Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -682,6 +682,8 @@
 def flto_EQ : Joined<["-"], "flto=">, Group<clang_ignored_gcc_optimization_f_Group>;
 def flto : Flag<["-"], "flto">, Flags<[CC1Option]>, Group<f_Group>;
 def fno_lto : Flag<["-"], "fno-lto">, Group<f_Group>;
+def fthinlto : Flag<["-"], "fthinlto">, Flags<[CC1Option]>, Group<f_Group>;
+def fno_thinlto : Flag<["-"], "fno-thinlto">, Group<f_Group>;
 def fmacro_backtrace_limit_EQ : Joined<["-"], "fmacro-backtrace-limit=">,
                                 Group<f_Group>, Flags<[DriverOption, CoreOption]>;
 def fmerge_all_constants : Flag<["-"], "fmerge-all-constants">, Group<f_Group>;
Index: include/clang/Frontend/CodeGenOptions.def
===================================================================
--- include/clang/Frontend/CodeGenOptions.def
+++ include/clang/Frontend/CodeGenOptions.def
@@ -72,7 +72,9 @@
 CODEGENOPT(InstrumentForProfiling , 1, 0) ///< Set when -pg is enabled.
 CODEGENOPT(LessPreciseFPMAD  , 1, 0) ///< Enable less precise MAD instructions to
                                      ///< be generated.
-CODEGENOPT(PrepareForLTO     , 1, 0) ///< Set when -flto is enabled on the
+CODEGENOPT(PrepareForLTO     , 1, 0) ///< Set when -flto or -fthinlto is enabled
+                                     ///< on the compile step.
+CODEGENOPT(EmitThinLTOIndex  , 1, 0) ///< Set when -fthinlto is enabled on the
                                      ///< compile step.
 CODEGENOPT(MergeAllConstants , 1, 1) ///< Merge identical constants.
 CODEGENOPT(MergeFunctions    , 1, 0) ///< Set when -fmerge-functions is enabled.
Index: lib/CodeGen/BackendUtil.cpp
===================================================================
--- lib/CodeGen/BackendUtil.cpp
+++ lib/CodeGen/BackendUtil.cpp
@@ -618,7 +618,8 @@
 
   case Backend_EmitBC:
     getPerModulePasses()->add(
-        createBitcodeWriterPass(*OS, CodeGenOpts.EmitLLVMUseLists));
+        createBitcodeWriterPass(*OS, CodeGenOpts.EmitLLVMUseLists,
+                                CodeGenOpts.EmitThinLTOIndex));
     break;
 
   case Backend_EmitLL:
Index: lib/Driver/Driver.cpp
===================================================================
--- lib/Driver/Driver.cpp
+++ lib/Driver/Driver.cpp
@@ -1583,7 +1583,8 @@
 }
 
 bool Driver::IsUsingLTO(const ArgList &Args) const {
-  return Args.hasFlag(options::OPT_flto, options::OPT_fno_lto, false);
+  return Args.hasFlag(options::OPT_flto, options::OPT_fno_lto, false) ||
+      Args.hasFlag(options::OPT_fthinlto, options::OPT_fno_thinlto, false);
 }
 
 void Driver::BuildJobs(Compilation &C) const {
Index: lib/Driver/Tools.cpp
===================================================================
--- lib/Driver/Tools.cpp
+++ lib/Driver/Tools.cpp
@@ -1648,6 +1648,9 @@
   std::string CPU = getCPUName(Args, ToolChain.getTriple());
   if (!CPU.empty())
     CmdArgs.push_back(Args.MakeArgString(Twine("-plugin-opt=mcpu=") + CPU));
+
+  if (Args.hasArg(options::OPT_fthinlto))
+    CmdArgs.push_back("-plugin-opt=thinlto");
 }
 
 /// This is a helper function for validating the optional refinement step
@@ -2761,6 +2764,8 @@
   // preprocessing, precompiling or assembling.
   Args.ClaimAllArgs(options::OPT_flto);
   Args.ClaimAllArgs(options::OPT_fno_lto);
+  Args.ClaimAllArgs(options::OPT_fthinlto);
+  Args.ClaimAllArgs(options::OPT_fno_thinlto);
 }
 
 static void appendUserToPath(SmallVectorImpl<char> &Result) {
@@ -3027,6 +3032,9 @@
            "Invalid action for clang tool.");
 
     if (JA.getType() == types::TY_LTO_IR || JA.getType() == types::TY_LTO_BC) {
+      // Note that both -flto and -fthinlto use the TY_LTO_* types.
+      // Setting -flto here will enable the PrepareForLTO codegen option,
+      // which we do want for -fthinlto compiles as well.
       CmdArgs.push_back("-flto");
     }
     if (JA.getType() == types::TY_Nothing) {
@@ -3059,6 +3067,10 @@
     // the use-list order, since serialization to bitcode is part of the flow.
     if (JA.getType() == types::TY_LLVM_BC)
       CmdArgs.push_back("-emit-llvm-uselists");
+
+    if (Args.hasArg(options::OPT_fthinlto)) {
+      CmdArgs.push_back("-fthinlto");
+    }
   }
 
   // We normally speed up the clang process a bit by skipping destructors at
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -516,6 +516,7 @@
   Opts.MergeFunctions = Args.hasArg(OPT_fmerge_functions);
 
   Opts.PrepareForLTO = Args.hasArg(OPT_flto);
+  Opts.EmitThinLTOIndex = Args.hasArg(OPT_fthinlto);
 
   Opts.MSVolatile = Args.hasArg(OPT_fms_volatile);