Index: clang/include/clang/Driver/Options.td
===================================================================
--- clang/include/clang/Driver/Options.td
+++ clang/include/clang/Driver/Options.td
@@ -1013,6 +1013,8 @@
 def fthinlto_index_EQ : Joined<["-"], "fthinlto-index=">,
   Flags<[CC1Option]>, Group<f_Group>,
   HelpText<"Perform ThinLTO importing using provided function summary index">;
+def femit_summary_index : Flag<["-"], "femit-summary-index">, Group<f_Group>,
+  Flags<[CC1Option]>, HelpText<"Emit a module summary (default for ThinLTO)">;
 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: clang/include/clang/Frontend/CodeGenOptions.def
===================================================================
--- clang/include/clang/Frontend/CodeGenOptions.def
+++ clang/include/clang/Frontend/CodeGenOptions.def
@@ -88,8 +88,10 @@
                                      ///< be generated.
 CODEGENOPT(PrepareForLTO     , 1, 0) ///< Set when -flto is enabled on the
                                      ///< compile step.
-CODEGENOPT(EmitSummaryIndex, 1, 0)   ///< Set when -flto=thin is enabled on the
+CODEGENOPT(PrepareForThinLTO , 1, 0) ///< Set when -flto=thin is enabled on the
                                      ///< compile step.
+CODEGENOPT(EmitSummaryIndex  , 1, 0) ///< Emit a module summary. This is enabled
+                                     ///< by default for ThinLTO.
 CODEGENOPT(LTOUnit, 1, 0) ///< Emit IR to support LTO unit features (CFI, whole
                           ///< program vtable opt).
 CODEGENOPT(IncrementalLinkerCompatible, 1, 0) ///< Emit an object file which can
Index: clang/lib/CodeGen/BackendUtil.cpp
===================================================================
--- clang/lib/CodeGen/BackendUtil.cpp
+++ clang/lib/CodeGen/BackendUtil.cpp
@@ -486,7 +486,7 @@
     PMBuilder.Inliner = createFunctionInliningPass(
         CodeGenOpts.OptimizationLevel, CodeGenOpts.OptimizeSize,
         (!CodeGenOpts.SampleProfileFile.empty() &&
-         CodeGenOpts.EmitSummaryIndex));
+         CodeGenOpts.PrepareForThinLTO));
   }
 
   PMBuilder.OptLevel = CodeGenOpts.OptimizationLevel;
@@ -497,7 +497,7 @@
 
   PMBuilder.DisableUnrollLoops = !CodeGenOpts.UnrollLoops;
   PMBuilder.MergeFunctions = CodeGenOpts.MergeFunctions;
-  PMBuilder.PrepareForThinLTO = CodeGenOpts.EmitSummaryIndex;
+  PMBuilder.PrepareForThinLTO = CodeGenOpts.PrepareForThinLTO;
   PMBuilder.PrepareForLTO = CodeGenOpts.PrepareForLTO;
   PMBuilder.RerollLoops = CodeGenOpts.RerollLoops;
 
@@ -738,7 +738,7 @@
     break;
 
   case Backend_EmitBC:
-    if (CodeGenOpts.EmitSummaryIndex) {
+    if (CodeGenOpts.PrepareForThinLTO) {
       if (!CodeGenOpts.ThinLinkBitcodeFile.empty()) {
         std::error_code EC;
         ThinLinkOS.reset(new llvm::raw_fd_ostream(
@@ -752,10 +752,13 @@
       }
       PerModulePasses.add(
           createWriteThinLTOBitcodePass(*OS, ThinLinkOS.get()));
-    }
-    else
+    } else {
+      if (CodeGenOpts.EmitSummaryIndex)
+         TheModule->addModuleFlag(Module::Error, "ThinLTO", uint32_t(0));
       PerModulePasses.add(
-          createBitcodeWriterPass(*OS, CodeGenOpts.EmitLLVMUseLists));
+          createBitcodeWriterPass(*OS, CodeGenOpts.EmitLLVMUseLists,
+                                  CodeGenOpts.EmitSummaryIndex));
+    }
     break;
 
   case Backend_EmitLL:
@@ -907,7 +910,7 @@
     break;
 
   case Backend_EmitBC:
-    if (CodeGenOpts.EmitSummaryIndex) {
+    if (CodeGenOpts.PrepareForThinLTO) {
       if (!CodeGenOpts.ThinLinkBitcodeFile.empty()) {
         std::error_code EC;
         ThinLinkOS.emplace(CodeGenOpts.ThinLinkBitcodeFile, EC,
@@ -921,8 +924,9 @@
       MPM.addPass(
           ThinLTOBitcodeWriterPass(*OS, ThinLinkOS ? &*ThinLinkOS : nullptr));
     } else {
+      if (CodeGenOpts.EmitSummaryIndex)
+         TheModule->addModuleFlag(Module::Error, "ThinLTO", uint32_t(0));
       MPM.addPass(BitcodeWriterPass(*OS, CodeGenOpts.EmitLLVMUseLists,
-                                    CodeGenOpts.EmitSummaryIndex,
                                     CodeGenOpts.EmitSummaryIndex));
     }
     break;
Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -2057,6 +2057,8 @@
     if (JA.getType() == types::TY_LLVM_BC)
       CmdArgs.push_back("-emit-llvm-uselists");
 
+    Args.AddLastArg(CmdArgs, options::OPT_femit_summary_index);
+
     if (D.isUsingLTO()) {
       Args.AddLastArg(CmdArgs, options::OPT_flto, options::OPT_flto_EQ);
 
Index: clang/lib/Frontend/CompilerInvocation.cpp
===================================================================
--- clang/lib/Frontend/CompilerInvocation.cpp
+++ clang/lib/Frontend/CompilerInvocation.cpp
@@ -649,8 +649,10 @@
   Opts.NoUseJumpTables = Args.hasArg(OPT_fno_jump_tables);
 
   Opts.PrepareForLTO = Args.hasArg(OPT_flto, OPT_flto_EQ);
+  Opts.EmitSummaryIndex = Args.hasArg(OPT_femit_summary_index);
   const Arg *A = Args.getLastArg(OPT_flto, OPT_flto_EQ);
-  Opts.EmitSummaryIndex = A && A->containsValue("thin");
+  if (A && A->containsValue("thin"))
+    Opts.PrepareForThinLTO = Opts.EmitSummaryIndex = true;
   Opts.LTOUnit = Args.hasFlag(OPT_flto_unit, OPT_fno_lto_unit, false);
   if (Arg *A = Args.getLastArg(OPT_fthinlto_index_EQ)) {
     if (IK.getLanguage() != InputKind::LLVM_IR)
Index: clang/test/CodeGen/emit-summary-index.c
===================================================================
--- /dev/null
+++ clang/test/CodeGen/emit-summary-index.c
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -flto=thin -emit-llvm-bc < %s | llvm-bcanalyzer -dump | FileCheck %s
+// ; Check that the -flto=thin option emits a ThinLTO summary
+// CHECK: <GLOBALVAL_SUMMARY_BLOCK
+//
+// RUN: %clang_cc1 -flto -emit-llvm-bc < %s | llvm-bcanalyzer -dump | FileCheck --check-prefix=LTO %s
+// ; Check that we do not emit a summary by default for regular LTO
+// LTO-NOT: GLOBALVAL_SUMMARY_BLOCK
+//
+// RUN: %clang -flto -femit-summary-index -o - -c %s | llvm-bcanalyzer -dump | FileCheck --check-prefix=LTOINDEX %s
+// ; Check that -femit-summary-index creates a summary with regular LTO
+// RUN: %clang -emit-llvm -femit-summary-index -o - -c %s | llvm-bcanalyzer -dump | FileCheck --check-prefix=LTOINDEX %s
+// ; Check that -femit-summary-index creates a summary with -emit-llvm
+// LTOINDEX: <FULL_LTO_GLOBALVAL_SUMMARY_BLOCK
+int main() {}
Index: clang/test/Misc/thinlto.c
===================================================================
--- clang/test/Misc/thinlto.c
+++ /dev/null
@@ -1,4 +0,0 @@
-// RUN: %clang_cc1 -flto=thin -emit-llvm-bc < %s | llvm-bcanalyzer -dump | FileCheck %s
-// ; Check that the -flto=thin option emits a summary
-// CHECK: <GLOBALVAL_SUMMARY_BLOCK
-int main() {}