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   assert(TM && "Failed to create target machine");
229   return TM;
230 }
231 
232 static void runNewPMPasses(const Config &Conf, Module &Mod, TargetMachine *TM,
233                            unsigned OptLevel, bool IsThinLTO,
234                            ModuleSummaryIndex *ExportSummary,
235                            const ModuleSummaryIndex *ImportSummary) {
236   auto FS = vfs::getRealFileSystem();
237   std::optional<PGOOptions> PGOOpt;
238   if (!Conf.SampleProfile.empty())
239     PGOOpt = PGOOptions(Conf.SampleProfile, "", Conf.ProfileRemapping,
240                         /*MemoryProfile=*/"", FS, PGOOptions::SampleUse,
241                         PGOOptions::NoCSAction, true);
242   else if (Conf.RunCSIRInstr) {
243     PGOOpt = PGOOptions("", Conf.CSIRProfile, Conf.ProfileRemapping,
244                         /*MemoryProfile=*/"", FS, PGOOptions::IRUse,
245                         PGOOptions::CSIRInstr, Conf.AddFSDiscriminator);
246   } else if (!Conf.CSIRProfile.empty()) {
247     PGOOpt = PGOOptions(Conf.CSIRProfile, "", Conf.ProfileRemapping,
248                         /*MemoryProfile=*/"", FS, PGOOptions::IRUse,
249                         PGOOptions::CSIRUse, Conf.AddFSDiscriminator);
250     NoPGOWarnMismatch = !Conf.PGOWarnMismatch;
251   } else if (Conf.AddFSDiscriminator) {
252     PGOOpt = PGOOptions("", "", "", /*MemoryProfile=*/"", nullptr,
253                         PGOOptions::NoAction, PGOOptions::NoCSAction, true);
254   }
255   TM->setPGOOption(PGOOpt);
256 
257   LoopAnalysisManager LAM;
258   FunctionAnalysisManager FAM;
259   CGSCCAnalysisManager CGAM;
260   ModuleAnalysisManager MAM;
261 
262   PassInstrumentationCallbacks PIC;
263   StandardInstrumentations SI(Mod.getContext(), Conf.DebugPassManager,
264                               Conf.VerifyEach);
265   SI.registerCallbacks(PIC, &MAM);
266   PassBuilder PB(TM, Conf.PTO, PGOOpt, &PIC);
267 
268   RegisterPassPlugins(Conf.PassPlugins, PB);
269 
270   std::unique_ptr<TargetLibraryInfoImpl> TLII(
271       new TargetLibraryInfoImpl(Triple(TM->getTargetTriple())));
272   if (Conf.Freestanding)
273     TLII->disableAllFunctions();
274   FAM.registerPass([&] { return TargetLibraryAnalysis(*TLII); });
275 
276   // Parse a custom AA pipeline if asked to.
277   if (!Conf.AAPipeline.empty()) {
278     AAManager AA;
279     if (auto Err = PB.parseAAPipeline(AA, Conf.AAPipeline)) {
280       report_fatal_error(Twine("unable to parse AA pipeline description '") +
281                          Conf.AAPipeline + "': " + toString(std::move(Err)));
282     }
283     // Register the AA manager first so that our version is the one used.
284     FAM.registerPass([&] { return std::move(AA); });
285   }
286 
287   // Register all the basic analyses with the managers.
288   PB.registerModuleAnalyses(MAM);
289   PB.registerCGSCCAnalyses(CGAM);
290   PB.registerFunctionAnalyses(FAM);
291   PB.registerLoopAnalyses(LAM);
292   PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
293 
294   ModulePassManager MPM;
295 
296   if (!Conf.DisableVerify)
297     MPM.addPass(VerifierPass());
298 
299   OptimizationLevel OL;
300 
301   switch (OptLevel) {
302   default:
303     llvm_unreachable("Invalid optimization level");
304   case 0:
305     OL = OptimizationLevel::O0;
306     break;
307   case 1:
308     OL = OptimizationLevel::O1;
309     break;
310   case 2:
311     OL = OptimizationLevel::O2;
312     break;
313   case 3:
314     OL = OptimizationLevel::O3;
315     break;
316   }
317 
318   // Parse a custom pipeline if asked to.
319   if (!Conf.OptPipeline.empty()) {
320     if (auto Err = PB.parsePassPipeline(MPM, Conf.OptPipeline)) {
321       report_fatal_error(Twine("unable to parse pass pipeline description '") +
322                          Conf.OptPipeline + "': " + toString(std::move(Err)));
323     }
324   } else if (Conf.UseDefaultPipeline) {
325     MPM.addPass(PB.buildPerModuleDefaultPipeline(OL));
326   } else if (IsThinLTO) {
327     MPM.addPass(PB.buildThinLTODefaultPipeline(OL, ImportSummary));
328   } else {
329     MPM.addPass(PB.buildLTODefaultPipeline(OL, ExportSummary));
330   }
331 
332   if (!Conf.DisableVerify)
333     MPM.addPass(VerifierPass());
334 
335   MPM.run(Mod, MAM);
336 }
337 
338 bool lto::opt(const Config &Conf, TargetMachine *TM, unsigned Task, Module &Mod,
339               bool IsThinLTO, ModuleSummaryIndex *ExportSummary,
340               const ModuleSummaryIndex *ImportSummary,
341               const std::vector<uint8_t> &CmdArgs) {
342   if (EmbedBitcode == LTOBitcodeEmbedding::EmbedPostMergePreOptimized) {
343     // FIXME: the motivation for capturing post-merge bitcode and command line
344     // is replicating the compilation environment from bitcode, without needing
345     // to understand the dependencies (the functions to be imported). This
346     // assumes a clang - based invocation, case in which we have the command
347     // line.
348     // It's not very clear how the above motivation would map in the
349     // linker-based case, so we currently don't plumb the command line args in
350     // that case.
351     if (CmdArgs.empty())
352       LLVM_DEBUG(
353           dbgs() << "Post-(Thin)LTO merge bitcode embedding was requested, but "
354                     "command line arguments are not available");
355     llvm::embedBitcodeInModule(Mod, llvm::MemoryBufferRef(),
356                                /*EmbedBitcode*/ true, /*EmbedCmdline*/ true,
357                                /*Cmdline*/ CmdArgs);
358   }
359   // FIXME: Plumb the combined index into the new pass manager.
360   runNewPMPasses(Conf, Mod, TM, Conf.OptLevel, IsThinLTO, ExportSummary,
361                  ImportSummary);
362   return !Conf.PostOptModuleHook || Conf.PostOptModuleHook(Task, Mod);
363 }
364 
365 static void codegen(const Config &Conf, TargetMachine *TM,
366                     AddStreamFn AddStream, unsigned Task, Module &Mod,
367                     const ModuleSummaryIndex &CombinedIndex) {
368   if (Conf.PreCodeGenModuleHook && !Conf.PreCodeGenModuleHook(Task, Mod))
369     return;
370 
371   if (EmbedBitcode == LTOBitcodeEmbedding::EmbedOptimized)
372     llvm::embedBitcodeInModule(Mod, llvm::MemoryBufferRef(),
373                                /*EmbedBitcode*/ true,
374                                /*EmbedCmdline*/ false,
375                                /*CmdArgs*/ std::vector<uint8_t>());
376 
377   std::unique_ptr<ToolOutputFile> DwoOut;
378   SmallString<1024> DwoFile(Conf.SplitDwarfOutput);
379   if (!Conf.DwoDir.empty()) {
380     std::error_code EC;
381     if (auto EC = llvm::sys::fs::create_directories(Conf.DwoDir))
382       report_fatal_error(Twine("Failed to create directory ") + Conf.DwoDir +
383                          ": " + EC.message());
384 
385     DwoFile = Conf.DwoDir;
386     sys::path::append(DwoFile, std::to_string(Task) + ".dwo");
387     TM->Options.MCOptions.SplitDwarfFile = std::string(DwoFile);
388   } else
389     TM->Options.MCOptions.SplitDwarfFile = Conf.SplitDwarfFile;
390 
391   if (!DwoFile.empty()) {
392     std::error_code EC;
393     DwoOut = std::make_unique<ToolOutputFile>(DwoFile, EC, sys::fs::OF_None);
394     if (EC)
395       report_fatal_error(Twine("Failed to open ") + DwoFile + ": " +
396                          EC.message());
397   }
398 
399   Expected<std::unique_ptr<CachedFileStream>> StreamOrErr =
400       AddStream(Task, Mod.getModuleIdentifier());
401   if (Error Err = StreamOrErr.takeError())
402     report_fatal_error(std::move(Err));
403   std::unique_ptr<CachedFileStream> &Stream = *StreamOrErr;
404   TM->Options.ObjectFilenameForDebug = Stream->ObjectPathName;
405 
406   legacy::PassManager CodeGenPasses;
407   TargetLibraryInfoImpl TLII(Triple(Mod.getTargetTriple()));
408   CodeGenPasses.add(new TargetLibraryInfoWrapperPass(TLII));
409   CodeGenPasses.add(
410       createImmutableModuleSummaryIndexWrapperPass(&CombinedIndex));
411   if (Conf.PreCodeGenPassesHook)
412     Conf.PreCodeGenPassesHook(CodeGenPasses);
413   if (TM->addPassesToEmitFile(CodeGenPasses, *Stream->OS,
414                               DwoOut ? &DwoOut->os() : nullptr,
415                               Conf.CGFileType))
416     report_fatal_error("Failed to setup codegen");
417   CodeGenPasses.run(Mod);
418 
419   if (DwoOut)
420     DwoOut->keep();
421 }
422 
423 static void splitCodeGen(const Config &C, TargetMachine *TM,
424                          AddStreamFn AddStream,
425                          unsigned ParallelCodeGenParallelismLevel, Module &Mod,
426                          const ModuleSummaryIndex &CombinedIndex) {
427   ThreadPool CodegenThreadPool(
428       heavyweight_hardware_concurrency(ParallelCodeGenParallelismLevel));
429   unsigned ThreadCount = 0;
430   const Target *T = &TM->getTarget();
431 
432   SplitModule(
433       Mod, ParallelCodeGenParallelismLevel,
434       [&](std::unique_ptr<Module> MPart) {
435         // We want to clone the module in a new context to multi-thread the
436         // codegen. We do it by serializing partition modules to bitcode
437         // (while still on the main thread, in order to avoid data races) and
438         // spinning up new threads which deserialize the partitions into
439         // separate contexts.
440         // FIXME: Provide a more direct way to do this in LLVM.
441         SmallString<0> BC;
442         raw_svector_ostream BCOS(BC);
443         WriteBitcodeToFile(*MPart, BCOS);
444 
445         // Enqueue the task
446         CodegenThreadPool.async(
447             [&](const SmallString<0> &BC, unsigned ThreadId) {
448               LTOLLVMContext Ctx(C);
449               Expected<std::unique_ptr<Module>> MOrErr = parseBitcodeFile(
450                   MemoryBufferRef(StringRef(BC.data(), BC.size()), "ld-temp.o"),
451                   Ctx);
452               if (!MOrErr)
453                 report_fatal_error("Failed to read bitcode");
454               std::unique_ptr<Module> MPartInCtx = std::move(MOrErr.get());
455 
456               std::unique_ptr<TargetMachine> TM =
457                   createTargetMachine(C, T, *MPartInCtx);
458 
459               codegen(C, TM.get(), AddStream, ThreadId, *MPartInCtx,
460                       CombinedIndex);
461             },
462             // Pass BC using std::move to ensure that it get moved rather than
463             // copied into the thread's context.
464             std::move(BC), ThreadCount++);
465       },
466       false);
467 
468   // Because the inner lambda (which runs in a worker thread) captures our local
469   // variables, we need to wait for the worker threads to terminate before we
470   // can leave the function scope.
471   CodegenThreadPool.wait();
472 }
473 
474 static Expected<const Target *> initAndLookupTarget(const Config &C,
475                                                     Module &Mod) {
476   if (!C.OverrideTriple.empty())
477     Mod.setTargetTriple(C.OverrideTriple);
478   else if (Mod.getTargetTriple().empty())
479     Mod.setTargetTriple(C.DefaultTriple);
480 
481   std::string Msg;
482   const Target *T = TargetRegistry::lookupTarget(Mod.getTargetTriple(), Msg);
483   if (!T)
484     return make_error<StringError>(Msg, inconvertibleErrorCode());
485   return T;
486 }
487 
488 Error lto::finalizeOptimizationRemarks(
489     std::unique_ptr<ToolOutputFile> DiagOutputFile) {
490   // Make sure we flush the diagnostic remarks file in case the linker doesn't
491   // call the global destructors before exiting.
492   if (!DiagOutputFile)
493     return Error::success();
494   DiagOutputFile->keep();
495   DiagOutputFile->os().flush();
496   return Error::success();
497 }
498 
499 Error lto::backend(const Config &C, AddStreamFn AddStream,
500                    unsigned ParallelCodeGenParallelismLevel, Module &Mod,
501                    ModuleSummaryIndex &CombinedIndex) {
502   Expected<const Target *> TOrErr = initAndLookupTarget(C, Mod);
503   if (!TOrErr)
504     return TOrErr.takeError();
505 
506   std::unique_ptr<TargetMachine> TM = createTargetMachine(C, *TOrErr, Mod);
507 
508   LLVM_DEBUG(dbgs() << "Running regular LTO\n");
509   if (!C.CodeGenOnly) {
510     if (!opt(C, TM.get(), 0, Mod, /*IsThinLTO=*/false,
511              /*ExportSummary=*/&CombinedIndex, /*ImportSummary=*/nullptr,
512              /*CmdArgs*/ std::vector<uint8_t>()))
513       return Error::success();
514   }
515 
516   if (ParallelCodeGenParallelismLevel == 1) {
517     codegen(C, TM.get(), AddStream, 0, Mod, CombinedIndex);
518   } else {
519     splitCodeGen(C, TM.get(), AddStream, ParallelCodeGenParallelismLevel, Mod,
520                  CombinedIndex);
521   }
522   return Error::success();
523 }
524 
525 static void dropDeadSymbols(Module &Mod, const GVSummaryMapTy &DefinedGlobals,
526                             const ModuleSummaryIndex &Index) {
527   std::vector<GlobalValue*> DeadGVs;
528   for (auto &GV : Mod.global_values())
529     if (GlobalValueSummary *GVS = DefinedGlobals.lookup(GV.getGUID()))
530       if (!Index.isGlobalValueLive(GVS)) {
531         DeadGVs.push_back(&GV);
532         convertToDeclaration(GV);
533       }
534 
535   // Now that all dead bodies have been dropped, delete the actual objects
536   // themselves when possible.
537   for (GlobalValue *GV : DeadGVs) {
538     GV->removeDeadConstantUsers();
539     // Might reference something defined in native object (i.e. dropped a
540     // non-prevailing IR def, but we need to keep the declaration).
541     if (GV->use_empty())
542       GV->eraseFromParent();
543   }
544 }
545 
546 Error lto::thinBackend(const Config &Conf, unsigned Task, AddStreamFn AddStream,
547                        Module &Mod, const ModuleSummaryIndex &CombinedIndex,
548                        const FunctionImporter::ImportMapTy &ImportList,
549                        const GVSummaryMapTy &DefinedGlobals,
550                        MapVector<StringRef, BitcodeModule> *ModuleMap,
551                        const std::vector<uint8_t> &CmdArgs) {
552   Expected<const Target *> TOrErr = initAndLookupTarget(Conf, Mod);
553   if (!TOrErr)
554     return TOrErr.takeError();
555 
556   std::unique_ptr<TargetMachine> TM = createTargetMachine(Conf, *TOrErr, Mod);
557 
558   // Setup optimization remarks.
559   auto DiagFileOrErr = lto::setupLLVMOptimizationRemarks(
560       Mod.getContext(), Conf.RemarksFilename, Conf.RemarksPasses,
561       Conf.RemarksFormat, Conf.RemarksWithHotness, Conf.RemarksHotnessThreshold,
562       Task);
563   if (!DiagFileOrErr)
564     return DiagFileOrErr.takeError();
565   auto DiagnosticOutputFile = std::move(*DiagFileOrErr);
566 
567   // Set the partial sample profile ratio in the profile summary module flag of
568   // the module, if applicable.
569   Mod.setPartialSampleProfileRatio(CombinedIndex);
570 
571   LLVM_DEBUG(dbgs() << "Running ThinLTO\n");
572   if (Conf.CodeGenOnly) {
573     codegen(Conf, TM.get(), AddStream, Task, Mod, CombinedIndex);
574     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
575   }
576 
577   if (Conf.PreOptModuleHook && !Conf.PreOptModuleHook(Task, Mod))
578     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
579 
580   auto OptimizeAndCodegen =
581       [&](Module &Mod, TargetMachine *TM,
582           std::unique_ptr<ToolOutputFile> DiagnosticOutputFile) {
583         if (!opt(Conf, TM, Task, Mod, /*IsThinLTO=*/true,
584                  /*ExportSummary=*/nullptr, /*ImportSummary=*/&CombinedIndex,
585                  CmdArgs))
586           return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
587 
588         codegen(Conf, TM, AddStream, Task, Mod, CombinedIndex);
589         return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
590       };
591 
592   if (ThinLTOAssumeMerged)
593     return OptimizeAndCodegen(Mod, TM.get(), std::move(DiagnosticOutputFile));
594 
595   // When linking an ELF shared object, dso_local should be dropped. We
596   // conservatively do this for -fpic.
597   bool ClearDSOLocalOnDeclarations =
598       TM->getTargetTriple().isOSBinFormatELF() &&
599       TM->getRelocationModel() != Reloc::Static &&
600       Mod.getPIELevel() == PIELevel::Default;
601   renameModuleForThinLTO(Mod, CombinedIndex, ClearDSOLocalOnDeclarations);
602 
603   dropDeadSymbols(Mod, DefinedGlobals, CombinedIndex);
604 
605   thinLTOFinalizeInModule(Mod, DefinedGlobals, /*PropagateAttrs=*/true);
606 
607   if (Conf.PostPromoteModuleHook && !Conf.PostPromoteModuleHook(Task, Mod))
608     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
609 
610   if (!DefinedGlobals.empty())
611     thinLTOInternalizeModule(Mod, DefinedGlobals);
612 
613   if (Conf.PostInternalizeModuleHook &&
614       !Conf.PostInternalizeModuleHook(Task, Mod))
615     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
616 
617   auto ModuleLoader = [&](StringRef Identifier) {
618     assert(Mod.getContext().isODRUniquingDebugTypes() &&
619            "ODR Type uniquing should be enabled on the context");
620     if (ModuleMap) {
621       auto I = ModuleMap->find(Identifier);
622       assert(I != ModuleMap->end());
623       return I->second.getLazyModule(Mod.getContext(),
624                                      /*ShouldLazyLoadMetadata=*/true,
625                                      /*IsImporting*/ true);
626     }
627 
628     ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> MBOrErr =
629         llvm::MemoryBuffer::getFile(Identifier);
630     if (!MBOrErr)
631       return Expected<std::unique_ptr<llvm::Module>>(make_error<StringError>(
632           Twine("Error loading imported file ") + Identifier + " : ",
633           MBOrErr.getError()));
634 
635     Expected<BitcodeModule> BMOrErr = findThinLTOModule(**MBOrErr);
636     if (!BMOrErr)
637       return Expected<std::unique_ptr<llvm::Module>>(make_error<StringError>(
638           Twine("Error loading imported file ") + Identifier + " : " +
639               toString(BMOrErr.takeError()),
640           inconvertibleErrorCode()));
641 
642     Expected<std::unique_ptr<Module>> MOrErr =
643         BMOrErr->getLazyModule(Mod.getContext(),
644                                /*ShouldLazyLoadMetadata=*/true,
645                                /*IsImporting*/ true);
646     if (MOrErr)
647       (*MOrErr)->setOwnedMemoryBuffer(std::move(*MBOrErr));
648     return MOrErr;
649   };
650 
651   FunctionImporter Importer(CombinedIndex, ModuleLoader,
652                             ClearDSOLocalOnDeclarations);
653   if (Error Err = Importer.importFunctions(Mod, ImportList).takeError())
654     return Err;
655 
656   // Do this after any importing so that imported code is updated.
657   updateMemProfAttributes(Mod, CombinedIndex);
658   updatePublicTypeTestCalls(Mod, CombinedIndex.withWholeProgramVisibility());
659 
660   if (Conf.PostImportModuleHook && !Conf.PostImportModuleHook(Task, Mod))
661     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
662 
663   return OptimizeAndCodegen(Mod, TM.get(), std::move(DiagnosticOutputFile));
664 }
665 
666 BitcodeModule *lto::findThinLTOModule(MutableArrayRef<BitcodeModule> BMs) {
667   if (ThinLTOAssumeMerged && BMs.size() == 1)
668     return BMs.begin();
669 
670   for (BitcodeModule &BM : BMs) {
671     Expected<BitcodeLTOInfo> LTOInfo = BM.getLTOInfo();
672     if (LTOInfo && LTOInfo->IsThinLTO)
673       return &BM;
674   }
675   return nullptr;
676 }
677 
678 Expected<BitcodeModule> lto::findThinLTOModule(MemoryBufferRef MBRef) {
679   Expected<std::vector<BitcodeModule>> BMsOrErr = getBitcodeModuleList(MBRef);
680   if (!BMsOrErr)
681     return BMsOrErr.takeError();
682 
683   // The bitcode file may contain multiple modules, we want the one that is
684   // marked as being the ThinLTO module.
685   if (const BitcodeModule *Bm = lto::findThinLTOModule(*BMsOrErr))
686     return *Bm;
687 
688   return make_error<StringError>("Could not find module summary",
689                                  inconvertibleErrorCode());
690 }
691 
692 bool lto::initImportList(const Module &M,
693                          const ModuleSummaryIndex &CombinedIndex,
694                          FunctionImporter::ImportMapTy &ImportList) {
695   if (ThinLTOAssumeMerged)
696     return true;
697   // We can simply import the values mentioned in the combined index, since
698   // we should only invoke this using the individual indexes written out
699   // via a WriteIndexesThinBackend.
700   for (const auto &GlobalList : CombinedIndex) {
701     // Ignore entries for undefined references.
702     if (GlobalList.second.SummaryList.empty())
703       continue;
704 
705     auto GUID = GlobalList.first;
706     for (const auto &Summary : GlobalList.second.SummaryList) {
707       // Skip the summaries for the importing module. These are included to
708       // e.g. record required linkage changes.
709       if (Summary->modulePath() == M.getModuleIdentifier())
710         continue;
711       // Add an entry to provoke importing by thinBackend.
712       ImportList[Summary->modulePath()].insert(GUID);
713     }
714   }
715   return true;
716 }
717