterator Start,
-                              sys::path::const_iterator End, Entry *From);
+                              sys::path::const_iterator End, Entry *From) const;
 
   /// Get the status of a given an \c Entry.
   ErrorOr<Status> status(const Twine &Path, Entry *E);
 
 public:
   /// Looks up \p Path in \c Roots.
-  ErrorOr<Entry *> lookupPath(const Twine &Path);
+  ErrorOr<Entry *> lookupPath(const Twine &Path) const;
 
   /// Parses \p Buffer, which is expected to be in YAML format and
   /// returns a virtual file system representing its contents.
@@ -1183,6 +1183,9 @@
   ErrorOr<Status> status(const Twine &Path) override;
   ErrorOr<std::unique_ptr<File>> openFileForRead(const Twine &Path) override;
 
+  std::error_code getRealPath(const Twine &Path,
+                              SmallVectorImpl<char> &Output) const override;
+
   llvm::ErrorOr<std::string> getCurrentWorkingDirectory() const override {
     return ExternalFS->getCurrentWorkingDirectory();
   }
@@ -1726,7 +1729,7 @@
   return FS.release();
 }
 
-ErrorOr<Entry *> RedirectingFileSystem::lookupPath(const Twine &Path_) {
+ErrorOr<Entry *> RedirectingFileSystem::lookupPath(const Twine &Path_) const {
   SmallString<256> Path;
   Path_.toVector(Path);
 
@@ -1757,7 +1760,8 @@
 
 ErrorOr<Entry *>
 RedirectingFileSystem::lookupPath(sys::path::const_iterator Start,
-                                  sys::path::const_iterator End, Entry *From) {
+                                  sys::path::const_iterator End,
+                                  Entry *From) const {
 #ifndef _WIN32
   assert(!isTraversalComponent(*Start) &&
          !isTraversalComponent(From->getName()) &&
@@ -1890,6 +1894,27 @@
       llvm::make_unique<FileWithFixedStatus>(std::move(*Result), S));
 }
 
+std::error_code
+RedirectingFileSystem::getRealPath(const Twine &Path,
+                                   SmallVectorImpl<char> &Output) const {
+  ErrorOr<Entry *> Result = lookupPath(Path);
+  if (!Result) {
+    if (IsFallthrough &&
+        Result.getError() == llvm::errc::no_such_file_or_directory) {
+      return ExternalFS->getRealPath(Path, Output);
+    }
+    return Result.getError();
+  }
+
+  if (auto *F = dyn_cast<RedirectingFileEntry>(*Result)) {
+    return ExternalFS->getRealPath(F->getExternalContentsPath(), Output);
+  }
+  // Even if there is a directory entry, fall back to ExternalFS if allowed,
+  // because directories don't have a single external contents path.
+  return IsFallthrough ? ExternalFS->getRealPath(Path, Output)
+                       : llvm::errc::invalid_argument;
+}
+
 IntrusiveRefCntPtr<FileSystem>
 vfs::getVFSFromYAML(std::unique_ptr<MemoryBuffer> Buffer,
                     SourceMgr::DiagHandlerTy DiagHandler,
Index: llvm/trunk/unittests/Support/VirtualFileSystemTest.cpp
===================================================================
--- llvm/trunk/unittests/Support/VirtualFileSystemTest.cpp
+++ llvm/trunk/unittests/Support/VirtualFileSystemTest.cpp
@@ -1775,3 +1775,49 @@
   checkContents(FS->dir_begin("//root/foo", EC),
                 {"//root/foo/a", "//root/foo/b"});
 }
+
+TEST_F(VFSFromYAMLTest, GetRealPath) {
+  IntrusiveRefCntPtr<DummyFileSystem> Lower(new DummyFileSystem());
+  Lower->addDirectory("/dir/");
+  Lower->addRegularFile("/foo");
+  Lower->addSymlink("/link");
+  IntrusiveRefCntPtr<vfs::FileSystem> FS = getFromYAMLString(
+      "{ 'use-external-names': false,\n"
+      "  'roots': [\n"
+      "{\n"
+      "  'type': 'directory',\n"
+      "  'name': '/root/',\n"
+      "  'contents': [ {\n"
+      "                  'type': 'file',\n"
+      "                  'name': 'bar',\n"
+      "                  'external-contents': '/link'\n"
+      "                }\n"
+      "              ]\n"
+      "},\n"
+      "{\n"
+      "  'type': 'directory',\n"
+      "  'name': '/dir/',\n"
+      "  'contents': []\n"
+      "}\n"
+      "]\n"
+      "}",
+      Lower);
+  ASSERT_TRUE(FS.get() != nullptr);
+
+  // Regular file present in underlying file system.
+  SmallString<16> RealPath;
+  EXPECT_FALSE(FS->getRealPath("/foo", RealPath));
+  EXPECT_EQ(RealPath.str(), "/foo");
+
+  // File present in YAML pointing to symlink in underlying file system.
+  EXPECT_FALSE(FS->getRealPath("/root/bar", RealPath));
+  EXPECT_EQ(RealPath.str(), "/symlink");
+
+  // Directories should fall back to the underlying file system is possible.
+  EXPECT_FALSE(FS->getRealPath("/dir/", RealPath));
+  EXPECT_EQ(RealPath.str(), "/dir/");
+
+  // Try a non-existing file.
+  EXPECT_EQ(FS->getRealPath("/non_existing", RealPath),
+            errc::no_such_file_or_directory);
+}