1 //===-LTOBackend.cpp - LLVM Link Time Optimizer Backend -------------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements the "backend" phase of LTO, i.e. it performs
11 // optimization and code generation on a loaded module. It is generally used
12 // internally by the LTO class but can also be used independently, for example
13 // to implement a standalone ThinLTO backend.
14 //
15 //===----------------------------------------------------------------------===//
16 
17 #include "llvm/LTO/LTOBackend.h"
18 #include "llvm/Analysis/AliasAnalysis.h"
19 #include "llvm/Analysis/CGSCCPassManager.h"
20 #include "llvm/Analysis/TargetLibraryInfo.h"
21 #include "llvm/Analysis/TargetTransformInfo.h"
22 #include "llvm/Bitcode/BitcodeReader.h"
23 #include "llvm/Bitcode/BitcodeWriter.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/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/raw_ostream.h"
37 #include "llvm/Support/TargetRegistry.h"
38 #include "llvm/Support/ThreadPool.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::F_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::F_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::F_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::F_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                         false, true);
160 
161   PassBuilder PB(TM, PGOOpt);
162   AAManager AA;
163 
164   // Parse a custom AA pipeline if asked to.
165   if (auto Err = PB.parseAAPipeline(AA, "default"))
166     report_fatal_error("Error parsing default AA pipeline");
167 
168   LoopAnalysisManager LAM(Conf.DebugPassManager);
169   FunctionAnalysisManager FAM(Conf.DebugPassManager);
170   CGSCCAnalysisManager CGAM(Conf.DebugPassManager);
171   ModuleAnalysisManager MAM(Conf.DebugPassManager);
172 
173   // Register the AA manager first so that our version is the one used.
174   FAM.registerPass([&] { return std::move(AA); });
175 
176   // Register all the basic analyses with the managers.
177   PB.registerModuleAnalyses(MAM);
178   PB.registerCGSCCAnalyses(CGAM);
179   PB.registerFunctionAnalyses(FAM);
180   PB.registerLoopAnalyses(LAM);
181   PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
182 
183   ModulePassManager MPM(Conf.DebugPassManager);
184   // FIXME (davide): verify the input.
185 
186   PassBuilder::OptimizationLevel OL;
187 
188   switch (OptLevel) {
189   default:
190     llvm_unreachable("Invalid optimization level");
191   case 0:
192     OL = PassBuilder::O0;
193     break;
194   case 1:
195     OL = PassBuilder::O1;
196     break;
197   case 2:
198     OL = PassBuilder::O2;
199     break;
200   case 3:
201     OL = PassBuilder::O3;
202     break;
203   }
204 
205   if (IsThinLTO)
206     MPM = PB.buildThinLTODefaultPipeline(OL, Conf.DebugPassManager,
207                                          ImportSummary);
208   else
209     MPM = PB.buildLTODefaultPipeline(OL, Conf.DebugPassManager, ExportSummary);
210   MPM.run(Mod, MAM);
211 
212   // FIXME (davide): verify the output.
213 }
214 
215 static void runNewPMCustomPasses(Module &Mod, TargetMachine *TM,
216                                  std::string PipelineDesc,
217                                  std::string AAPipelineDesc,
218                                  bool DisableVerify) {
219   PassBuilder PB(TM);
220   AAManager AA;
221 
222   // Parse a custom AA pipeline if asked to.
223   if (!AAPipelineDesc.empty())
224     if (auto Err = PB.parseAAPipeline(AA, AAPipelineDesc))
225       report_fatal_error("unable to parse AA pipeline description '" +
226                          AAPipelineDesc + "': " + toString(std::move(Err)));
227 
228   LoopAnalysisManager LAM;
229   FunctionAnalysisManager FAM;
230   CGSCCAnalysisManager CGAM;
231   ModuleAnalysisManager MAM;
232 
233   // Register the AA manager first so that our version is the one used.
234   FAM.registerPass([&] { return std::move(AA); });
235 
236   // Register all the basic analyses with the managers.
237   PB.registerModuleAnalyses(MAM);
238   PB.registerCGSCCAnalyses(CGAM);
239   PB.registerFunctionAnalyses(FAM);
240   PB.registerLoopAnalyses(LAM);
241   PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
242 
243   ModulePassManager MPM;
244 
245   // Always verify the input.
246   MPM.addPass(VerifierPass());
247 
248   // Now, add all the passes we've been requested to.
249   if (auto Err = PB.parsePassPipeline(MPM, PipelineDesc))
250     report_fatal_error("unable to parse pass pipeline description '" +
251                        PipelineDesc + "': " + toString(std::move(Err)));
252 
253   if (!DisableVerify)
254     MPM.addPass(VerifierPass());
255   MPM.run(Mod, MAM);
256 }
257 
258 static void runOldPMPasses(Config &Conf, Module &Mod, TargetMachine *TM,
259                            bool IsThinLTO, ModuleSummaryIndex *ExportSummary,
260                            const ModuleSummaryIndex *ImportSummary) {
261   legacy::PassManager passes;
262   passes.add(createTargetTransformInfoWrapperPass(TM->getTargetIRAnalysis()));
263 
264   PassManagerBuilder PMB;
265   PMB.LibraryInfo = new TargetLibraryInfoImpl(Triple(TM->getTargetTriple()));
266   PMB.Inliner = createFunctionInliningPass();
267   PMB.ExportSummary = ExportSummary;
268   PMB.ImportSummary = ImportSummary;
269   // Unconditionally verify input since it is not verified before this
270   // point and has unknown origin.
271   PMB.VerifyInput = true;
272   PMB.VerifyOutput = !Conf.DisableVerify;
273   PMB.LoopVectorize = true;
274   PMB.SLPVectorize = true;
275   PMB.OptLevel = Conf.OptLevel;
276   PMB.PGOSampleUse = Conf.SampleProfile;
277   if (IsThinLTO)
278     PMB.populateThinLTOPassManager(passes);
279   else
280     PMB.populateLTOPassManager(passes);
281   passes.run(Mod);
282 }
283 
284 bool opt(Config &Conf, TargetMachine *TM, unsigned Task, Module &Mod,
285          bool IsThinLTO, ModuleSummaryIndex *ExportSummary,
286          const ModuleSummaryIndex *ImportSummary) {
287   // FIXME: Plumb the combined index into the new pass manager.
288   if (!Conf.OptPipeline.empty())
289     runNewPMCustomPasses(Mod, TM, Conf.OptPipeline, Conf.AAPipeline,
290                          Conf.DisableVerify);
291   else if (Conf.UseNewPM)
292     runNewPMPasses(Conf, Mod, TM, Conf.OptLevel, IsThinLTO, ExportSummary,
293                    ImportSummary);
294   else
295     runOldPMPasses(Conf, Mod, TM, IsThinLTO, ExportSummary, ImportSummary);
296   return !Conf.PostOptModuleHook || Conf.PostOptModuleHook(Task, Mod);
297 }
298 
299 void codegen(Config &Conf, TargetMachine *TM, AddStreamFn AddStream,
300              unsigned Task, Module &Mod) {
301   if (Conf.PreCodeGenModuleHook && !Conf.PreCodeGenModuleHook(Task, Mod))
302     return;
303 
304   std::unique_ptr<ToolOutputFile> DwoOut;
305   SmallString<1024> DwoFile(Conf.DwoPath);
306   if (!Conf.DwoDir.empty()) {
307     std::error_code EC;
308     if (auto EC = llvm::sys::fs::create_directories(Conf.DwoDir))
309       report_fatal_error("Failed to create directory " + Conf.DwoDir + ": " +
310                          EC.message());
311 
312     DwoFile = Conf.DwoDir;
313     sys::path::append(DwoFile, std::to_string(Task) + ".dwo");
314   }
315 
316   if (!DwoFile.empty()) {
317     std::error_code EC;
318     TM->Options.MCOptions.SplitDwarfFile = DwoFile.str().str();
319     DwoOut = llvm::make_unique<ToolOutputFile>(DwoFile, EC, sys::fs::F_None);
320     if (EC)
321       report_fatal_error("Failed to open " + DwoFile + ": " + EC.message());
322   }
323 
324   auto Stream = AddStream(Task);
325   legacy::PassManager CodeGenPasses;
326   if (TM->addPassesToEmitFile(CodeGenPasses, *Stream->OS,
327                               DwoOut ? &DwoOut->os() : nullptr,
328                               Conf.CGFileType))
329     report_fatal_error("Failed to setup codegen");
330   CodeGenPasses.run(Mod);
331 
332   if (DwoOut)
333     DwoOut->keep();
334 }
335 
336 void splitCodeGen(Config &C, TargetMachine *TM, AddStreamFn AddStream,
337                   unsigned ParallelCodeGenParallelismLevel,
338                   std::unique_ptr<Module> Mod) {
339   ThreadPool CodegenThreadPool(ParallelCodeGenParallelismLevel);
340   unsigned ThreadCount = 0;
341   const Target *T = &TM->getTarget();
342 
343   SplitModule(
344       std::move(Mod), ParallelCodeGenParallelismLevel,
345       [&](std::unique_ptr<Module> MPart) {
346         // We want to clone the module in a new context to multi-thread the
347         // codegen. We do it by serializing partition modules to bitcode
348         // (while still on the main thread, in order to avoid data races) and
349         // spinning up new threads which deserialize the partitions into
350         // separate contexts.
351         // FIXME: Provide a more direct way to do this in LLVM.
352         SmallString<0> BC;
353         raw_svector_ostream BCOS(BC);
354         WriteBitcodeToFile(*MPart, BCOS);
355 
356         // Enqueue the task
357         CodegenThreadPool.async(
358             [&](const SmallString<0> &BC, unsigned ThreadId) {
359               LTOLLVMContext Ctx(C);
360               Expected<std::unique_ptr<Module>> MOrErr = parseBitcodeFile(
361                   MemoryBufferRef(StringRef(BC.data(), BC.size()), "ld-temp.o"),
362                   Ctx);
363               if (!MOrErr)
364                 report_fatal_error("Failed to read bitcode");
365               std::unique_ptr<Module> MPartInCtx = std::move(MOrErr.get());
366 
367               std::unique_ptr<TargetMachine> TM =
368                   createTargetMachine(C, T, *MPartInCtx);
369 
370               codegen(C, TM.get(), AddStream, ThreadId, *MPartInCtx);
371             },
372             // Pass BC using std::move to ensure that it get moved rather than
373             // copied into the thread's context.
374             std::move(BC), ThreadCount++);
375       },
376       false);
377 
378   // Because the inner lambda (which runs in a worker thread) captures our local
379   // variables, we need to wait for the worker threads to terminate before we
380   // can leave the function scope.
381   CodegenThreadPool.wait();
382 }
383 
384 Expected<const Target *> initAndLookupTarget(Config &C, Module &Mod) {
385   if (!C.OverrideTriple.empty())
386     Mod.setTargetTriple(C.OverrideTriple);
387   else if (Mod.getTargetTriple().empty())
388     Mod.setTargetTriple(C.DefaultTriple);
389 
390   std::string Msg;
391   const Target *T = TargetRegistry::lookupTarget(Mod.getTargetTriple(), Msg);
392   if (!T)
393     return make_error<StringError>(Msg, inconvertibleErrorCode());
394   return T;
395 }
396 
397 }
398 
399 static Error
400 finalizeOptimizationRemarks(std::unique_ptr<ToolOutputFile> DiagOutputFile) {
401   // Make sure we flush the diagnostic remarks file in case the linker doesn't
402   // call the global destructors before exiting.
403   if (!DiagOutputFile)
404     return Error::success();
405   DiagOutputFile->keep();
406   DiagOutputFile->os().flush();
407   return Error::success();
408 }
409 
410 Error lto::backend(Config &C, AddStreamFn AddStream,
411                    unsigned ParallelCodeGenParallelismLevel,
412                    std::unique_ptr<Module> Mod,
413                    ModuleSummaryIndex &CombinedIndex) {
414   Expected<const Target *> TOrErr = initAndLookupTarget(C, *Mod);
415   if (!TOrErr)
416     return TOrErr.takeError();
417 
418   std::unique_ptr<TargetMachine> TM = createTargetMachine(C, *TOrErr, *Mod);
419 
420   // Setup optimization remarks.
421   auto DiagFileOrErr = lto::setupOptimizationRemarks(
422       Mod->getContext(), C.RemarksFilename, C.RemarksWithHotness);
423   if (!DiagFileOrErr)
424     return DiagFileOrErr.takeError();
425   auto DiagnosticOutputFile = std::move(*DiagFileOrErr);
426 
427   if (!C.CodeGenOnly) {
428     if (!opt(C, TM.get(), 0, *Mod, /*IsThinLTO=*/false,
429              /*ExportSummary=*/&CombinedIndex, /*ImportSummary=*/nullptr))
430       return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
431   }
432 
433   if (ParallelCodeGenParallelismLevel == 1) {
434     codegen(C, TM.get(), AddStream, 0, *Mod);
435   } else {
436     splitCodeGen(C, TM.get(), AddStream, ParallelCodeGenParallelismLevel,
437                  std::move(Mod));
438   }
439   return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
440 }
441 
442 static void dropDeadSymbols(Module &Mod, const GVSummaryMapTy &DefinedGlobals,
443                             const ModuleSummaryIndex &Index) {
444   std::vector<GlobalValue*> DeadGVs;
445   for (auto &GV : Mod.global_values())
446     if (GlobalValueSummary *GVS = DefinedGlobals.lookup(GV.getGUID()))
447       if (!Index.isGlobalValueLive(GVS)) {
448         DeadGVs.push_back(&GV);
449         convertToDeclaration(GV);
450       }
451 
452   // Now that all dead bodies have been dropped, delete the actual objects
453   // themselves when possible.
454   for (GlobalValue *GV : DeadGVs) {
455     GV->removeDeadConstantUsers();
456     // Might reference something defined in native object (i.e. dropped a
457     // non-prevailing IR def, but we need to keep the declaration).
458     if (GV->use_empty())
459       GV->eraseFromParent();
460   }
461 }
462 
463 Error lto::thinBackend(Config &Conf, unsigned Task, AddStreamFn AddStream,
464                        Module &Mod, const ModuleSummaryIndex &CombinedIndex,
465                        const FunctionImporter::ImportMapTy &ImportList,
466                        const GVSummaryMapTy &DefinedGlobals,
467                        MapVector<StringRef, BitcodeModule> &ModuleMap) {
468   Expected<const Target *> TOrErr = initAndLookupTarget(Conf, Mod);
469   if (!TOrErr)
470     return TOrErr.takeError();
471 
472   std::unique_ptr<TargetMachine> TM = createTargetMachine(Conf, *TOrErr, Mod);
473 
474   // Setup optimization remarks.
475   auto DiagFileOrErr = lto::setupOptimizationRemarks(
476       Mod.getContext(), Conf.RemarksFilename, Conf.RemarksWithHotness, Task);
477   if (!DiagFileOrErr)
478     return DiagFileOrErr.takeError();
479   auto DiagnosticOutputFile = std::move(*DiagFileOrErr);
480 
481   if (Conf.CodeGenOnly) {
482     codegen(Conf, TM.get(), AddStream, Task, Mod);
483     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
484   }
485 
486   if (Conf.PreOptModuleHook && !Conf.PreOptModuleHook(Task, Mod))
487     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
488 
489   renameModuleForThinLTO(Mod, CombinedIndex);
490 
491   dropDeadSymbols(Mod, DefinedGlobals, CombinedIndex);
492 
493   thinLTOResolvePrevailingInModule(Mod, DefinedGlobals);
494 
495   if (Conf.PostPromoteModuleHook && !Conf.PostPromoteModuleHook(Task, Mod))
496     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
497 
498   if (!DefinedGlobals.empty())
499     thinLTOInternalizeModule(Mod, DefinedGlobals);
500 
501   if (Conf.PostInternalizeModuleHook &&
502       !Conf.PostInternalizeModuleHook(Task, Mod))
503     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
504 
505   auto ModuleLoader = [&](StringRef Identifier) {
506     assert(Mod.getContext().isODRUniquingDebugTypes() &&
507            "ODR Type uniquing should be enabled on the context");
508     auto I = ModuleMap.find(Identifier);
509     assert(I != ModuleMap.end());
510     return I->second.getLazyModule(Mod.getContext(),
511                                    /*ShouldLazyLoadMetadata=*/true,
512                                    /*IsImporting*/ true);
513   };
514 
515   FunctionImporter Importer(CombinedIndex, ModuleLoader);
516   if (Error Err = Importer.importFunctions(Mod, ImportList).takeError())
517     return Err;
518 
519   if (Conf.PostImportModuleHook && !Conf.PostImportModuleHook(Task, Mod))
520     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
521 
522   if (!opt(Conf, TM.get(), Task, Mod, /*IsThinLTO=*/true,
523            /*ExportSummary=*/nullptr, /*ImportSummary=*/&CombinedIndex))
524     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
525 
526   codegen(Conf, TM.get(), AddStream, Task, Mod);
527   return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
528 }
529