diff --git a/llvm/include/llvm/LTO/LTOBackend.h b/llvm/include/llvm/LTO/LTOBackend.h
--- a/llvm/include/llvm/LTO/LTOBackend.h
+++ b/llvm/include/llvm/LTO/LTOBackend.h
@@ -41,9 +41,11 @@
 
 /// Runs a regular LTO backend. The regular LTO backend can also act as the
 /// regular LTO phase of ThinLTO, which may need to access the combined index.
-Error backend(const Config &C, AddStreamFn AddStream,
-              unsigned ParallelCodeGenParallelismLevel,
-              std::unique_ptr<Module> M, ModuleSummaryIndex &CombinedIndex);
+/// If \p ParallelCodeGenParallelismLevel is 1, return the unmodified \p M.
+Expected<std::unique_ptr<Module>>
+backend(const Config &C, AddStreamFn AddStream,
+        unsigned ParallelCodeGenParallelismLevel, std::unique_ptr<Module> M,
+        ModuleSummaryIndex &CombinedIndex);
 
 /// Runs a ThinLTO backend.
 Error thinBackend(const Config &C, unsigned Task, AddStreamFn AddStream,
diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -1068,10 +1068,11 @@
   }
 
   if (!RegularLTO.EmptyCombinedModule || Conf.AlwaysEmitRegularLTOObj) {
-    if (Error Err = backend(
-            Conf, AddStream, RegularLTO.ParallelCodeGenParallelismLevel,
-            std::move(RegularLTO.CombinedModule), ThinLTO.CombinedIndex))
-      return Err;
+    auto Result =
+        backend(Conf, AddStream, RegularLTO.ParallelCodeGenParallelismLevel,
+                std::move(RegularLTO.CombinedModule), ThinLTO.CombinedIndex);
+    if (!Result)
+      return Result.takeError();
   }
 
   return finalizeOptimizationRemarks(std::move(*DiagFileOrErr));
diff --git a/llvm/lib/LTO/LTOBackend.cpp b/llvm/lib/LTO/LTOBackend.cpp
--- a/llvm/lib/LTO/LTOBackend.cpp
+++ b/llvm/lib/LTO/LTOBackend.cpp
@@ -531,10 +531,10 @@
   return Error::success();
 }
 
-Error lto::backend(const Config &C, AddStreamFn AddStream,
-                   unsigned ParallelCodeGenParallelismLevel,
-                   std::unique_ptr<Module> Mod,
-                   ModuleSummaryIndex &CombinedIndex) {
+Expected<std::unique_ptr<Module>>
+lto::backend(const Config &C, AddStreamFn AddStream,
+             unsigned ParallelCodeGenParallelismLevel,
+             std::unique_ptr<Module> Mod, ModuleSummaryIndex &CombinedIndex) {
   Expected<const Target *> TOrErr = initAndLookupTarget(C, *Mod);
   if (!TOrErr)
     return TOrErr.takeError();
@@ -545,16 +545,17 @@
     if (!opt(C, TM.get(), 0, *Mod, /*IsThinLTO=*/false,
              /*ExportSummary=*/&CombinedIndex, /*ImportSummary=*/nullptr,
              /*CmdArgs*/ std::vector<uint8_t>()))
-      return Error::success();
+      return {nullptr};
   }
 
   if (ParallelCodeGenParallelismLevel == 1) {
     codegen(C, TM.get(), AddStream, 0, *Mod, CombinedIndex);
+    return {std::move(Mod)};
   } else {
     splitCodeGen(C, TM.get(), AddStream, ParallelCodeGenParallelismLevel,
                  std::move(Mod), CombinedIndex);
   }
-  return Error::success();
+  return {nullptr};
 }
 
 static void dropDeadSymbols(Module &Mod, const GVSummaryMapTy &DefinedGlobals,