Index: clang/include/clang/Frontend/CodeGenOptions.def
===================================================================
--- clang/include/clang/Frontend/CodeGenOptions.def
+++ clang/include/clang/Frontend/CodeGenOptions.def
@@ -88,7 +88,7 @@
                                      ///< 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(LTOUnit, 1, 0) ///< Emit IR to support LTO unit features (CFI, whole
                           ///< program vtable opt).
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;
 
@@ -733,12 +733,20 @@
 
   std::unique_ptr<raw_fd_ostream> ThinLinkOS;
 
+  // Emit a module summary by default for Regular LTO except for ld64 targets
+  bool EmitLTOSummary =
+      (CodeGenOpts.PrepareForLTO && !CodeGenOpts.PrepareForThinLTO &&
+       llvm::Triple(TheModule->getTargetTriple()).getVendor() !=
+           llvm::Triple::Apple);
+  if (EmitLTOSummary)
+    TheModule->addModuleFlag(Module::Error, "ThinLTO", uint32_t(0));
+
   switch (Action) {
   case Backend_EmitNothing:
     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(
@@ -755,7 +763,8 @@
     }
     else
       PerModulePasses.add(
-          createBitcodeWriterPass(*OS, CodeGenOpts.EmitLLVMUseLists));
+          createBitcodeWriterPass(*OS, CodeGenOpts.EmitLLVMUseLists,
+                                  EmitLTOSummary));
     break;
 
   case Backend_EmitLL:
@@ -895,6 +904,14 @@
     }
   }
 
+  // Emit a module summary by default for Regular LTO except for ld64 targets
+  bool EmitLTOSummary =
+      (CodeGenOpts.PrepareForLTO && !CodeGenOpts.PrepareForThinLTO &&
+       llvm::Triple(TheModule->getTargetTriple()).getVendor() !=
+           llvm::Triple::Apple);
+  if (EmitLTOSummary)
+    TheModule->addModuleFlag(Module::Error, "ThinLTO", uint32_t(0));
+
   // FIXME: We still use the legacy pass manager to do code generation. We
   // create that pass manager here and use it as needed below.
   legacy::PassManager CodeGenPasses;
@@ -907,7 +924,7 @@
     break;
 
   case Backend_EmitBC:
-    if (CodeGenOpts.EmitSummaryIndex) {
+    if (CodeGenOpts.PrepareForThinLTO) {
       if (!CodeGenOpts.ThinLinkBitcodeFile.empty()) {
         std::error_code EC;
         ThinLinkOS.emplace(CodeGenOpts.ThinLinkBitcodeFile, EC,
@@ -922,8 +939,7 @@
           ThinLTOBitcodeWriterPass(*OS, ThinLinkOS ? &*ThinLinkOS : nullptr));
     } else {
       MPM.addPass(BitcodeWriterPass(*OS, CodeGenOpts.EmitLLVMUseLists,
-                                    CodeGenOpts.EmitSummaryIndex,
-                                    CodeGenOpts.EmitSummaryIndex));
+                                    EmitLTOSummary));
     }
     break;
 
Index: clang/lib/Frontend/CompilerInvocation.cpp
===================================================================
--- clang/lib/Frontend/CompilerInvocation.cpp
+++ clang/lib/Frontend/CompilerInvocation.cpp
@@ -649,11 +649,11 @@
   Opts.NoUseJumpTables = Args.hasArg(OPT_fno_jump_tables);
 
   Opts.PrepareForLTO = Args.hasArg(OPT_flto, OPT_flto_EQ);
-  Opts.EmitSummaryIndex = false;
+  Opts.PrepareForThinLTO = false;
   if (Arg *A = Args.getLastArg(OPT_flto_EQ)) {
     StringRef S = A->getValue();
     if (S == "thin")
-      Opts.EmitSummaryIndex = true;
+      Opts.PrepareForThinLTO = true;
     else if (S != "full")
       Diags.Report(diag::err_drv_invalid_value) << A->getAsString(Args) << S;
   }
Index: clang/test/CodeGen/emit-summary-index.c
===================================================================
--- /dev/null
+++ clang/test/CodeGen/emit-summary-index.c
@@ -0,0 +1,12 @@
+// 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 -triple x86_64-apple-darwin -emit-llvm-bc < %s | llvm-bcanalyzer -dump | FileCheck --check-prefix=LTO %s
+// ; Check that we do not emit a summary for regular LTO on Apple platforms
+// LTO-NOT: GLOBALVAL_SUMMARY_BLOCK
+//
+// RUN: %clang_cc1 -flto -triple x86_64-pc-linux-gnu -emit-llvm-bc < %s | llvm-bcanalyzer -dump | FileCheck --check-prefix=LTOINDEX %s
+// ; Check that we emit a summary for regular LTO by default elsewhere
+// 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() {}