diff --git a/llvm/include/llvm/LTO/LTO.h b/llvm/include/llvm/LTO/LTO.h
@@ -16,14 +16,27 @@
 #ifndef LLVM_LTO_LTO_H
 #define LLVM_LTO_LTO_H
 
+#include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/StringMap.h"
+#include "llvm/ADT/StringSet.h"
+#include "llvm/CodeGen/Analysis.h"
+#include "llvm/IR/DiagnosticInfo.h"
 #include "llvm/IR/ModuleSummaryIndex.h"
+#include "llvm/LTO/Config.h"
+#include "llvm/Linker/IRMover.h"
+#include "llvm/Object/IRObjectFile.h"
+#include "llvm/Support/thread.h"
+#include "llvm/Target/TargetOptions.h"
+#include "llvm/Transforms/IPO/FunctionImport.h"
 
 namespace llvm {
 
+class Error;
 class LLVMContext;
 class MemoryBufferRef;
 class Module;
+class Target;
+class raw_pwrite_stream;
 
 /// Helper to load a module from bitcode.
 std::unique_ptr<Module> loadModuleFromBuffer(const MemoryBufferRef &Buffer,
@@ -69,6 +82,319 @@ void thinLTOResolveWeakForLinkerInIndex(
 void thinLTOInternalizeAndPromoteInIndex(
     ModuleSummaryIndex &Index,
     function_ref<bool(StringRef, GlobalValue::GUID)> isExported);
-}
+
+namespace lto {
+
+class LTO;
+struct SymbolResolution;
+class ThinBackendProc;
+
+/// An input file. This is a wrapper for IRObjectFile that exposes only the
+/// information that an LTO client should need in order to do symbol resolution.
+class InputFile {
+  // FIXME: Remove LTO class friendship once we have bitcode symbol tables.
+  friend LTO;
+  InputFile() = default;
+
+  // FIXME: Remove the LLVMContext once we have bitcode symbol tables.
+  LLVMContext Ctx;
+  std::unique_ptr<object::IRObjectFile> Obj;
+
+public:
+  /// Create an InputFile.
+  static Expected<std::unique_ptr<InputFile>> create(MemoryBufferRef Object);
+
+  class symbol_iterator;
+
+  /// This is a wrapper for object::basic_symbol_iterator that exposes only the
+  /// information that an LTO client should need in order to do symbol
+  /// resolution.
+  ///
+  /// This object is ephemeral; it is only valid as long as an iterator obtained
+  /// from symbols() refers to it.
+  class Symbol {
+    friend symbol_iterator;
+    friend LTO;
+
+    object::basic_symbol_iterator I;
+    const GlobalValue *GV;
+    uint32_t Flags;
+    SmallString<64> Name;
+
+    bool shouldSkip() {
+      return !(Flags & object::BasicSymbolRef::SF_Global) ||
+             (Flags & object::BasicSymbolRef::SF_FormatSpecific);
+    }
+
+    void skip() {
+      const object::SymbolicFile *Obj = I->getObject();
+      auto E = Obj->symbol_end();
+      while (I != E) {
+        Flags = I->getFlags();
+        if (!shouldSkip())
+          break;
+        ++I;
+      }
+      if (I == E)
+        return;
+
+      Name.clear();
+      {
+        raw_svector_ostream OS(Name);
+        I->printName(OS);
+      }
+      GV = cast<object::IRObjectFile>(Obj)->getSymbolGV(I->getRawDataRefImpl());
+    }
+
+  public:
+    Symbol(object::basic_symbol_iterator I) : I(I) { skip(); }
+
+    StringRef getName() const { return Name; }
+    StringRef getIRName() const {
+      if (GV)
+        return GV->getName();
+      return StringRef();
+    }
+    uint32_t getFlags() const { return Flags; }
+    GlobalValue::VisibilityTypes getVisibility() const {
+      if (GV)
+        return GV->getVisibility();
+      return GlobalValue::DefaultVisibility;
+    }
+    bool canBeOmittedFromSymbolTable() const {
+      return GV && llvm::canBeOmittedFromSymbolTable(GV);
+    }
+    Expected<const Comdat *> getComdat() const {
+      const GlobalObject *GO;
+      if (auto *GA = dyn_cast<GlobalAlias>(GV)) {
+        GO = GA->getBaseObject();
+        if (!GO)
+          return make_error<StringError>("Unable to determine comdat of alias!",
+                                         inconvertibleErrorCode());
+      } else {
+        GO = cast<GlobalObject>(GV);
+      }
+      if (GV)
+        return GV->getComdat();
+      return nullptr;
+    }
+    size_t getCommonSize() const {
+      assert(Flags & object::BasicSymbolRef::SF_Common);
+      if (!GV)
+        return 0;
+      return GV->getParent()->getDataLayout().getTypeAllocSize(
+          GV->getType()->getElementType());
+    }
+    unsigned getCommonAlignment() const {
+      assert(Flags & object::BasicSymbolRef::SF_Common);
+      if (!GV)
+        return 0;
+      return GV->getAlignment();
+    }
+  };
+
+  class symbol_iterator {
+    Symbol Sym;
+
+  public:
+    symbol_iterator(object::basic_symbol_iterator I) : Sym(I) {}
+
+    symbol_iterator &operator++() {
+      ++Sym.I;
+      Sym.skip();
+      return *this;
+    }
+
+    symbol_iterator operator++(int) {
+      symbol_iterator I = *this;
+      ++*this;
+      return I;
+    }
+
+    const Symbol &operator*() const { return Sym; }
+    const Symbol *operator->() const { return &Sym; }
+
+    bool operator!=(const symbol_iterator &Other) const {
+      return Sym.I != Other.Sym.I;
+    }
+  };
+
+  /// A range over the symbols in this InputFile.
+  iterator_range<symbol_iterator> symbols() {
+    return llvm::make_range(symbol_iterator(Obj->symbol_begin()),
+                            symbol_iterator(Obj->symbol_end()));
+  }
+
+  StringRef getSourceFileName() const {
+    return Obj->getModule().getSourceFileName();
+  }
+};
+
+/// A ThinBackend defines what happens after the thin-link phase during ThinLTO.
+/// The details of this type definition aren't important; clients can only
+/// create a ThinBackend using one of the create*ThinBackend() functions below.
+typedef std::function<std::unique_ptr<ThinBackendProc>(
+    Config &C, ModuleSummaryIndex &CombinedIndex,
+    StringMap<GVSummaryMapTy> &ModuleToDefinedGVSummaries,
+    AddStreamFn AddStream)>
+    ThinBackend;
+
+/// This ThinBackend runs the individual backend jobs in-process.
+ThinBackend createInProcessThinBackend(unsigned ParallelismLevel);
+
+/// This ThinBackend writes individual module indexes to files, instead of
+/// running the individual backend jobs. This backend is for distributed builds
+/// where separate processes will invoke the real backends.
+///
+/// To find the path to write the index to, the backend checks if the path has a
+/// prefix of OldPrefix; if so, it replaces that prefix with NewPrefix. It then
+/// appends ".thinlto.bc" and writes the index to that path. If
+/// ShouldEmitImportsFiles is true it also writes a list of imported files to a
+/// similar path with ".imports" appended instead.
+ThinBackend createWriteIndexesThinBackend(std::string OldPrefix,
+                                          std::string NewPrefix,
+                                          bool ShouldEmitImportsFiles,
+                                          std::string LinkedObjectsFile);
+
+/// This class implements a resolution-based interface to LLVM's LTO
+/// functionality. It supports regular LTO, parallel LTO code generation and
+/// ThinLTO. You can use it from a linker in the following way:
+/// - Set hooks and code generation options (see lto::Config struct defined in
+///   Config.h), and use the lto::Config object to create an lto::LTO object.
+/// - Create lto::InputFile objects using lto::InputFile::create(), then use
+///   the symbols() function to enumerate its symbols and compute a resolution
+///   for each symbol (see SymbolResolution below).
+/// - After the linker has visited each input file (and each regular object
+///   file) and computed a resolution for each symbol, take each lto::InputFile
+///   and pass it and an array of symbol resolutions to the add() function.
+/// - Call the getMaxTasks() function to get an upper bound on the number of
+///   native object files that LTO may add to the link.
+/// - Call the run() function. This function will use the supplied AddStream
+///   function to add up to getMaxTasks() native object files to the link.
+class LTO {
+  friend InputFile;
+
+public:
+  /// Create an LTO object. A default constructed LTO object has a reasonable
+  /// production configuration, but you can customize it by passing arguments to
+  /// this constructor.
+  /// FIXME: We do currently require the DiagHandler field to be set in Conf.
+  /// Until that is fixed, a Config argument is required.
+  LTO(Config Conf, ThinBackend Backend = nullptr,
+      unsigned ParallelCodeGenParallelismLevel = 1);
+
+  /// Add an input file to the LTO link, using the provided symbol resolutions.
+  /// The symbol resolutions must appear in the enumeration order given by
+  /// InputFile::symbols().
+  Error add(std::unique_ptr<InputFile> Obj, ArrayRef<SymbolResolution> Res);
+
+  /// Returns an upper bound on the number of tasks that the client may expect.
+  /// This may only be called after all IR object files have been added. For a
+  /// full description of tasks see LTOBackend.h.
+  size_t getMaxTasks() const;
+
+  /// Runs the LTO pipeline. This function calls the supplied AddStream function
+  /// to add native object files to the link.
+  Error run(AddStreamFn AddStream);
+
+private:
+  Config Conf;
+
+  struct RegularLTOState {
+    RegularLTOState(unsigned ParallelCodeGenParallelismLevel, Config &Conf);
+
+    unsigned ParallelCodeGenParallelismLevel;
+    LTOLLVMContext Ctx;
+    bool HasModule = false;
+    std::unique_ptr<Module> CombinedModule;
+    IRMover Mover;
+  } RegularLTO;
+
+  struct ThinLTOState {
+    ThinLTOState(ThinBackend Backend);
+
+    ThinBackend Backend;
+    ModuleSummaryIndex CombinedIndex;
+    MapVector<StringRef, MemoryBufferRef> ModuleMap;
+    DenseMap<GlobalValue::GUID, StringRef> PrevailingModuleForGUID;
+  } ThinLTO;
+
+  // The global resolution for a particular (mangled) symbol name. This is in
+  // particular necessary to track whether each symbol can be internalized.
+  // Because any input file may introduce a new cross-partition reference, we
+  // cannot make any final internalization decisions until all input files have
+  // been added and the client has called run(). During run() we apply
+  // internalization decisions either directly to the module (for regular LTO)
+  // or to the combined index (for ThinLTO).
+  struct GlobalResolution {
+    /// The unmangled name of the global.
+    std::string IRName;
+
+    bool UnnamedAddr = true;
+
+    /// This field keeps track of the partition number of this global. The
+    /// regular LTO object is partition 0, while each ThinLTO object has its own
+    /// partition number from 1 onwards.
+    ///
+    /// Any global that is defined or used by more than one partition, or that
+    /// is referenced externally, may not be internalized.
+    ///
+    /// Partitions generally have a one-to-one correspondence with tasks, except
+    /// that we use partition 0 for all parallel LTO code generation partitions.
+    /// Any partitioning of the combined LTO object is done internally by the
+    /// LTO backend.
+    size_t Partition = Unknown;
+
+    /// Special partition numbers.
+    enum {
+      /// A partition number has not yet been assigned to this global.
+      Unknown = -1ull,
+
+      /// This global is either used by more than one partition or has an
+      /// external reference, and therefore cannot be internalized.
+      External = -2ull,
+    };
+  };
+
+  // Global mapping from mangled symbol names to resolutions.
+  StringMap<GlobalResolution> GlobalResolutions;
+
+  void writeToResolutionFile(InputFile *Input, ArrayRef<SymbolResolution> Res);
+
+  void addSymbolToGlobalRes(object::IRObjectFile *Obj,
+                            SmallPtrSet<GlobalValue *, 8> &Used,
+                            const InputFile::Symbol &Sym, SymbolResolution Res,
+                            size_t Partition);
+
+  Error addRegularLTO(std::unique_ptr<InputFile> Input,
+                      ArrayRef<SymbolResolution> Res);
+  Error addThinLTO(std::unique_ptr<InputFile> Input,
+                   ArrayRef<SymbolResolution> Res);
+
+  Error runRegularLTO(AddStreamFn AddStream);
+  Error runThinLTO(AddStreamFn AddStream);
+
+  mutable bool CalledGetMaxTasks = false;
+};
+
+/// The resolution for a symbol. The linker must provide a SymbolResolution for
+/// each global symbol based on its internal resolution of that symbol.
+struct SymbolResolution {
+  SymbolResolution()
+      : Prevailing(0), FinalDefinitionInLinkageUnit(0), VisibleToRegularObj(0) {
+  }
+  /// The linker has chosen this definition of the symbol.
+  unsigned Prevailing : 1;
+
+  /// The definition of this symbol is unpreemptable at runtime and is known to
+  /// be in this linkage unit.
+  unsigned FinalDefinitionInLinkageUnit : 1;
+
+  /// The definition of this symbol is visible outside of the LTO unit.
+  unsigned VisibleToRegularObj : 1;
+};
+
+} // namespace lto
+} // namespace llvm
 
 #endif
diff --git a/llvm/include/llvm/LTO/LTOBackend.h b/llvm/include/llvm/LTO/LTOBackend.h
@@ -0,0 +1,51 @@
+//===-LTOBackend.h - LLVM Link Time Optimizer Backend ---------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the "backend" phase of LTO, i.e. it performs
+// optimization and code generation on a loaded module. It is generally used
+// internally by the LTO class but can also be used independently, for example
+// to implement a standalone ThinLTO backend.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LTO_LTOBACKEND_H
+#define LLVM_LTO_LTOBACKEND_H
+
+#include "llvm/ADT/MapVector.h"
+#include "llvm/IR/DiagnosticInfo.h"
+#include "llvm/IR/ModuleSummaryIndex.h"
+#include "llvm/LTO/Config.h"
+#include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Target/TargetOptions.h"
+#include "llvm/Transforms/IPO/FunctionImport.h"
+
+namespace llvm {
+
+class Error;
+class Module;
+class Target;
+
+namespace lto {
+
+/// Runs a regular LTO backend.
+Error backend(Config &C, AddStreamFn AddStream,
+              unsigned ParallelCodeGenParallelismLevel,
+              std::unique_ptr<Module> M);
+
+/// Runs a ThinLTO backend.
+Error thinBackend(Config &C, size_t Task, AddStreamFn AddStream, Module &M,
+                  ModuleSummaryIndex &CombinedIndex,
+                  const FunctionImporter::ImportMapTy &ImportList,
+                  const GVSummaryMapTy &DefinedGlobals,
+                  MapVector<StringRef, MemoryBufferRef> &ModuleMap);
+
+}
+}
+
+#endif
diff --git a/llvm/lib/LTO/CMakeLists.txt b/llvm/lib/LTO/CMakeLists.txt
@@ -49,6 +49,7 @@ endif()
 
 add_llvm_library(LLVMLTO
   LTO.cpp
+  LTOBackend.cpp
   LTOModule.cpp
   LTOCodeGenerator.cpp
   UpdateCompilerUsed.cpp

diff --git a/llvm/lib/LTO/LLVMBuild.txt b/llvm/lib/LTO/LLVMBuild.txt
@@ -34,4 +34,4 @@ required_libraries =
  Scalar
  Support
  Target
- TransformUtils
+ TransformUtils
diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
diff --git a/llvm/lib/LTO/LTOBackend.cpp b/llvm/lib/LTO/LTOBackend.cpp
@@ -0,0 +1,277 @@
+//===-LTOBackend.cpp - LLVM Link Time Optimizer Backend -------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the "backend" phase of LTO, i.e. it performs
+// optimization and code generation on a loaded module. It is generally used
+// internally by the LTO class but can also be used independently, for example
+// to implement a standalone ThinLTO backend.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/LTO/LTOBackend.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/Bitcode/ReaderWriter.h"
+#include "llvm/IR/LegacyPassManager.h"
+#include "llvm/MC/SubtargetFeature.h"
+#include "llvm/Support/Error.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/TargetRegistry.h"
+#include "llvm/Support/ThreadPool.h"
+#include "llvm/Target/TargetMachine.h"
+#include "llvm/Transforms/IPO.h"
+#include "llvm/Transforms/IPO/PassManagerBuilder.h"
+#include "llvm/Transforms/Utils/FunctionImportUtils.h"
+#include "llvm/Transforms/Utils/SplitModule.h"
+
+using namespace llvm;
+using namespace lto;
+
+Error Config::addSaveTemps(std::string OutputFileName) {
+  ShouldDiscardValueNames = false;
+
+  std::error_code EC;
+  ResolutionFile = make_unique<raw_fd_ostream>(
+      OutputFileName + ".resolution.txt", EC, sys::fs::OpenFlags::F_Text);
+  if (EC)
+    return errorCodeToError(EC);
+
+  auto setHook = [&](std::string PathSuffix, ModuleHookFn &Hook) {
+    // Keep track of the hook provided by the linker, which also needs to run.
+    ModuleHookFn LinkerHook = Hook;
+    Hook = [=](size_t Task, Module &M) {
+      // If the linker's hook returned false, we need to pass that result
+      // through.
+      if (LinkerHook && !LinkerHook(Task, M))
+        return false;
+
+      std::string PathPrefix;
+      PathPrefix = OutputFileName;
+      if (Task != 0)
+        PathPrefix += "." + utostr(Task);
+      std::string Path = PathPrefix + "." + PathSuffix + ".bc";
+      std::error_code EC;
+      raw_fd_ostream OS(Path, EC, sys::fs::OpenFlags::F_None);
+      if (EC) {
+        // Because -save-temps is a debugging feature, we report the error
+        // directly and exit.
+        llvm::errs() << "failed to open " << Path << ": " << EC.message()
+                     << '\n';
+        exit(1);
+      }
+      WriteBitcodeToFile(&M, OS, /*ShouldPreserveUseListOrder=*/false);
+      return true;
+    };
+  };
+
+  setHook("0.preopt", PreOptModuleHook);
+  setHook("1.promote", PostPromoteModuleHook);
+  setHook("2.internalize", PostInternalizeModuleHook);
+  setHook("3.import", PostImportModuleHook);
+  setHook("4.opt", PostOptModuleHook);
+  setHook("5.precodegen", PreCodeGenModuleHook);
+
+  CombinedIndexHook = [=](const ModuleSummaryIndex &Index) {
+    std::string Path = OutputFileName + ".index.bc";
+    std::error_code EC;
+    raw_fd_ostream OS(Path, EC, sys::fs::OpenFlags::F_None);
+    if (EC) {
+      // Because -save-temps is a debugging feature, we report the error
+      // directly and exit.
+      llvm::errs() << "failed to open " << Path << ": " << EC.message() << '\n';
+      exit(1);
+    }
+    WriteIndexToFile(Index, OS);
+    return true;
+  };
+
+  return Error();
+}
+
+namespace {
+
+std::unique_ptr<TargetMachine>
+createTargetMachine(Config &C, StringRef TheTriple, const Target *TheTarget) {
+  SubtargetFeatures Features;
+  Features.getDefaultSubtargetFeatures(Triple(TheTriple));
+  for (const std::string &A : C.MAttrs)
+    Features.AddFeature(A);
+
+  return std::unique_ptr<TargetMachine>(TheTarget->createTargetMachine(
+      TheTriple, C.CPU, Features.getString(), C.Options, C.RelocModel,
+      C.CodeModel, C.CGOptLevel));
+}
+
+bool opt(Config &C, TargetMachine *TM, size_t Task, Module &M, bool IsThinLto) {
+  M.setDataLayout(TM->createDataLayout());
+
+  legacy::PassManager passes;
+  passes.add(createTargetTransformInfoWrapperPass(TM->getTargetIRAnalysis()));
+
+  PassManagerBuilder PMB;
+  PMB.LibraryInfo = new TargetLibraryInfoImpl(Triple(TM->getTargetTriple()));
+  PMB.Inliner = createFunctionInliningPass();
+  // Unconditionally verify input since it is not verified before this
+  // point and has unknown origin.
+  PMB.VerifyInput = true;
+  PMB.VerifyOutput = !C.DisableVerify;
+  PMB.LoopVectorize = true;
+  PMB.SLPVectorize = true;
+  PMB.OptLevel = C.OptLevel;
+  if (IsThinLto)
+    PMB.populateThinLTOPassManager(passes);
+  else
+    PMB.populateLTOPassManager(passes);
+  passes.run(M);
+
+  if (C.PostOptModuleHook && !C.PostOptModuleHook(Task, M))
+    return false;
+
+  return true;
+}
+
+void codegen(Config &C, TargetMachine *TM, AddStreamFn AddStream, size_t Task,
+             Module &M) {
+  if (C.PreCodeGenModuleHook && !C.PreCodeGenModuleHook(Task, M))
+    return;
+
+  std::unique_ptr<raw_pwrite_stream> OS = AddStream(Task);
+  legacy::PassManager CodeGenPasses;
+  if (TM->addPassesToEmitFile(CodeGenPasses, *OS,
+                              TargetMachine::CGFT_ObjectFile))
+    report_fatal_error("Failed to setup codegen");
+  CodeGenPasses.run(M);
+}
+
+void splitCodeGen(Config &C, TargetMachine *TM, AddStreamFn AddStream,
+                  unsigned ParallelCodeGenParallelismLevel,
+                  std::unique_ptr<Module> M) {
+  ThreadPool CodegenThreadPool(ParallelCodeGenParallelismLevel);
+  unsigned ThreadCount = 0;
+  const Target *T = &TM->getTarget();
+
+  SplitModule(
+      std::move(M), ParallelCodeGenParallelismLevel,
+      [&](std::unique_ptr<Module> MPart) {
+        // We want to clone the module in a new context to multi-thread the
+        // codegen. We do it by serializing partition modules to bitcode
+        // (while still on the main thread, in order to avoid data races) and
+        // spinning up new threads which deserialize the partitions into
+        // separate contexts.
+        // FIXME: Provide a more direct way to do this in LLVM.
+        SmallString<0> BC;
+        raw_svector_ostream BCOS(BC);
+        WriteBitcodeToFile(MPart.get(), BCOS);
+
+        // Enqueue the task
+        CodegenThreadPool.async(
+            [&](const SmallString<0> &BC, unsigned ThreadId) {
+              LTOLLVMContext Ctx(C);
+              ErrorOr<std::unique_ptr<Module>> MOrErr = parseBitcodeFile(
+                  MemoryBufferRef(StringRef(BC.data(), BC.size()), "ld-temp.o"),
+                  Ctx);
+              if (!MOrErr)
+                report_fatal_error("Failed to read bitcode");
+              std::unique_ptr<Module> MPartInCtx = std::move(MOrErr.get());
+
+              std::unique_ptr<TargetMachine> TM =
+                  createTargetMachine(C, MPartInCtx->getTargetTriple(), T);
+              codegen(C, TM.get(), AddStream, ThreadId, *MPartInCtx);
+            },
+            // Pass BC using std::move to ensure that it get moved rather than
+            // copied into the thread's context.
+            std::move(BC), ThreadCount++);
+      },
+      false);
+}
+
+Expected<const Target *> initAndLookupTarget(Config &C, Module &M) {
+  if (!C.OverrideTriple.empty())
+    M.setTargetTriple(C.OverrideTriple);
+  else if (M.getTargetTriple().empty())
+    M.setTargetTriple(C.DefaultTriple);
+
+  std::string Msg;
+  const Target *T = TargetRegistry::lookupTarget(M.getTargetTriple(), Msg);
+  if (!T)
+    return make_error<StringError>(Msg, inconvertibleErrorCode());
+  return T;
+}
+
+}
+
+Error lto::backend(Config &C, AddStreamFn AddStream,
+                   unsigned ParallelCodeGenParallelismLevel,
+                   std::unique_ptr<Module> M) {
+  Expected<const Target *> TOrErr = initAndLookupTarget(C, *M);
+  if (!TOrErr)
+    return TOrErr.takeError();
+
+  std::unique_ptr<TargetMachine> TM =
+      createTargetMachine(C, M->getTargetTriple(), *TOrErr);
+
+  if (!opt(C, TM.get(), 0, *M, /*IsThinLto=*/false))
+    return Error();
+
+  if (ParallelCodeGenParallelismLevel == 1)
+    codegen(C, TM.get(), AddStream, 0, *M);
+  else
+    splitCodeGen(C, TM.get(), AddStream, ParallelCodeGenParallelismLevel,
+                 std::move(M));
+  return Error();
+}
+
+Error lto::thinBackend(Config &C, size_t Task, AddStreamFn AddStream, Module &M,
+                       ModuleSummaryIndex &CombinedIndex,
+                       const FunctionImporter::ImportMapTy &ImportList,
+                       const GVSummaryMapTy &DefinedGlobals,
+                       MapVector<StringRef, MemoryBufferRef> &ModuleMap) {
+  Expected<const Target *> TOrErr = initAndLookupTarget(C, M);
+  if (!TOrErr)
+    return TOrErr.takeError();
+
+  std::unique_ptr<TargetMachine> TM =
+      createTargetMachine(C, M.getTargetTriple(), *TOrErr);
+
+  if (C.PreOptModuleHook && !C.PreOptModuleHook(Task, M))
+    return Error();
+
+  thinLTOResolveWeakForLinkerModule(M, DefinedGlobals);
+
+  renameModuleForThinLTO(M, CombinedIndex);
+
+  if (C.PostPromoteModuleHook && !C.PostPromoteModuleHook(Task, M))
+    return Error();
+
+  if (!DefinedGlobals.empty())
+    thinLTOInternalizeModule(M, DefinedGlobals);
+
+  if (C.PostInternalizeModuleHook && !C.PostInternalizeModuleHook(Task, M))
+    return Error();
+
+  auto ModuleLoader = [&](StringRef Identifier) {
+    return std::move(getLazyBitcodeModule(MemoryBuffer::getMemBuffer(
+                                              ModuleMap[Identifier], false),
+                                          M.getContext(),
+                                          /*ShouldLazyLoadMetadata=*/true)
+                         .get());
+  };
+
+  FunctionImporter Importer(CombinedIndex, ModuleLoader);
+  Importer.importFunctions(M, ImportList);
+
+  if (C.PostImportModuleHook && !C.PostImportModuleHook(Task, M))
+    return Error();
+
+  if (!opt(C, TM.get(), Task, M, /*IsThinLto=*/true))
+    return Error();
+
+  codegen(C, TM.get(), AddStream, Task, M);
+  return Error();
+}
diff --git a/llvm/lib/Object/IRObjectFile.cpp b/llvm/lib/Object/IRObjectFile.cpp
@@ -324,5 +324,5 @@ llvm::object::IRObjectFile::create(MemoryBufferRef Object,
     return EC;
 
   std::unique_ptr<Module> &M = MOrErr.get();
-  return llvm::make_unique<IRObjectFile>(Object, std::move(M));
+  return llvm::make_unique<IRObjectFile>(BCOrErr.get(), std::move(M));
 }
diff --git a/llvm/test/CMakeLists.txt b/llvm/test/CMakeLists.txt
@@ -43,6 +43,7 @@ set(LLVM_TEST_DEPENDS
           llvm-extract
           llvm-lib
           llvm-link
+          llvm-lto2
           llvm-mc
           llvm-mcmarkup
           llvm-nm

diff --git a/llvm/test/LTO/Resolution/X86/Inputs/alias-1.ll b/llvm/test/LTO/Resolution/X86/Inputs/alias-1.ll
@@ -0,0 +1,4 @@
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@a = global i32 42
diff --git a/llvm/test/LTO/Resolution/X86/Inputs/comdat.ll b/llvm/test/LTO/Resolution/X86/Inputs/comdat.ll
@@ -0,0 +1,28 @@
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+$c2 = comdat any
+$c1 = comdat any
+
+; This is only present in this file. The linker will keep $c1 from the first
+; file and this will be undefined.
+@will_be_undefined = global i32 1, comdat($c1)
+
+@v1 = weak_odr global i32 41, comdat($c2)
+define weak_odr protected i32 @f1(i8* %this) comdat($c2) {
+bb20:
+  store i8* %this, i8** null
+  br label %bb21
+bb21:
+  ret i32 41
+}
+
+@r21 = global i32* @v1
+@r22 = global i32(i8*)* @f1
+
+@a21 = alias i32, i32* @v1
+@a22 = alias i16, bitcast (i32* @v1 to i16*)
+
+@a23 = alias i32(i8*), i32(i8*)* @f1
+@a24 = alias i16, bitcast (i32(i8*)* @f1 to i16*)
+@a25 = alias i16, i16* @a24
diff --git a/llvm/test/LTO/Resolution/X86/alias.ll b/llvm/test/LTO/Resolution/X86/alias.ll
@@ -0,0 +1,22 @@
+; RUN: llvm-as %s -o %t1.o
+; RUN: llvm-as %p/Inputs/alias-1.ll -o %t2.o
+; RUN: llvm-lto2 -o %t3.o %t2.o %t1.o -r %t2.o,a,px -r %t1.o,a, -r %t1.o,b,px -save-temps
+; RUN: llvm-dis < %t3.o.0.preopt.bc -o - | FileCheck %s
+; RUN: FileCheck --check-prefix=RES %s < %t3.o.resolution.txt
+
+; CHECK-NOT: alias
+; CHECK: @a = global i32 42
+; CHECK-NEXT: @b = global i32 1
+; CHECK-NOT: alias
+
+; RES: 2.o{{$}}
+; RES: {{^}}-r={{.*}}2.o,a,px{{$}}
+; RES: 1.o{{$}}
+; RES: {{^}}-r={{.*}}1.o,b,px{{$}}
+; RES: {{^}}-r={{.*}}1.o,a,{{$}}
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@a = weak alias i32, i32* @b
+@b = global i32 1
diff --git a/llvm/test/LTO/Resolution/X86/comdat.ll b/llvm/test/LTO/Resolution/X86/comdat.ll
@@ -0,0 +1,86 @@
+; RUN: llvm-as %s -o %t.o
+; RUN: llvm-as %p/Inputs/comdat.ll -o %t2.o
+; RUN: llvm-lto2 -save-temps -o %t3.o %t.o %t2.o \
+; RUN:  -r=%t.o,f1,plx \
+; RUN:  -r=%t.o,v1,px \
+; RUN:  -r=%t.o,r11,px \
+; RUN:  -r=%t.o,r12,px \
+; RUN:  -r=%t.o,a11,px \
+; RUN:  -r=%t.o,a12,px \
+; RUN:  -r=%t.o,a13,px \
+; RUN:  -r=%t.o,a14,px \
+; RUN:  -r=%t.o,a15,px \
+; RUN:  -r=%t2.o,f1,l \
+; RUN:  -r=%t2.o,will_be_undefined, \
+; RUN:  -r=%t2.o,v1, \
+; RUN:  -r=%t2.o,r21,px \
+; RUN:  -r=%t2.o,r22,px \
+; RUN:  -r=%t2.o,a21,px \
+; RUN:  -r=%t2.o,a22,px \
+; RUN:  -r=%t2.o,a23,px \
+; RUN:  -r=%t2.o,a24,px \
+; RUN:  -r=%t2.o,a25,px
+; RUN: llvm-dis %t3.o.2.internalize.bc -o - | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+$c1 = comdat any
+
+@v1 = weak_odr global i32 42, comdat($c1)
+define weak_odr i32 @f1(i8*) comdat($c1) {
+bb10:
+  br label %bb11
+bb11:
+  ret i32 42
+}
+
+@r11 = global i32* @v1
+@r12 = global i32 (i8*)* @f1
+
+@a11 = alias i32, i32* @v1
+@a12 = alias i16, bitcast (i32* @v1 to i16*)
+
+@a13 = alias i32 (i8*), i32 (i8*)* @f1
+@a14 = alias i16, bitcast (i32 (i8*)* @f1 to i16*)
+@a15 = alias i16, i16* @a14
+
+; CHECK: $c1 = comdat any
+; CHECK: $c2 = comdat any
+
+; CHECK-DAG: @v1 = weak_odr global i32 42, comdat($c1)
+
+; CHECK-DAG: @r11 = global i32* @v1{{$}}
+; CHECK-DAG: @r12 = global i32 (i8*)* @f1{{$}}
+
+; CHECK-DAG: @r21 = global i32* @v1{{$}}
+; CHECK-DAG: @r22 = global i32 (i8*)* @f1{{$}}
+
+; CHECK-DAG: @v1.1 = internal global i32 41, comdat($c2)
+
+; CHECK-DAG: @a11 = alias i32, i32* @v1{{$}}
+; CHECK-DAG: @a12 = alias i16, bitcast (i32* @v1 to i16*)
+
+; CHECK-DAG: @a13 = alias i32 (i8*), i32 (i8*)* @f1{{$}}
+; CHECK-DAG: @a14 = alias i16, bitcast (i32 (i8*)* @f1 to i16*)
+
+; CHECK-DAG: @a21 = alias i32, i32* @v1.1{{$}}
+; CHECK-DAG: @a22 = alias i16, bitcast (i32* @v1.1 to i16*)
+
+; CHECK-DAG: @a23 = alias i32 (i8*), i32 (i8*)* @f1.2{{$}}
+; CHECK-DAG: @a24 = alias i16, bitcast (i32 (i8*)* @f1.2 to i16*)
+
+; CHECK:      define weak_odr i32 @f1(i8*) comdat($c1) {
+; CHECK-NEXT: bb10:
+; CHECK-NEXT:   br label %bb11{{$}}
+; CHECK:      bb11:
+; CHECK-NEXT:   ret i32 42
+; CHECK-NEXT: }
+
+; CHECK:      define internal i32 @f1.2(i8* %this) comdat($c2) {
+; CHECK-NEXT: bb20:
+; CHECK-NEXT:   store i8* %this, i8** null
+; CHECK-NEXT:   br label %bb21
+; CHECK:      bb21:
+; CHECK-NEXT:   ret i32 41
+; CHECK-NEXT: }
diff --git a/llvm/test/LTO/Resolution/X86/lit.local.cfg b/llvm/test/LTO/Resolution/X86/lit.local.cfg
@@ -0,0 +1,2 @@
+if not 'X86' in config.root.targets:
+  config.unsupported = True
diff --git a/llvm/test/lit.cfg b/llvm/test/lit.cfg
@@ -295,6 +295,7 @@ for pattern in [r"\bbugpoint\b(?!-)",
                 r"\bllvm-lib\b",
                 r"\bllvm-link\b",
                 r"\bllvm-lto\b",
+                r"\bllvm-lto2\b",
                 r"\bllvm-mc\b",
                 r"\bllvm-mcmarkup\b",
                 r"\bllvm-nm\b",

diff --git a/llvm/test/tools/gold/X86/coff.ll b/llvm/test/tools/gold/X86/coff.ll
@@ -16,7 +16,7 @@ define hidden void @g() {
   ret void
 }
 
-; CHECK: define internal void @h() local_unnamed_addr {
+; CHECK: define internal void @h() {
 define linkonce_odr void @h() local_unnamed_addr {
   ret void
 }
diff --git a/llvm/test/tools/gold/X86/comdat.ll b/llvm/test/tools/gold/X86/comdat.ll
@@ -1,8 +1,9 @@
-; RUN: llvm-as %s -o %t.o
+; RUN: llvm-as %s -o %t1.o
 ; RUN: llvm-as %p/Inputs/comdat.ll -o %t2.o
-; RUN: %gold -shared -o %t3.o -plugin %llvmshlibdir/LLVMgold.so %t.o %t2.o \
+; RUN: %gold -shared -o %t3.o -plugin %llvmshlibdir/LLVMgold.so %t1.o %t2.o \
 ; RUN:  -plugin-opt=save-temps
-; RUN: llvm-dis %t3.o.bc -o - | FileCheck %s
+; RUN: FileCheck --check-prefix=RES %s < %t3.o.resolution.txt
+; RUN: llvm-readobj -t %t3.o | FileCheck --check-prefix=OBJ %s
 
 $c1 = comdat any
 
@@ -24,42 +25,37 @@ bb11:
 @a14 = alias i16, bitcast (i32 (i8*)* @f1 to i16*)
 @a15 = alias i16, i16* @a14
 
-; CHECK: $c1 = comdat any
-; CHECK: $c2 = comdat any
-
-; CHECK-DAG: @v1 = weak_odr global i32 42, comdat($c1)
-
-; CHECK-DAG: @r11 = global i32* @v1{{$}}
-; CHECK-DAG: @r12 = global i32 (i8*)* @f1{{$}}
-
-; CHECK-DAG: @r21 = global i32* @v1{{$}}
-; CHECK-DAG: @r22 = global i32 (i8*)* @f1{{$}}
-
-; CHECK-DAG: @v1.1 = internal global i32 41, comdat($c2)
-
-; CHECK-DAG: @a11 = alias i32, i32* @v1{{$}}
-; CHECK-DAG: @a12 = alias i16, bitcast (i32* @v1 to i16*)
-
-; CHECK-DAG: @a13 = alias i32 (i8*), i32 (i8*)* @f1{{$}}
-; CHECK-DAG: @a14 = alias i16, bitcast (i32 (i8*)* @f1 to i16*)
-
-; CHECK-DAG: @a21 = alias i32, i32* @v1.1{{$}}
-; CHECK-DAG: @a22 = alias i16, bitcast (i32* @v1.1 to i16*)
-
-; CHECK-DAG: @a23 = alias i32 (i8*), i32 (i8*)* @f1.2{{$}}
-; CHECK-DAG: @a24 = alias i16, bitcast (i32 (i8*)* @f1.2 to i16*)
-
-; CHECK:      define weak_odr protected i32 @f1(i8*) comdat($c1) {
-; CHECK-NEXT: bb10:
-; CHECK-NEXT:   br label %bb11{{$}}
-; CHECK:      bb11:
-; CHECK-NEXT:   ret i32 42
-; CHECK-NEXT: }
-
-; CHECK:      define internal i32 @f1.2(i8* %this) comdat($c2) {
-; CHECK-NEXT: bb20:
-; CHECK-NEXT:   store i8* %this, i8** null
-; CHECK-NEXT:   br label %bb21
-; CHECK:      bb21:
-; CHECK-NEXT:   ret i32 41
-; CHECK-NEXT: }
+; gold's resolutions should tell us that our $c1 wins, and the other input's $c2
+; wins. f1 is also local due to having protected visibility in the other object.
+
+; RES: 1.o,f1,plx{{$}}
+; RES: 1.o,v1,px{{$}}
+; RES: 1.o,r11,px{{$}}
+; RES: 1.o,r12,px{{$}}
+; RES: 1.o,a11,px{{$}}
+; RES: 1.o,a12,px{{$}}
+; RES: 1.o,a13,px{{$}}
+; RES: 1.o,a14,px{{$}}
+; RES: 1.o,a15,px{{$}}
+
+; RES: 2.o,f1,l{{$}}
+; RES: 2.o,will_be_undefined,{{$}}
+; RES: 2.o,v1,{{$}}
+; RES: 2.o,r21,px{{$}}
+; RES: 2.o,r22,px{{$}}
+; RES: 2.o,a21,px{{$}}
+; RES: 2.o,a22,px{{$}}
+; RES: 2.o,a23,px{{$}}
+; RES: 2.o,a24,px{{$}}
+; RES: 2.o,a25,px{{$}}
+
+; f1's protected visibility should be reflected in the DSO.
+
+; OBJ:      Name: f1 (
+; OBJ-NEXT: Value:
+; OBJ-NEXT: Size:
+; OBJ-NEXT: Binding:
+; OBJ-NEXT: Type:
+; OBJ-NEXT: Other [
+; OBJ-NEXT:   STV_PROTECTED
+; OBJ-NEXT: ]
diff --git a/llvm/test/tools/gold/X86/common.ll b/llvm/test/tools/gold/X86/common.ll
@@ -11,31 +11,31 @@
 ; RUN: llvm-dis %t3.o -o - | FileCheck %s --check-prefix=A
 
 ; Shared library case, we merge @a as common and keep it for the symbol table.
-; A: @a = common global i32 0, align 8
+; A: @a = common global [4 x i8] zeroinitializer, align 8
 
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
 ; RUN:    --plugin-opt=emit-llvm \
 ; RUN:    -shared %t1.o %t2b.o -o %t3.o
 ; RUN: llvm-dis %t3.o -o - | FileCheck %s --check-prefix=B
 
 ; (i16 align 8) + (i8 align 16) = i16 align 16
-; B: @a = common global i16 0, align 16
+; B: @a = common global [2 x i8] zeroinitializer, align 16
 
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
 ; RUN:    --plugin-opt=emit-llvm \
 ; RUN:    -shared %t1.o %t2c.o -o %t3.o
 ; RUN: llvm-dis %t3.o -o - | FileCheck %s --check-prefix=C
 
 ; (i16 align 8) + (i8 align 1) = i16 align 8.
-; C: @a = common global i16 0, align 8
+; C: @a = common global [2 x i8] zeroinitializer, align 8
 
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
 ; RUN:    --plugin-opt=emit-llvm \
 ; RUN:    %t1.o %t2.o -o %t3.o
 ; RUN: llvm-dis %t3.o -o - | FileCheck --check-prefix=EXEC %s
 
 ; All IR case, we internalize a after merging.
-; EXEC: @a = internal global i32 0, align 8
+; EXEC: @a = internal global [4 x i8] zeroinitializer, align 8
 
 ; RUN: llc %p/Inputs/common.ll -o %t2native.o -filetype=obj
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
@@ -44,4 +44,4 @@
 ; RUN: llvm-dis %t3.o -o - | FileCheck --check-prefix=MIXED %s
 
 ; Mixed ELF and IR. We keep ours as common so the linker will finish the merge.
-; MIXED: @a = common global i16 0, align 8
+; MIXED: @a = common global [2 x i8] zeroinitializer, align 8
diff --git a/llvm/test/tools/gold/X86/emit-llvm.ll b/llvm/test/tools/gold/X86/emit-llvm.ll
@@ -2,17 +2,16 @@
 
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
 ; RUN:    --plugin-opt=emit-llvm \
-; RUN:    --plugin-opt=generate-api-file \
 ; RUN:    -shared %t.o -o %t2.o
 ; RUN: llvm-dis %t2.o -o - | FileCheck %s
-; RUN: FileCheck --check-prefix=API %s < %T/../apifile.txt
 
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
 ; RUN:     -m elf_x86_64 --plugin-opt=save-temps \
 ; RUN:    -shared %t.o -o %t3.o
-; RUN: llvm-dis %t3.o.bc -o - | FileCheck %s
-; RUN: llvm-dis %t3.o.opt.bc -o - | FileCheck --check-prefix=OPT %s
-; RUN: llvm-dis %t3.o.opt.bc -o - | FileCheck --check-prefix=OPT2 %s
+; RUN: FileCheck --check-prefix=RES %s < %t3.o.resolution.txt
+; RUN: llvm-dis %t3.o.2.internalize.bc -o - | FileCheck %s
+; RUN: llvm-dis %t3.o.4.opt.bc -o - | FileCheck --check-prefix=OPT %s
+; RUN: llvm-dis %t3.o.4.opt.bc -o - | FileCheck --check-prefix=OPT2 %s
 ; RUN: llvm-nm %t3.o.o | FileCheck --check-prefix=NM %s
 
 ; RUN: rm -f %t4.o
@@ -25,19 +24,19 @@
 
 target triple = "x86_64-unknown-linux-gnu"
 
-; CHECK-DAG: @g1 = linkonce_odr constant i32 32
+; CHECK-DAG: @g1 = weak_odr constant i32 32
 @g1 = linkonce_odr constant i32 32
 
-; CHECK-DAG: @g2 = internal local_unnamed_addr constant i32 32
+; CHECK-DAG: @g2 = internal constant i32 32
 @g2 = linkonce_odr local_unnamed_addr constant i32 32
 
 ; CHECK-DAG: @g3 = internal unnamed_addr constant i32 32
 @g3 = linkonce_odr unnamed_addr constant i32 32
 
-; CHECK-DAG: @g4 = linkonce_odr global i32 32
+; CHECK-DAG: @g4 = weak_odr global i32 32
 @g4 = linkonce_odr global i32 32
 
-; CHECK-DAG: @g5 = linkonce_odr local_unnamed_addr global i32 32
+; CHECK-DAG: @g5 = weak_odr global i32 32
 @g5 = linkonce_odr local_unnamed_addr global i32 32
 
 ; CHECK-DAG: @g6 = internal unnamed_addr global i32 32
@@ -75,8 +74,8 @@ define linkonce_odr void @f4() local_unnamed_addr {
   ret void
 }
 
-; CHECK-DAG: define linkonce_odr void @f5()
-; OPT-DAG: define linkonce_odr void @f5()
+; CHECK-DAG: define weak_odr void @f5()
+; OPT-DAG: define weak_odr void @f5()
 define linkonce_odr void @f5() {
   ret void
 }
@@ -97,15 +96,21 @@ define i32* @f8() {
   ret i32* @g8
 }
 
-; API: f1 PREVAILING_DEF_IRONLY
-; API: f2 PREVAILING_DEF_IRONLY
-; API: f3 PREVAILING_DEF_IRONLY_EXP
-; API: f4 PREVAILING_DEF_IRONLY_EXP
-; API: f5 PREVAILING_DEF_IRONLY_EXP
-; API: f6 PREVAILING_DEF_IRONLY_EXP
-; API: f7 PREVAILING_DEF_IRONLY_EXP
-; API: f8 PREVAILING_DEF_IRONLY_EXP
-; API: g7 UNDEF
-; API: g8 UNDEF
-; API: g9 PREVAILING_DEF_IRONLY_EXP
-; API: g10 PREVAILING_DEF_IRONLY_EXP
+; RES: .o,f1,pl{{$}}
+; RES: .o,f2,pl{{$}}
+; RES: .o,f3,px{{$}}
+; RES: .o,f4,p{{$}}
+; RES: .o,f5,px{{$}}
+; RES: .o,f6,p{{$}}
+; RES: .o,f7,px{{$}}
+; RES: .o,f8,px{{$}}
+; RES: .o,g1,px{{$}}
+; RES: .o,g2,p{{$}}
+; RES: .o,g3,p{{$}}
+; RES: .o,g4,px{{$}}
+; RES: .o,g5,px{{$}}
+; RES: .o,g6,p{{$}}
+; RES: .o,g7,{{$}}
+; RES: .o,g8,{{$}}
+; RES: .o,g9,px{{$}}
+; RES: .o,g10,px{{$}}
diff --git a/llvm/test/tools/gold/X86/opt-level.ll b/llvm/test/tools/gold/X86/opt-level.ll
@@ -1,13 +1,13 @@
 ; RUN: llvm-as -o %t.bc %s
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so -plugin-opt=save-temps \
 ; RUN:    -plugin-opt=O0 -r -o %t.o %t.bc
-; RUN: llvm-dis < %t.o.opt.bc -o - | FileCheck --check-prefix=CHECK-O0 %s
+; RUN: llvm-dis < %t.o.4.opt.bc -o - | FileCheck --check-prefix=CHECK-O0 %s
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so -plugin-opt=save-temps \
 ; RUN:    -plugin-opt=O1 -r -o %t.o %t.bc
-; RUN: llvm-dis < %t.o.opt.bc -o - | FileCheck --check-prefix=CHECK-O1 %s
+; RUN: llvm-dis < %t.o.4.opt.bc -o - | FileCheck --check-prefix=CHECK-O1 %s
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so -plugin-opt=save-temps \
 ; RUN:    -plugin-opt=O2 -r -o %t.o %t.bc
-; RUN: llvm-dis < %t.o.opt.bc -o - | FileCheck --check-prefix=CHECK-O2 %s
+; RUN: llvm-dis < %t.o.4.opt.bc -o - | FileCheck --check-prefix=CHECK-O2 %s
 
 ; CHECK-O0: define internal void @foo(
 ; CHECK-O1: define internal void @foo(

diff --git a/llvm/test/tools/gold/X86/parallel.ll b/llvm/test/tools/gold/X86/parallel.ll
@@ -1,7 +1,8 @@
 ; RUN: llvm-as -o %t.bc %s
+; RUN: rm -f %t.opt.bc0 %t.opt.bc1 %t.o0 %t.o1
 ; RUN: env LD_PRELOAD=%llvmshlibdir/LLVMgold.so %gold -plugin %llvmshlibdir/LLVMgold.so -u foo -u bar -plugin-opt jobs=2 -plugin-opt save-temps -m elf_x86_64 -o %t %t.bc
-; RUN: llvm-dis %t.opt.bc0 -o - | FileCheck --check-prefix=CHECK-BC0 %s
-; RUN: llvm-dis %t.opt.bc1 -o - | FileCheck --check-prefix=CHECK-BC1 %s
+; RUN: llvm-dis %t.5.precodegen.bc -o - | FileCheck --check-prefix=CHECK-BC0 %s
+; RUN: llvm-dis %t.1.5.precodegen.bc -o - | FileCheck --check-prefix=CHECK-BC1 %s
 ; RUN: llvm-nm %t.o0 | FileCheck --check-prefix=CHECK0 %s
 ; RUN: llvm-nm %t.o1 | FileCheck --check-prefix=CHECK1 %s
 

diff --git a/llvm/test/tools/gold/X86/slp-vectorize.ll b/llvm/test/tools/gold/X86/slp-vectorize.ll
@@ -3,7 +3,7 @@
 ; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold.so \
 ; RUN:    --plugin-opt=save-temps \
 ; RUN:    -shared %t.o -o %t2.o
-; RUN: llvm-dis %t2.o.opt.bc -o - | FileCheck %s
+; RUN: llvm-dis %t2.o.4.opt.bc -o - | FileCheck %s
 
 ; test that the vectorizer is run.
 ; CHECK: fadd <4 x float>

diff --git a/llvm/test/tools/gold/X86/start-lib-common.ll b/llvm/test/tools/gold/X86/start-lib-common.ll
@@ -19,4 +19,4 @@
 
 ; Check that the common symbol is not dropped completely, which was a regression
 ; in r262676.
-; CHECK: @x = common global i32 0
+; CHECK: @x = common global [4 x i8] zeroinitializer
diff --git a/llvm/test/tools/gold/X86/strip_names.ll b/llvm/test/tools/gold/X86/strip_names.ll
@@ -3,7 +3,7 @@
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
 ; RUN:    --plugin-opt=save-temps \
 ; RUN:    -shared %t.o -o %t2.o
-; RUN: llvm-dis %t2.o.bc -o - | FileCheck %s
+; RUN: llvm-dis %t2.o.2.internalize.bc -o - | FileCheck %s
 
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
 ; RUN:    --plugin-opt=emit-llvm \

diff --git a/llvm/test/tools/gold/X86/thinlto.ll b/llvm/test/tools/gold/X86/thinlto.ll
@@ -25,21 +25,23 @@
 ; RUN: llvm-bcanalyzer -dump %t2.o.thinlto.bc | FileCheck %s --check-prefix=BACKEND2
 ; RUN: not test -e %t3
 
-; Ensure gold generates an index as well as a binary by default in ThinLTO mode.
+; Ensure gold generates an index as well as a binary with save-temps in ThinLTO mode.
 ; First force single-threaded mode
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
+; RUN:    --plugin-opt=save-temps \
 ; RUN:    --plugin-opt=thinlto \
 ; RUN:    --plugin-opt=jobs=1 \
 ; RUN:    -shared %t.o %t2.o -o %t4
-; RUN: llvm-bcanalyzer -dump %t4.thinlto.bc | FileCheck %s --check-prefix=COMBINED
+; RUN: llvm-bcanalyzer -dump %t4.index.bc | FileCheck %s --check-prefix=COMBINED
 ; RUN: llvm-nm %t4 | FileCheck %s --check-prefix=NM
 
 ; Next force multi-threaded mode
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
+; RUN:    --plugin-opt=save-temps \
 ; RUN:    --plugin-opt=thinlto \
 ; RUN:    --plugin-opt=jobs=2 \
 ; RUN:    -shared %t.o %t2.o -o %t4
-; RUN: llvm-bcanalyzer -dump %t4.thinlto.bc | FileCheck %s --check-prefix=COMBINED
+; RUN: llvm-bcanalyzer -dump %t4.index.bc | FileCheck %s --check-prefix=COMBINED
 ; RUN: llvm-nm %t4 | FileCheck %s --check-prefix=NM
 
 ; Test --plugin-opt=obj-path to ensure unique object files generated.
@@ -48,8 +50,8 @@
 ; RUN:    --plugin-opt=jobs=2 \
 ; RUN:    --plugin-opt=obj-path=%t5.o \
 ; RUN:    -shared %t.o %t2.o -o %t4
-; RUN: llvm-nm %t5.o0 | FileCheck %s --check-prefix=NM2
 ; RUN: llvm-nm %t5.o1 | FileCheck %s --check-prefix=NM2
+; RUN: llvm-nm %t5.o2 | FileCheck %s --check-prefix=NM2
 
 ; NM: T f
 ; NM2: T {{f|g}}

diff --git a/llvm/test/tools/gold/X86/thinlto_alias.ll b/llvm/test/tools/gold/X86/thinlto_alias.ll
@@ -14,8 +14,14 @@
 ; RUN:     --plugin-opt=save-temps \
 ; RUN:     -o %t3.o %t2.o %t.o
 ; RUN: llvm-nm %t3.o | FileCheck %s
-; RUN: llvm-dis %t.o.opt.bc -o - | FileCheck --check-prefix=OPT %s
-; RUN: llvm-dis %t2.o.opt.bc -o - | FileCheck --check-prefix=OPT2 %s
+; RUN: llvm-dis %t.o.4.opt.bc -o - | FileCheck --check-prefix=OPT %s
+; RUN: llvm-dis %t2.o.4.opt.bc -o - | FileCheck --check-prefix=OPT2 %s
+
+; This does not currently pass because the gold plugin now uses the
+; combined summary rather than the IRMover to change the module's linkage
+; during the ThinLTO backend. The internalization step implemented by IRMover
+; for preempted symbols has not yet been implemented for the combined summary.
+; XFAIL: *
 
 ; CHECK-NOT: U f
 ; OPT: define hidden void @weakfunc.llvm.0()

diff --git a/llvm/test/tools/gold/X86/thinlto_internalize.ll b/llvm/test/tools/gold/X86/thinlto_internalize.ll
@@ -6,7 +6,7 @@
 ; RUN:     --plugin-opt=-import-instr-limit=0 \
 ; RUN:     --plugin-opt=save-temps \
 ; RUN:     -o %t3.o %t2.o %t.o
-; RUN: llvm-dis %t.o.opt.bc -o - | FileCheck %s
+; RUN: llvm-dis %t.o.4.opt.bc -o - | FileCheck %s
 
 ; f() should be internalized and eliminated after inlining
 ; CHECK-NOT: @f()

diff --git a/llvm/test/tools/gold/X86/thinlto_linkonceresolution.ll b/llvm/test/tools/gold/X86/thinlto_linkonceresolution.ll
@@ -14,13 +14,13 @@
 ; RUN:     -shared \
 ; RUN:     -o %t3.o %t2.o %t.o
 ; RUN: llvm-nm %t3.o | FileCheck %s
-; RUN: llvm-dis %t.o.opt.bc -o - | FileCheck --check-prefix=OPT %s
-; RUN: llvm-dis %t2.o.opt.bc -o - | FileCheck --check-prefix=OPT2 %s
+; RUN: llvm-dis %t.o.4.opt.bc -o - | FileCheck --check-prefix=OPT %s
+; RUN: llvm-dis %t2.o.4.opt.bc -o - | FileCheck --check-prefix=OPT2 %s
 
 ; Ensure that f() is defined in resulting object file, and also
 ; confirm the weak linkage directly in the saved opt bitcode files.
 ; CHECK-NOT: U f
-; OPT: declare hidden void @f()
+; OPT-NOT: @f()
 ; OPT2: define weak_odr hidden void @f()
 
 target triple = "x86_64-unknown-linux-gnu"

diff --git a/llvm/test/tools/gold/X86/thinlto_weak_resolution.ll b/llvm/test/tools/gold/X86/thinlto_weak_resolution.ll
@@ -13,12 +13,17 @@
 ; RUN: llvm-nm %t3.o | FileCheck %s
 ; CHECK: weakfunc
 
-; All of the preempted functions should have been eliminated (the plugin will
-; not link them in).
-; RUN: llvm-dis %t2.o.opt.bc -o - | FileCheck --check-prefix=OPT2 %s
+; Most of the preempted functions should have been eliminated (the plugin will
+; set linkage of odr functions to available_externally and linkonce functions
+; are removed by globaldce). FIXME: Need to introduce combined index linkage
+; that means "drop this function" so we can avoid importing linkonce functions
+; and drop weak functions.
+; RUN: llvm-dis %t2.o.4.opt.bc -o - | FileCheck --check-prefix=OPT2 %s
+; OPT2-NOT: @
+; OPT2: @weakfunc
 ; OPT2-NOT: @
 
-; RUN: llvm-dis %t.o.opt.bc -o - | FileCheck --check-prefix=OPT %s
+; RUN: llvm-dis %t.o.4.opt.bc -o - | FileCheck --check-prefix=OPT %s
 
 target triple = "x86_64-unknown-linux-gnu"
 

diff --git a/llvm/test/tools/gold/X86/type-merge2.ll b/llvm/test/tools/gold/X86/type-merge2.ll
@@ -3,7 +3,7 @@
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
 ; RUN:    --plugin-opt=save-temps \
 ; RUN:    -shared %t.o %t2.o -o %t3.o
-; RUN: llvm-dis %t3.o.bc -o - | FileCheck %s
+; RUN: llvm-dis %t3.o.2.internalize.bc -o - | FileCheck %s
 
 %zed = type { i8 }
 define void @foo()  {

diff --git a/llvm/test/tools/gold/X86/vectorize.ll b/llvm/test/tools/gold/X86/vectorize.ll
@@ -3,7 +3,7 @@
 ; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold.so \
 ; RUN:    --plugin-opt=save-temps \
 ; RUN:    -shared %t.o -o %t2.o
-; RUN: llvm-dis %t2.o.opt.bc -o - | FileCheck %s
+; RUN: llvm-dis %t2.o.4.opt.bc -o - | FileCheck %s
 
 ; test that the vectorizer is run.
 ; CHECK: fadd <4 x float>

diff --git a/llvm/test/tools/gold/X86/visibility.ll b/llvm/test/tools/gold/X86/visibility.ll
@@ -5,7 +5,7 @@
 ; RUN:    --plugin-opt=save-temps \
 ; RUN:    -shared %t.o %t2.o -o %t.so
 ; RUN: llvm-readobj -t %t.so | FileCheck %s
-; RUN: llvm-dis %t.so.bc -o - | FileCheck --check-prefix=IR %s
+; RUN: llvm-dis %t.so.2.internalize.bc -o - | FileCheck --check-prefix=IR %s
 
 ; CHECK:      Name: foo
 ; CHECK-NEXT: Value:
@@ -16,7 +16,7 @@
 ; CHECK-NEXT:   STV_PROTECTED
 ; CHECK-NEXT: ]
 
-; IR: define protected void @foo
+; IR: define void @foo
 
 define weak protected void @foo() {
   ret void

diff --git a/llvm/test/tools/llvm-lto2/errors.ll b/llvm/test/tools/llvm-lto2/errors.ll
@@ -0,0 +1,11 @@
+; RUN: llvm-as %s -o %t.bc
+; RUN: not llvm-lto2 -o %t2.o %t.bc 2>&1 | FileCheck --check-prefix=ERR1 %s
+; RUN: not llvm-lto2 -o %t2.o -r %t.bc,foo,p -r %t.bc,bar,p %t.bc 2>&1 | FileCheck --check-prefix=ERR2 %s
+; RUN: not llvm-lto2 -o %t2.o -r %t.bc,foo,q %t.bc 2>&1 | FileCheck --check-prefix=ERR3 %s
+; RUN: not llvm-lto2 -o %t2.o -r foo %t.bc 2>&1 | FileCheck --check-prefix=ERR4 %s
+
+; ERR1: missing symbol resolution for {{.*}}.bc,foo
+; ERR2: unused symbol resolution for {{.*}}.bc,bar
+; ERR3: invalid character q in resolution: {{.*}}.bc,foo
+; ERR4: invalid resolution: foo
+@foo = global i32 0
diff --git a/llvm/tools/gold/gold-plugin.cpp b/llvm/tools/gold/gold-plugin.cpp
diff --git a/llvm/tools/llvm-lto2/CMakeLists.txt b/llvm/tools/llvm-lto2/CMakeLists.txt
@@ -0,0 +1,10 @@
+set(LLVM_LINK_COMPONENTS
+  ${LLVM_TARGETS_TO_BUILD}
+  LTO
+  Object
+  Support
+  )
+
+add_llvm_tool(llvm-lto2
+  llvm-lto2.cpp
+  )
diff --git a/llvm/tools/llvm-lto2/LLVMBuild.txt b/llvm/tools/llvm-lto2/LLVMBuild.txt
@@ -0,0 +1,22 @@
+;===- ./tools/llvm-lto2/LLVMBuild.txt --------------------------*- Conf -*--===;
+;
+;                     The LLVM Compiler Infrastructure
+;
+; This file is distributed under the University of Illinois Open Source
+; License. See LICENSE.TXT for details.
+;
+;===------------------------------------------------------------------------===;
+;
+; This is an LLVMBuild description file for the components in this subdirectory.
+;
+; For more information on the LLVMBuild system, please see:
+;
+;   http://llvm.org/docs/LLVMBuild.html
+;
+;===------------------------------------------------------------------------===;
+
+[component_0]
+type = Tool
+name = llvm-lto2
+parent = Tools
+required_libraries = LTO Object all-targets
diff --git a/llvm/tools/llvm-lto2/llvm-lto2.cpp b/llvm/tools/llvm-lto2/llvm-lto2.cpp
@@ -0,0 +1,168 @@
+//===-- llvm-lto2: test harness for the resolution-based LTO interface ----===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This program takes in a list of bitcode files, links them and performs
+// link-time optimization according to the provided symbol resolutions using the
+// resolution-based LTO interface, and outputs one or more object files.
+//
+// This program is intended to eventually replace llvm-lto which uses the legacy
+// LTO interface.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/LTO/LTO.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/TargetSelect.h"
+
+using namespace llvm;
+using namespace lto;
+using namespace object;
+
+static cl::list<std::string> InputFilenames(cl::Positional, cl::OneOrMore,
+                                            cl::desc("<input bitcode files>"));
+
+static cl::opt<std::string> OutputFilename("o", cl::Required,
+                                           cl::desc("Output filename"),
+                                           cl::value_desc("filename"));
+
+static cl::opt<bool> SaveTemps("save-temps", cl::desc("Save temporary files"));
+
+static cl::list<std::string> SymbolResolutions(
+    "r",
+    cl::desc("Specify a symbol resolution: filename,symbolname,resolution\n"
+             "where \"resolution\" is a sequence (which may be empty) of the\n"
+             "following characters:\n"
+             " p - prevailing: the linker has chosen this definition of the\n"
+             "     symbol\n"
+             " l - local: the definition of this symbol is unpreemptable at\n"
+             "     runtime and is known to be in this linkage unit\n"
+             " x - externally visible: the definition of this symbol is\n"
+             "     visible outside of the LTO unit\n"
+             "A resolution for each symbol must be specified."),
+    cl::ZeroOrMore);
+
+static void check(Error E, std::string Msg) {
+  if (!E)
+    return;
+  handleAllErrors(std::move(E), [&](ErrorInfoBase &EIB) {
+    errs() << "llvm-lto: " << Msg << ": " << EIB.message().c_str() << '\n';
+  });
+  exit(1);
+}
+
+template <typename T> static T check(Expected<T> E, std::string Msg) {
+  if (E)
+    return std::move(*E);
+  check(E.takeError(), Msg);
+  return T();
+}
+
+static void check(std::error_code EC, std::string Msg) {
+  check(errorCodeToError(EC), Msg);
+}
+
+template <typename T> static T check(ErrorOr<T> E, std::string Msg) {
+  if (E)
+    return std::move(*E);
+  check(E.getError(), Msg);
+  return T();
+}
+
+int main(int argc, char **argv) {
+  InitializeAllTargets();
+  InitializeAllTargetMCs();
+  InitializeAllAsmPrinters();
+  InitializeAllAsmParsers();
+
+  cl::ParseCommandLineOptions(argc, argv, "Resolution-based LTO test harness");
+
+  std::map<std::pair<std::string, std::string>, SymbolResolution>
+      CommandLineResolutions;
+  for (std::string R : SymbolResolutions) {
+    StringRef Rest = R;
+    StringRef FileName, SymbolName;
+    std::tie(FileName, Rest) = Rest.split(',');
+    if (Rest.empty()) {
+      llvm::errs() << "invalid resolution: " << R << '\n';
+      return 1;
+    }
+    std::tie(SymbolName, Rest) = Rest.split(',');
+    SymbolResolution Res;
+    for (char C : Rest) {
+      if (C == 'p')
+        Res.Prevailing = true;
+      else if (C == 'l')
+        Res.FinalDefinitionInLinkageUnit = true;
+      else if (C == 'x')
+        Res.VisibleToRegularObj = true;
+      else
+        llvm::errs() << "invalid character " << C << " in resolution: " << R
+                     << '\n';
+    }
+    CommandLineResolutions[{FileName, SymbolName}] = Res;
+  }
+
+  std::vector<std::unique_ptr<MemoryBuffer>> MBs;
+
+  Config Conf;
+  Conf.DiagHandler = [](const DiagnosticInfo &) {
+    exit(1);
+  };
+
+  if (SaveTemps)
+    check(Conf.addSaveTemps(OutputFilename), "Config::addSaveTemps failed");
+
+  LTO Lto(std::move(Conf));
+
+  bool HasErrors = false;
+  for (std::string F : InputFilenames) {
+    std::unique_ptr<MemoryBuffer> MB = check(MemoryBuffer::getFile(F), F);
+    std::unique_ptr<InputFile> Input =
+        check(InputFile::create(MB->getMemBufferRef()), F);
+
+    std::vector<SymbolResolution> Res;
+    for (const InputFile::Symbol &Sym : Input->symbols()) {
+      auto I = CommandLineResolutions.find({F, Sym.getName()});
+      if (I == CommandLineResolutions.end()) {
+        llvm::errs() << argv[0] << ": missing symbol resolution for " << F
+                     << ',' << Sym.getName() << '\n';
+        HasErrors = true;
+      } else {
+        Res.push_back(I->second);
+        CommandLineResolutions.erase(I);
+      }
+    }
+
+    if (HasErrors)
+      continue;
+
+    MBs.push_back(std::move(MB));
+    check(Lto.add(std::move(Input), Res), F);
+  }
+
+  if (!CommandLineResolutions.empty()) {
+    HasErrors = true;
+    for (auto UnusedRes : CommandLineResolutions)
+      llvm::errs() << argv[0] << ": unused symbol resolution for "
+                   << UnusedRes.first.first << ',' << UnusedRes.first.second
+                   << '\n';
+  }
+  if (HasErrors)
+    return 1;
+
+  auto AddStream = [&](size_t Task) {
+    std::string Path = OutputFilename + "." + utostr(Task);
+    std::error_code EC;
+    auto S = make_unique<raw_fd_ostream>(Path, EC, sys::fs::F_None);
+    check(EC, Path);
+    return S;
+  };
+
+  check(Lto.run(AddStream), "LTO::run failed");
+}
-Original file line number
+Diff line change
@@ Expand Up / @@ -34,4 +34,4 @@ required_libraries = @@
      Scalar
      Support
      Target
-     TransformUtils
+     TransformUtils
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		if not 'X86' in config.root.targets:
		config.unsupported = True