Index: lib/CodeGen/BackendUtil.cpp
===================================================================
--- lib/CodeGen/BackendUtil.cpp
+++ lib/CodeGen/BackendUtil.cpp
@@ -896,7 +896,6 @@
 }
 
 static void runThinLTOBackend(ModuleSummaryIndex *CombinedIndex, Module *M,
-                              std::unique_ptr<raw_pwrite_stream> OS,
                               std::string SampleProfile) {
   StringMap<std::map<GlobalValue::GUID, GlobalValueSummary *>>
       ModuleToDefinedGVSummaries;
@@ -944,13 +943,11 @@
 
     OwnedImports.push_back(std::move(*MBOrErr));
   }
-  auto AddStream = [&](size_t Task) {
-    return llvm::make_unique<lto::NativeObjectStream>(std::move(OS));
-  };
   lto::Config Conf;
   Conf.SampleProfile = SampleProfile;
+  Conf.SkipCodeGen = true;
   if (Error E = thinBackend(
-          Conf, 0, AddStream, *M, *CombinedIndex, ImportList,
+          Conf, 0, nullptr, *M, *CombinedIndex, ImportList,
           ModuleToDefinedGVSummaries[M->getModuleIdentifier()], ModuleMap)) {
     handleAllErrors(std::move(E), [&](ErrorInfoBase &EIB) {
       errs() << "Error running ThinLTO backend: " << EIB.message() << '\n';
@@ -983,11 +980,8 @@
     // (LLVM will optionally ignore empty index files, returning null instead
     // of an error).
     bool DoThinLTOBackend = CombinedIndex != nullptr;
-    if (DoThinLTOBackend) {
-      runThinLTOBackend(CombinedIndex.get(), M, std::move(OS),
-                        CGOpts.SampleProfileFile);
-      return;
-    }
+    if (DoThinLTOBackend)
+      runThinLTOBackend(CombinedIndex.get(), M, CGOpts.SampleProfileFile);
   }
 
   EmitAssemblyHelper AsmHelper(Diags, HeaderOpts, CGOpts, TOpts, LOpts, M);
Index: test/CodeGen/function-sections.c
===================================================================
--- test/CodeGen/function-sections.c
+++ test/CodeGen/function-sections.c
@@ -9,6 +9,12 @@
 // RUN: %clang_cc1 -triple x86_64-pc-linux-gnu -S -fdata-sections -o - < %s | FileCheck %s --check-prefix=DATA_SECT
 // RUN: %clang_cc1 -triple x86_64-pc-linux-gnu -S -fno-data-sections -fdata-sections -o - < %s | FileCheck %s --check-prefix=DATA_SECT
 
+// Try again through a clang invocation of the ThinLTO backend.
+// RUN: %clang -O2 %s -flto=thin -c -o %t.o
+// RUN: llvm-lto -thinlto -o %t %t.o
+// RUN: %clang -O2 -x ir %t.o -fthinlto-index=%t.thinlto.bc -S -ffunction-sections -o - | FileCheck %s --check-prefix=FUNC_SECT
+// RUN: %clang -O2 -x ir %t.o -fthinlto-index=%t.thinlto.bc -S -fdata-sections -o - | FileCheck %s --check-prefix=DATA_SECT
+
 const int hello = 123;
 void world() {}
 
@@ -22,7 +28,7 @@
 // FUNC_SECT: section .rodata,
 // FUNC_SECT: hello:
 
-// DATA_SECT-NOT: section
+// DATA_SECT-NOT: .section
 // DATA_SECT: world:
 // DATA_SECT: .section .rodata.hello,
 // DATA_SECT: hello: