text: //===-LTOBackend.cpp - LLVM Link Time Optimizer Backend -------------------===//
  //
  // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
  // See https://llvm.org/LICENSE.txt for license information.
  // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
  //
  //===----------------------------------------------------------------------===//
  //
  // 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/AliasAnalysis.h"
  #include "llvm/Analysis/CGSCCPassManager.h"
  #include "llvm/Analysis/ModuleSummaryAnalysis.h"
  #include "llvm/Analysis/TargetLibraryInfo.h"
  #include "llvm/Analysis/TargetTransformInfo.h"
  #include "llvm/Bitcode/BitcodeReader.h"
  #include "llvm/Bitcode/BitcodeWriter.h"
  #include "llvm/IR/LLVMRemarkStreamer.h"
  #include "llvm/IR/LegacyPassManager.h"
  #include "llvm/IR/PassManager.h"
  #include "llvm/IR/Verifier.h"
  #include "llvm/LTO/LTO.h"
  #include "llvm/MC/SubtargetFeature.h"
  #include "llvm/MC/TargetRegistry.h"
  #include "llvm/Object/ModuleSymbolTable.h"
  #include "llvm/Passes/PassBuilder.h"
  #include "llvm/Passes/PassPlugin.h"
  #include "llvm/Passes/StandardInstrumentations.h"
  #include "llvm/Support/Error.h"
  #include "llvm/Support/FileSystem.h"
  #include "llvm/Support/MemoryBuffer.h"
  #include "llvm/Support/Path.h"
  #include "llvm/Support/Program.h"
  #include "llvm/Support/SmallVectorMemoryBuffer.h"
  #include "llvm/Support/ThreadPool.h"
  #include "llvm/Support/raw_ostream.h"
  #include "llvm/Target/TargetMachine.h"
  #include "llvm/Transforms/IPO.h"
  #include "llvm/Transforms/IPO/PassManagerBuilder.h"
  #include "llvm/Transforms/Scalar/LoopPassManager.h"
  #include "llvm/Transforms/Utils/FunctionImportUtils.h"
  #include "llvm/Transforms/Utils/SplitModule.h"
  
  using namespace llvm;
  using namespace lto;
  
  #define DEBUG_TYPE "lto-backend"
  
  enum class LTOBitcodeEmbedding {
    DoNotEmbed = 0,
    EmbedOptimized = 1,
    EmbedPostMergePreOptimized = 2
  };
  
  static cl::opt<LTOBitcodeEmbedding> EmbedBitcode(
      "lto-embed-bitcode", cl::init(LTOBitcodeEmbedding::DoNotEmbed),
      cl::values(clEnumValN(LTOBitcodeEmbedding::DoNotEmbed, "none",
                            "Do not embed"),
                 clEnumValN(LTOBitcodeEmbedding::EmbedOptimized, "optimized",
                            "Embed after all optimization passes"),
                 clEnumValN(LTOBitcodeEmbedding::EmbedPostMergePreOptimized,
                            "post-merge-pre-opt",
                            "Embed post merge, but before optimizations")),
      cl::desc("Embed LLVM bitcode in object files produced by LTO"));
  
  static cl::opt<bool> ThinLTOAssumeMerged(
      "thinlto-assume-merged", cl::init(false),
      cl::desc("Assume the input has already undergone ThinLTO function "
               "importing and the other pre-optimization pipeline changes."));
  
  namespace llvm {
  extern cl::opt<bool> NoPGOWarnMismatch;
  }
  
  [[noreturn]] static void reportOpenError(StringRef Path, Twine Msg) {
    errs() << "failed to open " << Path << ": " << Msg << '\n';
    errs().flush();
    exit(1);
  }
  
  Error Config::addSaveTemps(std::string OutputFileName,
                             bool UseInputModulePath) {
    ShouldDiscardValueNames = false;
  
    std::error_code EC;
    ResolutionFile =
        std::make_unique<raw_fd_ostream>(OutputFileName + "resolution.txt", EC,
                                         sys::fs::OpenFlags::OF_TextWithCRLF);
    if (EC) {
      ResolutionFile.reset();
      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 = [=](unsigned Task, const 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;
        // If this is the combined module (not a ThinLTO backend compile) or the
        // user hasn't requested using the input module's path, emit to a file
        // named from the provided OutputFileName with the Task ID appended.
        if (M.getModuleIdentifier() == "ld-temp.o" || !UseInputModulePath) {
          PathPrefix = OutputFileName;
          if (Task != (unsigned)-1)
            PathPrefix += utostr(Task) + ".";
        } else
          PathPrefix = M.getModuleIdentifier() + ".";
        std::string Path = PathPrefix + PathSuffix + ".bc";
        std::error_code EC;
        raw_fd_ostream OS(Path, EC, sys::fs::OpenFlags::OF_None);
        // Because -save-temps is a debugging feature, we report the error
        // directly and exit.
        if (EC)
          reportOpenError(Path, EC.message());
        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,
            const DenseSet<GlobalValue::GUID> &GUIDPreservedSymbols) {
          std::string Path = OutputFileName + "index.bc";
          std::error_code EC;
          raw_fd_ostream OS(Path, EC, sys::fs::OpenFlags::OF_None);
          // Because -save-temps is a debugging feature, we report the error
          // directly and exit.
          if (EC)
            reportOpenError(Path, EC.message());
          WriteIndexToFile(Index, OS);
  
          Path = OutputFileName + "index.dot";
          raw_fd_ostream OSDot(Path, EC, sys::fs::OpenFlags::OF_None);
          if (EC)
            reportOpenError(Path, EC.message());
          Index.exportToDot(OSDot, GUIDPreservedSymbols);
          return true;
        };
  
    return Error::success();
  }
  
  #define HANDLE_EXTENSION(Ext)                                                  \
    llvm::PassPluginLibraryInfo get##Ext##PluginInfo();
  #include "llvm/Support/Extension.def"
  
  static void RegisterPassPlugins(ArrayRef<std::string> PassPlugins,
                                  PassBuilder &PB) {
  #define HANDLE_EXTENSION(Ext)                                                  \
    get##Ext##PluginInfo().RegisterPassBuilderCallbacks(PB);
  #include "llvm/Support/Extension.def"
  
    // Load requested pass plugins and let them register pass builder callbacks
    for (auto &PluginFN : PassPlugins) {
      auto PassPlugin = PassPlugin::Load(PluginFN);
      if (!PassPlugin) {
        errs() << "Failed to load passes from '" << PluginFN
               << "'. Request ignored.\n";
        continue;
      }
  
      PassPlugin->registerPassBuilderCallbacks(PB);
    }
  }
  
  static std::unique_ptr<TargetMachine>
  createTargetMachine(const Config &Conf, const Target *TheTarget, Module &M) {
    StringRef TheTriple = M.getTargetTriple();
    SubtargetFeatures Features;
    Features.getDefaultSubtargetFeatures(Triple(TheTriple));
    for (const std::string &A : Conf.MAttrs)
      Features.AddFeature(A);
  
    Optional<Reloc::Model> RelocModel = None;
    if (Conf.RelocModel)
      RelocModel = *Conf.RelocModel;
    else if (M.getModuleFlag("PIC Level"))
      RelocModel =
          M.getPICLevel() == PICLevel::NotPIC ? Reloc::Static : Reloc::PIC_;
  
    Optional<CodeModel::Model> CodeModel;
    if (Conf.CodeModel)
      CodeModel = *Conf.CodeModel;
    else
      CodeModel = M.getCodeModel();
  
    std::unique_ptr<TargetMachine> TM(TheTarget->createTargetMachine(
        TheTriple, Conf.CPU, Features.getString(), Conf.Options, RelocModel,
        CodeModel, Conf.CGOptLevel));
    assert(TM && "Failed to create target machine");
    return TM;
  }
  
  static void runNewPMPasses(const Config &Conf, Module &Mod, TargetMachine *TM,
                             unsigned OptLevel, bool IsThinLTO,
                             ModuleSummaryIndex *ExportSummary,
                             const ModuleSummaryIndex *ImportSummary) {
    Optional<PGOOptions> PGOOpt;
    if (!Conf.SampleProfile.empty())
      PGOOpt = PGOOptions(Conf.SampleProfile, "", Conf.ProfileRemapping,
                          PGOOptions::SampleUse, PGOOptions::NoCSAction, true);
    else if (Conf.RunCSIRInstr) {
      PGOOpt = PGOOptions("", Conf.CSIRProfile, Conf.ProfileRemapping,
                          PGOOptions::IRUse, PGOOptions::CSIRInstr,
                          Conf.AddFSDiscriminator);
    } else if (!Conf.CSIRProfile.empty()) {
      PGOOpt = PGOOptions(Conf.CSIRProfile, "", Conf.ProfileRemapping,
                          PGOOptions::IRUse, PGOOptions::CSIRUse,
                          Conf.AddFSDiscriminator);
      NoPGOWarnMismatch = !Conf.PGOWarnMismatch;
    } else if (Conf.AddFSDiscriminator) {
      PGOOpt = PGOOptions("", "", "", PGOOptions::NoAction,
                          PGOOptions::NoCSAction, true);
    }
    if (TM)
      TM->setPGOOption(PGOOpt);
  
    LoopAnalysisManager LAM;
    FunctionAnalysisManager FAM;
    CGSCCAnalysisManager CGAM;
    ModuleAnalysisManager MAM;
  
    PassInstrumentationCallbacks PIC;
    StandardInstrumentations SI(Conf.DebugPassManager);
    SI.registerCallbacks(PIC, &FAM);
    PassBuilder PB(TM, Conf.PTO, PGOOpt, &PIC);
  
    RegisterPassPlugins(Conf.PassPlugins, PB);
  
    std::unique_ptr<TargetLibraryInfoImpl> TLII(
        new TargetLibraryInfoImpl(Triple(TM->getTargetTriple())));
    if (Conf.Freestanding)
      TLII->disableAllFunctions();
    FAM.registerPass([&] { return TargetLibraryAnalysis(*TLII); });
  
    // Parse a custom AA pipeline if asked to.
    if (!Conf.AAPipeline.empty()) {
      AAManager AA;
      if (auto Err = PB.parseAAPipeline(AA, Conf.AAPipeline)) {
        report_fatal_error(Twine("unable to parse AA pipeline description '") +
                           Conf.AAPipeline + "': " + toString(std::move(Err)));
      }
      // Register the AA manager first so that our version is the one used.
      FAM.registerPass([&] { return std::move(AA); });
    }
  
    // Register all the basic analyses with the managers.
    PB.registerModuleAnalyses(MAM);
    PB.registerCGSCCAnalyses(CGAM);
    PB.registerFunctionAnalyses(FAM);
    PB.registerLoopAnalyses(LAM);
    PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
  
    ModulePassManager MPM;
  
    if (!Conf.DisableVerify)
      MPM.addPass(VerifierPass());
  
    OptimizationLevel OL;
  
    switch (OptLevel) {
    default:
      llvm_unreachable("Invalid optimization level");
    case 0:
      OL = OptimizationLevel::O0;
      break;
    case 1:
      OL = OptimizationLevel::O1;
      break;
    case 2:
      OL = OptimizationLevel::O2;
      break;
    case 3:
      OL = OptimizationLevel::O3;
      break;
    }
  
    // Parse a custom pipeline if asked to.
    if (!Conf.OptPipeline.empty()) {
      if (auto Err = PB.parsePassPipeline(MPM, Conf.OptPipeline)) {
        report_fatal_error(Twine("unable to parse pass pipeline description '") +
                           Conf.OptPipeline + "': " + toString(std::move(Err)));
      }
    } else if (IsThinLTO) {
      MPM.addPass(PB.buildThinLTODefaultPipeline(OL, ImportSummary));
    } else {
      MPM.addPass(PB.buildLTODefaultPipeline(OL, ExportSummary));
    }
  
    if (!Conf.DisableVerify)
      MPM.addPass(VerifierPass());
  
    MPM.run(Mod, MAM);
  }
  
  static void runOldPMPasses(const Config &Conf, Module &Mod, TargetMachine *TM,
                             bool IsThinLTO, ModuleSummaryIndex *ExportSummary,
                             const ModuleSummaryIndex *ImportSummary) {
    legacy::PassManager passes;
    passes.add(createTargetTransformInfoWrapperPass(TM->getTargetIRAnalysis()));
  
    PassManagerBuilder PMB;
    PMB.LibraryInfo = new TargetLibraryInfoImpl(Triple(TM->getTargetTriple()));
    if (Conf.Freestanding)
      PMB.LibraryInfo->disableAllFunctions();
    PMB.Inliner = createFunctionInliningPass();
    PMB.ExportSummary = ExportSummary;
    PMB.ImportSummary = ImportSummary;
    // Unconditionally verify input since it is not verified before this
    // point and has unknown origin.
    PMB.VerifyInput = true;
    PMB.VerifyOutput = !Conf.DisableVerify;
    PMB.LoopVectorize = true;
    PMB.SLPVectorize = true;
    PMB.OptLevel = Conf.OptLevel;
    PMB.PGOSampleUse = Conf.SampleProfile;
    PMB.EnablePGOCSInstrGen = Conf.RunCSIRInstr;
    if (!Conf.RunCSIRInstr && !Conf.CSIRProfile.empty()) {
      PMB.EnablePGOCSInstrUse = true;
      PMB.PGOInstrUse = Conf.CSIRProfile;
    }
    if (IsThinLTO)
      PMB.populateThinLTOPassManager(passes);
    else
      PMB.populateLTOPassManager(passes);
    passes.run(Mod);
  }
  
  bool lto::opt(const Config &Conf, TargetMachine *TM, unsigned Task, Module &Mod,
                bool IsThinLTO, ModuleSummaryIndex *ExportSummary,
                const ModuleSummaryIndex *ImportSummary,
                const std::vector<uint8_t> &CmdArgs) {
    if (EmbedBitcode == LTOBitcodeEmbedding::EmbedPostMergePreOptimized) {
      // FIXME: the motivation for capturing post-merge bitcode and command line
      // is replicating the compilation environment from bitcode, without needing
      // to understand the dependencies (the functions to be imported). This
      // assumes a clang - based invocation, case in which we have the command
      // line.
      // It's not very clear how the above motivation would map in the
      // linker-based case, so we currently don't plumb the command line args in
      // that case.
      if (CmdArgs.empty())
        LLVM_DEBUG(
            dbgs() << "Post-(Thin)LTO merge bitcode embedding was requested, but "
                      "command line arguments are not available");
      llvm::EmbedBitcodeInModule(Mod, llvm::MemoryBufferRef(),
                                 /*EmbedBitcode*/ true, /*EmbedCmdline*/ true,
                                 /*Cmdline*/ CmdArgs);
    }
    // FIXME: Plumb the combined index into the new pass manager.
    if (Conf.UseNewPM || !Conf.OptPipeline.empty()) {
      runNewPMPasses(Conf, Mod, TM, Conf.OptLevel, IsThinLTO, ExportSummary,
                     ImportSummary);
    } else {
      runOldPMPasses(Conf, Mod, TM, IsThinLTO, ExportSummary, ImportSummary);
    }
    return !Conf.PostOptModuleHook || Conf.PostOptModuleHook(Task, Mod);
  }
  
  static void codegen(const Config &Conf, TargetMachine *TM,
                      AddStreamFn AddStream, unsigned Task, Module &Mod,
                      const ModuleSummaryIndex &CombinedIndex) {
    if (Conf.PreCodeGenModuleHook && !Conf.PreCodeGenModuleHook(Task, Mod))
      return;
  
    if (EmbedBitcode == LTOBitcodeEmbedding::EmbedOptimized)
      llvm::EmbedBitcodeInModule(Mod, llvm::MemoryBufferRef(),
                                 /*EmbedBitcode*/ true,
                                 /*EmbedCmdline*/ false,
                                 /*CmdArgs*/ std::vector<uint8_t>());
  
    std::unique_ptr<ToolOutputFile> DwoOut;
    SmallString<1024> DwoFile(Conf.SplitDwarfOutput);
    if (!Conf.DwoDir.empty()) {
      std::error_code EC;
      if (auto EC = llvm::sys::fs::create_directories(Conf.DwoDir))
        report_fatal_error(Twine("Failed to create directory ") + Conf.DwoDir +
                           ": " + EC.message());
  
      DwoFile = Conf.DwoDir;
      sys::path::append(DwoFile, std::to_string(Task) + ".dwo");
      TM->Options.MCOptions.SplitDwarfFile = std::string(DwoFile);
    } else
      TM->Options.MCOptions.SplitDwarfFile = Conf.SplitDwarfFile;
  
    if (!DwoFile.empty()) {
      std::error_code EC;
      DwoOut = std::make_unique<ToolOutputFile>(DwoFile, EC, sys::fs::OF_None);
      if (EC)
        report_fatal_error(Twine("Failed to open ") + DwoFile + ": " +
                           EC.message());
    }
  
    Expected<std::unique_ptr<CachedFileStream>> StreamOrErr = AddStream(Task);
    if (Error Err = StreamOrErr.takeError())
      report_fatal_error(std::move(Err));
    std::unique_ptr<CachedFileStream> &Stream = *StreamOrErr;
    legacy::PassManager CodeGenPasses;
    CodeGenPasses.add(
        createImmutableModuleSummaryIndexWrapperPass(&CombinedIndex));
    if (Conf.PreCodeGenPassesHook)
      Conf.PreCodeGenPassesHook(CodeGenPasses);
    if (TM->addPassesToEmitFile(CodeGenPasses, *Stream->OS,
                                DwoOut ? &DwoOut->os() : nullptr,
                                Conf.CGFileType))
      report_fatal_error("Failed to setup codegen");
    CodeGenPasses.run(Mod);
  
    if (DwoOut)
      DwoOut->keep();
  }
  
  static void splitCodeGen(const Config &C, TargetMachine *TM,
                           AddStreamFn AddStream,
                           unsigned ParallelCodeGenParallelismLevel, Module &Mod,
                           const ModuleSummaryIndex &CombinedIndex) {
    ThreadPool CodegenThreadPool(
        heavyweight_hardware_concurrency(ParallelCodeGenParallelismLevel));
    unsigned ThreadCount = 0;
    const Target *T = &TM->getTarget();
  
    SplitModule(
        Mod, 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, BCOS);
  
          // Enqueue the task
          CodegenThreadPool.async(
              [&](const SmallString<0> &BC, unsigned ThreadId) {
                LTOLLVMContext Ctx(C);
                Expected<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, T, *MPartInCtx);
  
                codegen(C, TM.get(), AddStream, ThreadId, *MPartInCtx,
                        CombinedIndex);
              },
              // Pass BC using std::move to ensure that it get moved rather than
              // copied into the thread's context.
              std::move(BC), ThreadCount++);
        },
        false);
  
    // Because the inner lambda (which runs in a worker thread) captures our local
    // variables, we need to wait for the worker threads to terminate before we
    // can leave the function scope.
    CodegenThreadPool.wait();
  }
  
  static Expected<const Target *> initAndLookupTarget(const Config &C,
                                                      Module &Mod) {
    if (!C.OverrideTriple.empty())
      Mod.setTargetTriple(C.OverrideTriple);
    else if (Mod.getTargetTriple().empty())
      Mod.setTargetTriple(C.DefaultTriple);
  
    std::string Msg;
    const Target *T = TargetRegistry::lookupTarget(Mod.getTargetTriple(), Msg);
    if (!T)
      return make_error<StringError>(Msg, inconvertibleErrorCode());
    return T;
  }
  
  Error lto::finalizeOptimizationRemarks(
      std::unique_ptr<ToolOutputFile> DiagOutputFile) {
    // Make sure we flush the diagnostic remarks file in case the linker doesn't
    // call the global destructors before exiting.
    if (!DiagOutputFile)
      return Error::success();
    DiagOutputFile->keep();
    DiagOutputFile->os().flush();
    return Error::success();
  }
  
  Error lto::backend(const Config &C, AddStreamFn AddStream,
                     unsigned ParallelCodeGenParallelismLevel, Module &Mod,
                     ModuleSummaryIndex &CombinedIndex) {
    Expected<const Target *> TOrErr = initAndLookupTarget(C, Mod);
    if (!TOrErr)
      return TOrErr.takeError();
  
    std::unique_ptr<TargetMachine> TM = createTargetMachine(C, *TOrErr, Mod);
  
    if (!C.CodeGenOnly) {
      if (!opt(C, TM.get(), 0, Mod, /*IsThinLTO=*/false,
               /*ExportSummary=*/&CombinedIndex, /*ImportSummary=*/nullptr,
               /*CmdArgs*/ std::vector<uint8_t>()))
        return Error::success();
    }
  
    if (ParallelCodeGenParallelismLevel == 1) {
      codegen(C, TM.get(), AddStream, 0, Mod, CombinedIndex);
    } else {
      splitCodeGen(C, TM.get(), AddStream, ParallelCodeGenParallelismLevel, Mod,
                   CombinedIndex);
    }
    return Error::success();
  }
  
  static void dropDeadSymbols(Module &Mod, const GVSummaryMapTy &DefinedGlobals,
                              const ModuleSummaryIndex &Index) {
    std::vector<GlobalValue*> DeadGVs;
    for (auto &GV : Mod.global_values())
      if (GlobalValueSummary *GVS = DefinedGlobals.lookup(GV.getGUID()))
        if (!Index.isGlobalValueLive(GVS)) {
          DeadGVs.push_back(&GV);
          convertToDeclaration(GV);
        }
  
    // Now that all dead bodies have been dropped, delete the actual objects
    // themselves when possible.
    for (GlobalValue *GV : DeadGVs) {
      GV->removeDeadConstantUsers();
      // Might reference something defined in native object (i.e. dropped a
      // non-prevailing IR def, but we need to keep the declaration).
      if (GV->use_empty())
        GV->eraseFromParent();
    }
  }
  
  Error lto::thinBackend(const Config &Conf, unsigned Task, AddStreamFn AddStream,
                         Module &Mod, const ModuleSummaryIndex &CombinedIndex,
                         const FunctionImporter::ImportMapTy &ImportList,
                         const GVSummaryMapTy &DefinedGlobals,
                         MapVector<StringRef, BitcodeModule> *ModuleMap,
                         const std::vector<uint8_t> &CmdArgs) {
    Expected<const Target *> TOrErr = initAndLookupTarget(Conf, Mod);
    if (!TOrErr)
      return TOrErr.takeError();
  
    std::unique_ptr<TargetMachine> TM = createTargetMachine(Conf, *TOrErr, Mod);
  
    // Setup optimization remarks.
    auto DiagFileOrErr = lto::setupLLVMOptimizationRemarks(
        Mod.getContext(), Conf.RemarksFilename, Conf.RemarksPasses,
        Conf.RemarksFormat, Conf.RemarksWithHotness, Conf.RemarksHotnessThreshold,
        Task);
    if (!DiagFileOrErr)
      return DiagFileOrErr.takeError();
    auto DiagnosticOutputFile = std::move(*DiagFileOrErr);
  
    // Set the partial sample profile ratio in the profile summary module flag of
    // the module, if applicable.
    Mod.setPartialSampleProfileRatio(CombinedIndex);
  
    if (Conf.CodeGenOnly) {
      codegen(Conf, TM.get(), AddStream, Task, Mod, CombinedIndex);
      return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
    }
  
    if (Conf.PreOptModuleHook && !Conf.PreOptModuleHook(Task, Mod))
      return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
  
    auto OptimizeAndCodegen =
        [&](Module &Mod, TargetMachine *TM,
            std::unique_ptr<ToolOutputFile> DiagnosticOutputFile) {
          if (!opt(Conf, TM, Task, Mod, /*IsThinLTO=*/true,
                   /*ExportSummary=*/nullptr, /*ImportSummary=*/&CombinedIndex,
                   CmdArgs))
            return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
  
          codegen(Conf, TM, AddStream, Task, Mod, CombinedIndex);
          return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
        };
  
    if (ThinLTOAssumeMerged)
      return OptimizeAndCodegen(Mod, TM.get(), std::move(DiagnosticOutputFile));
  
    // When linking an ELF shared object, dso_local should be dropped. We
    // conservatively do this for -fpic.
    bool ClearDSOLocalOnDeclarations =
        TM->getTargetTriple().isOSBinFormatELF() &&
        TM->getRelocationModel() != Reloc::Static &&
        Mod.getPIELevel() == PIELevel::Default;
    renameModuleForThinLTO(Mod, CombinedIndex, ClearDSOLocalOnDeclarations);
  
    dropDeadSymbols(Mod, DefinedGlobals, CombinedIndex);
  
    thinLTOFinalizeInModule(Mod, DefinedGlobals, /*PropagateAttrs=*/true);
  
    if (Conf.PostPromoteModuleHook && !Conf.PostPromoteModuleHook(Task, Mod))
      return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
  
    if (!DefinedGlobals.empty())
      thinLTOInternalizeModule(Mod, DefinedGlobals);
  
    if (Conf.PostInternalizeModuleHook &&
        !Conf.PostInternalizeModuleHook(Task, Mod))
      return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
  
    auto ModuleLoader = [&](StringRef Identifier) {
      assert(Mod.getContext().isODRUniquingDebugTypes() &&
             "ODR Type uniquing should be enabled on the context");
      if (ModuleMap) {
        auto I = ModuleMap->find(Identifier);
        assert(I != ModuleMap->end());
        return I->second.getLazyModule(Mod.getContext(),
                                       /*ShouldLazyLoadMetadata=*/true,
                                       /*IsImporting*/ true);
      }
  
      ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> MBOrErr =
          llvm::MemoryBuffer::getFile(Identifier);
      if (!MBOrErr)
        return Expected<std::unique_ptr<llvm::Module>>(make_error<StringError>(
            Twine("Error loading imported file ") + Identifier + " : ",
            MBOrErr.getError()));
  
      Expected<BitcodeModule> BMOrErr = findThinLTOModule(**MBOrErr);
      if (!BMOrErr)
        return Expected<std::unique_ptr<llvm::Module>>(make_error<StringError>(
            Twine("Error loading imported file ") + Identifier + " : " +
                toString(BMOrErr.takeError()),
            inconvertibleErrorCode()));
  
      Expected<std::unique_ptr<Module>> MOrErr =
          BMOrErr->getLazyModule(Mod.getContext(),
                                 /*ShouldLazyLoadMetadata=*/true,
                                 /*IsImporting*/ true);
      if (MOrErr)
        (*MOrErr)->setOwnedMemoryBuffer(std::move(*MBOrErr));
      return MOrErr;
    };
  
    FunctionImporter Importer(CombinedIndex, ModuleLoader,
                              ClearDSOLocalOnDeclarations);
    if (Error Err = Importer.importFunctions(Mod, ImportList).takeError())
      return Err;
  
    if (Conf.PostImportModuleHook && !Conf.PostImportModuleHook(Task, Mod))
      return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
  
    return OptimizeAndCodegen(Mod, TM.get(), std::move(DiagnosticOutputFile));
  }
  
  BitcodeModule *lto::findThinLTOModule(MutableArrayRef<BitcodeModule> BMs) {
    if (ThinLTOAssumeMerged && BMs.size() == 1)
      return BMs.begin();
  
    for (BitcodeModule &BM : BMs) {
      Expected<BitcodeLTOInfo> LTOInfo = BM.getLTOInfo();
      if (LTOInfo && LTOInfo->IsThinLTO)
        return &BM;
    }
    return nullptr;
  }
  
  Expected<BitcodeModule> lto::findThinLTOModule(MemoryBufferRef MBRef) {
    Expected<std::vector<BitcodeModule>> BMsOrErr = getBitcodeModuleList(MBRef);
    if (!BMsOrErr)
      return BMsOrErr.takeError();
  
    // The bitcode file may contain multiple modules, we want the one that is
    // marked as being the ThinLTO module.
    if (const BitcodeModule *Bm = lto::findThinLTOModule(*BMsOrErr))
      return *Bm;
  
    return make_error<StringError>("Could not find module summary",
                                   inconvertibleErrorCode());
  }
  
  bool lto::initImportList(const Module &M,
                           const ModuleSummaryIndex &CombinedIndex,
                           FunctionImporter::ImportMapTy &ImportList) {
    if (ThinLTOAssumeMerged)
      return true;
    // We can simply import the values mentioned in the combined index, since
    // we should only invoke this using the individual indexes written out
    // via a WriteIndexesThinBackend.
    for (const auto &GlobalList : CombinedIndex) {
      // Ignore entries for undefined references.
      if (GlobalList.second.SummaryList.empty())
        continue;
  
      auto GUID = GlobalList.first;
      for (const auto &Summary : GlobalList.second.SummaryList) {
        // Skip the summaries for the importing module. These are included to
        // e.g. record required linkage changes.
        if (Summary->modulePath() == M.getModuleIdentifier())
          continue;
        // Add an entry to provoke importing by thinBackend.
        ImportList[Summary->modulePath()].insert(GUID);
      }
    }
    return true;
  }
  