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/ModuleSummaryAnalysis.h"
20 #include "llvm/Analysis/TargetLibraryInfo.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/TargetRegistry.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/ThreadPool.h"
39 #include "llvm/Support/ToolOutputFile.h"
40 #include "llvm/Support/VirtualFileSystem.h"
41 #include "llvm/Support/raw_ostream.h"
42 #include "llvm/Target/TargetMachine.h"
43 #include "llvm/TargetParser/SubtargetFeature.h"
44 #include "llvm/Transforms/IPO/WholeProgramDevirt.h"
45 #include "llvm/Transforms/Scalar/LoopPassManager.h"
46 #include "llvm/Transforms/Utils/FunctionImportUtils.h"
47 #include "llvm/Transforms/Utils/SplitModule.h"
48 #include <optional>
49 
50 using namespace llvm;
51 using namespace lto;
52 
53 #define DEBUG_TYPE "lto-backend"
54 
55 enum class LTOBitcodeEmbedding {
56   DoNotEmbed = 0,
57   EmbedOptimized = 1,
58   EmbedPostMergePreOptimized = 2
59 };
60 
61 static cl::opt<LTOBitcodeEmbedding> EmbedBitcode(
62     "lto-embed-bitcode", cl::init(LTOBitcodeEmbedding::DoNotEmbed),
63     cl::values(clEnumValN(LTOBitcodeEmbedding::DoNotEmbed, "none",
64                           "Do not embed"),
65                clEnumValN(LTOBitcodeEmbedding::EmbedOptimized, "optimized",
66                           "Embed after all optimization passes"),
67                clEnumValN(LTOBitcodeEmbedding::EmbedPostMergePreOptimized,
68                           "post-merge-pre-opt",
69                           "Embed post merge, but before optimizations")),
70     cl::desc("Embed LLVM bitcode in object files produced by LTO"));
71 
72 static cl::opt<bool> ThinLTOAssumeMerged(
73     "thinlto-assume-merged", cl::init(false),
74     cl::desc("Assume the input has already undergone ThinLTO function "
75              "importing and the other pre-optimization pipeline changes."));
76 
77 namespace llvm {
78 extern cl::opt<bool> NoPGOWarnMismatch;
79 }
80 
81 [[noreturn]] static void reportOpenError(StringRef Path, Twine Msg) {
82   errs() << "failed to open " << Path << ": " << Msg << '\n';
83   errs().flush();
84   exit(1);
85 }
86 
87 Error Config::addSaveTemps(std::string OutputFileName, bool UseInputModulePath,
88                            const DenseSet<StringRef> &SaveTempsArgs) {
89   ShouldDiscardValueNames = false;
90 
91   std::error_code EC;
92   if (SaveTempsArgs.empty() || SaveTempsArgs.contains("resolution")) {
93     ResolutionFile =
94         std::make_unique<raw_fd_ostream>(OutputFileName + "resolution.txt", EC,
95                                          sys::fs::OpenFlags::OF_TextWithCRLF);
96     if (EC) {
97       ResolutionFile.reset();
98       return errorCodeToError(EC);
99     }
100   }
101 
102   auto setHook = [&](std::string PathSuffix, ModuleHookFn &Hook) {
103     // Keep track of the hook provided by the linker, which also needs to run.
104     ModuleHookFn LinkerHook = Hook;
105     Hook = [=](unsigned Task, const Module &M) {
106       // If the linker's hook returned false, we need to pass that result
107       // through.
108       if (LinkerHook && !LinkerHook(Task, M))
109         return false;
110 
111       std::string PathPrefix;
112       // If this is the combined module (not a ThinLTO backend compile) or the
113       // user hasn't requested using the input module's path, emit to a file
114       // named from the provided OutputFileName with the Task ID appended.
115       if (M.getModuleIdentifier() == "ld-temp.o" || !UseInputModulePath) {
116         PathPrefix = OutputFileName;
117         if (Task != (unsigned)-1)
118           PathPrefix += utostr(Task) + ".";
119       } else
120         PathPrefix = M.getModuleIdentifier() + ".";
121       std::string Path = PathPrefix + PathSuffix + ".bc";
122       std::error_code EC;
123       raw_fd_ostream OS(Path, EC, sys::fs::OpenFlags::OF_None);
124       // Because -save-temps is a debugging feature, we report the error
125       // directly and exit.
126       if (EC)
127         reportOpenError(Path, EC.message());
128       WriteBitcodeToFile(M, OS, /*ShouldPreserveUseListOrder=*/false);
129       return true;
130     };
131   };
132 
133   auto SaveCombinedIndex =
134       [=](const ModuleSummaryIndex &Index,
135           const DenseSet<GlobalValue::GUID> &GUIDPreservedSymbols) {
136         std::string Path = OutputFileName + "index.bc";
137         std::error_code EC;
138         raw_fd_ostream OS(Path, EC, sys::fs::OpenFlags::OF_None);
139         // Because -save-temps is a debugging feature, we report the error
140         // directly and exit.
141         if (EC)
142           reportOpenError(Path, EC.message());
143         writeIndexToFile(Index, OS);
144 
145         Path = OutputFileName + "index.dot";
146         raw_fd_ostream OSDot(Path, EC, sys::fs::OpenFlags::OF_None);
147         if (EC)
148           reportOpenError(Path, EC.message());
149         Index.exportToDot(OSDot, GUIDPreservedSymbols);
150         return true;
151       };
152 
153   if (SaveTempsArgs.empty()) {
154     setHook("0.preopt", PreOptModuleHook);
155     setHook("1.promote", PostPromoteModuleHook);
156     setHook("2.internalize", PostInternalizeModuleHook);
157     setHook("3.import", PostImportModuleHook);
158     setHook("4.opt", PostOptModuleHook);
159     setHook("5.precodegen", PreCodeGenModuleHook);
160     CombinedIndexHook = SaveCombinedIndex;
161   } else {
162     if (SaveTempsArgs.contains("preopt"))
163       setHook("0.preopt", PreOptModuleHook);
164     if (SaveTempsArgs.contains("promote"))
165       setHook("1.promote", PostPromoteModuleHook);
166     if (SaveTempsArgs.contains("internalize"))
167       setHook("2.internalize", PostInternalizeModuleHook);
168     if (SaveTempsArgs.contains("import"))
169       setHook("3.import", PostImportModuleHook);
170     if (SaveTempsArgs.contains("opt"))
171       setHook("4.opt", PostOptModuleHook);
172     if (SaveTempsArgs.contains("precodegen"))
173       setHook("5.precodegen", PreCodeGenModuleHook);
174     if (SaveTempsArgs.contains("combinedindex"))
175       CombinedIndexHook = SaveCombinedIndex;
176   }
177 
178   return Error::success();
179 }
180 
181 #define HANDLE_EXTENSION(Ext)                                                  \
182   llvm::PassPluginLibraryInfo get##Ext##PluginInfo();
183 #include "llvm/Support/Extension.def"
184 
185 static void RegisterPassPlugins(ArrayRef<std::string> PassPlugins,
186                                 PassBuilder &PB) {
187 #define HANDLE_EXTENSION(Ext)                                                  \
188   get##Ext##PluginInfo().RegisterPassBuilderCallbacks(PB);
189 #include "llvm/Support/Extension.def"
190 
191   // Load requested pass plugins and let them register pass builder callbacks
192   for (auto &PluginFN : PassPlugins) {
193     auto PassPlugin = PassPlugin::Load(PluginFN);
194     if (!PassPlugin) {
195       errs() << "Failed to load passes from '" << PluginFN
196              << "'. Request ignored.\n";
197       continue;
198     }
199 
200     PassPlugin->registerPassBuilderCallbacks(PB);
201   }
202 }
203 
204 static std::unique_ptr<TargetMachine>
205 createTargetMachine(const Config &Conf, const Target *TheTarget, Module &M) {
206   StringRef TheTriple = M.getTargetTriple();
207   SubtargetFeatures Features;
208   Features.getDefaultSubtargetFeatures(Triple(TheTriple));
209   for (const std::string &A : Conf.MAttrs)
210     Features.AddFeature(A);
211 
212   std::optional<Reloc::Model> RelocModel;
213   if (Conf.RelocModel)
214     RelocModel = *Conf.RelocModel;
215   else if (M.getModuleFlag("PIC Level"))
216     RelocModel =
217         M.getPICLevel() == PICLevel::NotPIC ? Reloc::Static : Reloc::PIC_;
218 
219   std::optional<CodeModel::Model> CodeModel;
220   if (Conf.CodeModel)
221     CodeModel = *Conf.CodeModel;
222   else
223     CodeModel = M.getCodeModel();
224 
225   std::unique_ptr<TargetMachine> TM(TheTarget->createTargetMachine(
226       TheTriple, Conf.CPU, Features.getString(), Conf.Options, RelocModel,
227       CodeModel, Conf.CGOptLevel));
228 
229   assert(TM && "Failed to create target machine");
230 
231   if (std::optional<uint64_t> LargeDataThreshold = M.getLargeDataThreshold())
232     TM->setLargeDataThreshold(*LargeDataThreshold);
233 
234   return TM;
235 }
236 
237 static void runNewPMPasses(const Config &Conf, Module &Mod, TargetMachine *TM,
238                            unsigned OptLevel, bool IsThinLTO,
239                            ModuleSummaryIndex *ExportSummary,
240                            const ModuleSummaryIndex *ImportSummary) {
241   auto FS = vfs::getRealFileSystem();
242   std::optional<PGOOptions> PGOOpt;
243   if (!Conf.SampleProfile.empty())
244     PGOOpt = PGOOptions(Conf.SampleProfile, "", Conf.ProfileRemapping,
245                         /*MemoryProfile=*/"", FS, PGOOptions::SampleUse,
246                         PGOOptions::NoCSAction, true);
247   else if (Conf.RunCSIRInstr) {
248     PGOOpt = PGOOptions("", Conf.CSIRProfile, Conf.ProfileRemapping,
249                         /*MemoryProfile=*/"", FS, PGOOptions::IRUse,
250                         PGOOptions::CSIRInstr, Conf.AddFSDiscriminator);
251   } else if (!Conf.CSIRProfile.empty()) {
252     PGOOpt = PGOOptions(Conf.CSIRProfile, "", Conf.ProfileRemapping,
253                         /*MemoryProfile=*/"", FS, PGOOptions::IRUse,
254                         PGOOptions::CSIRUse, Conf.AddFSDiscriminator);
255     NoPGOWarnMismatch = !Conf.PGOWarnMismatch;
256   } else if (Conf.AddFSDiscriminator) {
257     PGOOpt = PGOOptions("", "", "", /*MemoryProfile=*/"", nullptr,
258                         PGOOptions::NoAction, PGOOptions::NoCSAction, true);
259   }
260   TM->setPGOOption(PGOOpt);
261 
262   LoopAnalysisManager LAM;
263   FunctionAnalysisManager FAM;
264   CGSCCAnalysisManager CGAM;
265   ModuleAnalysisManager MAM;
266 
267   PassInstrumentationCallbacks PIC;
268   StandardInstrumentations SI(Mod.getContext(), Conf.DebugPassManager,
269                               Conf.VerifyEach);
270   SI.registerCallbacks(PIC, &MAM);
271   PassBuilder PB(TM, Conf.PTO, PGOOpt, &PIC);
272 
273   RegisterPassPlugins(Conf.PassPlugins, PB);
274 
275   std::unique_ptr<TargetLibraryInfoImpl> TLII(
276       new TargetLibraryInfoImpl(Triple(TM->getTargetTriple())));
277   if (Conf.Freestanding)
278     TLII->disableAllFunctions();
279   FAM.registerPass([&] { return TargetLibraryAnalysis(*TLII); });
280 
281   // Parse a custom AA pipeline if asked to.
282   if (!Conf.AAPipeline.empty()) {
283     AAManager AA;
284     if (auto Err = PB.parseAAPipeline(AA, Conf.AAPipeline)) {
285       report_fatal_error(Twine("unable to parse AA pipeline description '") +
286                          Conf.AAPipeline + "': " + toString(std::move(Err)));
287     }
288     // Register the AA manager first so that our version is the one used.
289     FAM.registerPass([&] { return std::move(AA); });
290   }
291 
292   // Register all the basic analyses with the managers.
293   PB.registerModuleAnalyses(MAM);
294   PB.registerCGSCCAnalyses(CGAM);
295   PB.registerFunctionAnalyses(FAM);
296   PB.registerLoopAnalyses(LAM);
297   PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
298 
299   ModulePassManager MPM;
300 
301   if (!Conf.DisableVerify)
302     MPM.addPass(VerifierPass());
303 
304   OptimizationLevel OL;
305 
306   switch (OptLevel) {
307   default:
308     llvm_unreachable("Invalid optimization level");
309   case 0:
310     OL = OptimizationLevel::O0;
311     break;
312   case 1:
313     OL = OptimizationLevel::O1;
314     break;
315   case 2:
316     OL = OptimizationLevel::O2;
317     break;
318   case 3:
319     OL = OptimizationLevel::O3;
320     break;
321   }
322 
323   // Parse a custom pipeline if asked to.
324   if (!Conf.OptPipeline.empty()) {
325     if (auto Err = PB.parsePassPipeline(MPM, Conf.OptPipeline)) {
326       report_fatal_error(Twine("unable to parse pass pipeline description '") +
327                          Conf.OptPipeline + "': " + toString(std::move(Err)));
328     }
329   } else if (Conf.UseDefaultPipeline) {
330     MPM.addPass(PB.buildPerModuleDefaultPipeline(OL));
331   } else if (IsThinLTO) {
332     MPM.addPass(PB.buildThinLTODefaultPipeline(OL, ImportSummary));
333   } else {
334     MPM.addPass(PB.buildLTODefaultPipeline(OL, ExportSummary));
335   }
336 
337   if (!Conf.DisableVerify)
338     MPM.addPass(VerifierPass());
339 
340   MPM.run(Mod, MAM);
341 }
342 
343 bool lto::opt(const Config &Conf, TargetMachine *TM, unsigned Task, Module &Mod,
344               bool IsThinLTO, ModuleSummaryIndex *ExportSummary,
345               const ModuleSummaryIndex *ImportSummary,
346               const std::vector<uint8_t> &CmdArgs) {
347   if (EmbedBitcode == LTOBitcodeEmbedding::EmbedPostMergePreOptimized) {
348     // FIXME: the motivation for capturing post-merge bitcode and command line
349     // is replicating the compilation environment from bitcode, without needing
350     // to understand the dependencies (the functions to be imported). This
351     // assumes a clang - based invocation, case in which we have the command
352     // line.
353     // It's not very clear how the above motivation would map in the
354     // linker-based case, so we currently don't plumb the command line args in
355     // that case.
356     if (CmdArgs.empty())
357       LLVM_DEBUG(
358           dbgs() << "Post-(Thin)LTO merge bitcode embedding was requested, but "
359                     "command line arguments are not available");
360     llvm::embedBitcodeInModule(Mod, llvm::MemoryBufferRef(),
361                                /*EmbedBitcode*/ true, /*EmbedCmdline*/ true,
362                                /*Cmdline*/ CmdArgs);
363   }
364   // FIXME: Plumb the combined index into the new pass manager.
365   runNewPMPasses(Conf, Mod, TM, Conf.OptLevel, IsThinLTO, ExportSummary,
366                  ImportSummary);
367   return !Conf.PostOptModuleHook || Conf.PostOptModuleHook(Task, Mod);
368 }
369 
370 static void codegen(const Config &Conf, TargetMachine *TM,
371                     AddStreamFn AddStream, unsigned Task, Module &Mod,
372                     const ModuleSummaryIndex &CombinedIndex) {
373   if (Conf.PreCodeGenModuleHook && !Conf.PreCodeGenModuleHook(Task, Mod))
374     return;
375 
376   if (EmbedBitcode == LTOBitcodeEmbedding::EmbedOptimized)
377     llvm::embedBitcodeInModule(Mod, llvm::MemoryBufferRef(),
378                                /*EmbedBitcode*/ true,
379                                /*EmbedCmdline*/ false,
380                                /*CmdArgs*/ std::vector<uint8_t>());
381 
382   std::unique_ptr<ToolOutputFile> DwoOut;
383   SmallString<1024> DwoFile(Conf.SplitDwarfOutput);
384   if (!Conf.DwoDir.empty()) {
385     std::error_code EC;
386     if (auto EC = llvm::sys::fs::create_directories(Conf.DwoDir))
387       report_fatal_error(Twine("Failed to create directory ") + Conf.DwoDir +
388                          ": " + EC.message());
389 
390     DwoFile = Conf.DwoDir;
391     sys::path::append(DwoFile, std::to_string(Task) + ".dwo");
392     TM->Options.MCOptions.SplitDwarfFile = std::string(DwoFile);
393   } else
394     TM->Options.MCOptions.SplitDwarfFile = Conf.SplitDwarfFile;
395 
396   if (!DwoFile.empty()) {
397     std::error_code EC;
398     DwoOut = std::make_unique<ToolOutputFile>(DwoFile, EC, sys::fs::OF_None);
399     if (EC)
400       report_fatal_error(Twine("Failed to open ") + DwoFile + ": " +
401                          EC.message());
402   }
403 
404   Expected<std::unique_ptr<CachedFileStream>> StreamOrErr =
405       AddStream(Task, Mod.getModuleIdentifier());
406   if (Error Err = StreamOrErr.takeError())
407     report_fatal_error(std::move(Err));
408   std::unique_ptr<CachedFileStream> &Stream = *StreamOrErr;
409   TM->Options.ObjectFilenameForDebug = Stream->ObjectPathName;
410 
411   legacy::PassManager CodeGenPasses;
412   TargetLibraryInfoImpl TLII(Triple(Mod.getTargetTriple()));
413   CodeGenPasses.add(new TargetLibraryInfoWrapperPass(TLII));
414   CodeGenPasses.add(
415       createImmutableModuleSummaryIndexWrapperPass(&CombinedIndex));
416   if (Conf.PreCodeGenPassesHook)
417     Conf.PreCodeGenPassesHook(CodeGenPasses);
418   if (TM->addPassesToEmitFile(CodeGenPasses, *Stream->OS,
419                               DwoOut ? &DwoOut->os() : nullptr,
420                               Conf.CGFileType))
421     report_fatal_error("Failed to setup codegen");
422   CodeGenPasses.run(Mod);
423 
424   if (DwoOut)
425     DwoOut->keep();
426 }
427 
428 static void splitCodeGen(const Config &C, TargetMachine *TM,
429                          AddStreamFn AddStream,
430                          unsigned ParallelCodeGenParallelismLevel, Module &Mod,
431                          const ModuleSummaryIndex &CombinedIndex) {
432   ThreadPool CodegenThreadPool(
433       heavyweight_hardware_concurrency(ParallelCodeGenParallelismLevel));
434   unsigned ThreadCount = 0;
435   const Target *T = &TM->getTarget();
436 
437   SplitModule(
438       Mod, ParallelCodeGenParallelismLevel,
439       [&](std::unique_ptr<Module> MPart) {
440         // We want to clone the module in a new context to multi-thread the
441         // codegen. We do it by serializing partition modules to bitcode
442         // (while still on the main thread, in order to avoid data races) and
443         // spinning up new threads which deserialize the partitions into
444         // separate contexts.
445         // FIXME: Provide a more direct way to do this in LLVM.
446         SmallString<0> BC;
447         raw_svector_ostream BCOS(BC);
448         WriteBitcodeToFile(*MPart, BCOS);
449 
450         // Enqueue the task
451         CodegenThreadPool.async(
452             [&](const SmallString<0> &BC, unsigned ThreadId) {
453               LTOLLVMContext Ctx(C);
454               Expected<std::unique_ptr<Module>> MOrErr = parseBitcodeFile(
455                   MemoryBufferRef(StringRef(BC.data(), BC.size()), "ld-temp.o"),
456                   Ctx);
457               if (!MOrErr)
458                 report_fatal_error("Failed to read bitcode");
459               std::unique_ptr<Module> MPartInCtx = std::move(MOrErr.get());
460 
461               std::unique_ptr<TargetMachine> TM =
462                   createTargetMachine(C, T, *MPartInCtx);
463 
464               codegen(C, TM.get(), AddStream, ThreadId, *MPartInCtx,
465                       CombinedIndex);
466             },
467             // Pass BC using std::move to ensure that it get moved rather than
468             // copied into the thread's context.
469             std::move(BC), ThreadCount++);
470       },
471       false);
472 
473   // Because the inner lambda (which runs in a worker thread) captures our local
474   // variables, we need to wait for the worker threads to terminate before we
475   // can leave the function scope.
476   CodegenThreadPool.wait();
477 }
478 
479 static Expected<const Target *> initAndLookupTarget(const Config &C,
480                                                     Module &Mod) {
481   if (!C.OverrideTriple.empty())
482     Mod.setTargetTriple(C.OverrideTriple);
483   else if (Mod.getTargetTriple().empty())
484     Mod.setTargetTriple(C.DefaultTriple);
485 
486   std::string Msg;
487   const Target *T = TargetRegistry::lookupTarget(Mod.getTargetTriple(), Msg);
488   if (!T)
489     return make_error<StringError>(Msg, inconvertibleErrorCode());
490   return T;
491 }
492 
493 Error lto::finalizeOptimizationRemarks(
494     std::unique_ptr<ToolOutputFile> DiagOutputFile) {
495   // Make sure we flush the diagnostic remarks file in case the linker doesn't
496   // call the global destructors before exiting.
497   if (!DiagOutputFile)
498     return Error::success();
499   DiagOutputFile->keep();
500   DiagOutputFile->os().flush();
501   return Error::success();
502 }
503 
504 Error lto::backend(const Config &C, AddStreamFn AddStream,
505                    unsigned ParallelCodeGenParallelismLevel, Module &Mod,
506                    ModuleSummaryIndex &CombinedIndex) {
507   Expected<const Target *> TOrErr = initAndLookupTarget(C, Mod);
508   if (!TOrErr)
509     return TOrErr.takeError();
510 
511   std::unique_ptr<TargetMachine> TM = createTargetMachine(C, *TOrErr, Mod);
512 
513   LLVM_DEBUG(dbgs() << "Running regular LTO\n");
514   if (!C.CodeGenOnly) {
515     if (!opt(C, TM.get(), 0, Mod, /*IsThinLTO=*/false,
516              /*ExportSummary=*/&CombinedIndex, /*ImportSummary=*/nullptr,
517              /*CmdArgs*/ std::vector<uint8_t>()))
518       return Error::success();
519   }
520 
521   if (ParallelCodeGenParallelismLevel == 1) {
522     codegen(C, TM.get(), AddStream, 0, Mod, CombinedIndex);
523   } else {
524     splitCodeGen(C, TM.get(), AddStream, ParallelCodeGenParallelismLevel, Mod,
525                  CombinedIndex);
526   }
527   return Error::success();
528 }
529 
530 static void dropDeadSymbols(Module &Mod, const GVSummaryMapTy &DefinedGlobals,
531                             const ModuleSummaryIndex &Index) {
532   std::vector<GlobalValue*> DeadGVs;
533   for (auto &GV : Mod.global_values())
534     if (GlobalValueSummary *GVS = DefinedGlobals.lookup(GV.getGUID()))
535       if (!Index.isGlobalValueLive(GVS)) {
536         DeadGVs.push_back(&GV);
537         convertToDeclaration(GV);
538       }
539 
540   // Now that all dead bodies have been dropped, delete the actual objects
541   // themselves when possible.
542   for (GlobalValue *GV : DeadGVs) {
543     GV->removeDeadConstantUsers();
544     // Might reference something defined in native object (i.e. dropped a
545     // non-prevailing IR def, but we need to keep the declaration).
546     if (GV->use_empty())
547       GV->eraseFromParent();
548   }
549 }
550 
551 Error lto::thinBackend(const Config &Conf, unsigned Task, AddStreamFn AddStream,
552                        Module &Mod, const ModuleSummaryIndex &CombinedIndex,
553                        const FunctionImporter::ImportMapTy &ImportList,
554                        const GVSummaryMapTy &DefinedGlobals,
555                        MapVector<StringRef, BitcodeModule> *ModuleMap,
556                        const std::vector<uint8_t> &CmdArgs) {
557   Expected<const Target *> TOrErr = initAndLookupTarget(Conf, Mod);
558   if (!TOrErr)
559     return TOrErr.takeError();
560 
561   std::unique_ptr<TargetMachine> TM = createTargetMachine(Conf, *TOrErr, Mod);
562 
563   // Setup optimization remarks.
564   auto DiagFileOrErr = lto::setupLLVMOptimizationRemarks(
565       Mod.getContext(), Conf.RemarksFilename, Conf.RemarksPasses,
566       Conf.RemarksFormat, Conf.RemarksWithHotness, Conf.RemarksHotnessThreshold,
567       Task);
568   if (!DiagFileOrErr)
569     return DiagFileOrErr.takeError();
570   auto DiagnosticOutputFile = std::move(*DiagFileOrErr);
571 
572   // Set the partial sample profile ratio in the profile summary module flag of
573   // the module, if applicable.
574   Mod.setPartialSampleProfileRatio(CombinedIndex);
575 
576   LLVM_DEBUG(dbgs() << "Running ThinLTO\n");
577   if (Conf.CodeGenOnly) {
578     codegen(Conf, TM.get(), AddStream, Task, Mod, CombinedIndex);
579     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
580   }
581 
582   if (Conf.PreOptModuleHook && !Conf.PreOptModuleHook(Task, Mod))
583     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
584 
585   auto OptimizeAndCodegen =
586       [&](Module &Mod, TargetMachine *TM,
587           std::unique_ptr<ToolOutputFile> DiagnosticOutputFile) {
588         if (!opt(Conf, TM, Task, Mod, /*IsThinLTO=*/true,
589                  /*ExportSummary=*/nullptr, /*ImportSummary=*/&CombinedIndex,
590                  CmdArgs))
591           return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
592 
593         codegen(Conf, TM, AddStream, Task, Mod, CombinedIndex);
594         return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
595       };
596 
597   if (ThinLTOAssumeMerged)
598     return OptimizeAndCodegen(Mod, TM.get(), std::move(DiagnosticOutputFile));
599 
600   // When linking an ELF shared object, dso_local should be dropped. We
601   // conservatively do this for -fpic.
602   bool ClearDSOLocalOnDeclarations =
603       TM->getTargetTriple().isOSBinFormatELF() &&
604       TM->getRelocationModel() != Reloc::Static &&
605       Mod.getPIELevel() == PIELevel::Default;
606   renameModuleForThinLTO(Mod, CombinedIndex, ClearDSOLocalOnDeclarations);
607 
608   dropDeadSymbols(Mod, DefinedGlobals, CombinedIndex);
609 
610   thinLTOFinalizeInModule(Mod, DefinedGlobals, /*PropagateAttrs=*/true);
611 
612   if (Conf.PostPromoteModuleHook && !Conf.PostPromoteModuleHook(Task, Mod))
613     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
614 
615   if (!DefinedGlobals.empty())
616     thinLTOInternalizeModule(Mod, DefinedGlobals);
617 
618   if (Conf.PostInternalizeModuleHook &&
619       !Conf.PostInternalizeModuleHook(Task, Mod))
620     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
621 
622   auto ModuleLoader = [&](StringRef Identifier) {
623     assert(Mod.getContext().isODRUniquingDebugTypes() &&
624            "ODR Type uniquing should be enabled on the context");
625     if (ModuleMap) {
626       auto I = ModuleMap->find(Identifier);
627       assert(I != ModuleMap->end());
628       return I->second.getLazyModule(Mod.getContext(),
629                                      /*ShouldLazyLoadMetadata=*/true,
630                                      /*IsImporting*/ true);
631     }
632 
633     ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> MBOrErr =
634         llvm::MemoryBuffer::getFile(Identifier);
635     if (!MBOrErr)
636       return Expected<std::unique_ptr<llvm::Module>>(make_error<StringError>(
637           Twine("Error loading imported file ") + Identifier + " : ",
638           MBOrErr.getError()));
639 
640     Expected<BitcodeModule> BMOrErr = findThinLTOModule(**MBOrErr);
641     if (!BMOrErr)
642       return Expected<std::unique_ptr<llvm::Module>>(make_error<StringError>(
643           Twine("Error loading imported file ") + Identifier + " : " +
644               toString(BMOrErr.takeError()),
645           inconvertibleErrorCode()));
646 
647     Expected<std::unique_ptr<Module>> MOrErr =
648         BMOrErr->getLazyModule(Mod.getContext(),
649                                /*ShouldLazyLoadMetadata=*/true,
650                                /*IsImporting*/ true);
651     if (MOrErr)
652       (*MOrErr)->setOwnedMemoryBuffer(std::move(*MBOrErr));
653     return MOrErr;
654   };
655 
656   FunctionImporter Importer(CombinedIndex, ModuleLoader,
657                             ClearDSOLocalOnDeclarations);
658   if (Error Err = Importer.importFunctions(Mod, ImportList).takeError())
659     return Err;
660 
661   // Do this after any importing so that imported code is updated.
662   updateMemProfAttributes(Mod, CombinedIndex);
663   updatePublicTypeTestCalls(Mod, CombinedIndex.withWholeProgramVisibility());
664 
665   if (Conf.PostImportModuleHook && !Conf.PostImportModuleHook(Task, Mod))
666     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
667 
668   return OptimizeAndCodegen(Mod, TM.get(), std::move(DiagnosticOutputFile));
669 }
670 
671 BitcodeModule *lto::findThinLTOModule(MutableArrayRef<BitcodeModule> BMs) {
672   if (ThinLTOAssumeMerged && BMs.size() == 1)
673     return BMs.begin();
674 
675   for (BitcodeModule &BM : BMs) {
676     Expected<BitcodeLTOInfo> LTOInfo = BM.getLTOInfo();
677     if (LTOInfo && LTOInfo->IsThinLTO)
678       return &BM;
679   }
680   return nullptr;
681 }
682 
683 Expected<BitcodeModule> lto::findThinLTOModule(MemoryBufferRef MBRef) {
684   Expected<std::vector<BitcodeModule>> BMsOrErr = getBitcodeModuleList(MBRef);
685   if (!BMsOrErr)
686     return BMsOrErr.takeError();
687 
688   // The bitcode file may contain multiple modules, we want the one that is
689   // marked as being the ThinLTO module.
690   if (const BitcodeModule *Bm = lto::findThinLTOModule(*BMsOrErr))
691     return *Bm;
692 
693   return make_error<StringError>("Could not find module summary",
694                                  inconvertibleErrorCode());
695 }
696 
697 bool lto::initImportList(const Module &M,
698                          const ModuleSummaryIndex &CombinedIndex,
699                          FunctionImporter::ImportMapTy &ImportList) {
700   if (ThinLTOAssumeMerged)
701     return true;
702   // We can simply import the values mentioned in the combined index, since
703   // we should only invoke this using the individual indexes written out
704   // via a WriteIndexesThinBackend.
705   for (const auto &GlobalList : CombinedIndex) {
706     // Ignore entries for undefined references.
707     if (GlobalList.second.SummaryList.empty())
708       continue;
709 
710     auto GUID = GlobalList.first;
711     for (const auto &Summary : GlobalList.second.SummaryList) {
712       // Skip the summaries for the importing module. These are included to
713       // e.g. record required linkage changes.
714       if (Summary->modulePath() == M.getModuleIdentifier())
715         continue;
716       // Add an entry to provoke importing by thinBackend.
717       ImportList[Summary->modulePath()].insert(GUID);
718     }
719   }
720   return true;
721 }
722