Revert "Resolution-based LTO API."

This reverts commit r278330.

I made a change to the save temps output that is causing issues with the
bots. Didn't realize this because I had older output files sitting on
disk in my test output directory.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278331 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 944 insertions, 227 deletions

diff --git a/tools/gold/gold-plugin.cpp b/tools/gold/gold-plugin.cpp
index eb7f7aa8d05b..c8b02e6fa14b 100644
--- a/tools/gold/gold-plugin.cpp
+++ b/tools/gold/gold-plugin.cpp
@@ -12,22 +12,44 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "llvm/ADT/StringSet.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
 #include "llvm/Bitcode/ReaderWriter.h"
+#include "llvm/CodeGen/Analysis.h"
 #include "llvm/CodeGen/CommandFlags.h"
+#include "llvm/CodeGen/ParallelCG.h"
 #include "llvm/Config/config.h" // plugin-api.h requires HAVE_STDINT_H
+#include "llvm/IR/AutoUpgrade.h"
 #include "llvm/IR/Constants.h"
+#include "llvm/IR/DiagnosticInfo.h"
 #include "llvm/IR/DiagnosticPrinter.h"
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/LegacyPassManager.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/Verifier.h"
 #include "llvm/LTO/LTO.h"
-#include "llvm/Support/CommandLine.h"
+#include "llvm/Linker/IRMover.h"
+#include "llvm/MC/SubtargetFeature.h"
+#include "llvm/Object/IRObjectFile.h"
+#include "llvm/Object/ModuleSummaryIndexObjectFile.h"
+#include "llvm/Support/Host.h"
 #include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/TargetRegistry.h"
 #include "llvm/Support/TargetSelect.h"
+#include "llvm/Support/ThreadPool.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/Support/thread.h"
+#include "llvm/Transforms/IPO.h"
+#include "llvm/Transforms/IPO/FunctionImport.h"
+#include "llvm/Transforms/IPO/PassManagerBuilder.h"
+#include "llvm/Transforms/Utils/FunctionImportUtils.h"
+#include "llvm/Transforms/Utils/GlobalStatus.h"
+#include "llvm/Transforms/Utils/ValueMapper.h"
 #include <list>
-#include <map>
 #include <plugin-api.h>
-#include <string>
 #include <system_error>
 #include <utility>
 #include <vector>
@@ -39,7 +61,6 @@
 #define LDPT_GET_SYMBOLS_V3 28
 
 using namespace llvm;
-using namespace lto;
 
 static ld_plugin_status discard_message(int level, const char *format, ...) {
   // Die loudly. Recent versions of Gold pass ld_plugin_message as the first
@@ -85,17 +106,38 @@ struct PluginInputFile {
 };
 
 struct ResolutionInfo {
-  bool CanOmitFromDynSym = true;
-  bool DefaultVisibility = true;
+  uint64_t CommonSize = 0;
+  unsigned CommonAlign = 0;
+  bool IsLinkonceOdr = true;
+  GlobalValue::UnnamedAddr UnnamedAddr = GlobalValue::UnnamedAddr::Global;
+  GlobalValue::VisibilityTypes Visibility = GlobalValue::DefaultVisibility;
+  bool CommonInternal = false;
+  bool UseCommon = false;
 };
 
-struct CommonResolution {
-  bool Prevailing = false;
-  bool VisibleToRegularObj = false;
-  uint64_t Size = 0;
-  unsigned Align = 0;
-};
+/// Class to own information used by a task or during its cleanup for a
+/// ThinLTO backend instantiation.
+class ThinLTOTaskInfo {
+  /// The output stream the task will codegen into.
+  std::unique_ptr<raw_fd_ostream> OS;
+
+  /// The file name corresponding to the output stream, used during cleanup.
+  std::string Filename;
+
+  /// Flag indicating whether the output file is a temp file that must be
+  /// added to the cleanup list during cleanup.
+  bool TempOutFile;
+
+public:
+  ThinLTOTaskInfo(std::unique_ptr<raw_fd_ostream> OS, std::string Filename,
+                  bool TempOutFile)
+      : OS(std::move(OS)), Filename(std::move(Filename)),
+        TempOutFile(TempOutFile) {}
 
+  /// Performs task related cleanup activities that must be done
+  /// single-threaded (i.e. call backs to gold).
+  void cleanup();
+};
 }
 
 static ld_plugin_add_symbols add_symbols = nullptr;
@@ -103,16 +145,14 @@ static ld_plugin_get_symbols get_symbols = nullptr;
 static ld_plugin_add_input_file add_input_file = nullptr;
 static ld_plugin_set_extra_library_path set_extra_library_path = nullptr;
 static ld_plugin_get_view get_view = nullptr;
-static bool IsExecutable = false;
 static Optional<Reloc::Model> RelocationModel;
 static std::string output_name = "";
 static std::list<claimed_file> Modules;
 static DenseMap<int, void *> FDToLeaderHandle;
 static StringMap<ResolutionInfo> ResInfo;
-static std::map<std::string, CommonResolution> Commons;
 static std::vector<std::string> Cleanup;
 static llvm::TargetOptions TargetOpts;
-static size_t MaxTasks;
+static std::string DefaultTriple = sys::getDefaultTargetTriple();
 
 namespace options {
   enum OutputType {
@@ -121,6 +161,7 @@ namespace options {
     OT_BC_ONLY,
     OT_SAVE_TEMPS
   };
+  static bool generate_api_file = false;
   static OutputType TheOutputType = OT_NORMAL;
   static unsigned OptLevel = 2;
   // Default parallelism of 0 used to indicate that user did not specify.
@@ -172,6 +213,8 @@ namespace options {
   // Additional options to pass into the code generator.
   // Note: This array will contain all plugin options which are not claimed
   // as plugin exclusive to pass to the code generator.
+  // For example, "generate-api-file" and "as"options are for the plugin
+  // use only and will not be passed.
   static std::vector<const char *> extra;
 
   static void process_plugin_option(const char *opt_)
@@ -180,7 +223,9 @@ namespace options {
       return;
     llvm::StringRef opt = opt_;
 
-    if (opt.startswith("mcpu=")) {
+    if (opt == "generate-api-file") {
+      generate_api_file = true;
+    } else if (opt.startswith("mcpu=")) {
       mcpu = opt.substr(strlen("mcpu="));
     } else if (opt.startswith("extra-library-path=")) {
       extra_library_path = opt.substr(strlen("extra_library_path="));
@@ -262,15 +307,10 @@ ld_plugin_status onload(ld_plugin_tv *tv) {
       switch (tv->tv_u.tv_val) {
       case LDPO_REL: // .o
       case LDPO_DYN: // .so
-        IsExecutable = false;
-        RelocationModel = Reloc::PIC_;
-        break;
       case LDPO_PIE: // position independent executable
-        IsExecutable = true;
         RelocationModel = Reloc::PIC_;
         break;
       case LDPO_EXEC: // .exe
-        IsExecutable = true;
         RelocationModel = Reloc::Static;
         break;
       default:
@@ -364,6 +404,20 @@ ld_plugin_status onload(ld_plugin_tv *tv) {
   return LDPS_OK;
 }
 
+static const GlobalObject *getBaseObject(const GlobalValue &GV) {
+  if (auto *GA = dyn_cast<GlobalAlias>(&GV))
+    return GA->getBaseObject();
+  return cast<GlobalObject>(&GV);
+}
+
+static bool shouldSkip(uint32_t Symflags) {
+  if (!(Symflags & object::BasicSymbolRef::SF_Global))
+    return true;
+  if (Symflags & object::BasicSymbolRef::SF_FormatSpecific)
+    return true;
+  return false;
+}
+
 static void diagnosticHandler(const DiagnosticInfo &DI) {
   if (const auto *BDI = dyn_cast<BitcodeDiagnosticInfo>(&DI)) {
     std::error_code EC = BDI->getError();
@@ -393,18 +447,21 @@ static void diagnosticHandler(const DiagnosticInfo &DI) {
   message(Level, "LLVM gold plugin: %s",  ErrStorage.c_str());
 }
 
-static void check(Error E, std::string Msg = "LLVM gold plugin") {
-  handleAllErrors(std::move(E), [&](ErrorInfoBase &EIB) {
-    message(LDPL_FATAL, "%s: %s", Msg.c_str(), EIB.message().c_str());
-    return Error::success();
-  });
+static void diagnosticHandlerForContext(const DiagnosticInfo &DI,
+                                        void *Context) {
+  diagnosticHandler(DI);
 }
 
-template <typename T> static T check(Expected<T> E) {
-  if (E)
-    return std::move(*E);
-  check(E.takeError());
-  return T();
+static GlobalValue::VisibilityTypes
+getMinVisibility(GlobalValue::VisibilityTypes A,
+                 GlobalValue::VisibilityTypes B) {
+  if (A == GlobalValue::HiddenVisibility)
+    return A;
+  if (B == GlobalValue::HiddenVisibility)
+    return B;
+  if (A == GlobalValue::ProtectedVisibility)
+    return A;
+  return B;
 }
 
 /// Called by gold to see whether this file is one that our plugin can handle.
@@ -412,6 +469,7 @@ template <typename T> static T check(Expected<T> E) {
 /// possible.
 static ld_plugin_status claim_file_hook(const ld_plugin_input_file *file,
                                         int *claimed) {
+  LLVMContext Context;
   MemoryBufferRef BufferRef;
   std::unique_ptr<MemoryBuffer> Buffer;
   if (get_view) {
@@ -440,25 +498,22 @@ static ld_plugin_status claim_file_hook(const ld_plugin_input_file *file,
     BufferRef = Buffer->getMemBufferRef();
   }
 
-  *claimed = 1;
+  Context.setDiagnosticHandler(diagnosticHandlerForContext);
+  ErrorOr<std::unique_ptr<object::IRObjectFile>> ObjOrErr =
+      object::IRObjectFile::create(BufferRef, Context);
+  std::error_code EC = ObjOrErr.getError();
+  if (EC == object::object_error::invalid_file_type ||
+      EC == object::object_error::bitcode_section_not_found)
+    return LDPS_OK;
 
-  Expected<std::unique_ptr<InputFile>> ObjOrErr = InputFile::create(BufferRef);
-  if (!ObjOrErr) {
-    handleAllErrors(ObjOrErr.takeError(), [&](const ErrorInfoBase &EI) {
-      std::error_code EC = EI.convertToErrorCode();
-      if (EC == object::object_error::invalid_file_type ||
-          EC == object::object_error::bitcode_section_not_found)
-        *claimed = 0;
-      else
-        message(LDPL_ERROR,
-                "LLVM gold plugin has failed to create LTO module: %s",
-                EI.message().c_str());
-    });
+  *claimed = 1;
 
-    return *claimed ? LDPS_ERR : LDPS_OK;
+  if (EC) {
+    message(LDPL_ERROR, "LLVM gold plugin has failed to create LTO module: %s",
+            EC.message().c_str());
+    return LDPS_ERR;
   }
-
-  std::unique_ptr<InputFile> Obj = std::move(*ObjOrErr);
+  std::unique_ptr<object::IRObjectFile> Obj = std::move(*ObjOrErr);
 
   Modules.resize(Modules.size() + 1);
   claimed_file &cf = Modules.back();
@@ -480,52 +535,72 @@ static ld_plugin_status claim_file_hook(const ld_plugin_input_file *file,
   cf.name = file->name;
   if (file->offset)
     cf.name += ".llvm." + std::to_string(file->offset) + "." +
-               sys::path::filename(Obj->getSourceFileName()).str();
+               sys::path::filename(Obj->getModule().getSourceFileName()).str();
 
   for (auto &Sym : Obj->symbols()) {
     uint32_t Symflags = Sym.getFlags();
+    if (shouldSkip(Symflags))
+      continue;
 
     cf.syms.push_back(ld_plugin_symbol());
     ld_plugin_symbol &sym = cf.syms.back();
     sym.version = nullptr;
-    StringRef Name = Sym.getName();
-    sym.name = strdup(Name.str().c_str());
 
-    ResolutionInfo &Res = ResInfo[Name];
+    SmallString<64> Name;
+    {
+      raw_svector_ostream OS(Name);
+      Sym.printName(OS);
+    }
+    sym.name = strdup(Name.c_str());
+
+    const GlobalValue *GV = Obj->getSymbolGV(Sym.getRawDataRefImpl());
 
-    Res.CanOmitFromDynSym &= Sym.canBeOmittedFromSymbolTable();
+    ResolutionInfo &Res = ResInfo[sym.name];
 
     sym.visibility = LDPV_DEFAULT;
-    GlobalValue::VisibilityTypes Vis = Sym.getVisibility();
-    if (Vis != GlobalValue::DefaultVisibility)
-      Res.DefaultVisibility = false;
-    switch (Vis) {
-    case GlobalValue::DefaultVisibility:
-      break;
-    case GlobalValue::HiddenVisibility:
-      sym.visibility = LDPV_HIDDEN;
-      break;
-    case GlobalValue::ProtectedVisibility:
-      sym.visibility = LDPV_PROTECTED;
-      break;
+    if (GV) {
+      Res.UnnamedAddr =
+          GlobalValue::getMinUnnamedAddr(Res.UnnamedAddr, GV->getUnnamedAddr());
+      Res.IsLinkonceOdr &= GV->hasLinkOnceLinkage();
+      Res.Visibility = getMinVisibility(Res.Visibility, GV->getVisibility());
+      switch (GV->getVisibility()) {
+      case GlobalValue::DefaultVisibility:
+        break;
+      case GlobalValue::HiddenVisibility:
+        sym.visibility = LDPV_HIDDEN;
+        break;
+      case GlobalValue::ProtectedVisibility:
+        sym.visibility = LDPV_PROTECTED;
+        break;
+      }
     }
 
     if (Symflags & object::BasicSymbolRef::SF_Undefined) {
       sym.def = LDPK_UNDEF;
-      if (Symflags & object::BasicSymbolRef::SF_Weak)
+      if (GV && GV->hasExternalWeakLinkage())
         sym.def = LDPK_WEAKUNDEF;
-    } else if (Symflags & object::BasicSymbolRef::SF_Common)
-      sym.def = LDPK_COMMON;
-    else if (Symflags & object::BasicSymbolRef::SF_Weak)
-      sym.def = LDPK_WEAKDEF;
-    else
+    } else {
       sym.def = LDPK_DEF;
+      if (GV) {
+        assert(!GV->hasExternalWeakLinkage() &&
+               !GV->hasAvailableExternallyLinkage() && "Not a declaration!");
+        if (GV->hasCommonLinkage())
+          sym.def = LDPK_COMMON;
+        else if (GV->isWeakForLinker())
+          sym.def = LDPK_WEAKDEF;
+      }
+    }
 
     sym.size = 0;
     sym.comdat_key = nullptr;
-    const Comdat *C = check(Sym.getComdat());
-    if (C)
-      sym.comdat_key = strdup(C->getName().str().c_str());
+    if (GV) {
+      const GlobalObject *Base = getBaseObject(*GV);
+      if (!Base)
+        message(LDPL_FATAL, "Unable to determine comdat of alias!");
+      const Comdat *C = Base->getComdat();
+      if (C)
+        sym.comdat_key = strdup(C->getName().str().c_str());
+    }
 
     sym.resolution = LDPR_UNKNOWN;
   }
@@ -540,6 +615,39 @@ static ld_plugin_status claim_file_hook(const ld_plugin_input_file *file,
   return LDPS_OK;
 }
 
+static void internalize(GlobalValue &GV) {
+  if (GV.isDeclarationForLinker())
+    return; // We get here if there is a matching asm definition.
+  if (!GV.hasLocalLinkage())
+    GV.setLinkage(GlobalValue::InternalLinkage);
+}
+
+static const char *getResolutionName(ld_plugin_symbol_resolution R) {
+  switch (R) {
+  case LDPR_UNKNOWN:
+    return "UNKNOWN";
+  case LDPR_UNDEF:
+    return "UNDEF";
+  case LDPR_PREVAILING_DEF:
+    return "PREVAILING_DEF";
+  case LDPR_PREVAILING_DEF_IRONLY:
+    return "PREVAILING_DEF_IRONLY";
+  case LDPR_PREEMPTED_REG:
+    return "PREEMPTED_REG";
+  case LDPR_PREEMPTED_IR:
+    return "PREEMPTED_IR";
+  case LDPR_RESOLVED_IR:
+    return "RESOLVED_IR";
+  case LDPR_RESOLVED_EXEC:
+    return "RESOLVED_EXEC";
+  case LDPR_RESOLVED_DYN:
+    return "RESOLVED_DYN";
+  case LDPR_PREVAILING_DEF_IRONLY_EXP:
+    return "PREVAILING_DEF_IRONLY_EXP";
+  }
+  llvm_unreachable("Unknown resolution");
+}
+
 static void freeSymName(ld_plugin_symbol &Sym) {
   free(Sym.name);
   free(Sym.comdat_key);
@@ -563,27 +671,120 @@ static const void *getSymbolsAndView(claimed_file &F) {
   return View;
 }
 
-static void addModule(LTO &Lto, claimed_file &F, const void *View) {
+static std::unique_ptr<ModuleSummaryIndex>
+getModuleSummaryIndexForFile(claimed_file &F) {
+  const void *View = getSymbolsAndView(F);
+  if (!View)
+    return nullptr;
+
   MemoryBufferRef BufferRef(StringRef((const char *)View, F.filesize), F.name);
-  Expected<std::unique_ptr<InputFile>> ObjOrErr = InputFile::create(BufferRef);
 
-  if (!ObjOrErr)
+  // Don't bother trying to build an index if there is no summary information
+  // in this bitcode file.
+  if (!object::ModuleSummaryIndexObjectFile::hasGlobalValueSummaryInMemBuffer(
+          BufferRef, diagnosticHandler))
+    return std::unique_ptr<ModuleSummaryIndex>(nullptr);
+
+  ErrorOr<std::unique_ptr<object::ModuleSummaryIndexObjectFile>> ObjOrErr =
+      object::ModuleSummaryIndexObjectFile::create(BufferRef,
+                                                   diagnosticHandler);
+
+  if (std::error_code EC = ObjOrErr.getError())
+    message(LDPL_FATAL,
+            "Could not read module summary index bitcode from file : %s",
+            EC.message().c_str());
+
+  object::ModuleSummaryIndexObjectFile &Obj = **ObjOrErr;
+
+  return Obj.takeIndex();
+}
+
+static std::unique_ptr<Module>
+getModuleForFile(LLVMContext &Context, claimed_file &F, const void *View,
+                 StringRef Name, raw_fd_ostream *ApiFile,
+                 StringSet<> &Internalize, std::vector<GlobalValue *> &Keep,
+                 StringMap<unsigned> &Realign) {
+  MemoryBufferRef BufferRef(StringRef((const char *)View, F.filesize), Name);
+  ErrorOr<std::unique_ptr<object::IRObjectFile>> ObjOrErr =
+      object::IRObjectFile::create(BufferRef, Context);
+
+  if (std::error_code EC = ObjOrErr.getError())
     message(LDPL_FATAL, "Could not read bitcode from file : %s",
-            toString(ObjOrErr.takeError()).c_str());
+            EC.message().c_str());
+
+  object::IRObjectFile &Obj = **ObjOrErr;
+
+  Module &M = Obj.getModule();
 
-  InputFile &Obj = **ObjOrErr;
+  M.materializeMetadata();
+  UpgradeDebugInfo(M);
+
+  SmallPtrSet<GlobalValue *, 8> Used;
+  collectUsedGlobalVariables(M, Used, /*CompilerUsed*/ false);
 
   unsigned SymNum = 0;
-  std::vector<SymbolResolution> Resols(F.syms.size());
   for (auto &ObjSym : Obj.symbols()) {
+    GlobalValue *GV = Obj.getSymbolGV(ObjSym.getRawDataRefImpl());
+    if (GV && GV->hasAppendingLinkage())
+      Keep.push_back(GV);
+
+    if (shouldSkip(ObjSym.getFlags()))
+      continue;
     ld_plugin_symbol &Sym = F.syms[SymNum];
-    SymbolResolution &R = Resols[SymNum];
     ++SymNum;
 
     ld_plugin_symbol_resolution Resolution =
         (ld_plugin_symbol_resolution)Sym.resolution;
 
+    if (options::generate_api_file)
+      *ApiFile << Sym.name << ' ' << getResolutionName(Resolution) << '\n';
+
+    if (!GV) {
+      freeSymName(Sym);
+      continue; // Asm symbol.
+    }
+
     ResolutionInfo &Res = ResInfo[Sym.name];
+    if (Resolution == LDPR_PREVAILING_DEF_IRONLY_EXP && !Res.IsLinkonceOdr)
+      Resolution = LDPR_PREVAILING_DEF;
+
+    GV->setUnnamedAddr(Res.UnnamedAddr);
+    GV->setVisibility(Res.Visibility);
+
+    // Override gold's resolution for common symbols. We want the largest
+    // one to win.
+    if (GV->hasCommonLinkage()) {
+      if (Resolution == LDPR_PREVAILING_DEF_IRONLY)
+        Res.CommonInternal = true;
+
+      if (Resolution == LDPR_PREVAILING_DEF_IRONLY ||
+          Resolution == LDPR_PREVAILING_DEF)
+        Res.UseCommon = true;
+
+      const DataLayout &DL = GV->getParent()->getDataLayout();
+      uint64_t Size = DL.getTypeAllocSize(GV->getType()->getElementType());
+      unsigned Align = GV->getAlignment();
+
+      if (Res.UseCommon && Size >= Res.CommonSize) {
+        // Take GV.
+        if (Res.CommonInternal)
+          Resolution = LDPR_PREVAILING_DEF_IRONLY;
+        else
+          Resolution = LDPR_PREVAILING_DEF;
+        cast<GlobalVariable>(GV)->setAlignment(
+            std::max(Res.CommonAlign, Align));
+      } else {
+        // Do not take GV, it's smaller than what we already have in the
+        // combined module.
+        Resolution = LDPR_PREEMPTED_IR;
+        if (Align > Res.CommonAlign)
+          // Need to raise the alignment though.
+          Realign[Sym.name] = Align;
+      }
+
+      Res.CommonSize = std::max(Res.CommonSize, Size);
+      Res.CommonAlign = std::max(Res.CommonAlign, Align);
+    }
 
     switch (Resolution) {
     case LDPR_UNKNOWN:
@@ -594,48 +795,57 @@ static void addModule(LTO &Lto, claimed_file &F, const void *View) {
     case LDPR_RESOLVED_DYN:
     case LDPR_PREEMPTED_IR:
     case LDPR_PREEMPTED_REG:
+      break;
+
     case LDPR_UNDEF:
+      if (!GV->isDeclarationForLinker())
+        assert(GV->hasComdat());
       break;
 
-    case LDPR_PREVAILING_DEF_IRONLY:
-      R.Prevailing = true;
+    case LDPR_PREVAILING_DEF_IRONLY: {
+      Keep.push_back(GV);
+      // The IR linker has to be able to map this value to a declaration,
+      // so we can only internalize after linking.
+      if (!Used.count(GV))
+        Internalize.insert(GV->getName());
       break;
+    }
 
     case LDPR_PREVAILING_DEF:
-      R.Prevailing = true;
-      R.VisibleToRegularObj = true;
+      Keep.push_back(GV);
+      // There is a non IR use, so we have to force optimizations to keep this.
+      switch (GV->getLinkage()) {
+      default:
+        break;
+      case GlobalValue::LinkOnceAnyLinkage:
+        GV->setLinkage(GlobalValue::WeakAnyLinkage);
+        break;
+      case GlobalValue::LinkOnceODRLinkage:
+        GV->setLinkage(GlobalValue::WeakODRLinkage);
+        break;
+      }
       break;
 
-    case LDPR_PREVAILING_DEF_IRONLY_EXP:
-      R.Prevailing = true;
-      if (!Res.CanOmitFromDynSym)
-        R.VisibleToRegularObj = true;
+    case LDPR_PREVAILING_DEF_IRONLY_EXP: {
+      Keep.push_back(GV);
+      if (canBeOmittedFromSymbolTable(GV))
+        Internalize.insert(GV->getName());
       break;
     }
-
-    if (Resolution != LDPR_RESOLVED_DYN && Resolution != LDPR_UNDEF &&
-        (IsExecutable || !Res.DefaultVisibility))
-      R.FinalDefinitionInLinkageUnit = true;
-
-    if (ObjSym.getFlags() & object::BasicSymbolRef::SF_Common) {
-      // We ignore gold's resolution for common symbols. A common symbol with
-      // the correct size and alignment is added to the module by the pre-opt
-      // module hook if any common symbol prevailed.
-      CommonResolution &CommonRes = Commons[ObjSym.getIRName()];
-      if (R.Prevailing) {
-        CommonRes.Prevailing = true;
-        CommonRes.VisibleToRegularObj = R.VisibleToRegularObj;
-      }
-      CommonRes.Size = std::max(CommonRes.Size, ObjSym.getCommonSize());
-      CommonRes.Align = std::max(CommonRes.Align, ObjSym.getCommonAlignment());
-      R.Prevailing = false;
     }
 
     freeSymName(Sym);
   }
 
-  check(Lto.add(std::move(*ObjOrErr), Resols),
-        std::string("Failed to link module ") + F.name);
+  return Obj.takeModule();
+}
+
+static void saveBCFile(StringRef Path, Module &M) {
+  std::error_code EC;
+  raw_fd_ostream OS(Path, EC, sys::fs::OpenFlags::F_None);
+  if (EC)
+    message(LDPL_FATAL, "Failed to write the output file.");
+  WriteBitcodeToFile(&M, OS, /* ShouldPreserveUseListOrder */ false);
 }
 
 static void recordFile(std::string Filename, bool TempOutFile) {
@@ -647,6 +857,190 @@ static void recordFile(std::string Filename, bool TempOutFile) {
     Cleanup.push_back(Filename.c_str());
 }
 
+void ThinLTOTaskInfo::cleanup() {
+  // Close the output file descriptor before we pass it to gold.
+  OS->close();
+
+  recordFile(Filename, TempOutFile);
+}
+
+namespace {
+/// Class to manage optimization and code generation for a module, possibly
+/// in a thread (ThinLTO).
+class CodeGen {
+  /// The module for which this will generate code.
+  std::unique_ptr<llvm::Module> M;
+
+  /// The output stream to generate code into.
+  raw_fd_ostream *OS;
+
+  /// The task ID when this was invoked in a thread (ThinLTO).
+  int TaskID;
+
+  /// The module summary index for ThinLTO tasks.
+  const ModuleSummaryIndex *CombinedIndex;
+
+  /// The target machine for generating code for this module.
+  std::unique_ptr<TargetMachine> TM;
+
+  /// Filename to use as base when save-temps is enabled, used to get
+  /// a unique and identifiable save-temps output file for each ThinLTO backend.
+  std::string SaveTempsFilename;
+
+  /// Map from a module name to the corresponding buffer holding a view of the
+  /// bitcode provided via the get_view gold callback.
+  StringMap<MemoryBufferRef> *ModuleMap;
+
+  // Functions to import into this module.
+  FunctionImporter::ImportMapTy *ImportList;
+
+  // Map of globals defined in this module to their summary.
+  std::map<GlobalValue::GUID, GlobalValueSummary *> *DefinedGlobals;
+
+public:
+  /// Constructor used by full LTO.
+  CodeGen(std::unique_ptr<llvm::Module> M)
+      : M(std::move(M)), OS(nullptr), TaskID(-1), CombinedIndex(nullptr),
+        ModuleMap(nullptr) {
+    initTargetMachine();
+  }
+  /// Constructor used by ThinLTO.
+  CodeGen(std::unique_ptr<llvm::Module> M, raw_fd_ostream *OS, int TaskID,
+          const ModuleSummaryIndex *CombinedIndex, std::string Filename,
+          StringMap<MemoryBufferRef> *ModuleMap,
+          FunctionImporter::ImportMapTy *ImportList,
+          std::map<GlobalValue::GUID, GlobalValueSummary *> *DefinedGlobals)
+      : M(std::move(M)), OS(OS), TaskID(TaskID), CombinedIndex(CombinedIndex),
+        SaveTempsFilename(std::move(Filename)), ModuleMap(ModuleMap),
+        ImportList(ImportList), DefinedGlobals(DefinedGlobals) {
+    assert(options::thinlto == !!CombinedIndex &&
+           "Expected module summary index iff performing ThinLTO");
+    initTargetMachine();
+  }
+
+  /// Invoke LTO passes and the code generator for the module.
+  void runAll();
+
+  /// Invoke the actual code generation to emit Module's object to file.
+  void runCodegenPasses();
+
+private:
+  const Target *TheTarget;
+  std::string TripleStr;
+  std::string FeaturesString;
+  TargetOptions Options;
+
+  /// Create a target machine for the module. Must be unique for each
+  /// module/task.
+  void initTargetMachine();
+
+  std::unique_ptr<TargetMachine> createTargetMachine();
+
+  /// Run all LTO passes on the module.
+  void runLTOPasses();
+
+  /// Sets up output files necessary to perform optional multi-threaded
+  /// split code generation, and invokes the code generation implementation.
+  /// If BCFileName is not empty, saves bitcode for module partitions into
+  /// {BCFileName}0 .. {BCFileName}N.
+  void runSplitCodeGen(const SmallString<128> &BCFilename);
+};
+}
+
+static SubtargetFeatures getFeatures(Triple &TheTriple) {
+  SubtargetFeatures Features;
+  Features.getDefaultSubtargetFeatures(TheTriple);
+  for (const std::string &A : MAttrs)
+    Features.AddFeature(A);
+  return Features;
+}
+
+static CodeGenOpt::Level getCGOptLevel() {
+  switch (options::OptLevel) {
+  case 0:
+    return CodeGenOpt::None;
+  case 1:
+    return CodeGenOpt::Less;
+  case 2:
+    return CodeGenOpt::Default;
+  case 3:
+    return CodeGenOpt::Aggressive;
+  }
+  llvm_unreachable("Invalid optimization level");
+}
+
+void CodeGen::initTargetMachine() {
+  TripleStr = M->getTargetTriple();
+  Triple TheTriple(TripleStr);
+
+  std::string ErrMsg;
+  TheTarget = TargetRegistry::lookupTarget(TripleStr, ErrMsg);
+  if (!TheTarget)
+    message(LDPL_FATAL, "Target not found: %s", ErrMsg.c_str());
+
+  SubtargetFeatures Features = getFeatures(TheTriple);
+  FeaturesString = Features.getString();
+  Options = InitTargetOptionsFromCodeGenFlags();
+
+  // Disable the new X86 relax relocations since gold might not support them.
+  // FIXME: Check the gold version or add a new option to enable them.
+  Options.RelaxELFRelocations = false;
+
+  TM = createTargetMachine();
+}
+
+std::unique_ptr<TargetMachine> CodeGen::createTargetMachine() {
+  CodeGenOpt::Level CGOptLevel = getCGOptLevel();
+
+  return std::unique_ptr<TargetMachine>(TheTarget->createTargetMachine(
+      TripleStr, options::mcpu, FeaturesString, Options, RelocationModel,
+      CodeModel::Default, CGOptLevel));
+}
+
+void CodeGen::runLTOPasses() {
+  M->setDataLayout(TM->createDataLayout());
+
+  if (CombinedIndex) {
+    // Apply summary-based LinkOnce/Weak resolution decisions.
+    thinLTOResolveWeakForLinkerModule(*M, *DefinedGlobals);
+
+    // Apply summary-based internalization decisions. Skip if there are no
+    // defined globals from the summary since not only is it unnecessary, but
+    // if this module did not have a summary section the internalizer will
+    // assert if it finds any definitions in this module that aren't in the
+    // DefinedGlobals set.
+    if (!DefinedGlobals->empty())
+      thinLTOInternalizeModule(*M, *DefinedGlobals);
+
+    // Create a loader that will parse the bitcode from the buffers
+    // in the ModuleMap.
+    ModuleLoader Loader(M->getContext(), *ModuleMap);
+
+    // Perform function importing.
+    FunctionImporter Importer(*CombinedIndex, Loader);
+    Importer.importFunctions(*M, *ImportList);
+  }
+
+  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 = !options::DisableVerify;
+  PMB.LoopVectorize = true;
+  PMB.SLPVectorize = true;
+  PMB.OptLevel = options::OptLevel;
+  if (options::thinlto)
+    PMB.populateThinLTOPassManager(passes);
+  else
+    PMB.populateLTOPassManager(passes);
+  passes.run(*M);
+}
+
 /// Open a file and return the new file descriptor given a base input
 /// file name, a flag indicating whether a temp file should be generated,
 /// and an optional task id. The new filename generated is
@@ -672,44 +1066,220 @@ static int openOutputFile(SmallString<128> InFilename, bool TempOutFile,
   return FD;
 }
 
-/// Add all required common symbols to M, which is expected to be the first
-/// combined module.
-static void addCommons(Module &M) {
-  for (auto &I : Commons) {
-    if (!I.second.Prevailing)
-      continue;
-    ArrayType *Ty =
-        ArrayType::get(Type::getInt8Ty(M.getContext()), I.second.Size);
-    GlobalVariable *OldGV = M.getNamedGlobal(I.first);
-    auto *GV = new GlobalVariable(M, Ty, false, GlobalValue::CommonLinkage,
-                                  ConstantAggregateZero::get(Ty), "");
-    GV->setAlignment(I.second.Align);
-    if (OldGV) {
-      OldGV->replaceAllUsesWith(ConstantExpr::getBitCast(GV, OldGV->getType()));
-      GV->takeName(OldGV);
-      OldGV->eraseFromParent();
-    } else {
-      GV->setName(I.first);
+void CodeGen::runCodegenPasses() {
+  assert(OS && "Output stream must be set before emitting to file");
+  legacy::PassManager CodeGenPasses;
+  if (TM->addPassesToEmitFile(CodeGenPasses, *OS,
+                              TargetMachine::CGFT_ObjectFile))
+    report_fatal_error("Failed to setup codegen");
+  CodeGenPasses.run(*M);
+}
+
+void CodeGen::runSplitCodeGen(const SmallString<128> &BCFilename) {
+  SmallString<128> Filename;
+  // Note that openOutputFile will append a unique ID for each task
+  if (!options::obj_path.empty())
+    Filename = options::obj_path;
+  else if (options::TheOutputType == options::OT_SAVE_TEMPS)
+    Filename = output_name + ".o";
+
+  // Note that the default parallelism is 1 instead of the
+  // hardware_concurrency, as there are behavioral differences between
+  // parallelism levels (e.g. symbol ordering will be different, and some uses
+  // of inline asm currently have issues with parallelism >1).
+  unsigned int MaxThreads = options::Parallelism ? options::Parallelism : 1;
+
+  std::vector<SmallString<128>> Filenames(MaxThreads);
+  std::vector<SmallString<128>> BCFilenames(MaxThreads);
+  bool TempOutFile = Filename.empty();
+  {
+    // Open a file descriptor for each backend task. This is done in a block
+    // so that the output file descriptors are closed before gold opens them.
+    std::list<llvm::raw_fd_ostream> OSs;
+    std::vector<llvm::raw_pwrite_stream *> OSPtrs(MaxThreads);
+    for (unsigned I = 0; I != MaxThreads; ++I) {
+      int FD = openOutputFile(Filename, TempOutFile, Filenames[I],
+                              // Only append ID if there are multiple tasks.
+                              MaxThreads > 1 ? I : -1);
+      OSs.emplace_back(FD, true);
+      OSPtrs[I] = &OSs.back();
+    }
+
+    std::list<llvm::raw_fd_ostream> BCOSs;
+    std::vector<llvm::raw_pwrite_stream *> BCOSPtrs;
+    if (!BCFilename.empty() && MaxThreads > 1) {
+      for (unsigned I = 0; I != MaxThreads; ++I) {
+        int FD = openOutputFile(BCFilename, false, BCFilenames[I], I);
+        BCOSs.emplace_back(FD, true);
+        BCOSPtrs.push_back(&BCOSs.back());
+      }
     }
-    // We may only internalize commons if there is a single LTO task because
-    // other native object files may require the common.
-    if (MaxTasks == 1 && !I.second.VisibleToRegularObj)
-      GV->setLinkage(GlobalValue::InternalLinkage);
+
+    // Run backend tasks.
+    splitCodeGen(std::move(M), OSPtrs, BCOSPtrs,
+                 [&]() { return createTargetMachine(); });
   }
+
+  for (auto &Filename : Filenames)
+    recordFile(Filename.c_str(), TempOutFile);
 }
 
-static CodeGenOpt::Level getCGOptLevel() {
-  switch (options::OptLevel) {
-  case 0:
-    return CodeGenOpt::None;
-  case 1:
-    return CodeGenOpt::Less;
-  case 2:
-    return CodeGenOpt::Default;
-  case 3:
-    return CodeGenOpt::Aggressive;
+void CodeGen::runAll() {
+  runLTOPasses();
+
+  SmallString<128> OptFilename;
+  if (options::TheOutputType == options::OT_SAVE_TEMPS) {
+    OptFilename = output_name;
+    // If the CodeGen client provided a filename, use it. Always expect
+    // a provided filename if we are in a task (i.e. ThinLTO backend).
+    assert(!SaveTempsFilename.empty() || TaskID == -1);
+    if (!SaveTempsFilename.empty())
+      OptFilename = SaveTempsFilename;
+    OptFilename += ".opt.bc";
+    saveBCFile(OptFilename, *M);
+  }
+
+  // If we are already in a thread (i.e. ThinLTO), just perform
+  // codegen passes directly.
+  if (TaskID >= 0)
+    runCodegenPasses();
+  // Otherwise attempt split code gen.
+  else
+    runSplitCodeGen(OptFilename);
+}
+
+/// Links the module in \p View from file \p F into the combined module
+/// saved in the IRMover \p L.
+static void linkInModule(LLVMContext &Context, IRMover &L, claimed_file &F,
+                         const void *View, StringRef Name,
+                         raw_fd_ostream *ApiFile, StringSet<> &Internalize,
+                         bool SetName = false) {
+  std::vector<GlobalValue *> Keep;
+  StringMap<unsigned> Realign;
+  std::unique_ptr<Module> M = getModuleForFile(Context, F, View, Name, ApiFile,
+                                               Internalize, Keep, Realign);
+  if (!M.get())
+    return;
+  if (!options::triple.empty())
+    M->setTargetTriple(options::triple.c_str());
+  else if (M->getTargetTriple().empty()) {
+    M->setTargetTriple(DefaultTriple);
+  }
+
+  // For ThinLTO we want to propagate the source file name to ensure
+  // we can create the correct global identifiers matching those in the
+  // original module.
+  if (SetName)
+    L.getModule().setSourceFileName(M->getSourceFileName());
+
+  if (Error E = L.move(std::move(M), Keep,
+                       [](GlobalValue &, IRMover::ValueAdder) {})) {
+    handleAllErrors(std::move(E), [&](const llvm::ErrorInfoBase &EIB) {
+      message(LDPL_FATAL, "Failed to link module %s: %s", Name.str().c_str(),
+              EIB.message().c_str());
+    });
+  }
+
+  for (const auto &I : Realign) {
+    GlobalValue *Dst = L.getModule().getNamedValue(I.first());
+    if (!Dst)
+      continue;
+    cast<GlobalVariable>(Dst)->setAlignment(I.second);
   }
-  llvm_unreachable("Invalid optimization level");
+}
+
+/// Perform the ThinLTO backend on a single module, invoking the LTO and codegen
+/// pipelines.
+static void thinLTOBackendTask(claimed_file &F, const void *View,
+                               StringRef Name, raw_fd_ostream *ApiFile,
+                               const ModuleSummaryIndex &CombinedIndex,
+                               raw_fd_ostream *OS, unsigned TaskID,
+                               StringMap<MemoryBufferRef> &ModuleMap,
+                               FunctionImporter::ImportMapTy &ImportList,
+                               std::map<GlobalValue::GUID, GlobalValueSummary *> &DefinedGlobals) {
+  // Need to use a separate context for each task
+  LLVMContext Context;
+  Context.setDiscardValueNames(options::TheOutputType !=
+                               options::OT_SAVE_TEMPS);
+  Context.enableDebugTypeODRUniquing(); // Merge debug info types.
+  Context.setDiagnosticHandler(diagnosticHandlerForContext, nullptr, true);
+
+  std::unique_ptr<llvm::Module> NewModule(new llvm::Module(Name, Context));
+  IRMover L(*NewModule.get());
+
+  StringSet<> Dummy;
+  linkInModule(Context, L, F, View, Name, ApiFile, Dummy, true);
+  if (renameModuleForThinLTO(*NewModule, CombinedIndex))
+    message(LDPL_FATAL, "Failed to rename module for ThinLTO");
+
+  CodeGen codeGen(std::move(NewModule), OS, TaskID, &CombinedIndex, Name,
+                  &ModuleMap, &ImportList, &DefinedGlobals);
+  codeGen.runAll();
+}
+
+/// Launch each module's backend pipeline in a separate task in a thread pool.
+static void
+thinLTOBackends(raw_fd_ostream *ApiFile,
+                const ModuleSummaryIndex &CombinedIndex,
+                StringMap<MemoryBufferRef> &ModuleMap,
+                StringMap<FunctionImporter::ImportMapTy> &ImportLists,
+  StringMap<std::map<GlobalValue::GUID, GlobalValueSummary *>>
+      &ModuleToDefinedGVSummaries) {
+  unsigned TaskCount = 0;
+  std::vector<ThinLTOTaskInfo> Tasks;
+  Tasks.reserve(Modules.size());
+  unsigned int MaxThreads = options::Parallelism
+                                ? options::Parallelism
+                                : thread::hardware_concurrency();
+
+  // Create ThreadPool in nested scope so that threads will be joined
+  // on destruction.
+  {
+    ThreadPool ThinLTOThreadPool(MaxThreads);
+    for (claimed_file &F : Modules) {
+      // Do all the gold callbacks in the main thread, since gold is not thread
+      // safe by default.
+      const void *View = getSymbolsAndView(F);
+      if (!View)
+        continue;
+
+      SmallString<128> Filename;
+      if (!options::obj_path.empty())
+        // Note that openOutputFile will append a unique ID for each task
+        Filename = options::obj_path;
+      else if (options::TheOutputType == options::OT_SAVE_TEMPS) {
+        // Use the input file name so that we get a unique and identifiable
+        // output file for each ThinLTO backend task.
+        Filename = F.name;
+        Filename += ".thinlto.o";
+      }
+      bool TempOutFile = Filename.empty();
+
+      SmallString<128> NewFilename;
+      int FD = openOutputFile(Filename, TempOutFile, NewFilename,
+                              // Only append the TaskID if we will use the
+                              // non-unique obj_path.
+                              !options::obj_path.empty() ? TaskCount : -1);
+      TaskCount++;
+      std::unique_ptr<raw_fd_ostream> OS =
+          llvm::make_unique<raw_fd_ostream>(FD, true);
+
+      // Enqueue the task
+      ThinLTOThreadPool.async(thinLTOBackendTask, std::ref(F), View, F.name,
+                              ApiFile, std::ref(CombinedIndex), OS.get(),
+                              TaskCount, std::ref(ModuleMap),
+                              std::ref(ImportLists[F.name]),
+                              std::ref(ModuleToDefinedGVSummaries[F.name]));
+
+      // Record the information needed by the task or during its cleanup
+      // to a ThinLTOTaskInfo instance. For information needed by the task
+      // the unique_ptr ownership is transferred to the ThinLTOTaskInfo.
+      Tasks.emplace_back(std::move(OS), NewFilename.c_str(), TempOutFile);
+    }
+  }
+
+  for (auto &Task : Tasks)
+    Task.cleanup();
 }
 
 /// Parse the thinlto_prefix_replace option into the \p OldPrefix and
@@ -723,130 +1293,266 @@ static void getThinLTOOldAndNewPrefix(std::string &OldPrefix,
   NewPrefix = Split.second.str();
 }
 
-static std::unique_ptr<LTO> createLTO() {
-  Config Conf;
-  ThinBackend Backend;
-  unsigned ParallelCodeGenParallelismLevel = 1;
+/// Given the original \p Path to an output file, replace any path
+/// prefix matching \p OldPrefix with \p NewPrefix. Also, create the
+/// resulting directory if it does not yet exist.
+static std::string getThinLTOOutputFile(const std::string &Path,
+                                        const std::string &OldPrefix,
+                                        const std::string &NewPrefix) {
+  if (OldPrefix.empty() && NewPrefix.empty())
+    return Path;
+  SmallString<128> NewPath(Path);
+  llvm::sys::path::replace_path_prefix(NewPath, OldPrefix, NewPrefix);
+  StringRef ParentPath = llvm::sys::path::parent_path(NewPath.str());
+  if (!ParentPath.empty()) {
+    // Make sure the new directory exists, creating it if necessary.
+    if (std::error_code EC = llvm::sys::fs::create_directories(ParentPath))
+      llvm::errs() << "warning: could not create directory '" << ParentPath
+                   << "': " << EC.message() << '\n';
+  }
+  return NewPath.str();
+}
 
-  Conf.CPU = options::mcpu;
-  Conf.Options = InitTargetOptionsFromCodeGenFlags();
+/// Perform ThinLTO link, which creates the combined index file.
+/// Also, either launch backend threads or (under thinlto-index-only)
+/// emit individual index files for distributed backends and exit.
+static ld_plugin_status thinLTOLink(raw_fd_ostream *ApiFile) {
+  // Map from a module name to the corresponding buffer holding a view of the
+  // bitcode provided via the get_view gold callback.
+  StringMap<MemoryBufferRef> ModuleMap;
+  // Map to own RAII objects that manage the file opening and releasing
+  // interfaces with gold.
+  DenseMap<void *, std::unique_ptr<PluginInputFile>> HandleToInputFile;
+
+  // Keep track of symbols that must not be internalized because they
+  // are referenced outside of a single IR module.
+  DenseSet<GlobalValue::GUID> Preserve;
+
+  // Keep track of the prevailing copy for each GUID, for use in resolving
+  // weak linkages.
+  DenseMap<GlobalValue::GUID, const GlobalValueSummary *> PrevailingCopy;
+
+  ModuleSummaryIndex CombinedIndex;
+  uint64_t NextModuleId = 0;
+  for (claimed_file &F : Modules) {
+    if (!HandleToInputFile.count(F.leader_handle))
+      HandleToInputFile.insert(std::make_pair(
+          F.leader_handle, llvm::make_unique<PluginInputFile>(F.handle)));
+    // Pass this into getModuleSummaryIndexForFile
+    const void *View = getSymbolsAndView(F);
+    if (!View)
+      continue;
 
-  // Disable the new X86 relax relocations since gold might not support them.
-  // FIXME: Check the gold version or add a new option to enable them.
-  Conf.Options.RelaxELFRelocations = false;
-
-  Conf.MAttrs = MAttrs;
-  Conf.RelocModel = *RelocationModel;
-  Conf.CGOptLevel = getCGOptLevel();
-  Conf.DisableVerify = options::DisableVerify;
-  Conf.OptLevel = options::OptLevel;
-  if (options::Parallelism) {
-    if (options::thinlto)
-      Backend = createInProcessThinBackend(options::Parallelism);
-    else
-      ParallelCodeGenParallelismLevel = options::Parallelism;
-  }
-  if (options::thinlto_index_only) {
-    std::string OldPrefix, NewPrefix;
-    getThinLTOOldAndNewPrefix(OldPrefix, NewPrefix);
-    Backend = createWriteIndexesThinBackend(
-        OldPrefix, NewPrefix, options::thinlto_emit_imports_files,
-        options::thinlto_linked_objects_file);
-  }
+    MemoryBufferRef ModuleBuffer(StringRef((const char *)View, F.filesize),
+                                 F.name);
+    assert(ModuleMap.find(ModuleBuffer.getBufferIdentifier()) ==
+               ModuleMap.end() &&
+           "Expect unique Buffer Identifier");
+    ModuleMap[ModuleBuffer.getBufferIdentifier()] = ModuleBuffer;
+
+    std::unique_ptr<ModuleSummaryIndex> Index = getModuleSummaryIndexForFile(F);
+
+    // Use gold's symbol resolution information to identify symbols referenced
+    // by more than a single IR module (i.e. referenced by multiple IR modules
+    // or by a non-IR module). Cross references introduced by importing are
+    // checked separately via the export lists. Also track the prevailing copy
+    // for later symbol resolution.
+    for (auto &Sym : F.syms) {
+      ld_plugin_symbol_resolution Resolution =
+          (ld_plugin_symbol_resolution)Sym.resolution;
+      GlobalValue::GUID SymGUID = GlobalValue::getGUID(Sym.name);
+      if (Resolution != LDPR_PREVAILING_DEF_IRONLY)
+        Preserve.insert(SymGUID);
+
+      if (Index && (Resolution == LDPR_PREVAILING_DEF ||
+                    Resolution == LDPR_PREVAILING_DEF_IRONLY ||
+                    Resolution == LDPR_PREVAILING_DEF_IRONLY_EXP))
+        PrevailingCopy[SymGUID] = Index->getGlobalValueSummary(SymGUID);
+    }
 
-  Conf.OverrideTriple = options::triple;
-  Conf.DefaultTriple = sys::getDefaultTargetTriple();
+    // Skip files without a module summary.
+    if (Index)
+      CombinedIndex.mergeFrom(std::move(Index), ++NextModuleId);
+  }
 
-  Conf.DiagHandler = diagnosticHandler;
+  // Collect for each module the list of function it defines (GUID ->
+  // Summary).
+  StringMap<std::map<GlobalValue::GUID, GlobalValueSummary *>>
+      ModuleToDefinedGVSummaries(NextModuleId);
+  CombinedIndex.collectDefinedGVSummariesPerModule(ModuleToDefinedGVSummaries);
+
+  StringMap<FunctionImporter::ImportMapTy> ImportLists(NextModuleId);
+  StringMap<FunctionImporter::ExportSetTy> ExportLists(NextModuleId);
+  ComputeCrossModuleImport(CombinedIndex, ModuleToDefinedGVSummaries,
+                           ImportLists, ExportLists);
+
+  auto isPrevailing = [&](GlobalValue::GUID GUID, const GlobalValueSummary *S) {
+    const auto &Prevailing = PrevailingCopy.find(GUID);
+    assert(Prevailing != PrevailingCopy.end());
+    return Prevailing->second == S;
+  };
 
-  Conf.PreOptModuleHook = [](size_t Task, Module &M) {
-    if (Task == 0)
-      addCommons(M);
-    return true;
+  // Callback for internalization, to prevent internalization of symbols
+  // that were not candidates initially, and those that are being imported
+  // (which introduces new cross references).
+  auto isExported = [&](StringRef ModuleIdentifier, GlobalValue::GUID GUID) {
+    const auto &ExportList = ExportLists.find(ModuleIdentifier);
+    return (ExportList != ExportLists.end() &&
+            ExportList->second.count(GUID)) ||
+           Preserve.count(GUID);
   };
 
-  switch (options::TheOutputType) {
-  case options::OT_NORMAL:
-    break;
+  thinLTOResolveWeakForLinkerInIndex(
+      CombinedIndex, isPrevailing,
+      [](StringRef ModuleIdentifier, GlobalValue::GUID GUID,
+         GlobalValue::LinkageTypes NewLinkage) {});
 
-  case options::OT_DISABLE:
-    Conf.PreOptModuleHook = [](size_t Task, Module &M) { return false; };
-    break;
+  // Use global summary-based analysis to identify symbols that can be
+  // internalized (because they aren't exported or preserved as per callback).
+  // Changes are made in the index, consumed in the ThinLTO backends.
+  thinLTOInternalizeAndPromoteInIndex(CombinedIndex, isExported);
 
-  case options::OT_BC_ONLY:
-    Conf.PostInternalizeModuleHook = [](size_t Task, Module &M) {
+  if (options::thinlto_emit_imports_files && !options::thinlto_index_only)
+    message(LDPL_WARNING,
+            "thinlto-emit-imports-files ignored unless thinlto-index-only");
+
+  if (options::thinlto_index_only) {
+    // If the thinlto-prefix-replace option was specified, parse it and
+    // extract the old and new prefixes.
+    std::string OldPrefix, NewPrefix;
+    getThinLTOOldAndNewPrefix(OldPrefix, NewPrefix);
+
+    // If the user requested a list of objects gold included in the link,
+    // create and open the requested file.
+    raw_fd_ostream *ObjFileOS = nullptr;
+    if (!options::thinlto_linked_objects_file.empty()) {
       std::error_code EC;
-      raw_fd_ostream OS(output_name, EC, sys::fs::OpenFlags::F_None);
+      ObjFileOS = new raw_fd_ostream(options::thinlto_linked_objects_file, EC,
+                                     sys::fs::OpenFlags::F_None);
       if (EC)
-        message(LDPL_FATAL, "Failed to write the output file.");
-      WriteBitcodeToFile(&M, OS, /* ShouldPreserveUseListOrder */ false);
-      return false;
-    };
-    break;
+        message(LDPL_FATAL, "Unable to open %s for writing: %s",
+                options::thinlto_linked_objects_file.c_str(),
+                EC.message().c_str());
+    }
+    // For each input bitcode file, generate an individual index that
+    // contains summaries only for its own global values, and for any that
+    // should be imported.
+    for (claimed_file &F : Modules) {
+      std::error_code EC;
 
-  case options::OT_SAVE_TEMPS:
-    check(Conf.addSaveTemps(output_name));
-    break;
+      std::string NewModulePath =
+          getThinLTOOutputFile(F.name, OldPrefix, NewPrefix);
+
+      if (!options::thinlto_linked_objects_file.empty()) {
+        // If gold included any symbols from ths file in the link, emit path
+        // to the final object file, which should be included in the final
+        // native link.
+        if (get_symbols(F.handle, F.syms.size(), F.syms.data()) !=
+            LDPS_NO_SYMS) {
+          assert(ObjFileOS);
+          *ObjFileOS << NewModulePath << "\n";
+        }
+      }
+
+      raw_fd_ostream OS((Twine(NewModulePath) + ".thinlto.bc").str(), EC,
+                        sys::fs::OpenFlags::F_None);
+      if (EC)
+        message(LDPL_FATAL, "Unable to open %s.thinlto.bc for writing: %s",
+                NewModulePath.c_str(), EC.message().c_str());
+      // Build a map of module to the GUIDs and summary objects that should
+      // be written to its index.
+      std::map<std::string, GVSummaryMapTy> ModuleToSummariesForIndex;
+      gatherImportedSummariesForModule(F.name, ModuleToDefinedGVSummaries,
+                                       ImportLists, ModuleToSummariesForIndex);
+      WriteIndexToFile(CombinedIndex, OS, &ModuleToSummariesForIndex);
+
+      if (options::thinlto_emit_imports_files) {
+        if ((EC = EmitImportsFiles(F.name,
+                                   (Twine(NewModulePath) + ".imports").str(),
+                                   ImportLists)))
+          message(LDPL_FATAL, "Unable to open %s.imports",
+                  NewModulePath.c_str(), EC.message().c_str());
+      }
+    }
+
+    if (ObjFileOS)
+      ObjFileOS->close();
+
+    cleanup_hook();
+    exit(0);
+  }
+
+  // Create OS in nested scope so that it will be closed on destruction.
+  {
+    std::error_code EC;
+    raw_fd_ostream OS(output_name + ".thinlto.bc", EC,
+                      sys::fs::OpenFlags::F_None);
+    if (EC)
+      message(LDPL_FATAL, "Unable to open %s.thinlto.bc for writing: %s",
+              output_name.data(), EC.message().c_str());
+    WriteIndexToFile(CombinedIndex, OS);
   }
 
-  return make_unique<LTO>(std::move(Conf), Backend,
-                          ParallelCodeGenParallelismLevel);
+  thinLTOBackends(ApiFile, CombinedIndex, ModuleMap, ImportLists,
+                  ModuleToDefinedGVSummaries);
+  return LDPS_OK;
 }
 
 /// gold informs us that all symbols have been read. At this point, we use
 /// get_symbols to see if any of our definitions have been overridden by a
 /// native object file. Then, perform optimization and codegen.
-static ld_plugin_status allSymbolsReadHook() {
+static ld_plugin_status allSymbolsReadHook(raw_fd_ostream *ApiFile) {
   if (Modules.empty())
     return LDPS_OK;
 
   if (unsigned NumOpts = options::extra.size())
     cl::ParseCommandLineOptions(NumOpts, &options::extra[0]);
 
-  std::unique_ptr<LTO> Lto = createLTO();
+  if (options::thinlto)
+    return thinLTOLink(ApiFile);
+
+  LLVMContext Context;
+  Context.setDiscardValueNames(options::TheOutputType !=
+                               options::OT_SAVE_TEMPS);
+  Context.enableDebugTypeODRUniquing(); // Merge debug info types.
+  Context.setDiagnosticHandler(diagnosticHandlerForContext, nullptr, true);
 
+  std::unique_ptr<Module> Combined(new Module("ld-temp.o", Context));
+  IRMover L(*Combined);
+
+  StringSet<> Internalize;
   for (claimed_file &F : Modules) {
+    // RAII object to manage the file opening and releasing interfaces with
+    // gold.
     PluginInputFile InputFile(F.handle);
     const void *View = getSymbolsAndView(F);
     if (!View)
       continue;
-    addModule(*Lto, F, View);
+    linkInModule(Context, L, F, View, F.name, ApiFile, Internalize);
   }
 
-  SmallString<128> Filename;
-  // Note that openOutputFile will append a unique ID for each task
-  if (!options::obj_path.empty())
-    Filename = options::obj_path;
-  else if (options::TheOutputType == options::OT_SAVE_TEMPS)
-    Filename = output_name + ".o";
-  bool SaveTemps = !Filename.empty();
-
-  MaxTasks = Lto->getMaxTasks();
-  std::vector<uintptr_t> IsTemporary(MaxTasks);
-  std::vector<SmallString<128>> Filenames(MaxTasks);
-
-  auto AddStream = [&](size_t Task) {
-    int FD = openOutputFile(Filename, /*TempOutFile=*/!SaveTemps,
-                            Filenames[Task], MaxTasks > 1 ? Task : -1);
-    IsTemporary[Task] = !SaveTemps;
-
-    return make_unique<llvm::raw_fd_ostream>(FD, true);
-  };
-
-  check(Lto->run(AddStream));
+  for (const auto &Name : Internalize) {
+    GlobalValue *GV = Combined->getNamedValue(Name.first());
+    if (GV)
+      internalize(*GV);
+  }
 
-  if (options::TheOutputType == options::OT_DISABLE ||
-      options::TheOutputType == options::OT_BC_ONLY)
+  if (options::TheOutputType == options::OT_DISABLE)
     return LDPS_OK;
 
-  if (options::thinlto_index_only) {
-    cleanup_hook();
-    exit(0);
+  if (options::TheOutputType != options::OT_NORMAL) {
+    std::string path;
+    if (options::TheOutputType == options::OT_BC_ONLY)
+      path = output_name;
+    else
+      path = output_name + ".bc";
+    saveBCFile(path, *Combined);
+    if (options::TheOutputType == options::OT_BC_ONLY)
+      return LDPS_OK;
   }
 
-  for (unsigned I = 0; I != MaxTasks; ++I)
-    if (!Filenames[I].empty())
-      recordFile(Filenames[I].str(), IsTemporary[I]);
+  CodeGen codeGen(std::move(Combined));
+  codeGen.runAll();
 
   if (!options::extra_library_path.empty() &&
       set_extra_library_path(options::extra_library_path.c_str()) != LDPS_OK)
@@ -856,7 +1562,18 @@ static ld_plugin_status allSymbolsReadHook() {
 }
 
 static ld_plugin_status all_symbols_read_hook(void) {
-  ld_plugin_status Ret = allSymbolsReadHook();
+  ld_plugin_status Ret;
+  if (!options::generate_api_file) {
+    Ret = allSymbolsReadHook(nullptr);
+  } else {
+    std::error_code EC;
+    raw_fd_ostream ApiFile("apifile.txt", EC, sys::fs::F_None);
+    if (EC)
+      message(LDPL_FATAL, "Unable to open apifile.txt for writing: %s",
+              EC.message().c_str());
+    Ret = allSymbolsReadHook(&ApiFile);
+  }
+
   llvm_shutdown();
 
   if (options::TheOutputType == options::OT_BC_ONLY ||