Index: lib/LTO/LTOBackend.cpp
===================================================================
--- lib/LTO/LTOBackend.cpp
+++ lib/LTO/LTOBackend.cpp
@@ -42,6 +42,10 @@
 using namespace llvm;
 using namespace lto;
 
+cl::opt<bool> LTOUseNewPM("lto-use-new-pm",
+                          cl::desc("Run passes using the new pass manager"),
+                          cl::init(false), cl::Hidden);
+
 LLVM_ATTRIBUTE_NORETURN static void reportOpenError(StringRef Path, Twine Msg) {
   errs() << "failed to open " << Path << ": " << Msg << '\n';
   errs().flush();
@@ -124,6 +128,56 @@
       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 +247,20 @@
 
 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
+  // There's still no ThinLTO pipeline hooked up in the new pass manager,
+  // once there is one, we can just remove this.
+  if (LTOUseNewPM && IsThinLTO)
+    report_fatal_error("ThinLTO not supported with the new PM yet!");
+
+  if (LTOUseNewPM || !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);
 }
 
Index: test/ThinLTO/X86/error-newpm.ll
===================================================================
--- /dev/null
+++ test/ThinLTO/X86/error-newpm.ll
@@ -0,0 +1,22 @@
+; RUN: opt -module-summary %s -o %t1.bc
+; RUN: not llvm-lto2 %t1.bc -o %t.o \
+; RUN:     -r=%t1.bc,_foo,pxl \
+; RUN:     -r=%t1.bc,_bar,pl \
+; RUN:     -r=%t1.bc,_linkonce_func,pl \
+; RUN:     -lto-use-new-pm 2>&1 | FileCheck %s
+
+; CHECK: ThinLTO not supported with the new PM yet!
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.11.0"
+
+define void @foo() {
+    ret void
+}
+define void @bar() {
+    call void @linkonce_func()
+	ret void
+}
+define linkonce void @linkonce_func() {
+    ret void
+}
Index: test/tools/llvm-lto/thinlto.ll
===================================================================
--- test/tools/llvm-lto/thinlto.ll
+++ test/tools/llvm-lto/thinlto.ll
@@ -27,3 +27,6 @@
 entry:
   ret void
 }
+
+; We don't (yet) accept ThinLTO + new pass manager.
+; RUN: not llvm-lto -thinlto
Index: test/tools/llvm-lto2/X86/pipeline.ll
===================================================================
--- test/tools/llvm-lto2/X86/pipeline.ll
+++ test/tools/llvm-lto2/X86/pipeline.ll
@@ -6,6 +6,10 @@
 ; RUN:  -aa-pipeline basic-aa
 ; RUN: llvm-dis < %t.o.0.4.opt.bc | FileCheck %s --check-prefix=CUSTOM
 
+; Try the new pass manager LTO default pipeline (make sure the option
+; is accepted).
+; RUN: llvm-lto2 %t1.bc -o %t.o -lto-use-new-pm -r %t1.bc,patatino,px
+
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"