Index: include/clang/Basic/FileSystemOptions.h
===================================================================
--- include/clang/Basic/FileSystemOptions.h
+++ include/clang/Basic/FileSystemOptions.h
@@ -25,6 +25,9 @@
   /// \brief If set, paths are resolved as if the working directory was
   /// set to the value of WorkingDir.
   std::string WorkingDir;
+
+  /// If set, store the absolute path to files.
+  bool AbsolutePaths = false;
 };
 
 } // end namespace clang
Index: include/clang/Driver/CC1Options.td
===================================================================
--- include/clang/Driver/CC1Options.td
+++ include/clang/Driver/CC1Options.td
@@ -653,6 +653,8 @@
            "implicit extern \"C\" semantics; these are assumed to not be "
            "user-provided and are used to model system and standard headers' "
            "paths.">;
+def fabsolute_paths : Flag<["-"], "fabsolute-paths">,
+  HelpText<"Use absolute paths in diagnostics and __FILE__">;
 
 //===----------------------------------------------------------------------===//
 // Preprocessor Options
Index: include/clang/Driver/CLCompatOptions.td
===================================================================
--- include/clang/Driver/CLCompatOptions.td
+++ include/clang/Driver/CLCompatOptions.td
@@ -64,6 +64,9 @@
 def _SLASH_D : CLJoinedOrSeparate<"D">, HelpText<"Define macro">,
   MetaVarName<"<macro[=value]>">, Alias<D>;
 def _SLASH_E : CLFlag<"E">, HelpText<"Preprocess to stdout">, Alias<E>;
+def _SLASH_FC : CLFlag<"FC">,
+  HelpText<"Use absolute paths in diagnostics and __FILE__">,
+  Alias<fabsolute_paths>;
 def _SLASH_fp_except : CLFlag<"fp:except">, HelpText<"">, Alias<ftrapping_math>;
 def _SLASH_fp_except_ : CLFlag<"fp:except-">,
   HelpText<"">, Alias<fno_trapping_math>;
@@ -292,7 +295,6 @@
 def _SLASH_d2Zi_PLUS : CLIgnoredFlag<"d2Zi+">;
 def _SLASH_errorReport : CLIgnoredJoined<"errorReport">;
 def _SLASH_Fd : CLIgnoredJoined<"Fd">;
-def _SLASH_FC : CLIgnoredFlag<"FC">;
 def _SLASH_FS : CLIgnoredFlag<"FS">, HelpText<"Force synchronous PDB writes">;
 def _SLASH_GF : CLIgnoredFlag<"GF">;
 def _SLASH_kernel_ : CLIgnoredFlag<"kernel-">;
Index: lib/Basic/FileManager.cpp
===================================================================
--- lib/Basic/FileManager.cpp
+++ lib/Basic/FileManager.cpp
@@ -216,6 +216,14 @@
                                       bool CacheFailure) {
   ++NumFileLookups;
 
+  SmallVector<char, 128> AbsoluteFilename(Filename.begin(), Filename.end());
+  if (FileSystemOpts.AbsolutePaths) {
+    makeAbsolutePath(AbsoluteFilename);
+    llvm::sys::path::remove_dots(AbsoluteFilename, true);
+    llvm::sys::path::native(AbsoluteFilename);
+    Filename = StringRef(AbsoluteFilename.data(), AbsoluteFilename.size());
+  }
+
   // See if there is already an entry in the map.
   auto &NamedFileEnt =
       *SeenFileEntries.insert(std::make_pair(Filename, nullptr)).first;
Index: lib/Driver/Tools.cpp
===================================================================
--- lib/Driver/Tools.cpp
+++ lib/Driver/Tools.cpp
@@ -6013,6 +6013,8 @@
     A->claim();
   }
 
+  Args.AddAllArgs(CmdArgs, options::OPT_fabsolute_paths);
+
   // Forward -Xclang arguments to -cc1, and -mllvm arguments to the LLVM option
   // parser.
   Args.AddAllArgValues(CmdArgs, options::OPT_Xclang);
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -1047,6 +1047,7 @@
 
 static void ParseFileSystemArgs(FileSystemOptions &Opts, ArgList &Args) {
   Opts.WorkingDir = Args.getLastArgValue(OPT_working_directory);
+  Opts.AbsolutePaths = Args.hasArg(OPT_fabsolute_paths);
 }
 
 /// Parse the argument to the -ftest-module-file-extension
Index: test/Driver/cl-options.c
===================================================================
--- test/Driver/cl-options.c
+++ test/Driver/cl-options.c
@@ -32,6 +32,9 @@
 // EP: "-P"
 // EP: "-o" "-"
 
+// RUN: %clang_cl /FC -### -- %s 2>&1 | FileCheck -check-prefix=FC %s
+// FC: "-fabsolute-paths"
+
 // RUN: %clang_cl /fp:fast /fp:except -### -- %s 2>&1 | FileCheck -check-prefix=fpexcept %s
 // fpexcept-NOT: -menable-unsafe-fp-math
 
@@ -280,7 +283,6 @@
 // RUN:    /d2FastFail \
 // RUN:    /d2Zi+ \
 // RUN:    /errorReport:foo \
-// RUN:    /FC \
 // RUN:    /Fdfoo \
 // RUN:    /FS \
 // RUN:    /Gd \
Index: test/Frontend/Inputs/absolute-paths.h
===================================================================
--- /dev/null
+++ test/Frontend/Inputs/absolute-paths.h
@@ -0,0 +1,3 @@
+int f() {
+  // Oops, no return.
+}
Index: test/Frontend/absolute-paths.c
===================================================================
--- /dev/null
+++ test/Frontend/absolute-paths.c
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -fsyntax-only -I %S/Inputs/SystemHeaderPrefix/.. %s 2>&1 | FileCheck -check-prefix=NORMAL -check-prefix=CHECK %s
+// RUN: %clang_cc1 -fsyntax-only -I %S/Inputs/SystemHeaderPrefix/.. -fabsolute-paths %s 2>&1 | FileCheck -check-prefix=ABSOLUTE -check-prefix=CHECK %s
+
+#include "absolute-paths.h"
+
+// Check whether the diagnostic from the header above includes the dummy
+// directory in the path.
+
+// NORMAL: SystemHeaderPrefix
+// ABSOLUTE-NOT: SystemHeaderPrefix
+
+// CHECK: warning: control reaches end of non-void function