Index: lib/LTO/LTOBackend.cpp
===================================================================
--- lib/LTO/LTOBackend.cpp
+++ lib/LTO/LTOBackend.cpp
@@ -498,13 +498,23 @@
 
   std::unique_ptr<TargetMachine> TM = createTargetMachine(Conf, *TOrErr, Mod);
 
+  // Setup optimization remarks.
+  auto DiagFileOrErr = lto::setupOptimizationRemarks(
+      Mod.getContext(), Conf.RemarksFilename, Conf.RemarksWithHotness, Task);
+  if (!DiagFileOrErr)
+    return DiagFileOrErr.takeError();
+  auto DiagnosticOutputFile = std::move(*DiagFileOrErr);
+
   if (Conf.CodeGenOnly) {
     codegen(Conf, TM.get(), AddStream, Task, Mod);
+    finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
     return Error::success();
   }
 
-  if (Conf.PreOptModuleHook && !Conf.PreOptModuleHook(Task, Mod))
+  if (Conf.PreOptModuleHook && !Conf.PreOptModuleHook(Task, Mod)) {
+    finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
     return Error::success();
+  }
 
   renameModuleForThinLTO(Mod, CombinedIndex);
 
@@ -512,15 +522,19 @@
 
   thinLTOResolveWeakForLinkerModule(Mod, DefinedGlobals);
 
-  if (Conf.PostPromoteModuleHook && !Conf.PostPromoteModuleHook(Task, Mod))
+  if (Conf.PostPromoteModuleHook && !Conf.PostPromoteModuleHook(Task, Mod)) {
+    finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
     return Error::success();
+  }
 
   if (!DefinedGlobals.empty())
     thinLTOInternalizeModule(Mod, DefinedGlobals);
 
   if (Conf.PostInternalizeModuleHook &&
-      !Conf.PostInternalizeModuleHook(Task, Mod))
+      !Conf.PostInternalizeModuleHook(Task, Mod)) {
+    finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
     return Error::success();
+  }
 
   auto ModuleLoader = [&](StringRef Identifier) {
     assert(Mod.getContext().isODRUniquingDebugTypes() &&
@@ -536,13 +550,18 @@
   if (Error Err = Importer.importFunctions(Mod, ImportList).takeError())
     return Err;
 
-  if (Conf.PostImportModuleHook && !Conf.PostImportModuleHook(Task, Mod))
+  if (Conf.PostImportModuleHook && !Conf.PostImportModuleHook(Task, Mod)) {
+    finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
     return Error::success();
+  }
 
   if (!opt(Conf, TM.get(), Task, Mod, /*IsThinLTO=*/true,
-           /*ExportSummary=*/nullptr, /*ImportSummary=*/&CombinedIndex))
+           /*ExportSummary=*/nullptr, /*ImportSummary=*/&CombinedIndex)) {
+    finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
     return Error::success();
+  }
 
   codegen(Conf, TM.get(), AddStream, Task, Mod);
+  finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
   return Error::success();
 }
Index: test/LTO/Resolution/X86/diagnostic-handler-remarks.ll
===================================================================
--- test/LTO/Resolution/X86/diagnostic-handler-remarks.ll
+++ test/LTO/Resolution/X86/diagnostic-handler-remarks.ll
@@ -1,5 +1,7 @@
-; RUN: llvm-as < %s >%t.bc
+; Test of LTO with opt remarks YAML output.
 
+; First try with Regular LTO
+; RUN: llvm-as < %s >%t.bc
 ; RUN: rm -f %t.yaml
 ; RUN: llvm-lto2 run -pass-remarks-output=%t.yaml \
 ; RUN:           -r %t.bc,tinkywinky,p \
@@ -7,6 +9,15 @@
 ; RUN:           -r %t.bc,main,px -o %t.o %t.bc
 ; RUN: cat %t.yaml | FileCheck %s -check-prefix=YAML
 
+; Try again with ThinLTO
+; RUN: opt -module-summary %s -o %t.bc
+; RUN: rm -f %t.thin.1.yaml
+; RUN: llvm-lto2 run -pass-remarks-output=%t \
+; RUN:           -r %t.bc,tinkywinky,p \
+; RUN:           -r %t.bc,patatino,px \
+; RUN:           -r %t.bc,main,px -o %t.o %t.bc
+; RUN: cat %t.thin.1.yaml | FileCheck %s -check-prefix=YAML
+
 ; YAML:      --- !Passed
 ; YAML-NEXT: Pass:            inline
 ; YAML-NEXT: Name:            Inlined