Index: lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
===================================================================
--- lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
+++ lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
@@ -201,13 +201,19 @@
     function_ref<AAResults &(Function &)> AARGetter, Module &M) {
   std::string ModuleId = getUniqueModuleId(&M);
   if (ModuleId.empty()) {
-    // We couldn't generate a module ID for this module, just write it out as a
-    // regular LTO module.
-    WriteBitcodeToFile(M, OS);
+    // We couldn't generate a module ID for this module, write it out as a
+    // regular LTO module with an index for summary-based dead stripping.
+    ProfileSummaryInfo PSI(M);
+    M.addModuleFlag(Module::Error, "ThinLTO", uint32_t(0));
+    ModuleSummaryIndex Index = buildModuleSummaryIndex(M, nullptr, &PSI);
+    WriteBitcodeToFile(M, OS, /*ShouldPreserveUseListOrder=*/false, &Index);
+
     if (ThinLinkOS)
       // We don't have a ThinLTO part, but still write the module to the
       // ThinLinkOS if requested so that the expected output file is produced.
-      WriteBitcodeToFile(M, *ThinLinkOS);
+      WriteBitcodeToFile(M, *ThinLinkOS, /*ShouldPreserveUseListOrder=*/false,
+                         &Index);
+
     return;
   }
 
Index: test/Transforms/ThinLTOBitcodeWriter/unsplittable.ll
===================================================================
--- test/Transforms/ThinLTOBitcodeWriter/unsplittable.ll
+++ test/Transforms/ThinLTOBitcodeWriter/unsplittable.ll
@@ -5,6 +5,7 @@
 ; copy of the regular module.
 ; RUN: diff %t %t2
 
+; BCA: <FULL_LTO_GLOBALVAL_SUMMARY_BLOCK
 ; BCA-NOT: <GLOBALVAL_SUMMARY_BLOCK
 
 ; CHECK: @llvm.global_ctors = appending global
@@ -27,4 +28,7 @@
   ret void
 }
 
+; CHECK: !llvm.module.flags = !{![[FLAG:[0-9]+]]}
+; CHECK: ![[FLAG]] = !{i32 1, !"ThinLTO", i32 0}
+
 !0 = !{i32 0, !"typeid"}