Index: include/llvm/LTO/Config.h
===================================================================
--- include/llvm/LTO/Config.h
+++ include/llvm/LTO/Config.h
@@ -47,6 +47,11 @@
   /// Disable entirely the optimizer, including importing for ThinLTO
   bool CodeGenOnly = false;
 
+  /// If this field is set, the new pass manager is used to run passes.
+  /// This will go away once the new pass manager will be the default and
+  /// the old pass manager will be removed.
+  bool OptUseNewPM = false;
+
   /// If this field is set, the set of passes run in the middle-end optimizer
   /// will be the one specified by the string. Only works with the new pass
   /// manager as the old one doesn't have this ability.
Index: lib/LTO/LTOBackend.cpp
===================================================================
--- lib/LTO/LTOBackend.cpp
+++ lib/LTO/LTOBackend.cpp
@@ -124,6 +124,57 @@
       Conf.CodeModel, Conf.CGOptLevel));
 }
 
+static void runNewPMPasses(Module &Mod, TargetMachine *TM,
+                           unsigned OptLevel) {
+  PassBuilder PB(TM);
+  AAManager AA;
+
+  // Parse a custom AA pipeline if asked to.
+  assert(PB.parseAAPipeline(AA, "default"));
+
+  LoopAnalysisManager LAM;
+  FunctionAnalysisManager FAM;
+  CGSCCAnalysisManager CGAM;
+  ModuleAnalysisManager MAM;
+
+  // Register the AA manager first so that our version is the one used.
+  FAM.registerPass([&] { return std::move(AA); });
+
+  // Register all the basic analyses with the managers.
+  PB.registerModuleAnalyses(MAM);
+  PB.registerCGSCCAnalyses(CGAM);
+  PB.registerFunctionAnalyses(FAM);
+  PB.registerLoopAnalyses(LAM);
+  PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
+
+  ModulePassManager MPM;
+  // FIXME (davide): verify the input.
+
+  PassBuilder::OptimizationLevel OL;
+
+  switch (OptLevel) {
+    default:
+      llvm_unreachable("Invalid optimization level");
+    case 0:
+      OL = PassBuilder::O0;
+      break;
+    case 1:
+      OL = PassBuilder::O1;
+      break;
+    case 2:
+      OL = PassBuilder::O2;
+      break;
+    case 3:
+      OL = PassBuilder::O3;
+      break;
+  }
+
+  MPM = PB.buildLTODefaultPipeline(OL, false /* DebugLogging */);
+  MPM.run(Mod, MAM);
+
+  // FIXME (davide): verify the output.
+}
+
 static void runNewPMCustomPasses(Module &Mod, TargetMachine *TM,
                                  std::string PipelineDesc,
                                  std::string AAPipelineDesc,
@@ -193,12 +244,16 @@
 
 bool opt(Config &Conf, TargetMachine *TM, unsigned Task, Module &Mod,
          bool IsThinLTO, ModuleSummaryIndex &CombinedIndex) {
-  if (Conf.OptPipeline.empty())
-    runOldPMPasses(Conf, Mod, TM, IsThinLTO, CombinedIndex);
-  else
+  if (Conf.OptUseNewPM || !Conf.OptPipeline.empty()) {
     // FIXME: Plumb the combined index into the new pass manager.
-    runNewPMCustomPasses(Mod, TM, Conf.OptPipeline, Conf.AAPipeline,
-                         Conf.DisableVerify);
+    if (Conf.OptPipeline.empty())
+      runNewPMPasses(Mod, TM, Conf.OptLevel);
+    else
+      runNewPMCustomPasses(Mod, TM, Conf.OptPipeline, Conf.AAPipeline,
+                           Conf.DisableVerify);
+  }
+  else
+    runOldPMPasses(Conf, Mod, TM, IsThinLTO, CombinedIndex);
   return !Conf.PostOptModuleHook || Conf.PostOptModuleHook(Task, Mod);
 }