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/LoopPassManager.h"
21 #include "llvm/Analysis/TargetLibraryInfo.h"
22 #include "llvm/Analysis/TargetTransformInfo.h"
23 #include "llvm/Bitcode/BitcodeReader.h"
24 #include "llvm/Bitcode/BitcodeWriter.h"
25 #include "llvm/IR/LegacyPassManager.h"
26 #include "llvm/IR/PassManager.h"
27 #include "llvm/IR/Verifier.h"
28 #include "llvm/LTO/LTO.h"
29 #include "llvm/LTO/legacy/UpdateCompilerUsed.h"
30 #include "llvm/MC/SubtargetFeature.h"
31 #include "llvm/Passes/PassBuilder.h"
32 #include "llvm/Support/Error.h"
33 #include "llvm/Support/FileSystem.h"
34 #include "llvm/Support/TargetRegistry.h"
35 #include "llvm/Support/ThreadPool.h"
36 #include "llvm/Target/TargetMachine.h"
37 #include "llvm/Transforms/IPO.h"
38 #include "llvm/Transforms/IPO/PassManagerBuilder.h"
39 #include "llvm/Transforms/Utils/FunctionImportUtils.h"
40 #include "llvm/Transforms/Utils/SplitModule.h"
41 
42 using namespace llvm;
43 using namespace lto;
44 
45 LLVM_ATTRIBUTE_NORETURN static void reportOpenError(StringRef Path, Twine Msg) {
46   errs() << "failed to open " << Path << ": " << Msg << '\n';
47   errs().flush();
48   exit(1);
49 }
50 
51 Error Config::addSaveTemps(std::string OutputFileName,
52                            bool UseInputModulePath) {
53   ShouldDiscardValueNames = false;
54 
55   std::error_code EC;
56   ResolutionFile = llvm::make_unique<raw_fd_ostream>(
57       OutputFileName + "resolution.txt", EC, sys::fs::OpenFlags::F_Text);
58   if (EC)
59     return errorCodeToError(EC);
60 
61   auto setHook = [&](std::string PathSuffix, ModuleHookFn &Hook) {
62     // Keep track of the hook provided by the linker, which also needs to run.
63     ModuleHookFn LinkerHook = Hook;
64     Hook = [=](unsigned Task, const Module &M) {
65       // If the linker's hook returned false, we need to pass that result
66       // through.
67       if (LinkerHook && !LinkerHook(Task, M))
68         return false;
69 
70       std::string PathPrefix;
71       // If this is the combined module (not a ThinLTO backend compile) or the
72       // user hasn't requested using the input module's path, emit to a file
73       // named from the provided OutputFileName with the Task ID appended.
74       if (M.getModuleIdentifier() == "ld-temp.o" || !UseInputModulePath) {
75         PathPrefix = OutputFileName + utostr(Task);
76       } else
77         PathPrefix = M.getModuleIdentifier();
78       std::string Path = PathPrefix + "." + PathSuffix + ".bc";
79       std::error_code EC;
80       raw_fd_ostream OS(Path, EC, sys::fs::OpenFlags::F_None);
81       // Because -save-temps is a debugging feature, we report the error
82       // directly and exit.
83       if (EC)
84         reportOpenError(Path, EC.message());
85       WriteBitcodeToFile(&M, OS, /*ShouldPreserveUseListOrder=*/false);
86       return true;
87     };
88   };
89 
90   setHook("0.preopt", PreOptModuleHook);
91   setHook("1.promote", PostPromoteModuleHook);
92   setHook("2.internalize", PostInternalizeModuleHook);
93   setHook("3.import", PostImportModuleHook);
94   setHook("4.opt", PostOptModuleHook);
95   setHook("5.precodegen", PreCodeGenModuleHook);
96 
97   CombinedIndexHook = [=](const ModuleSummaryIndex &Index) {
98     std::string Path = OutputFileName + "index.bc";
99     std::error_code EC;
100     raw_fd_ostream OS(Path, EC, sys::fs::OpenFlags::F_None);
101     // Because -save-temps is a debugging feature, we report the error
102     // directly and exit.
103     if (EC)
104       reportOpenError(Path, EC.message());
105     WriteIndexToFile(Index, OS);
106     return true;
107   };
108 
109   return Error::success();
110 }
111 
112 namespace {
113 
114 std::unique_ptr<TargetMachine>
115 createTargetMachine(Config &Conf, StringRef TheTriple,
116                     const Target *TheTarget) {
117   SubtargetFeatures Features;
118   Features.getDefaultSubtargetFeatures(Triple(TheTriple));
119   for (const std::string &A : Conf.MAttrs)
120     Features.AddFeature(A);
121 
122   return std::unique_ptr<TargetMachine>(TheTarget->createTargetMachine(
123       TheTriple, Conf.CPU, Features.getString(), Conf.Options, Conf.RelocModel,
124       Conf.CodeModel, Conf.CGOptLevel));
125 }
126 
127 static void runNewPMCustomPasses(Module &Mod, TargetMachine *TM,
128                                  std::string PipelineDesc,
129                                  std::string AAPipelineDesc,
130                                  bool DisableVerify) {
131   PassBuilder PB(TM);
132   AAManager AA;
133 
134   // Parse a custom AA pipeline if asked to.
135   if (!AAPipelineDesc.empty())
136     if (!PB.parseAAPipeline(AA, AAPipelineDesc))
137       report_fatal_error("unable to parse AA pipeline description: " +
138                          AAPipelineDesc);
139 
140   LoopAnalysisManager LAM;
141   FunctionAnalysisManager FAM;
142   CGSCCAnalysisManager CGAM;
143   ModuleAnalysisManager MAM;
144 
145   // Register the AA manager first so that our version is the one used.
146   FAM.registerPass([&] { return std::move(AA); });
147 
148   // Register all the basic analyses with the managers.
149   PB.registerModuleAnalyses(MAM);
150   PB.registerCGSCCAnalyses(CGAM);
151   PB.registerFunctionAnalyses(FAM);
152   PB.registerLoopAnalyses(LAM);
153   PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
154 
155   ModulePassManager MPM;
156 
157   // Always verify the input.
158   MPM.addPass(VerifierPass());
159 
160   // Now, add all the passes we've been requested to.
161   if (!PB.parsePassPipeline(MPM, PipelineDesc))
162     report_fatal_error("unable to parse pass pipeline description: " +
163                        PipelineDesc);
164 
165   if (!DisableVerify)
166     MPM.addPass(VerifierPass());
167   MPM.run(Mod, MAM);
168 }
169 
170 static void runOldPMPasses(Config &Conf, Module &Mod, TargetMachine *TM,
171                            bool IsThinLto) {
172   legacy::PassManager passes;
173   passes.add(createTargetTransformInfoWrapperPass(TM->getTargetIRAnalysis()));
174 
175   PassManagerBuilder PMB;
176   PMB.LibraryInfo = new TargetLibraryInfoImpl(Triple(TM->getTargetTriple()));
177   PMB.Inliner = createFunctionInliningPass();
178   // Unconditionally verify input since it is not verified before this
179   // point and has unknown origin.
180   PMB.VerifyInput = true;
181   PMB.VerifyOutput = !Conf.DisableVerify;
182   PMB.LoopVectorize = true;
183   PMB.SLPVectorize = true;
184   PMB.OptLevel = Conf.OptLevel;
185   if (IsThinLto)
186     PMB.populateThinLTOPassManager(passes);
187   else
188     PMB.populateLTOPassManager(passes);
189   passes.run(Mod);
190 }
191 
192 bool opt(Config &Conf, TargetMachine *TM, unsigned Task, Module &Mod,
193          bool IsThinLto) {
194   Mod.setDataLayout(TM->createDataLayout());
195   if (Conf.OptPipeline.empty())
196     runOldPMPasses(Conf, Mod, TM, IsThinLto);
197   else
198     runNewPMCustomPasses(Mod, TM, Conf.OptPipeline, Conf.AAPipeline,
199                          Conf.DisableVerify);
200   return !Conf.PostOptModuleHook || Conf.PostOptModuleHook(Task, Mod);
201 }
202 
203 void codegen(Config &Conf, TargetMachine *TM, AddStreamFn AddStream,
204              unsigned Task, Module &Mod) {
205   if (Conf.PreCodeGenModuleHook && !Conf.PreCodeGenModuleHook(Task, Mod))
206     return;
207 
208   auto Stream = AddStream(Task);
209   legacy::PassManager CodeGenPasses;
210   if (TM->addPassesToEmitFile(CodeGenPasses, *Stream->OS,
211                               TargetMachine::CGFT_ObjectFile))
212     report_fatal_error("Failed to setup codegen");
213   CodeGenPasses.run(Mod);
214 }
215 
216 void splitCodeGen(Config &C, TargetMachine *TM, AddStreamFn AddStream,
217                   unsigned ParallelCodeGenParallelismLevel,
218                   std::unique_ptr<Module> Mod) {
219   ThreadPool CodegenThreadPool(ParallelCodeGenParallelismLevel);
220   unsigned ThreadCount = 0;
221   const Target *T = &TM->getTarget();
222 
223   SplitModule(
224       std::move(Mod), ParallelCodeGenParallelismLevel,
225       [&](std::unique_ptr<Module> MPart) {
226         // We want to clone the module in a new context to multi-thread the
227         // codegen. We do it by serializing partition modules to bitcode
228         // (while still on the main thread, in order to avoid data races) and
229         // spinning up new threads which deserialize the partitions into
230         // separate contexts.
231         // FIXME: Provide a more direct way to do this in LLVM.
232         SmallString<0> BC;
233         raw_svector_ostream BCOS(BC);
234         WriteBitcodeToFile(MPart.get(), BCOS);
235 
236         // Enqueue the task
237         CodegenThreadPool.async(
238             [&](const SmallString<0> &BC, unsigned ThreadId) {
239               LTOLLVMContext Ctx(C);
240               Expected<std::unique_ptr<Module>> MOrErr = parseBitcodeFile(
241                   MemoryBufferRef(StringRef(BC.data(), BC.size()), "ld-temp.o"),
242                   Ctx);
243               if (!MOrErr)
244                 report_fatal_error("Failed to read bitcode");
245               std::unique_ptr<Module> MPartInCtx = std::move(MOrErr.get());
246 
247               std::unique_ptr<TargetMachine> TM =
248                   createTargetMachine(C, MPartInCtx->getTargetTriple(), T);
249 
250               codegen(C, TM.get(), AddStream, ThreadId, *MPartInCtx);
251             },
252             // Pass BC using std::move to ensure that it get moved rather than
253             // copied into the thread's context.
254             std::move(BC), ThreadCount++);
255       },
256       false);
257 
258   // Because the inner lambda (which runs in a worker thread) captures our local
259   // variables, we need to wait for the worker threads to terminate before we
260   // can leave the function scope.
261   CodegenThreadPool.wait();
262 }
263 
264 Expected<const Target *> initAndLookupTarget(Config &C, Module &Mod) {
265   if (!C.OverrideTriple.empty())
266     Mod.setTargetTriple(C.OverrideTriple);
267   else if (Mod.getTargetTriple().empty())
268     Mod.setTargetTriple(C.DefaultTriple);
269 
270   std::string Msg;
271   const Target *T = TargetRegistry::lookupTarget(Mod.getTargetTriple(), Msg);
272   if (!T)
273     return make_error<StringError>(Msg, inconvertibleErrorCode());
274   return T;
275 }
276 
277 }
278 
279 static void handleAsmUndefinedRefs(Module &Mod, TargetMachine &TM) {
280   // Collect the list of undefined symbols used in asm and update
281   // llvm.compiler.used to prevent optimization to drop these from the output.
282   StringSet<> AsmUndefinedRefs;
283   object::IRObjectFile::CollectAsmUndefinedRefs(
284       Triple(Mod.getTargetTriple()), Mod.getModuleInlineAsm(),
285       [&AsmUndefinedRefs](StringRef Name, object::BasicSymbolRef::Flags Flags) {
286         if (Flags & object::BasicSymbolRef::SF_Undefined)
287           AsmUndefinedRefs.insert(Name);
288       });
289   updateCompilerUsed(Mod, TM, AsmUndefinedRefs);
290 }
291 
292 Error lto::backend(Config &C, AddStreamFn AddStream,
293                    unsigned ParallelCodeGenParallelismLevel,
294                    std::unique_ptr<Module> Mod) {
295   Expected<const Target *> TOrErr = initAndLookupTarget(C, *Mod);
296   if (!TOrErr)
297     return TOrErr.takeError();
298 
299   std::unique_ptr<TargetMachine> TM =
300       createTargetMachine(C, Mod->getTargetTriple(), *TOrErr);
301 
302   handleAsmUndefinedRefs(*Mod, *TM);
303 
304   if (!C.CodeGenOnly)
305     if (!opt(C, TM.get(), 0, *Mod, /*IsThinLto=*/false))
306       return Error::success();
307 
308   if (ParallelCodeGenParallelismLevel == 1) {
309     codegen(C, TM.get(), AddStream, 0, *Mod);
310   } else {
311     splitCodeGen(C, TM.get(), AddStream, ParallelCodeGenParallelismLevel,
312                  std::move(Mod));
313   }
314   return Error::success();
315 }
316 
317 Error lto::thinBackend(Config &Conf, unsigned Task, AddStreamFn AddStream,
318                        Module &Mod, ModuleSummaryIndex &CombinedIndex,
319                        const FunctionImporter::ImportMapTy &ImportList,
320                        const GVSummaryMapTy &DefinedGlobals,
321                        MapVector<StringRef, MemoryBufferRef> &ModuleMap) {
322   Expected<const Target *> TOrErr = initAndLookupTarget(Conf, Mod);
323   if (!TOrErr)
324     return TOrErr.takeError();
325 
326   std::unique_ptr<TargetMachine> TM =
327       createTargetMachine(Conf, Mod.getTargetTriple(), *TOrErr);
328 
329   handleAsmUndefinedRefs(Mod, *TM);
330 
331   if (Conf.CodeGenOnly) {
332     codegen(Conf, TM.get(), AddStream, Task, Mod);
333     return Error::success();
334   }
335 
336   if (Conf.PreOptModuleHook && !Conf.PreOptModuleHook(Task, Mod))
337     return Error::success();
338 
339   renameModuleForThinLTO(Mod, CombinedIndex);
340 
341   thinLTOResolveWeakForLinkerModule(Mod, DefinedGlobals);
342 
343   if (Conf.PostPromoteModuleHook && !Conf.PostPromoteModuleHook(Task, Mod))
344     return Error::success();
345 
346   if (!DefinedGlobals.empty())
347     thinLTOInternalizeModule(Mod, DefinedGlobals);
348 
349   if (Conf.PostInternalizeModuleHook &&
350       !Conf.PostInternalizeModuleHook(Task, Mod))
351     return Error::success();
352 
353   auto ModuleLoader = [&](StringRef Identifier) {
354     assert(Mod.getContext().isODRUniquingDebugTypes() &&
355            "ODR Type uniquing should be enabled on the context");
356     return getLazyBitcodeModule(ModuleMap[Identifier], Mod.getContext(),
357                                 /*ShouldLazyLoadMetadata=*/true);
358   };
359 
360   FunctionImporter Importer(CombinedIndex, ModuleLoader);
361   if (Error Err = Importer.importFunctions(Mod, ImportList).takeError())
362     return Err;
363 
364   if (Conf.PostImportModuleHook && !Conf.PostImportModuleHook(Task, Mod))
365     return Error::success();
366 
367   if (!opt(Conf, TM.get(), Task, Mod, /*IsThinLto=*/true))
368     return Error::success();
369 
370   codegen(Conf, TM.get(), AddStream, Task, Mod);
371   return Error::success();
372 }
373