text: //===-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/AliasAnalysis.h"
  #include "llvm/Analysis/CGSCCPassManager.h"
  #include "llvm/Analysis/TargetLibraryInfo.h"
  #include "llvm/Analysis/TargetTransformInfo.h"
  #include "llvm/Bitcode/BitcodeReader.h"
  #include "llvm/Bitcode/BitcodeWriter.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/Object/ModuleSymbolTable.h"
  #include "llvm/Passes/PassBuilder.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/raw_ostream.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/Scalar/LoopPassManager.h"
  #include "llvm/Transforms/Utils/FunctionImportUtils.h"
  #include "llvm/Transforms/Utils/SplitModule.h"
  
  using namespace llvm;
  using namespace lto;
  
  LLVM_ATTRIBUTE_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 = llvm::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 = [=](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::F_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) {
      std::string Path = OutputFileName + "index.bc";
      std::error_code EC;
      raw_fd_ostream OS(Path, EC, sys::fs::OpenFlags::F_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::F_None);
      if (EC)
        reportOpenError(Path, EC.message());
      Index.exportToDot(OSDot);
      return true;
    };
  
    return Error::success();
  }
  
  namespace {
  
  std::unique_ptr<TargetMachine>
  createTargetMachine(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);
  
    Reloc::Model RelocModel;
    if (Conf.RelocModel)
      RelocModel = *Conf.RelocModel;
    else
      RelocModel =
          M.getPICLevel() == PICLevel::NotPIC ? Reloc::Static : Reloc::PIC_;
  
    Optional<CodeModel::Model> CodeModel;
    if (Conf.CodeModel)
      CodeModel = *Conf.CodeModel;
    else
      CodeModel = M.getCodeModel();
  
    return std::unique_ptr<TargetMachine>(TheTarget->createTargetMachine(
        TheTriple, Conf.CPU, Features.getString(), Conf.Options, RelocModel,
        CodeModel, Conf.CGOptLevel));
  }
  
  static void runNewPMPasses(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, false, true);
  
    PassBuilder PB(TM, PGOOpt);
    AAManager AA;
  
    // Parse a custom AA pipeline if asked to.
    if (!PB.parseAAPipeline(AA, "default"))
      report_fatal_error("Error parsing default AA pipeline");
  
    LoopAnalysisManager LAM(Conf.DebugPassManager);
    FunctionAnalysisManager FAM(Conf.DebugPassManager);
    CGSCCAnalysisManager CGAM(Conf.DebugPassManager);
    ModuleAnalysisManager MAM(Conf.DebugPassManager);
  
    // 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(Conf.DebugPassManager);
    // FIXME (davide): verify the input.
  
    PassBuilder::OptimizationLevel OL;
  
    switch (OptLevel) {
    default:
      llvm_unreachable("Invalid optimization level");
    case 0:
      OL = PassBuilder::O0;
      break;
    case 1:
      OL = PassBuilder::O1;
      break;
    case 2:
      OL = PassBuilder::O2;
      break;
    case 3:
      OL = PassBuilder::O3;
      break;
    }
  
    if (IsThinLTO)
      MPM = PB.buildThinLTODefaultPipeline(OL, Conf.DebugPassManager,
                                           ImportSummary);
    else
      MPM = PB.buildLTODefaultPipeline(OL, Conf.DebugPassManager, ExportSummary);
    MPM.run(Mod, MAM);
  
    // FIXME (davide): verify the output.
  }
  
  static void runNewPMCustomPasses(Module &Mod, TargetMachine *TM,
                                   std::string PipelineDesc,
                                   std::string AAPipelineDesc,
                                   bool DisableVerify) {
    PassBuilder PB(TM);
    AAManager AA;
  
    // Parse a custom AA pipeline if asked to.
    if (!AAPipelineDesc.empty())
      if (!PB.parseAAPipeline(AA, AAPipelineDesc))
        report_fatal_error("unable to parse AA pipeline description: " +
                           AAPipelineDesc);
  
    LoopAnalysisManager LAM;
    FunctionAnalysisManager FAM;
    CGSCCAnalysisManager CGAM;
    ModuleAnalysisManager MAM;
  
    // Register the AA manager first so that our version is the one used.
    FAM.registerPass([&] { return std::move(AA); });
  
    // Register all the basic analyses with the managers.
    PB.registerModuleAnalyses(MAM);
    PB.registerCGSCCAnalyses(CGAM);
    PB.registerFunctionAnalyses(FAM);
    PB.registerLoopAnalyses(LAM);
    PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
  
    ModulePassManager MPM;
  
    // Always verify the input.
    MPM.addPass(VerifierPass());
  
    // Now, add all the passes we've been requested to.
    if (!PB.parsePassPipeline(MPM, PipelineDesc))
      report_fatal_error("unable to parse pass pipeline description: " +
                         PipelineDesc);
  
    if (!DisableVerify)
      MPM.addPass(VerifierPass());
    MPM.run(Mod, MAM);
  }
  
  static void runOldPMPasses(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()));
    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;
    if (IsThinLTO)
      PMB.populateThinLTOPassManager(passes);
    else
      PMB.populateLTOPassManager(passes);
    passes.run(Mod);
  }
  
  bool opt(Config &Conf, TargetMachine *TM, unsigned Task, Module &Mod,
           bool IsThinLTO, ModuleSummaryIndex *ExportSummary,
           const ModuleSummaryIndex *ImportSummary) {
    // FIXME: Plumb the combined index into the new pass manager.
    if (!Conf.OptPipeline.empty())
      runNewPMCustomPasses(Mod, TM, Conf.OptPipeline, Conf.AAPipeline,
                           Conf.DisableVerify);
    else if (Conf.UseNewPM)
      runNewPMPasses(Conf, Mod, TM, Conf.OptLevel, IsThinLTO, ExportSummary,
                     ImportSummary);
    else
      runOldPMPasses(Conf, Mod, TM, IsThinLTO, ExportSummary, ImportSummary);
    return !Conf.PostOptModuleHook || Conf.PostOptModuleHook(Task, Mod);
  }
  
  void codegen(Config &Conf, TargetMachine *TM, AddStreamFn AddStream,
               unsigned Task, Module &Mod) {
    if (Conf.PreCodeGenModuleHook && !Conf.PreCodeGenModuleHook(Task, Mod))
      return;
  
    std::unique_ptr<ToolOutputFile> DwoOut;
    SmallString<1024> DwoFile(Conf.DwoPath);
    if (!Conf.DwoDir.empty()) {
      std::error_code EC;
      if (auto EC = llvm::sys::fs::create_directories(Conf.DwoDir))
        report_fatal_error("Failed to create directory " + Conf.DwoDir + ": " +
                           EC.message());
  
      DwoFile = Conf.DwoDir;
      sys::path::append(DwoFile, std::to_string(Task) + ".dwo");
    }
  
    if (!DwoFile.empty()) {
      std::error_code EC;
      TM->Options.MCOptions.SplitDwarfFile = DwoFile.str().str();
      DwoOut = llvm::make_unique<ToolOutputFile>(DwoFile, EC, sys::fs::F_None);
      if (EC)
        report_fatal_error("Failed to open " + DwoFile + ": " + EC.message());
    }
  
    auto Stream = AddStream(Task);
    legacy::PassManager 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();
  }
  
  void splitCodeGen(Config &C, TargetMachine *TM, AddStreamFn AddStream,
                    unsigned ParallelCodeGenParallelismLevel,
                    std::unique_ptr<Module> Mod) {
    ThreadPool CodegenThreadPool(ParallelCodeGenParallelismLevel);
    unsigned ThreadCount = 0;
    const Target *T = &TM->getTarget();
  
    SplitModule(
        std::move(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);
              },
              // 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();
  }
  
  Expected<const Target *> initAndLookupTarget(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;
  }
  
  }
  
  static Error
  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(Config &C, AddStreamFn AddStream,
                     unsigned ParallelCodeGenParallelismLevel,
                     std::unique_ptr<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);
  
    // Setup optimization remarks.
    auto DiagFileOrErr = lto::setupOptimizationRemarks(
        Mod->getContext(), C.RemarksFilename, C.RemarksWithHotness);
    if (!DiagFileOrErr)
      return DiagFileOrErr.takeError();
    auto DiagnosticOutputFile = std::move(*DiagFileOrErr);
  
    if (!C.CodeGenOnly) {
      if (!opt(C, TM.get(), 0, *Mod, /*IsThinLTO=*/false,
               /*ExportSummary=*/&CombinedIndex, /*ImportSummary=*/nullptr))
        return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
    }
  
    if (ParallelCodeGenParallelismLevel == 1) {
      codegen(C, TM.get(), AddStream, 0, *Mod);
    } else {
      splitCodeGen(C, TM.get(), AddStream, ParallelCodeGenParallelismLevel,
                   std::move(Mod));
    }
    return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
  }
  
  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(Config &Conf, unsigned Task, AddStreamFn AddStream,
                         Module &Mod, const ModuleSummaryIndex &CombinedIndex,
                         const FunctionImporter::ImportMapTy &ImportList,
                         const GVSummaryMapTy &DefinedGlobals,
                         MapVector<StringRef, BitcodeModule> &ModuleMap) {
    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::setupOptimizationRemarks(
        Mod.getContext(), Conf.RemarksFilename, Conf.RemarksWithHotness, Task);
    if (!DiagFileOrErr)
      return DiagFileOrErr.takeError();
    auto DiagnosticOutputFile = std::move(*DiagFileOrErr);
  
    if (Conf.CodeGenOnly) {
      codegen(Conf, TM.get(), AddStream, Task, Mod);
      return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
    }
  
    if (Conf.PreOptModuleHook && !Conf.PreOptModuleHook(Task, Mod))
      return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
  
    renameModuleForThinLTO(Mod, CombinedIndex);
  
    dropDeadSymbols(Mod, DefinedGlobals, CombinedIndex);
  
    thinLTOResolveWeakForLinkerModule(Mod, DefinedGlobals);
  
    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");
      auto I = ModuleMap.find(Identifier);
      assert(I != ModuleMap.end());
      return I->second.getLazyModule(Mod.getContext(),
                                     /*ShouldLazyLoadMetadata=*/true,
                                     /*IsImporting*/ true);
    };
  
    FunctionImporter Importer(CombinedIndex, ModuleLoader);
    if (Error Err = Importer.importFunctions(Mod, ImportList).takeError())
      return Err;
  
    if (Conf.PostImportModuleHook && !Conf.PostImportModuleHook(Task, Mod))
      return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
  
    if (!opt(Conf, TM.get(), Task, Mod, /*IsThinLTO=*/true,
             /*ExportSummary=*/nullptr, /*ImportSummary=*/&CombinedIndex))
      return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
  
    codegen(Conf, TM.get(), AddStream, Task, Mod);
    return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
  }
  