lib/LTO/LTOBackend.cpp

   1 //===-LTOBackend.cpp - LLVM Link Time Optimizer Backend -------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements the "backend" phase of LTO, i.e. it performs
  10 // optimization and code generation on a loaded module. It is generally used
  11 // internally by the LTO class but can also be used independently, for example
  12 // to implement a standalone ThinLTO backend.
  13 //
  14 //===----------------------------------------------------------------------===//
  15
  16 #include "llvm/LTO/LTOBackend.h"
  17 #include "llvm/Analysis/AliasAnalysis.h"
  18 #include "llvm/Analysis/CGSCCPassManager.h"
  19 #include "llvm/Analysis/TargetLibraryInfo.h"
  20 #include "llvm/Analysis/TargetTransformInfo.h"
  21 #include "llvm/Bitcode/BitcodeReader.h"
  22 #include "llvm/Bitcode/BitcodeWriter.h"
  23 #include "llvm/IR/LegacyPassManager.h"
  24 #include "llvm/IR/PassManager.h"
  25 #include "llvm/IR/RemarkStreamer.h"
  26 #include "llvm/IR/Verifier.h"
  27 #include "llvm/LTO/LTO.h"
  28 #include "llvm/MC/SubtargetFeature.h"
  29 #include "llvm/Object/ModuleSymbolTable.h"
  30 #include "llvm/Passes/PassBuilder.h"
  31 #include "llvm/Support/Error.h"
  32 #include "llvm/Support/FileSystem.h"
  33 #include "llvm/Support/MemoryBuffer.h"
  34 #include "llvm/Support/Path.h"
  35 #include "llvm/Support/Program.h"
  36 #include "llvm/Support/TargetRegistry.h"
  37 #include "llvm/Support/ThreadPool.h"
  38 #include "llvm/Support/raw_ostream.h"
  39 #include "llvm/Target/TargetMachine.h"
  40 #include "llvm/Transforms/IPO.h"
  41 #include "llvm/Transforms/IPO/PassManagerBuilder.h"
  42 #include "llvm/Transforms/Scalar/LoopPassManager.h"
  43 #include "llvm/Transforms/Utils/FunctionImportUtils.h"
  44 #include "llvm/Transforms/Utils/SplitModule.h"
  45
  46 using namespace llvm;
  47 using namespace lto;
  48
  49 LLVM_ATTRIBUTE_NORETURN static void reportOpenError(StringRef Path, Twine Msg) {
  50   errs() << "failed to open " << Path << ": " << Msg << '\n';
  51   errs().flush();
  52   exit(1);
  53 }
  54
  55 Error Config::addSaveTemps(std::string OutputFileName,
  56                            bool UseInputModulePath) {
  57   ShouldDiscardValueNames = false;
  58
  59   std::error_code EC;
  60   ResolutionFile = llvm::make_unique<raw_fd_ostream>(
  61       OutputFileName + "resolution.txt", EC, sys::fs::OpenFlags::OF_Text);
  62   if (EC)
  63     return errorCodeToError(EC);
  64
  65   auto setHook = [&](std::string PathSuffix, ModuleHookFn &Hook) {
  66     // Keep track of the hook provided by the linker, which also needs to run.
  67     ModuleHookFn LinkerHook = Hook;
  68     Hook = [=](unsigned Task, const Module &M) {
  69       // If the linker's hook returned false, we need to pass that result
  70       // through.
  71       if (LinkerHook && !LinkerHook(Task, M))
  72         return false;
  73
  74       std::string PathPrefix;
  75       // If this is the combined module (not a ThinLTO backend compile) or the
  76       // user hasn't requested using the input module's path, emit to a file
  77       // named from the provided OutputFileName with the Task ID appended.
  78       if (M.getModuleIdentifier() == "ld-temp.o" || !UseInputModulePath) {
  79         PathPrefix = OutputFileName;
  80         if (Task != (unsigned)-1)
  81           PathPrefix += utostr(Task) + ".";
  82       } else
  83         PathPrefix = M.getModuleIdentifier() + ".";
  84       std::string Path = PathPrefix + PathSuffix + ".bc";
  85       std::error_code EC;
  86       raw_fd_ostream OS(Path, EC, sys::fs::OpenFlags::OF_None);
  87       // Because -save-temps is a debugging feature, we report the error
  88       // directly and exit.
  89       if (EC)
  90         reportOpenError(Path, EC.message());
  91       WriteBitcodeToFile(M, OS, /*ShouldPreserveUseListOrder=*/false);
  92       return true;
  93     };
  94   };
  95
  96   setHook("0.preopt", PreOptModuleHook);
  97   setHook("1.promote", PostPromoteModuleHook);
  98   setHook("2.internalize", PostInternalizeModuleHook);
  99   setHook("3.import", PostImportModuleHook);
 100   setHook("4.opt", PostOptModuleHook);
 101   setHook("5.precodegen", PreCodeGenModuleHook);
 102
 103   CombinedIndexHook = [=](const ModuleSummaryIndex &Index) {
 104     std::string Path = OutputFileName + "index.bc";
 105     std::error_code EC;
 106     raw_fd_ostream OS(Path, EC, sys::fs::OpenFlags::OF_None);
 107     // Because -save-temps is a debugging feature, we report the error
 108     // directly and exit.
 109     if (EC)
 110       reportOpenError(Path, EC.message());
 111     WriteIndexToFile(Index, OS);
 112
 113     Path = OutputFileName + "index.dot";
 114     raw_fd_ostream OSDot(Path, EC, sys::fs::OpenFlags::OF_None);
 115     if (EC)
 116       reportOpenError(Path, EC.message());
 117     Index.exportToDot(OSDot);
 118     return true;
 119   };
 120
 121   return Error::success();
 122 }
 123
 124 namespace {
 125
 126 std::unique_ptr<TargetMachine>
 127 createTargetMachine(Config &Conf, const Target *TheTarget, Module &M) {
 128   StringRef TheTriple = M.getTargetTriple();
 129   SubtargetFeatures Features;
 130   Features.getDefaultSubtargetFeatures(Triple(TheTriple));
 131   for (const std::string &A : Conf.MAttrs)
 132     Features.AddFeature(A);
 133
 134   Reloc::Model RelocModel;
 135   if (Conf.RelocModel)
 136     RelocModel = *Conf.RelocModel;
 137   else
 138     RelocModel =
 139         M.getPICLevel() == PICLevel::NotPIC ? Reloc::Static : Reloc::PIC_;
 140
 141   Optional<CodeModel::Model> CodeModel;
 142   if (Conf.CodeModel)
 143     CodeModel = *Conf.CodeModel;
 144   else
 145     CodeModel = M.getCodeModel();
 146
 147   return std::unique_ptr<TargetMachine>(TheTarget->createTargetMachine(
 148       TheTriple, Conf.CPU, Features.getString(), Conf.Options, RelocModel,
 149       CodeModel, Conf.CGOptLevel));
 150 }
 151
 152 static void runNewPMPasses(Config &Conf, Module &Mod, TargetMachine *TM,
 153                            unsigned OptLevel, bool IsThinLTO,
 154                            ModuleSummaryIndex *ExportSummary,
 155                            const ModuleSummaryIndex *ImportSummary) {
 156   Optional<PGOOptions> PGOOpt;
 157   if (!Conf.SampleProfile.empty())
 158     PGOOpt = PGOOptions(Conf.SampleProfile, "", Conf.ProfileRemapping,
 159                         PGOOptions::SampleUse, PGOOptions::NoCSAction, true);
 160   else if (Conf.RunCSIRInstr) {
 161     PGOOpt = PGOOptions("", Conf.CSIRProfile, Conf.ProfileRemapping,
 162                         PGOOptions::IRUse, PGOOptions::CSIRInstr);
 163   } else if (!Conf.CSIRProfile.empty()) {
 164     PGOOpt = PGOOptions(Conf.CSIRProfile, "", Conf.ProfileRemapping,
 165                         PGOOptions::IRUse, PGOOptions::CSIRUse);
 166   }
 167
 168   PassBuilder PB(TM, PipelineTuningOptions(), PGOOpt);
 169   AAManager AA;
 170
 171   // Parse a custom AA pipeline if asked to.
 172   if (auto Err = PB.parseAAPipeline(AA, "default"))
 173     report_fatal_error("Error parsing default AA pipeline");
 174
 175   LoopAnalysisManager LAM(Conf.DebugPassManager);
 176   FunctionAnalysisManager FAM(Conf.DebugPassManager);
 177   CGSCCAnalysisManager CGAM(Conf.DebugPassManager);
 178   ModuleAnalysisManager MAM(Conf.DebugPassManager);
 179
 180   // Register the AA manager first so that our version is the one used.
 181   FAM.registerPass([&] { return std::move(AA); });
 182
 183   // Register all the basic analyses with the managers.
 184   PB.registerModuleAnalyses(MAM);
 185   PB.registerCGSCCAnalyses(CGAM);
 186   PB.registerFunctionAnalyses(FAM);
 187   PB.registerLoopAnalyses(LAM);
 188   PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
 189
 190   ModulePassManager MPM(Conf.DebugPassManager);
 191   // FIXME (davide): verify the input.
 192
 193   PassBuilder::OptimizationLevel OL;
 194
 195   switch (OptLevel) {
 196   default:
 197     llvm_unreachable("Invalid optimization level");
 198   case 0:
 199     OL = PassBuilder::O0;
 200     break;
 201   case 1:
 202     OL = PassBuilder::O1;
 203     break;
 204   case 2:
 205     OL = PassBuilder::O2;
 206     break;
 207   case 3:
 208     OL = PassBuilder::O3;
 209     break;
 210   }
 211
 212   if (IsThinLTO)
 213     MPM = PB.buildThinLTODefaultPipeline(OL, Conf.DebugPassManager,
 214                                          ImportSummary);
 215   else
 216     MPM = PB.buildLTODefaultPipeline(OL, Conf.DebugPassManager, ExportSummary);
 217   MPM.run(Mod, MAM);
 218
 219   // FIXME (davide): verify the output.
 220 }
 221
 222 static void runNewPMCustomPasses(Module &Mod, TargetMachine *TM,
 223                                  std::string PipelineDesc,
 224                                  std::string AAPipelineDesc,
 225                                  bool DisableVerify) {
 226   PassBuilder PB(TM);
 227   AAManager AA;
 228
 229   // Parse a custom AA pipeline if asked to.
 230   if (!AAPipelineDesc.empty())
 231     if (auto Err = PB.parseAAPipeline(AA, AAPipelineDesc))
 232       report_fatal_error("unable to parse AA pipeline description '" +
 233                          AAPipelineDesc + "': " + toString(std::move(Err)));
 234
 235   LoopAnalysisManager LAM;
 236   FunctionAnalysisManager FAM;
 237   CGSCCAnalysisManager CGAM;
 238   ModuleAnalysisManager MAM;
 239
 240   // Register the AA manager first so that our version is the one used.
 241   FAM.registerPass([&] { return std::move(AA); });
 242
 243   // Register all the basic analyses with the managers.
 244   PB.registerModuleAnalyses(MAM);
 245   PB.registerCGSCCAnalyses(CGAM);
 246   PB.registerFunctionAnalyses(FAM);
 247   PB.registerLoopAnalyses(LAM);
 248   PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
 249
 250   ModulePassManager MPM;
 251
 252   // Always verify the input.
 253   MPM.addPass(VerifierPass());
 254
 255   // Now, add all the passes we've been requested to.
 256   if (auto Err = PB.parsePassPipeline(MPM, PipelineDesc))
 257     report_fatal_error("unable to parse pass pipeline description '" +
 258                        PipelineDesc + "': " + toString(std::move(Err)));
 259
 260   if (!DisableVerify)
 261     MPM.addPass(VerifierPass());
 262   MPM.run(Mod, MAM);
 263 }
 264
 265 static void runOldPMPasses(Config &Conf, Module &Mod, TargetMachine *TM,
 266                            bool IsThinLTO, ModuleSummaryIndex *ExportSummary,
 267                            const ModuleSummaryIndex *ImportSummary) {
 268   legacy::PassManager passes;
 269   passes.add(createTargetTransformInfoWrapperPass(TM->getTargetIRAnalysis()));
 270
 271   PassManagerBuilder PMB;
 272   PMB.LibraryInfo = new TargetLibraryInfoImpl(Triple(TM->getTargetTriple()));
 273   PMB.Inliner = createFunctionInliningPass();
 274   PMB.ExportSummary = ExportSummary;
 275   PMB.ImportSummary = ImportSummary;
 276   // Unconditionally verify input since it is not verified before this
 277   // point and has unknown origin.
 278   PMB.VerifyInput = true;
 279   PMB.VerifyOutput = !Conf.DisableVerify;
 280   PMB.LoopVectorize = true;
 281   PMB.SLPVectorize = true;
 282   PMB.OptLevel = Conf.OptLevel;
 283   PMB.PGOSampleUse = Conf.SampleProfile;
 284   PMB.EnablePGOCSInstrGen = Conf.RunCSIRInstr;
 285   if (!Conf.RunCSIRInstr && !Conf.CSIRProfile.empty()) {
 286     PMB.EnablePGOCSInstrUse = true;
 287     PMB.PGOInstrUse = Conf.CSIRProfile;
 288   }
 289   if (IsThinLTO)
 290     PMB.populateThinLTOPassManager(passes);
 291   else
 292     PMB.populateLTOPassManager(passes);
 293   passes.run(Mod);
 294 }
 295
 296 bool opt(Config &Conf, TargetMachine *TM, unsigned Task, Module &Mod,
 297          bool IsThinLTO, ModuleSummaryIndex *ExportSummary,
 298          const ModuleSummaryIndex *ImportSummary) {
 299   // FIXME: Plumb the combined index into the new pass manager.
 300   if (!Conf.OptPipeline.empty())
 301     runNewPMCustomPasses(Mod, TM, Conf.OptPipeline, Conf.AAPipeline,
 302                          Conf.DisableVerify);
 303   else if (Conf.UseNewPM)
 304     runNewPMPasses(Conf, Mod, TM, Conf.OptLevel, IsThinLTO, ExportSummary,
 305                    ImportSummary);
 306   else
 307     runOldPMPasses(Conf, Mod, TM, IsThinLTO, ExportSummary, ImportSummary);
 308   return !Conf.PostOptModuleHook || Conf.PostOptModuleHook(Task, Mod);
 309 }
 310
 311 void codegen(Config &Conf, TargetMachine *TM, AddStreamFn AddStream,
 312              unsigned Task, Module &Mod) {
 313   if (Conf.PreCodeGenModuleHook && !Conf.PreCodeGenModuleHook(Task, Mod))
 314     return;
 315
 316   std::unique_ptr<ToolOutputFile> DwoOut;
 317   SmallString<1024> DwoFile(Conf.SplitDwarfOutput);
 318   if (!Conf.DwoDir.empty()) {
 319     std::error_code EC;
 320     if (auto EC = llvm::sys::fs::create_directories(Conf.DwoDir))
 321       report_fatal_error("Failed to create directory " + Conf.DwoDir + ": " +
 322                          EC.message());
 323
 324     DwoFile = Conf.DwoDir;
 325     sys::path::append(DwoFile, std::to_string(Task) + ".dwo");
 326     TM->Options.MCOptions.SplitDwarfFile = DwoFile.str().str();
 327   } else
 328     TM->Options.MCOptions.SplitDwarfFile = Conf.SplitDwarfFile;
 329
 330   if (!DwoFile.empty()) {
 331     std::error_code EC;
 332     DwoOut = llvm::make_unique<ToolOutputFile>(DwoFile, EC, sys::fs::OF_None);
 333     if (EC)
 334       report_fatal_error("Failed to open " + DwoFile + ": " + EC.message());
 335   }
 336
 337   auto Stream = AddStream(Task);
 338   legacy::PassManager CodeGenPasses;
 339   if (TM->addPassesToEmitFile(CodeGenPasses, *Stream->OS,
 340                               DwoOut ? &DwoOut->os() : nullptr,
 341                               Conf.CGFileType))
 342     report_fatal_error("Failed to setup codegen");
 343   CodeGenPasses.run(Mod);
 344
 345   if (DwoOut)
 346     DwoOut->keep();
 347 }
 348
 349 void splitCodeGen(Config &C, TargetMachine *TM, AddStreamFn AddStream,
 350                   unsigned ParallelCodeGenParallelismLevel,
 351                   std::unique_ptr<Module> Mod) {
 352   ThreadPool CodegenThreadPool(ParallelCodeGenParallelismLevel);
 353   unsigned ThreadCount = 0;
 354   const Target *T = &TM->getTarget();
 355
 356   SplitModule(
 357       std::move(Mod), ParallelCodeGenParallelismLevel,
 358       [&](std::unique_ptr<Module> MPart) {
 359         // We want to clone the module in a new context to multi-thread the
 360         // codegen. We do it by serializing partition modules to bitcode
 361         // (while still on the main thread, in order to avoid data races) and
 362         // spinning up new threads which deserialize the partitions into
 363         // separate contexts.
 364         // FIXME: Provide a more direct way to do this in LLVM.
 365         SmallString<0> BC;
 366         raw_svector_ostream BCOS(BC);
 367         WriteBitcodeToFile(*MPart, BCOS);
 368
 369         // Enqueue the task
 370         CodegenThreadPool.async(
 371             [&](const SmallString<0> &BC, unsigned ThreadId) {
 372               LTOLLVMContext Ctx(C);
 373               Expected<std::unique_ptr<Module>> MOrErr = parseBitcodeFile(
 374                   MemoryBufferRef(StringRef(BC.data(), BC.size()), "ld-temp.o"),
 375                   Ctx);
 376               if (!MOrErr)
 377                 report_fatal_error("Failed to read bitcode");
 378               std::unique_ptr<Module> MPartInCtx = std::move(MOrErr.get());
 379
 380               std::unique_ptr<TargetMachine> TM =
 381                   createTargetMachine(C, T, *MPartInCtx);
 382
 383               codegen(C, TM.get(), AddStream, ThreadId, *MPartInCtx);
 384             },
 385             // Pass BC using std::move to ensure that it get moved rather than
 386             // copied into the thread's context.
 387             std::move(BC), ThreadCount++);
 388       },
 389       false);
 390
 391   // Because the inner lambda (which runs in a worker thread) captures our local
 392   // variables, we need to wait for the worker threads to terminate before we
 393   // can leave the function scope.
 394   CodegenThreadPool.wait();
 395 }
 396
 397 Expected<const Target *> initAndLookupTarget(Config &C, Module &Mod) {
 398   if (!C.OverrideTriple.empty())
 399     Mod.setTargetTriple(C.OverrideTriple);
 400   else if (Mod.getTargetTriple().empty())
 401     Mod.setTargetTriple(C.DefaultTriple);
 402
 403   std::string Msg;
 404   const Target *T = TargetRegistry::lookupTarget(Mod.getTargetTriple(), Msg);
 405   if (!T)
 406     return make_error<StringError>(Msg, inconvertibleErrorCode());
 407   return T;
 408 }
 409
 410 }
 411
 412 static Error
 413 finalizeOptimizationRemarks(std::unique_ptr<ToolOutputFile> DiagOutputFile) {
 414   // Make sure we flush the diagnostic remarks file in case the linker doesn't
 415   // call the global destructors before exiting.
 416   if (!DiagOutputFile)
 417     return Error::success();
 418   DiagOutputFile->keep();
 419   DiagOutputFile->os().flush();
 420   return Error::success();
 421 }
 422
 423 Error lto::backend(Config &C, AddStreamFn AddStream,
 424                    unsigned ParallelCodeGenParallelismLevel,
 425                    std::unique_ptr<Module> Mod,
 426                    ModuleSummaryIndex &CombinedIndex) {
 427   Expected<const Target *> TOrErr = initAndLookupTarget(C, *Mod);
 428   if (!TOrErr)
 429     return TOrErr.takeError();
 430
 431   std::unique_ptr<TargetMachine> TM = createTargetMachine(C, *TOrErr, *Mod);
 432
 433   // Setup optimization remarks.
 434   auto DiagFileOrErr = lto::setupOptimizationRemarks(
 435       Mod->getContext(), C.RemarksFilename, C.RemarksPasses, C.RemarksFormat,
 436       C.RemarksWithHotness);
 437   if (!DiagFileOrErr)
 438     return DiagFileOrErr.takeError();
 439   auto DiagnosticOutputFile = std::move(*DiagFileOrErr);
 440
 441   if (!C.CodeGenOnly) {
 442     if (!opt(C, TM.get(), 0, *Mod, /*IsThinLTO=*/false,
 443              /*ExportSummary=*/&CombinedIndex, /*ImportSummary=*/nullptr))
 444       return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
 445   }
 446
 447   if (ParallelCodeGenParallelismLevel == 1) {
 448     codegen(C, TM.get(), AddStream, 0, *Mod);
 449   } else {
 450     splitCodeGen(C, TM.get(), AddStream, ParallelCodeGenParallelismLevel,
 451                  std::move(Mod));
 452   }
 453   return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
 454 }
 455
 456 static void dropDeadSymbols(Module &Mod, const GVSummaryMapTy &DefinedGlobals,
 457                             const ModuleSummaryIndex &Index) {
 458   std::vector<GlobalValue*> DeadGVs;
 459   for (auto &GV : Mod.global_values())
 460     if (GlobalValueSummary *GVS = DefinedGlobals.lookup(GV.getGUID()))
 461       if (!Index.isGlobalValueLive(GVS)) {
 462         DeadGVs.push_back(&GV);
 463         convertToDeclaration(GV);
 464       }
 465
 466   // Now that all dead bodies have been dropped, delete the actual objects
 467   // themselves when possible.
 468   for (GlobalValue *GV : DeadGVs) {
 469     GV->removeDeadConstantUsers();
 470     // Might reference something defined in native object (i.e. dropped a
 471     // non-prevailing IR def, but we need to keep the declaration).
 472     if (GV->use_empty())
 473       GV->eraseFromParent();
 474   }
 475 }
 476
 477 Error lto::thinBackend(Config &Conf, unsigned Task, AddStreamFn AddStream,
 478                        Module &Mod, const ModuleSummaryIndex &CombinedIndex,
 479                        const FunctionImporter::ImportMapTy &ImportList,
 480                        const GVSummaryMapTy &DefinedGlobals,
 481                        MapVector<StringRef, BitcodeModule> &ModuleMap) {
 482   Expected<const Target *> TOrErr = initAndLookupTarget(Conf, Mod);
 483   if (!TOrErr)
 484     return TOrErr.takeError();
 485
 486   std::unique_ptr<TargetMachine> TM = createTargetMachine(Conf, *TOrErr, Mod);
 487
 488   // Setup optimization remarks.
 489   auto DiagFileOrErr = lto::setupOptimizationRemarks(
 490       Mod.getContext(), Conf.RemarksFilename, Conf.RemarksPasses,
 491       Conf.RemarksFormat, Conf.RemarksWithHotness, Task);
 492   if (!DiagFileOrErr)
 493     return DiagFileOrErr.takeError();
 494   auto DiagnosticOutputFile = std::move(*DiagFileOrErr);
 495
 496   if (Conf.CodeGenOnly) {
 497     codegen(Conf, TM.get(), AddStream, Task, Mod);
 498     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
 499   }
 500
 501   if (Conf.PreOptModuleHook && !Conf.PreOptModuleHook(Task, Mod))
 502     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
 503
 504   renameModuleForThinLTO(Mod, CombinedIndex);
 505
 506   dropDeadSymbols(Mod, DefinedGlobals, CombinedIndex);
 507
 508   thinLTOResolvePrevailingInModule(Mod, DefinedGlobals);
 509
 510   if (Conf.PostPromoteModuleHook && !Conf.PostPromoteModuleHook(Task, Mod))
 511     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
 512
 513   if (!DefinedGlobals.empty())
 514     thinLTOInternalizeModule(Mod, DefinedGlobals);
 515
 516   if (Conf.PostInternalizeModuleHook &&
 517       !Conf.PostInternalizeModuleHook(Task, Mod))
 518     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
 519
 520   auto ModuleLoader = [&](StringRef Identifier) {
 521     assert(Mod.getContext().isODRUniquingDebugTypes() &&
 522            "ODR Type uniquing should be enabled on the context");
 523     auto I = ModuleMap.find(Identifier);
 524     assert(I != ModuleMap.end());
 525     return I->second.getLazyModule(Mod.getContext(),
 526                                    /*ShouldLazyLoadMetadata=*/true,
 527                                    /*IsImporting*/ true);
 528   };
 529
 530   FunctionImporter Importer(CombinedIndex, ModuleLoader);
 531   if (Error Err = Importer.importFunctions(Mod, ImportList).takeError())
 532     return Err;
 533
 534   if (Conf.PostImportModuleHook && !Conf.PostImportModuleHook(Task, Mod))
 535     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
 536
 537   if (!opt(Conf, TM.get(), Task, Mod, /*IsThinLTO=*/true,
 538            /*ExportSummary=*/nullptr, /*ImportSummary=*/&CombinedIndex))
 539     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
 540
 541   codegen(Conf, TM.get(), AddStream, Task, Mod);
 542   return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
 543 }