diff --git a/lld/MachO/CMakeLists.txt b/lld/MachO/CMakeLists.txt
--- a/lld/MachO/CMakeLists.txt
+++ b/lld/MachO/CMakeLists.txt
@@ -13,6 +13,7 @@
   ExportTrie.cpp
   InputFiles.cpp
   InputSection.cpp
+  LTO.cpp
   MergedOutputSection.cpp
   ObjC.cpp
   OutputSection.cpp
@@ -27,8 +28,11 @@
   ${LLVM_TARGETS_TO_BUILD}
   BinaryFormat
   Core
+  LTO
+  MC
   Object
   Option
+  Passes
   Support
   TextAPI
 
diff --git a/lld/MachO/Config.h b/lld/MachO/Config.h
--- a/lld/MachO/Config.h
+++ b/lld/MachO/Config.h
@@ -39,6 +39,7 @@
   bool isPic = false;
   bool headerPadMaxInstallNames = false;
   bool searchDylibsFirst = false;
+  bool saveTemps = false;
   uint32_t headerPad;
   llvm::StringRef installName;
   llvm::StringRef outputFile;
diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
--- a/lld/MachO/Driver.cpp
+++ b/lld/MachO/Driver.cpp
@@ -10,6 +10,7 @@
 #include "Config.h"
 #include "DriverUtils.h"
 #include "InputFiles.h"
+#include "LTO.h"
 #include "ObjC.h"
 #include "OutputSection.h"
 #include "OutputSegment.h"
@@ -30,6 +31,7 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/BinaryFormat/MachO.h"
 #include "llvm/BinaryFormat/Magic.h"
+#include "llvm/LTO/LTO.h"
 #include "llvm/Object/Archive.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Option/Option.h"
@@ -37,6 +39,7 @@
 #include "llvm/Support/Host.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/TargetSelect.h"
 
 #include <algorithm>
 
@@ -316,16 +319,18 @@
     newFile = make<DylibFile>(mbref);
     break;
   case file_magic::tapi_file: {
-    Optional<DylibFile *> dylibFile = makeDylibFromTAPI(mbref);
-    if (!dylibFile)
-      return nullptr;
-    newFile = *dylibFile;
+    if (Optional<DylibFile *> dylibFile = makeDylibFromTAPI(mbref))
+      newFile = *dylibFile;
     break;
   }
+  case file_magic::bitcode:
+    newFile = make<BitcodeFile>(mbref);
+    break;
   default:
     error(path + ": unhandled file type");
   }
-  inputFiles.push_back(newFile);
+  if (newFile)
+    inputFiles.push_back(newFile);
   return newFile;
 }
 
@@ -455,6 +460,27 @@
   return false;
 }
 
+// This function is called on startup. We need this for LTO since
+// LTO calls LLVM functions to compile bitcode files to native code.
+// Technically this can be delayed until we read bitcode files, but
+// we don't bother to do lazily because the initialization is fast.
+static void initLLVM() {
+  InitializeAllTargets();
+  InitializeAllTargetMCs();
+  InitializeAllAsmPrinters();
+  InitializeAllAsmParsers();
+}
+
+static void compileBitcodeFiles() {
+  BitcodeCompiler lto;
+  for (InputFile *file : inputFiles)
+    if (auto *bitcodeFile = dyn_cast<BitcodeFile>(file))
+      lto.add(*bitcodeFile);
+
+  for (ObjFile *file : lto.compile())
+    inputFiles.push_back(file);
+}
+
 // Replaces common symbols with defined symbols residing in __common sections.
 // This function must be called after all symbol names are resolved (i.e. after
 // all InputFiles have been loaded.) As a result, later operations won't see
@@ -612,6 +638,8 @@
     config->searchDylibsFirst =
         (arg && arg->getOption().getID() == OPT_search_dylibs_first);
 
+  config->saveTemps = args.hasArg(OPT_save_temps);
+
   if (args.hasArg(OPT_v)) {
     message(getLLDVersion());
     message(StringRef("Library search paths:") +
@@ -692,6 +720,8 @@
       error("-sub_library " + searchName + " does not match a supplied dylib");
   }
 
+  initLLVM();
+  compileBitcodeFiles();
   replaceCommonSymbols();
 
   StringRef orderFile = args.getLastArgValue(OPT_order_file);
diff --git a/lld/MachO/InputFiles.h b/lld/MachO/InputFiles.h
--- a/lld/MachO/InputFiles.h
+++ b/lld/MachO/InputFiles.h
@@ -24,6 +24,12 @@
 #include <map>
 #include <vector>
 
+namespace llvm {
+namespace lto {
+class InputFile;
+} // namespace lto
+} // namespace llvm
+
 namespace lld {
 namespace macho {
 
@@ -40,9 +46,10 @@
 public:
   enum Kind {
     ObjKind,
+    OpaqueKind,
     DylibKind,
     ArchiveKind,
-    OpaqueKind,
+    BitcodeKind,
   };
 
   virtual ~InputFile() = default;
@@ -133,6 +140,14 @@
   llvm::DenseSet<uint64_t> seen;
 };
 
+class BitcodeFile : public InputFile {
+public:
+  explicit BitcodeFile(MemoryBufferRef mb);
+  static bool classof(const InputFile *f) { return f->kind() == BitcodeKind; }
+
+  std::unique_ptr<llvm::lto::InputFile> obj;
+};
+
 extern std::vector<InputFile *> inputFiles;
 
 llvm::Optional<MemoryBufferRef> readFile(StringRef path);
diff --git a/lld/MachO/InputFiles.cpp b/lld/MachO/InputFiles.cpp
--- a/lld/MachO/InputFiles.cpp
+++ b/lld/MachO/InputFiles.cpp
@@ -60,6 +60,7 @@
 #include "lld/Common/Memory.h"
 #include "llvm/ADT/iterator.h"
 #include "llvm/BinaryFormat/MachO.h"
+#include "llvm/LTO/LTO.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"
@@ -594,6 +595,11 @@
                      file->subsections.end());
 }
 
+BitcodeFile::BitcodeFile(MemoryBufferRef mbref)
+    : InputFile(BitcodeKind, mbref) {
+  obj = check(lto::InputFile::create(mbref));
+}
+
 // Returns "<internal>" or "baz.o".
 std::string lld::toString(const InputFile *file) {
   return file ? std::string(file->getName()) : "<internal>";
diff --git a/lld/MachO/LTO.h b/lld/MachO/LTO.h
new file mode 100644
--- /dev/null
+++ b/lld/MachO/LTO.h
@@ -0,0 +1,43 @@
+//===- LTO.h ----------------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLD_MACHO_LTO_H
+#define LLD_MACHO_LTO_H
+
+#include "llvm/ADT/SmallString.h"
+#include <memory>
+#include <vector>
+
+namespace llvm {
+namespace lto {
+class LTO;
+} // namespace lto
+} // namespace llvm
+
+namespace lld {
+namespace macho {
+
+class BitcodeFile;
+class ObjFile;
+
+class BitcodeCompiler {
+public:
+  BitcodeCompiler();
+
+  void add(BitcodeFile &f);
+  std::vector<ObjFile *> compile();
+
+private:
+  std::unique_ptr<llvm::lto::LTO> ltoObj;
+  std::vector<llvm::SmallString<0>> buf;
+};
+
+} // namespace macho
+} // namespace lld
+
+#endif
diff --git a/lld/MachO/LTO.cpp b/lld/MachO/LTO.cpp
new file mode 100644
--- /dev/null
+++ b/lld/MachO/LTO.cpp
@@ -0,0 +1,82 @@
+//===- LTO.cpp ------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "LTO.h"
+#include "Config.h"
+#include "InputFiles.h"
+
+#include "lld/Common/ErrorHandler.h"
+#include "lld/Common/Strings.h"
+#include "lld/Common/TargetOptionsCommandFlags.h"
+#include "llvm/LTO/LTO.h"
+#include "llvm/Support/raw_ostream.h"
+
+using namespace lld;
+using namespace lld::macho;
+using namespace llvm;
+
+static lto::Config createConfig() {
+  lto::Config c;
+  c.Options = initTargetOptionsFromCodeGenFlags();
+  return c;
+}
+
+BitcodeCompiler::BitcodeCompiler() {
+  auto backend =
+      lto::createInProcessThinBackend(llvm::heavyweight_hardware_concurrency());
+  ltoObj = std::make_unique<lto::LTO>(createConfig(), backend);
+}
+
+void BitcodeCompiler::add(BitcodeFile &f) {
+  ArrayRef<lto::InputFile::Symbol> objSyms = f.obj->symbols();
+  std::vector<lto::SymbolResolution> resols;
+  resols.reserve(objSyms.size());
+
+  // Provide a resolution to the LTO API for each symbol.
+  for (const lto::InputFile::Symbol &objSym : objSyms) {
+    resols.emplace_back();
+    lto::SymbolResolution &r = resols.back();
+
+    // Ideally we shouldn't check for SF_Undefined but currently IRObjectFile
+    // reports two symbols for module ASM defined. Without this check, lld
+    // flags an undefined in IR with a definition in ASM as prevailing.
+    // Once IRObjectFile is fixed to report only one symbol this hack can
+    // be removed.
+    r.Prevailing = !objSym.isUndefined();
+
+    // TODO: set the other resolution configs properly
+    r.VisibleToRegularObj = true;
+  }
+  checkError(ltoObj->add(std::move(f.obj), resols));
+}
+
+// Merge all the bitcode files we have seen, codegen the result
+// and return the resulting ObjectFile(s).
+std::vector<ObjFile *> BitcodeCompiler::compile() {
+  unsigned maxTasks = ltoObj->getMaxTasks();
+  buf.resize(maxTasks);
+
+  checkError(ltoObj->run([&](size_t task) {
+    return std::make_unique<lto::NativeObjectStream>(
+        std::make_unique<raw_svector_ostream>(buf[task]));
+  }));
+
+  if (config->saveTemps) {
+    if (!buf[0].empty())
+      saveBuffer(buf[0], config->outputFile + ".lto.o");
+    for (unsigned i = 1; i != maxTasks; ++i)
+      saveBuffer(buf[i], config->outputFile + Twine(i) + ".lto.o");
+  }
+
+  std::vector<ObjFile *> ret;
+  for (unsigned i = 0; i != maxTasks; ++i)
+    if (!buf[i].empty())
+      ret.push_back(make<ObjFile>(MemoryBufferRef(buf[i], "lto.tmp")));
+
+  return ret;
+}
diff --git a/lld/MachO/Options.td b/lld/MachO/Options.td
--- a/lld/MachO/Options.td
+++ b/lld/MachO/Options.td
@@ -453,6 +453,9 @@
      HelpText<"Dump dependency info">,
      Flags<[HelpHidden]>,
      Group<grp_introspect>;
+def save_temps : Flag<["-"], "save-temps">,
+     HelpText<"Save temporary files instead of deleting them">,
+     Group<grp_introspect>;
 
 def grp_symtab : OptionGroup<"symtab">, HelpText<"SYMBOL TABLE OPTIMIZATIONS">;
 
@@ -1233,10 +1236,6 @@
      HelpText<"This option is undocumented in ld64">,
      Flags<[HelpHidden]>,
      Group<grp_undocumented>;
-def save_temps : Flag<["-"], "save-temps">,
-     HelpText<"This option is undocumented in ld64">,
-     Flags<[HelpHidden]>,
-     Group<grp_undocumented>;
 def simulator_support : Flag<["-"], "simulator_support">,
      HelpText<"This option is undocumented in ld64">,
      Flags<[HelpHidden]>,
diff --git a/lld/test/MachO/lto-archive.ll b/lld/test/MachO/lto-archive.ll
new file mode 100644
--- /dev/null
+++ b/lld/test/MachO/lto-archive.ll
@@ -0,0 +1,28 @@
+; REQUIRES: x86
+; RUN: split-file %s %t
+; RUN: llvm-as %t/foo.ll -o %t/foo.o
+; RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/test.s -o %t/test.o
+; RUN: rm -f %t/foo.a
+; RUN: llvm-ar rcs %t/foo.a %t/foo.o
+; RUN: %lld -save-temps -lSystem %t/test.o %t/foo.a -o %t/test
+; RUN: llvm-objdump -d --no-show-raw-insn %t/test | FileCheck %s
+
+; CHECK:      <_main>:
+; CHECK-NEXT: callq _foo
+; CHECK-NEXT: retq
+
+;--- foo.ll
+
+target triple = "x86_64-apple-macosx10.15.0"
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+define void @foo() {
+  ret void
+}
+
+;--- test.s
+
+.globl _main
+_main:
+  callq _foo
+  ret
diff --git a/lld/test/MachO/lto-save-temps.ll b/lld/test/MachO/lto-save-temps.ll
new file mode 100644
--- /dev/null
+++ b/lld/test/MachO/lto-save-temps.ll
@@ -0,0 +1,16 @@
+; REQUIRES: x86
+; RUN: mkdir -p %t
+; RUN: llvm-as %s -o %t/test.o
+; RUN: %lld -save-temps %t/test.o -o %t/test
+; RUN: llvm-objdump -d --no-show-raw-insn %t/test.lto.o | FileCheck %s
+; RUN: llvm-objdump -d --no-show-raw-insn %t/test | FileCheck %s
+
+; CHECK:      <_main>:
+; CHECK-NEXT: retq
+
+target triple = "x86_64-apple-macosx10.15.0"
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+define void @main() {
+  ret void
+}
diff --git a/lld/test/MachO/thinlto-save-temps.ll b/lld/test/MachO/thinlto-save-temps.ll
new file mode 100644
--- /dev/null
+++ b/lld/test/MachO/thinlto-save-temps.ll
@@ -0,0 +1,37 @@
+; REQUIRES: x86
+; RUN: rm -rf %t; split-file %s %t
+; RUN: opt -module-summary %t/foo.ll -o %t/foo.o
+; RUN: opt -module-summary %t/test.ll -o %t/test.o
+; RUN: %lld -save-temps %t/foo.o %t/test.o -o %t/test
+; RUN: llvm-objdump -d --no-show-raw-insn %t/test1.lto.o | FileCheck %s --check-prefix=FOO
+; RUN: llvm-objdump -d --no-show-raw-insn %t/test2.lto.o | FileCheck %s --check-prefix=MAIN
+; RUN: llvm-objdump -d --no-show-raw-insn %t/test | FileCheck %s
+
+; FOO:      <_foo>:
+; FOO-NEXT: retq
+
+; MAIN:      <_main>:
+; MAIN-NEXT: retq
+
+; CHECK:      <_foo>:
+; CHECK-NEXT: retq
+; CHECK:      <_main>:
+; CHECK-NEXT: retq
+
+;--- foo.ll
+
+target triple = "x86_64-apple-macosx10.15.0"
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+define void @foo() {
+  ret void
+}
+
+;--- test.ll
+
+target triple = "x86_64-apple-macosx10.15.0"
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+define void @main() {
+  ret void
+}