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/LLVMRemarkStreamer.h"
24 #include "llvm/IR/LegacyPassManager.h"
25 #include "llvm/IR/PassManager.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/Passes/PassPlugin.h"
32 #include "llvm/Passes/StandardInstrumentations.h"
33 #include "llvm/Support/Error.h"
34 #include "llvm/Support/FileSystem.h"
35 #include "llvm/Support/MemoryBuffer.h"
36 #include "llvm/Support/Path.h"
37 #include "llvm/Support/Program.h"
38 #include "llvm/Support/SmallVectorMemoryBuffer.h"
39 #include "llvm/Support/TargetRegistry.h"
40 #include "llvm/Support/ThreadPool.h"
41 #include "llvm/Support/raw_ostream.h"
42 #include "llvm/Target/TargetMachine.h"
43 #include "llvm/Transforms/IPO.h"
44 #include "llvm/Transforms/IPO/PassManagerBuilder.h"
45 #include "llvm/Transforms/Scalar/LoopPassManager.h"
46 #include "llvm/Transforms/Utils/FunctionImportUtils.h"
47 #include "llvm/Transforms/Utils/SplitModule.h"
48 
49 using namespace llvm;
50 using namespace lto;
51 
52 LLVM_ATTRIBUTE_NORETURN static void reportOpenError(StringRef Path, Twine Msg) {
53   errs() << "failed to open " << Path << ": " << Msg << '\n';
54   errs().flush();
55   exit(1);
56 }
57 
58 Error Config::addSaveTemps(std::string OutputFileName,
59                            bool UseInputModulePath) {
60   ShouldDiscardValueNames = false;
61 
62   std::error_code EC;
63   ResolutionFile = std::make_unique<raw_fd_ostream>(
64       OutputFileName + "resolution.txt", EC, sys::fs::OpenFlags::OF_Text);
65   if (EC) {
66     ResolutionFile.reset();
67     return errorCodeToError(EC);
68   }
69 
70   auto setHook = [&](std::string PathSuffix, ModuleHookFn &Hook) {
71     // Keep track of the hook provided by the linker, which also needs to run.
72     ModuleHookFn LinkerHook = Hook;
73     Hook = [=](unsigned Task, const Module &M) {
74       // If the linker's hook returned false, we need to pass that result
75       // through.
76       if (LinkerHook && !LinkerHook(Task, M))
77         return false;
78 
79       std::string PathPrefix;
80       // If this is the combined module (not a ThinLTO backend compile) or the
81       // user hasn't requested using the input module's path, emit to a file
82       // named from the provided OutputFileName with the Task ID appended.
83       if (M.getModuleIdentifier() == "ld-temp.o" || !UseInputModulePath) {
84         PathPrefix = OutputFileName;
85         if (Task != (unsigned)-1)
86           PathPrefix += utostr(Task) + ".";
87       } else
88         PathPrefix = M.getModuleIdentifier() + ".";
89       std::string Path = PathPrefix + PathSuffix + ".bc";
90       std::error_code EC;
91       raw_fd_ostream OS(Path, EC, sys::fs::OpenFlags::OF_None);
92       // Because -save-temps is a debugging feature, we report the error
93       // directly and exit.
94       if (EC)
95         reportOpenError(Path, EC.message());
96       WriteBitcodeToFile(M, OS, /*ShouldPreserveUseListOrder=*/false);
97       return true;
98     };
99   };
100 
101   setHook("0.preopt", PreOptModuleHook);
102   setHook("1.promote", PostPromoteModuleHook);
103   setHook("2.internalize", PostInternalizeModuleHook);
104   setHook("3.import", PostImportModuleHook);
105   setHook("4.opt", PostOptModuleHook);
106   setHook("5.precodegen", PreCodeGenModuleHook);
107 
108   CombinedIndexHook =
109       [=](const ModuleSummaryIndex &Index,
110           const DenseSet<GlobalValue::GUID> &GUIDPreservedSymbols) {
111         std::string Path = OutputFileName + "index.bc";
112         std::error_code EC;
113         raw_fd_ostream OS(Path, EC, sys::fs::OpenFlags::OF_None);
114         // Because -save-temps is a debugging feature, we report the error
115         // directly and exit.
116         if (EC)
117           reportOpenError(Path, EC.message());
118         WriteIndexToFile(Index, OS);
119 
120         Path = OutputFileName + "index.dot";
121         raw_fd_ostream OSDot(Path, EC, sys::fs::OpenFlags::OF_None);
122         if (EC)
123           reportOpenError(Path, EC.message());
124         Index.exportToDot(OSDot, GUIDPreservedSymbols);
125         return true;
126       };
127 
128   return Error::success();
129 }
130 
131 #define HANDLE_EXTENSION(Ext)                                                  \
132   llvm::PassPluginLibraryInfo get##Ext##PluginInfo();
133 #include "llvm/Support/Extension.def"
134 
135 static void RegisterPassPlugins(ArrayRef<std::string> PassPlugins,
136                                 PassBuilder &PB) {
137 #define HANDLE_EXTENSION(Ext)                                                  \
138   get##Ext##PluginInfo().RegisterPassBuilderCallbacks(PB);
139 #include "llvm/Support/Extension.def"
140 
141   // Load requested pass plugins and let them register pass builder callbacks
142   for (auto &PluginFN : PassPlugins) {
143     auto PassPlugin = PassPlugin::Load(PluginFN);
144     if (!PassPlugin) {
145       errs() << "Failed to load passes from '" << PluginFN
146              << "'. Request ignored.\n";
147       continue;
148     }
149 
150     PassPlugin->registerPassBuilderCallbacks(PB);
151   }
152 }
153 
154 namespace {
155 
156 std::unique_ptr<TargetMachine>
157 createTargetMachine(const Config &Conf, const Target *TheTarget, Module &M) {
158   StringRef TheTriple = M.getTargetTriple();
159   SubtargetFeatures Features;
160   Features.getDefaultSubtargetFeatures(Triple(TheTriple));
161   for (const std::string &A : Conf.MAttrs)
162     Features.AddFeature(A);
163 
164   Reloc::Model RelocModel;
165   if (Conf.RelocModel)
166     RelocModel = *Conf.RelocModel;
167   else
168     RelocModel =
169         M.getPICLevel() == PICLevel::NotPIC ? Reloc::Static : Reloc::PIC_;
170 
171   Optional<CodeModel::Model> CodeModel;
172   if (Conf.CodeModel)
173     CodeModel = *Conf.CodeModel;
174   else
175     CodeModel = M.getCodeModel();
176 
177   return std::unique_ptr<TargetMachine>(TheTarget->createTargetMachine(
178       TheTriple, Conf.CPU, Features.getString(), Conf.Options, RelocModel,
179       CodeModel, Conf.CGOptLevel));
180 }
181 
182 static void runNewPMPasses(const Config &Conf, Module &Mod, TargetMachine *TM,
183                            unsigned OptLevel, bool IsThinLTO,
184                            ModuleSummaryIndex *ExportSummary,
185                            const ModuleSummaryIndex *ImportSummary) {
186   Optional<PGOOptions> PGOOpt;
187   if (!Conf.SampleProfile.empty())
188     PGOOpt = PGOOptions(Conf.SampleProfile, "", Conf.ProfileRemapping,
189                         PGOOptions::SampleUse, PGOOptions::NoCSAction, true);
190   else if (Conf.RunCSIRInstr) {
191     PGOOpt = PGOOptions("", Conf.CSIRProfile, Conf.ProfileRemapping,
192                         PGOOptions::IRUse, PGOOptions::CSIRInstr);
193   } else if (!Conf.CSIRProfile.empty()) {
194     PGOOpt = PGOOptions(Conf.CSIRProfile, "", Conf.ProfileRemapping,
195                         PGOOptions::IRUse, PGOOptions::CSIRUse);
196   }
197 
198   PassInstrumentationCallbacks PIC;
199   StandardInstrumentations SI;
200   SI.registerCallbacks(PIC);
201   PassBuilder PB(TM, Conf.PTO, PGOOpt, &PIC);
202   AAManager AA;
203 
204   // Parse a custom AA pipeline if asked to.
205   if (auto Err = PB.parseAAPipeline(AA, "default"))
206     report_fatal_error("Error parsing default AA pipeline");
207 
208   RegisterPassPlugins(Conf.PassPlugins, PB);
209 
210   LoopAnalysisManager LAM(Conf.DebugPassManager);
211   FunctionAnalysisManager FAM(Conf.DebugPassManager);
212   CGSCCAnalysisManager CGAM(Conf.DebugPassManager);
213   ModuleAnalysisManager MAM(Conf.DebugPassManager);
214 
215   // Register the AA manager first so that our version is the one used.
216   FAM.registerPass([&] { return std::move(AA); });
217 
218   // Register all the basic analyses with the managers.
219   PB.registerModuleAnalyses(MAM);
220   PB.registerCGSCCAnalyses(CGAM);
221   PB.registerFunctionAnalyses(FAM);
222   PB.registerLoopAnalyses(LAM);
223   PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
224 
225   ModulePassManager MPM(Conf.DebugPassManager);
226   // FIXME (davide): verify the input.
227 
228   PassBuilder::OptimizationLevel OL;
229 
230   switch (OptLevel) {
231   default:
232     llvm_unreachable("Invalid optimization level");
233   case 0:
234     OL = PassBuilder::OptimizationLevel::O0;
235     break;
236   case 1:
237     OL = PassBuilder::OptimizationLevel::O1;
238     break;
239   case 2:
240     OL = PassBuilder::OptimizationLevel::O2;
241     break;
242   case 3:
243     OL = PassBuilder::OptimizationLevel::O3;
244     break;
245   }
246 
247   if (IsThinLTO)
248     MPM = PB.buildThinLTODefaultPipeline(OL, Conf.DebugPassManager,
249                                          ImportSummary);
250   else
251     MPM = PB.buildLTODefaultPipeline(OL, Conf.DebugPassManager, ExportSummary);
252   MPM.run(Mod, MAM);
253 
254   // FIXME (davide): verify the output.
255 }
256 
257 static void runNewPMCustomPasses(const Config &Conf, Module &Mod,
258                                  TargetMachine *TM, std::string PipelineDesc,
259                                  std::string AAPipelineDesc,
260                                  bool DisableVerify) {
261   PassBuilder PB(TM);
262   AAManager AA;
263 
264   // Parse a custom AA pipeline if asked to.
265   if (!AAPipelineDesc.empty())
266     if (auto Err = PB.parseAAPipeline(AA, AAPipelineDesc))
267       report_fatal_error("unable to parse AA pipeline description '" +
268                          AAPipelineDesc + "': " + toString(std::move(Err)));
269 
270   RegisterPassPlugins(Conf.PassPlugins, PB);
271 
272   LoopAnalysisManager LAM;
273   FunctionAnalysisManager FAM;
274   CGSCCAnalysisManager CGAM;
275   ModuleAnalysisManager MAM;
276 
277   // Register the AA manager first so that our version is the one used.
278   FAM.registerPass([&] { return std::move(AA); });
279 
280   // Register all the basic analyses with the managers.
281   PB.registerModuleAnalyses(MAM);
282   PB.registerCGSCCAnalyses(CGAM);
283   PB.registerFunctionAnalyses(FAM);
284   PB.registerLoopAnalyses(LAM);
285   PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
286 
287   ModulePassManager MPM;
288 
289   // Always verify the input.
290   MPM.addPass(VerifierPass());
291 
292   // Now, add all the passes we've been requested to.
293   if (auto Err = PB.parsePassPipeline(MPM, PipelineDesc))
294     report_fatal_error("unable to parse pass pipeline description '" +
295                        PipelineDesc + "': " + toString(std::move(Err)));
296 
297   if (!DisableVerify)
298     MPM.addPass(VerifierPass());
299   MPM.run(Mod, MAM);
300 }
301 
302 static void runOldPMPasses(const Config &Conf, Module &Mod, TargetMachine *TM,
303                            bool IsThinLTO, ModuleSummaryIndex *ExportSummary,
304                            const ModuleSummaryIndex *ImportSummary) {
305   legacy::PassManager passes;
306   passes.add(createTargetTransformInfoWrapperPass(TM->getTargetIRAnalysis()));
307 
308   PassManagerBuilder PMB;
309   PMB.LibraryInfo = new TargetLibraryInfoImpl(Triple(TM->getTargetTriple()));
310   PMB.Inliner = createFunctionInliningPass();
311   PMB.ExportSummary = ExportSummary;
312   PMB.ImportSummary = ImportSummary;
313   // Unconditionally verify input since it is not verified before this
314   // point and has unknown origin.
315   PMB.VerifyInput = true;
316   PMB.VerifyOutput = !Conf.DisableVerify;
317   PMB.LoopVectorize = true;
318   PMB.SLPVectorize = true;
319   PMB.OptLevel = Conf.OptLevel;
320   PMB.PGOSampleUse = Conf.SampleProfile;
321   PMB.EnablePGOCSInstrGen = Conf.RunCSIRInstr;
322   if (!Conf.RunCSIRInstr && !Conf.CSIRProfile.empty()) {
323     PMB.EnablePGOCSInstrUse = true;
324     PMB.PGOInstrUse = Conf.CSIRProfile;
325   }
326   if (IsThinLTO)
327     PMB.populateThinLTOPassManager(passes);
328   else
329     PMB.populateLTOPassManager(passes);
330   passes.run(Mod);
331 }
332 
333 bool opt(const Config &Conf, TargetMachine *TM, unsigned Task, Module &Mod,
334          bool IsThinLTO, ModuleSummaryIndex *ExportSummary,
335          const ModuleSummaryIndex *ImportSummary) {
336   // FIXME: Plumb the combined index into the new pass manager.
337   if (!Conf.OptPipeline.empty())
338     runNewPMCustomPasses(Conf, Mod, TM, Conf.OptPipeline, Conf.AAPipeline,
339                          Conf.DisableVerify);
340   else if (Conf.UseNewPM)
341     runNewPMPasses(Conf, Mod, TM, Conf.OptLevel, IsThinLTO, ExportSummary,
342                    ImportSummary);
343   else
344     runOldPMPasses(Conf, Mod, TM, IsThinLTO, ExportSummary, ImportSummary);
345   return !Conf.PostOptModuleHook || Conf.PostOptModuleHook(Task, Mod);
346 }
347 
348 static cl::opt<bool> EmbedBitcode(
349     "lto-embed-bitcode", cl::init(false),
350     cl::desc("Embed LLVM bitcode in object files produced by LTO"));
351 
352 static void EmitBitcodeSection(Module &M, const Config &Conf) {
353   if (!EmbedBitcode)
354     return;
355   SmallVector<char, 0> Buffer;
356   raw_svector_ostream OS(Buffer);
357   WriteBitcodeToFile(M, OS);
358 
359   std::unique_ptr<MemoryBuffer> Buf(
360       new SmallVectorMemoryBuffer(std::move(Buffer)));
361   llvm::EmbedBitcodeInModule(M, Buf->getMemBufferRef(), /*EmbedBitcode*/ true,
362                              /*EmbedMarker*/ false, /*CmdArgs*/ nullptr);
363 }
364 
365 void codegen(const Config &Conf, TargetMachine *TM, AddStreamFn AddStream,
366              unsigned Task, Module &Mod) {
367   if (Conf.PreCodeGenModuleHook && !Conf.PreCodeGenModuleHook(Task, Mod))
368     return;
369 
370   EmitBitcodeSection(Mod, Conf);
371 
372   std::unique_ptr<ToolOutputFile> DwoOut;
373   SmallString<1024> DwoFile(Conf.SplitDwarfOutput);
374   if (!Conf.DwoDir.empty()) {
375     std::error_code EC;
376     if (auto EC = llvm::sys::fs::create_directories(Conf.DwoDir))
377       report_fatal_error("Failed to create directory " + Conf.DwoDir + ": " +
378                          EC.message());
379 
380     DwoFile = Conf.DwoDir;
381     sys::path::append(DwoFile, std::to_string(Task) + ".dwo");
382     TM->Options.MCOptions.SplitDwarfFile = std::string(DwoFile);
383   } else
384     TM->Options.MCOptions.SplitDwarfFile = Conf.SplitDwarfFile;
385 
386   if (!DwoFile.empty()) {
387     std::error_code EC;
388     DwoOut = std::make_unique<ToolOutputFile>(DwoFile, EC, sys::fs::OF_None);
389     if (EC)
390       report_fatal_error("Failed to open " + DwoFile + ": " + EC.message());
391   }
392 
393   auto Stream = AddStream(Task);
394   legacy::PassManager CodeGenPasses;
395   if (TM->addPassesToEmitFile(CodeGenPasses, *Stream->OS,
396                               DwoOut ? &DwoOut->os() : nullptr,
397                               Conf.CGFileType))
398     report_fatal_error("Failed to setup codegen");
399   CodeGenPasses.run(Mod);
400 
401   if (DwoOut)
402     DwoOut->keep();
403 }
404 
405 void splitCodeGen(const Config &C, TargetMachine *TM, AddStreamFn AddStream,
406                   unsigned ParallelCodeGenParallelismLevel,
407                   std::unique_ptr<Module> Mod) {
408   ThreadPool CodegenThreadPool(
409       heavyweight_hardware_concurrency(ParallelCodeGenParallelismLevel));
410   unsigned ThreadCount = 0;
411   const Target *T = &TM->getTarget();
412 
413   SplitModule(
414       std::move(Mod), ParallelCodeGenParallelismLevel,
415       [&](std::unique_ptr<Module> MPart) {
416         // We want to clone the module in a new context to multi-thread the
417         // codegen. We do it by serializing partition modules to bitcode
418         // (while still on the main thread, in order to avoid data races) and
419         // spinning up new threads which deserialize the partitions into
420         // separate contexts.
421         // FIXME: Provide a more direct way to do this in LLVM.
422         SmallString<0> BC;
423         raw_svector_ostream BCOS(BC);
424         WriteBitcodeToFile(*MPart, BCOS);
425 
426         // Enqueue the task
427         CodegenThreadPool.async(
428             [&](const SmallString<0> &BC, unsigned ThreadId) {
429               LTOLLVMContext Ctx(C);
430               Expected<std::unique_ptr<Module>> MOrErr = parseBitcodeFile(
431                   MemoryBufferRef(StringRef(BC.data(), BC.size()), "ld-temp.o"),
432                   Ctx);
433               if (!MOrErr)
434                 report_fatal_error("Failed to read bitcode");
435               std::unique_ptr<Module> MPartInCtx = std::move(MOrErr.get());
436 
437               std::unique_ptr<TargetMachine> TM =
438                   createTargetMachine(C, T, *MPartInCtx);
439 
440               codegen(C, TM.get(), AddStream, ThreadId, *MPartInCtx);
441             },
442             // Pass BC using std::move to ensure that it get moved rather than
443             // copied into the thread's context.
444             std::move(BC), ThreadCount++);
445       },
446       false);
447 
448   // Because the inner lambda (which runs in a worker thread) captures our local
449   // variables, we need to wait for the worker threads to terminate before we
450   // can leave the function scope.
451   CodegenThreadPool.wait();
452 }
453 
454 Expected<const Target *> initAndLookupTarget(const Config &C, Module &Mod) {
455   if (!C.OverrideTriple.empty())
456     Mod.setTargetTriple(C.OverrideTriple);
457   else if (Mod.getTargetTriple().empty())
458     Mod.setTargetTriple(C.DefaultTriple);
459 
460   std::string Msg;
461   const Target *T = TargetRegistry::lookupTarget(Mod.getTargetTriple(), Msg);
462   if (!T)
463     return make_error<StringError>(Msg, inconvertibleErrorCode());
464   return T;
465 }
466 }
467 
468 Error lto::finalizeOptimizationRemarks(
469     std::unique_ptr<ToolOutputFile> DiagOutputFile) {
470   // Make sure we flush the diagnostic remarks file in case the linker doesn't
471   // call the global destructors before exiting.
472   if (!DiagOutputFile)
473     return Error::success();
474   DiagOutputFile->keep();
475   DiagOutputFile->os().flush();
476   return Error::success();
477 }
478 
479 Error lto::backend(const Config &C, AddStreamFn AddStream,
480                    unsigned ParallelCodeGenParallelismLevel,
481                    std::unique_ptr<Module> Mod,
482                    ModuleSummaryIndex &CombinedIndex) {
483   Expected<const Target *> TOrErr = initAndLookupTarget(C, *Mod);
484   if (!TOrErr)
485     return TOrErr.takeError();
486 
487   std::unique_ptr<TargetMachine> TM = createTargetMachine(C, *TOrErr, *Mod);
488 
489   if (!C.CodeGenOnly) {
490     if (!opt(C, TM.get(), 0, *Mod, /*IsThinLTO=*/false,
491              /*ExportSummary=*/&CombinedIndex, /*ImportSummary=*/nullptr))
492       return Error::success();
493   }
494 
495   if (ParallelCodeGenParallelismLevel == 1) {
496     codegen(C, TM.get(), AddStream, 0, *Mod);
497   } else {
498     splitCodeGen(C, TM.get(), AddStream, ParallelCodeGenParallelismLevel,
499                  std::move(Mod));
500   }
501   return Error::success();
502 }
503 
504 static void dropDeadSymbols(Module &Mod, const GVSummaryMapTy &DefinedGlobals,
505                             const ModuleSummaryIndex &Index) {
506   std::vector<GlobalValue*> DeadGVs;
507   for (auto &GV : Mod.global_values())
508     if (GlobalValueSummary *GVS = DefinedGlobals.lookup(GV.getGUID()))
509       if (!Index.isGlobalValueLive(GVS)) {
510         DeadGVs.push_back(&GV);
511         convertToDeclaration(GV);
512       }
513 
514   // Now that all dead bodies have been dropped, delete the actual objects
515   // themselves when possible.
516   for (GlobalValue *GV : DeadGVs) {
517     GV->removeDeadConstantUsers();
518     // Might reference something defined in native object (i.e. dropped a
519     // non-prevailing IR def, but we need to keep the declaration).
520     if (GV->use_empty())
521       GV->eraseFromParent();
522   }
523 }
524 
525 Error lto::thinBackend(const Config &Conf, unsigned Task, AddStreamFn AddStream,
526                        Module &Mod, const ModuleSummaryIndex &CombinedIndex,
527                        const FunctionImporter::ImportMapTy &ImportList,
528                        const GVSummaryMapTy &DefinedGlobals,
529                        MapVector<StringRef, BitcodeModule> &ModuleMap) {
530   Expected<const Target *> TOrErr = initAndLookupTarget(Conf, Mod);
531   if (!TOrErr)
532     return TOrErr.takeError();
533 
534   std::unique_ptr<TargetMachine> TM = createTargetMachine(Conf, *TOrErr, Mod);
535 
536   // Setup optimization remarks.
537   auto DiagFileOrErr = lto::setupLLVMOptimizationRemarks(
538       Mod.getContext(), Conf.RemarksFilename, Conf.RemarksPasses,
539       Conf.RemarksFormat, Conf.RemarksWithHotness, Task);
540   if (!DiagFileOrErr)
541     return DiagFileOrErr.takeError();
542   auto DiagnosticOutputFile = std::move(*DiagFileOrErr);
543 
544   if (Conf.CodeGenOnly) {
545     codegen(Conf, TM.get(), AddStream, Task, Mod);
546     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
547   }
548 
549   if (Conf.PreOptModuleHook && !Conf.PreOptModuleHook(Task, Mod))
550     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
551 
552   // When linking an ELF shared object, dso_local should be dropped. We
553   // conservatively do this for -fpic.
554   bool ClearDSOLocalOnDeclarations =
555       TM->getTargetTriple().isOSBinFormatELF() &&
556       TM->getRelocationModel() != Reloc::Static &&
557       Mod.getPIELevel() == PIELevel::Default;
558   renameModuleForThinLTO(Mod, CombinedIndex, ClearDSOLocalOnDeclarations);
559 
560   dropDeadSymbols(Mod, DefinedGlobals, CombinedIndex);
561 
562   thinLTOResolvePrevailingInModule(Mod, DefinedGlobals);
563 
564   if (Conf.PostPromoteModuleHook && !Conf.PostPromoteModuleHook(Task, Mod))
565     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
566 
567   if (!DefinedGlobals.empty())
568     thinLTOInternalizeModule(Mod, DefinedGlobals);
569 
570   if (Conf.PostInternalizeModuleHook &&
571       !Conf.PostInternalizeModuleHook(Task, Mod))
572     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
573 
574   auto ModuleLoader = [&](StringRef Identifier) {
575     assert(Mod.getContext().isODRUniquingDebugTypes() &&
576            "ODR Type uniquing should be enabled on the context");
577     auto I = ModuleMap.find(Identifier);
578     assert(I != ModuleMap.end());
579     return I->second.getLazyModule(Mod.getContext(),
580                                    /*ShouldLazyLoadMetadata=*/true,
581                                    /*IsImporting*/ true);
582   };
583 
584   FunctionImporter Importer(CombinedIndex, ModuleLoader,
585                             ClearDSOLocalOnDeclarations);
586   if (Error Err = Importer.importFunctions(Mod, ImportList).takeError())
587     return Err;
588 
589   if (Conf.PostImportModuleHook && !Conf.PostImportModuleHook(Task, Mod))
590     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
591 
592   if (!opt(Conf, TM.get(), Task, Mod, /*IsThinLTO=*/true,
593            /*ExportSummary=*/nullptr, /*ImportSummary=*/&CombinedIndex))
594     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
595 
596   codegen(Conf, TM.get(), AddStream, Task, Mod);
597   return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
598 }
599