Index: llvm/trunk/include/llvm/Support/VirtualFileSystem.h
===================================================================
--- llvm/trunk/include/llvm/Support/VirtualFileSystem.h
+++ llvm/trunk/include/llvm/Support/VirtualFileSystem.h
@@ -279,6 +279,9 @@
   /// Check whether a file exists. Provided for convenience.
   bool exists(const Twine &Path);
 
+  /// Is the file mounted on a local filesystem?
+  virtual std::error_code isLocal(const Twine &Path, bool &Result);
+
   /// Make \a Path an absolute path.
   ///
   /// Makes \a Path absolute using the current directory if it is not already.
@@ -326,6 +329,7 @@
   directory_iterator dir_begin(const Twine &Dir, std::error_code &EC) override;
   llvm::ErrorOr<std::string> getCurrentWorkingDirectory() const override;
   std::error_code setCurrentWorkingDirectory(const Twine &Path) override;
+  std::error_code isLocal(const Twine &Path, bool &Result) override;
   std::error_code getRealPath(const Twine &Path,
                               SmallVectorImpl<char> &Output) const override;
 
@@ -463,7 +467,7 @@
   /// system.
   std::error_code getRealPath(const Twine &Path,
                               SmallVectorImpl<char> &Output) const override;
-
+  std::error_code isLocal(const Twine &Path, bool &Result) override;
   std::error_code setCurrentWorkingDirectory(const Twine &Path) override;
 };
 
Index: llvm/trunk/lib/Support/VirtualFileSystem.cpp
===================================================================
--- llvm/trunk/lib/Support/VirtualFileSystem.cpp
+++ llvm/trunk/lib/Support/VirtualFileSystem.cpp
@@ -136,6 +136,10 @@
   return errc::operation_not_permitted;
 }
 
+std::error_code FileSystem::isLocal(const Twine &Path, bool &Result) {
+  return errc::operation_not_permitted;
+}
+
 bool FileSystem::exists(const Twine &Path) {
   auto Status = status(Path);
   return Status && Status->exists();
@@ -233,6 +237,7 @@
 
   llvm::ErrorOr<std::string> getCurrentWorkingDirectory() const override;
   std::error_code setCurrentWorkingDirectory(const Twine &Path) override;
+  std::error_code isLocal(const Twine &Path, bool &Result) override;
   std::error_code getRealPath(const Twine &Path,
                               SmallVectorImpl<char> &Output) const override;
 
@@ -288,6 +293,10 @@
   return std::error_code();
 }
 
+std::error_code RealFileSystem::isLocal(const Twine &Path, bool &Result) {
+  return llvm::sys::fs::is_local(Path, Result);
+}
+
 std::error_code
 RealFileSystem::getRealPath(const Twine &Path,
                             SmallVectorImpl<char> &Output) const {
@@ -377,6 +386,13 @@
   return {};
 }
 
+std::error_code OverlayFileSystem::isLocal(const Twine &Path, bool &Result) {
+  for (auto &FS : FSList)
+    if (FS->exists(Path))
+      return FS->isLocal(Path, Result);
+  return errc::no_such_file_or_directory;
+}
+
 std::error_code
 OverlayFileSystem::getRealPath(const Twine &Path,
                                SmallVectorImpl<char> &Output) const {
@@ -913,6 +929,11 @@
   return {};
 }
 
+std::error_code InMemoryFileSystem::isLocal(const Twine &Path, bool &Result) {
+  Result = false;
+  return {};
+}
+
 } // namespace vfs
 } // namespace llvm
 
@@ -1170,6 +1191,10 @@
     return ExternalFS->setCurrentWorkingDirectory(Path);
   }
 
+  std::error_code isLocal(const Twine &Path, bool &Result) override {
+    return ExternalFS->isLocal(Path, Result);
+  }
+
   directory_iterator dir_begin(const Twine &Dir, std::error_code &EC) override {
     ErrorOr<Entry *> E = lookupPath(Dir);
     if (!E) {
Index: llvm/trunk/unittests/Support/VirtualFileSystemTest.cpp
===================================================================
--- llvm/trunk/unittests/Support/VirtualFileSystemTest.cpp
+++ llvm/trunk/unittests/Support/VirtualFileSystemTest.cpp
@@ -885,6 +885,17 @@
             getPosixPath(NormalizedFS.getCurrentWorkingDirectory().get()));
 }
 
+TEST_F(InMemoryFileSystemTest, IsLocal) {
+  FS.setCurrentWorkingDirectory("/b");
+  FS.addFile("c", 0, MemoryBuffer::getMemBuffer(""));
+
+  std::error_code EC;
+  bool IsLocal = true;
+  EC = FS.isLocal("c", IsLocal);
+  ASSERT_FALSE(EC);
+  ASSERT_FALSE(IsLocal);
+}
+
 #if !defined(_WIN32)
 TEST_F(InMemoryFileSystemTest, GetRealPath) {
   SmallString<16> Path;