Index: include/clang/Basic/DiagnosticDriverKinds.td
===================================================================
--- include/clang/Basic/DiagnosticDriverKinds.td
+++ include/clang/Basic/DiagnosticDriverKinds.td
@@ -91,6 +91,8 @@
   "there is no external assembler that can be used on this platform">;
 def err_drv_unable_to_remove_file : Error<
   "unable to remove file: %0">;
+def err_drv_unable_to_set_working_directory : Error <
+  "unable to set working directory: %0">;
 def err_drv_command_failure : Error<
   "unable to execute command: %0">;
 def err_drv_invalid_darwin_version : Error<
Index: lib/Driver/Driver.cpp
===================================================================
--- lib/Driver/Driver.cpp
+++ lib/Driver/Driver.cpp
@@ -133,7 +133,7 @@
 
   // Provide a sane fallback if no VFS is specified.
   if (!this->VFS)
-    this->VFS = llvm::vfs::getRealFileSystem();
+    this->VFS = llvm::vfs::createPhysicalFileSystem().release();
 
   Name = llvm::sys::path::filename(ClangExecutable);
   Dir = llvm::sys::path::parent_path(ClangExecutable);
@@ -1005,6 +1005,11 @@
     }
   }
 
+  // Check for working directory option before accessing any files
+  if (Arg *WD = Args.getLastArg(options::OPT_working_directory))
+    if (std::error_code EC = VFS->setCurrentWorkingDirectory(WD->getValue()))
+      Diag(diag::err_drv_unable_to_set_working_directory) << WD->getValue();
+
   // FIXME: This stuff needs to go into the Compilation, not the driver.
   bool CCCPrintPhases;
 
@@ -1984,20 +1989,11 @@
   if (Value == "-")
     return true;
 
-  SmallString<64> Path(Value);
-  if (Arg *WorkDir = Args.getLastArg(options::OPT_working_directory)) {
-    if (!llvm::sys::path::is_absolute(Path)) {
-      SmallString<64> Directory(WorkDir->getValue());
-      llvm::sys::path::append(Directory, Value);
-      Path.assign(Directory);
-    }
-  }
-
-  if (getVFS().exists(Path))
+  if (getVFS().exists(Value))
     return true;
 
   if (IsCLMode()) {
-    if (!llvm::sys::path::is_absolute(Twine(Path)) &&
+    if (!llvm::sys::path::is_absolute(Twine(Value)) &&
         llvm::sys::Process::FindInEnvPath("LIB", Value))
       return true;
 
@@ -2023,12 +2019,12 @@
     if (getOpts().findNearest(Value, Nearest, IncludedFlagsBitmask,
                               ExcludedFlagsBitmask) <= 1) {
       Diag(clang::diag::err_drv_no_such_file_with_suggestion)
-          << Path << Nearest;
+          << Value << Nearest;
       return false;
     }
   }
 
-  Diag(clang::diag::err_drv_no_such_file) << Path;
+  Diag(clang::diag::err_drv_no_such_file) << Value;
   return false;
 }
 
Index: lib/Driver/ToolChains/Clang.cpp
===================================================================
--- lib/Driver/ToolChains/Clang.cpp
+++ lib/Driver/ToolChains/Clang.cpp
@@ -616,11 +616,11 @@
 }
 
 /// Add a CC1 option to specify the debug compilation directory.
-static void addDebugCompDirArg(const ArgList &Args, ArgStringList &CmdArgs) {
-  SmallString<128> cwd;
-  if (!llvm::sys::fs::current_path(cwd)) {
+static void addDebugCompDirArg(const ArgList &Args, ArgStringList &CmdArgs,
+                               const llvm::vfs::FileSystem &VFS) {
+  if (llvm::ErrorOr<std::string> CWD = VFS.getCurrentWorkingDirectory()) {
     CmdArgs.push_back("-fdebug-compilation-dir");
-    CmdArgs.push_back(Args.MakeArgString(cwd));
+    CmdArgs.push_back(Args.MakeArgString(*CWD));
   }
 }
 
@@ -885,13 +885,8 @@
       else
         OutputFilename = llvm::sys::path::filename(Output.getBaseInput());
       SmallString<128> CoverageFilename = OutputFilename;
-      if (llvm::sys::path::is_relative(CoverageFilename)) {
-        SmallString<128> Pwd;
-        if (!llvm::sys::fs::current_path(Pwd)) {
-          llvm::sys::path::append(Pwd, CoverageFilename);
-          CoverageFilename.swap(Pwd);
-        }
-      }
+      if (llvm::sys::path::is_relative(CoverageFilename))
+        (void)D.getVFS().makeAbsolute(CoverageFilename);
       llvm::sys::path::replace_extension(CoverageFilename, "gcno");
       CmdArgs.push_back(Args.MakeArgString(CoverageFilename));
 
@@ -4354,7 +4349,7 @@
     CmdArgs.push_back("-fno-autolink");
 
   // Add in -fdebug-compilation-dir if necessary.
-  addDebugCompDirArg(Args, CmdArgs);
+  addDebugCompDirArg(Args, CmdArgs, D.getVFS());
 
   addDebugPrefixMapArg(D, Args, CmdArgs);
 
@@ -6065,7 +6060,7 @@
     DebugInfoKind = (WantDebug ? codegenoptions::LimitedDebugInfo
                                : codegenoptions::NoDebugInfo);
     // Add the -fdebug-compilation-dir flag if needed.
-    addDebugCompDirArg(Args, CmdArgs);
+    addDebugCompDirArg(Args, CmdArgs, C.getDriver().getVFS());
 
     addDebugPrefixMapArg(getToolChain().getDriver(), Args, CmdArgs);
 
Index: test/Driver/working-directory.c
===================================================================
--- test/Driver/working-directory.c
+++ test/Driver/working-directory.c
@@ -1,3 +1,11 @@
 // RUN: %clang -### -working-directory /no/such/dir/ input 2>&1 | FileCheck %s
+// RUN: %clang -### -working-directory %p/Inputs no_such_file.cpp -c 2>&1 | FileCheck %s --check-prefix=CHECK_NO_FILE
+// RUN: %clang -### -working-directory %p/Inputs pchfile.cpp -c 2>&1 | FileCheck %s --check-prefix=CHECK_WORKS
 
-//CHECK: no such file or directory: '/no/such/dir/input'
+// CHECK: unable to set working directory: /no/such/dir/
+
+// CHECK_NO_FILE: no such file or directory: 'no_such_file.cpp'
+
+// CHECK_WORKS: "-coverage-notes-file" "{{[^"]+}}test{{/|\\\\}}Driver{{/|\\\\}}Inputs{{/|\\\\}}pchfile.gcno"
+// CHECK_WORKS: "-working-directory" "{{[^"]+}}test{{/|\\\\}}Driver{{/|\\\\}}Inputs"
+// CHECK_WORKS: "-fdebug-compilation-dir" "{{[^"]+}}test{{/|\\\\}}Driver{{/|\\\\}}Inputs"