Index: cfe/trunk/include/clang/Basic/VirtualFileSystem.h
===================================================================
--- cfe/trunk/include/clang/Basic/VirtualFileSystem.h
+++ cfe/trunk/include/clang/Basic/VirtualFileSystem.h
@@ -248,6 +248,12 @@
   /// Get the working directory of this file system.
   virtual llvm::ErrorOr<std::string> getCurrentWorkingDirectory() const = 0;
 
+  /// Gets real path of \p Path e.g. collapse all . and .. patterns, resolve
+  /// symlinks. For real file system, this uses `llvm::sys::fs::real_path`.
+  /// This returns errc::operation_not_permitted if not implemented by subclass.
+  virtual std::error_code getRealPath(const Twine &Path,
+                                      SmallVectorImpl<char> &Output) const;
+
   /// Check whether a file exists. Provided for convenience.
   bool exists(const Twine &Path);
 
Index: cfe/trunk/lib/Basic/FileManager.cpp
===================================================================
--- cfe/trunk/lib/Basic/FileManager.cpp
+++ cfe/trunk/lib/Basic/FileManager.cpp
@@ -534,23 +534,9 @@
 
   StringRef CanonicalName(Dir->getName());
 
-#ifdef LLVM_ON_UNIX
-  char CanonicalNameBuf[PATH_MAX];
-  if (realpath(Dir->getName().str().c_str(), CanonicalNameBuf))
+  SmallString<4096> CanonicalNameBuf;
+  if (!FS->getRealPath(Dir->getName(), CanonicalNameBuf))
     CanonicalName = StringRef(CanonicalNameBuf).copy(CanonicalNameStorage);
-#else
-  SmallString<256> CanonicalNameBuf(CanonicalName);
-  llvm::sys::fs::make_absolute(CanonicalNameBuf);
-  llvm::sys::path::native(CanonicalNameBuf);
-  // We've run into needing to remove '..' here in the wild though, so
-  // remove it.
-  // On Windows, symlinks are significantly less prevalent, so removing
-  // '..' is pretty safe.
-  // Ideally we'd have an equivalent of `realpath` and could implement
-  // sys::fs::canonical across all the platforms.
-  llvm::sys::path::remove_dots(CanonicalNameBuf, /* remove_dot_dot */ true);
-  CanonicalName = StringRef(CanonicalNameBuf).copy(CanonicalNameStorage);
-#endif
 
   CanonicalDirNames.insert(std::make_pair(Dir, CanonicalName));
   return CanonicalName;
Index: cfe/trunk/lib/Basic/VirtualFileSystem.cpp
===================================================================
--- cfe/trunk/lib/Basic/VirtualFileSystem.cpp
+++ cfe/trunk/lib/Basic/VirtualFileSystem.cpp
@@ -139,6 +139,11 @@
   return llvm::sys::fs::make_absolute(WorkingDir.get(), Path);
 }
 
+std::error_code FileSystem::getRealPath(const Twine &Path,
+                                        SmallVectorImpl<char> &Output) const {
+  return errc::operation_not_permitted;
+}
+
 bool FileSystem::exists(const Twine &Path) {
   auto Status = status(Path);
   return Status && Status->exists();
@@ -236,6 +241,8 @@
 
   llvm::ErrorOr<std::string> getCurrentWorkingDirectory() const override;
   std::error_code setCurrentWorkingDirectory(const Twine &Path) override;
+  std::error_code getRealPath(const Twine &Path,
+                              SmallVectorImpl<char> &Output) const override;
 };
 
 } // namespace
@@ -274,6 +281,12 @@
   return llvm::sys::fs::set_current_path(Path);
 }
 
+std::error_code
+RealFileSystem::getRealPath(const Twine &Path,
+                            SmallVectorImpl<char> &Output) const {
+  return llvm::sys::fs::real_path(Path, Output);
+}
+
 IntrusiveRefCntPtr<FileSystem> vfs::getRealFileSystem() {
   static IntrusiveRefCntPtr<FileSystem> FS = new RealFileSystem();
   return FS;