llvm/lib/CodeGen/DwarfEHPrepare.cpp

   1 //===- DwarfEHPrepare - Prepare exception handling for code generation ----===//
   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 pass mulches exception handling code into a form adapted to code
  10 // generation. Required if using dwarf exception handling.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "llvm/CodeGen/DwarfEHPrepare.h"
  15 #include "llvm/ADT/BitVector.h"
  16 #include "llvm/ADT/SmallVector.h"
  17 #include "llvm/ADT/Statistic.h"
  18 #include "llvm/Analysis/CFG.h"
  19 #include "llvm/Analysis/DomTreeUpdater.h"
  20 #include "llvm/Analysis/TargetTransformInfo.h"
  21 #include "llvm/CodeGen/RuntimeLibcalls.h"
  22 #include "llvm/CodeGen/TargetLowering.h"
  23 #include "llvm/CodeGen/TargetPassConfig.h"
  24 #include "llvm/CodeGen/TargetSubtargetInfo.h"
  25 #include "llvm/IR/BasicBlock.h"
  26 #include "llvm/IR/Constants.h"
  27 #include "llvm/IR/DebugInfoMetadata.h"
  28 #include "llvm/IR/DerivedTypes.h"
  29 #include "llvm/IR/Dominators.h"
  30 #include "llvm/IR/EHPersonalities.h"
  31 #include "llvm/IR/Function.h"
  32 #include "llvm/IR/Instructions.h"
  33 #include "llvm/IR/Module.h"
  34 #include "llvm/IR/Type.h"
  35 #include "llvm/InitializePasses.h"
  36 #include "llvm/Pass.h"
  37 #include "llvm/Support/Casting.h"
  38 #include "llvm/Target/TargetMachine.h"
  39 #include "llvm/TargetParser/Triple.h"
  40 #include "llvm/Transforms/Utils/Local.h"
  41 #include <cstddef>
  42
  43 using namespace llvm;
  44
  45 #define DEBUG_TYPE "dwarf-eh-prepare"
  46
  47 STATISTIC(NumResumesLowered, "Number of resume calls lowered");
  48 STATISTIC(NumCleanupLandingPadsUnreachable,
  49           "Number of cleanup landing pads found unreachable");
  50 STATISTIC(NumCleanupLandingPadsRemaining,
  51           "Number of cleanup landing pads remaining");
  52 STATISTIC(NumNoUnwind, "Number of functions with nounwind");
  53 STATISTIC(NumUnwind, "Number of functions with unwind");
  54
  55 namespace {
  56
  57 class DwarfEHPrepare {
  58   CodeGenOptLevel OptLevel;
  59
  60   Function &F;
  61   const TargetLowering &TLI;
  62   DomTreeUpdater *DTU;
  63   const TargetTransformInfo *TTI;
  64   const Triple &TargetTriple;
  65
  66   /// Return the exception object from the value passed into
  67   /// the 'resume' instruction (typically an aggregate). Clean up any dead
  68   /// instructions, including the 'resume' instruction.
  69   Value *GetExceptionObject(ResumeInst *RI);
  70
  71   /// Replace resumes that are not reachable from a cleanup landing pad with
  72   /// unreachable and then simplify those blocks.
  73   size_t
  74   pruneUnreachableResumes(SmallVectorImpl<ResumeInst *> &Resumes,
  75                           SmallVectorImpl<LandingPadInst *> &CleanupLPads);
  76
  77   /// Convert the ResumeInsts that are still present
  78   /// into calls to the appropriate _Unwind_Resume function.
  79   bool InsertUnwindResumeCalls();
  80
  81 public:
  82   DwarfEHPrepare(CodeGenOptLevel OptLevel_, Function &F_,
  83                  const TargetLowering &TLI_, DomTreeUpdater *DTU_,
  84                  const TargetTransformInfo *TTI_, const Triple &TargetTriple_)
  85       : OptLevel(OptLevel_), F(F_), TLI(TLI_), DTU(DTU_), TTI(TTI_),
  86         TargetTriple(TargetTriple_) {}
  87
  88   bool run();
  89 };
  90
  91 } // namespace
  92
  93 Value *DwarfEHPrepare::GetExceptionObject(ResumeInst *RI) {
  94   Value *V = RI->getOperand(0);
  95   Value *ExnObj = nullptr;
  96   InsertValueInst *SelIVI = dyn_cast<InsertValueInst>(V);
  97   LoadInst *SelLoad = nullptr;
  98   InsertValueInst *ExcIVI = nullptr;
  99   bool EraseIVIs = false;
 100
 101   if (SelIVI) {
 102     if (SelIVI->getNumIndices() == 1 && *SelIVI->idx_begin() == 1) {
 103       ExcIVI = dyn_cast<InsertValueInst>(SelIVI->getOperand(0));
 104       if (ExcIVI && isa<UndefValue>(ExcIVI->getOperand(0)) &&
 105           ExcIVI->getNumIndices() == 1 && *ExcIVI->idx_begin() == 0) {
 106         ExnObj = ExcIVI->getOperand(1);
 107         SelLoad = dyn_cast<LoadInst>(SelIVI->getOperand(1));
 108         EraseIVIs = true;
 109       }
 110     }
 111   }
 112
 113   if (!ExnObj)
 114     ExnObj = ExtractValueInst::Create(RI->getOperand(0), 0, "exn.obj", RI);
 115
 116   RI->eraseFromParent();
 117
 118   if (EraseIVIs) {
 119     if (SelIVI->use_empty())
 120       SelIVI->eraseFromParent();
 121     if (ExcIVI->use_empty())
 122       ExcIVI->eraseFromParent();
 123     if (SelLoad && SelLoad->use_empty())
 124       SelLoad->eraseFromParent();
 125   }
 126
 127   return ExnObj;
 128 }
 129
 130 size_t DwarfEHPrepare::pruneUnreachableResumes(
 131     SmallVectorImpl<ResumeInst *> &Resumes,
 132     SmallVectorImpl<LandingPadInst *> &CleanupLPads) {
 133   assert(DTU && "Should have DomTreeUpdater here.");
 134
 135   BitVector ResumeReachable(Resumes.size());
 136   size_t ResumeIndex = 0;
 137   for (auto *RI : Resumes) {
 138     for (auto *LP : CleanupLPads) {
 139       if (isPotentiallyReachable(LP, RI, nullptr, &DTU->getDomTree())) {
 140         ResumeReachable.set(ResumeIndex);
 141         break;
 142       }
 143     }
 144     ++ResumeIndex;
 145   }
 146
 147   // If everything is reachable, there is no change.
 148   if (ResumeReachable.all())
 149     return Resumes.size();
 150
 151   LLVMContext &Ctx = F.getContext();
 152
 153   // Otherwise, insert unreachable instructions and call simplifycfg.
 154   size_t ResumesLeft = 0;
 155   for (size_t I = 0, E = Resumes.size(); I < E; ++I) {
 156     ResumeInst *RI = Resumes[I];
 157     if (ResumeReachable[I]) {
 158       Resumes[ResumesLeft++] = RI;
 159     } else {
 160       BasicBlock *BB = RI->getParent();
 161       new UnreachableInst(Ctx, RI);
 162       RI->eraseFromParent();
 163       simplifyCFG(BB, *TTI, DTU);
 164     }
 165   }
 166   Resumes.resize(ResumesLeft);
 167   return ResumesLeft;
 168 }
 169
 170 bool DwarfEHPrepare::InsertUnwindResumeCalls() {
 171   SmallVector<ResumeInst *, 16> Resumes;
 172   SmallVector<LandingPadInst *, 16> CleanupLPads;
 173   if (F.doesNotThrow())
 174     NumNoUnwind++;
 175   else
 176     NumUnwind++;
 177   for (BasicBlock &BB : F) {
 178     if (auto *RI = dyn_cast<ResumeInst>(BB.getTerminator()))
 179       Resumes.push_back(RI);
 180     if (auto *LP = BB.getLandingPadInst())
 181       if (LP->isCleanup())
 182         CleanupLPads.push_back(LP);
 183   }
 184
 185   NumCleanupLandingPadsRemaining += CleanupLPads.size();
 186
 187   if (Resumes.empty())
 188     return false;
 189
 190   // Check the personality, don't do anything if it's scope-based.
 191   EHPersonality Pers = classifyEHPersonality(F.getPersonalityFn());
 192   if (isScopedEHPersonality(Pers))
 193     return false;
 194
 195   LLVMContext &Ctx = F.getContext();
 196
 197   size_t ResumesLeft = Resumes.size();
 198   if (OptLevel != CodeGenOptLevel::None) {
 199     ResumesLeft = pruneUnreachableResumes(Resumes, CleanupLPads);
 200 #if LLVM_ENABLE_STATS
 201     unsigned NumRemainingLPs = 0;
 202     for (BasicBlock &BB : F) {
 203       if (auto *LP = BB.getLandingPadInst())
 204         if (LP->isCleanup())
 205           NumRemainingLPs++;
 206     }
 207     NumCleanupLandingPadsUnreachable += CleanupLPads.size() - NumRemainingLPs;
 208     NumCleanupLandingPadsRemaining -= CleanupLPads.size() - NumRemainingLPs;
 209 #endif
 210   }
 211
 212   if (ResumesLeft == 0)
 213     return true; // We pruned them all.
 214
 215   // RewindFunction - _Unwind_Resume or the target equivalent.
 216   FunctionCallee RewindFunction;
 217   CallingConv::ID RewindFunctionCallingConv;
 218   FunctionType *FTy;
 219   const char *RewindName;
 220   bool DoesRewindFunctionNeedExceptionObject;
 221
 222   if ((Pers == EHPersonality::GNU_CXX || Pers == EHPersonality::GNU_CXX_SjLj) &&
 223       TargetTriple.isTargetEHABICompatible()) {
 224     RewindName = TLI.getLibcallName(RTLIB::CXA_END_CLEANUP);
 225     FTy = FunctionType::get(Type::getVoidTy(Ctx), false);
 226     RewindFunctionCallingConv =
 227         TLI.getLibcallCallingConv(RTLIB::CXA_END_CLEANUP);
 228     DoesRewindFunctionNeedExceptionObject = false;
 229   } else {
 230     RewindName = TLI.getLibcallName(RTLIB::UNWIND_RESUME);
 231     FTy = FunctionType::get(Type::getVoidTy(Ctx), PointerType::getUnqual(Ctx),
 232                             false);
 233     RewindFunctionCallingConv = TLI.getLibcallCallingConv(RTLIB::UNWIND_RESUME);
 234     DoesRewindFunctionNeedExceptionObject = true;
 235   }
 236   RewindFunction = F.getParent()->getOrInsertFunction(RewindName, FTy);
 237
 238   // Create the basic block where the _Unwind_Resume call will live.
 239   if (ResumesLeft == 1) {
 240     // Instead of creating a new BB and PHI node, just append the call to
 241     // _Unwind_Resume to the end of the single resume block.
 242     ResumeInst *RI = Resumes.front();
 243     BasicBlock *UnwindBB = RI->getParent();
 244     Value *ExnObj = GetExceptionObject(RI);
 245     llvm::SmallVector<Value *, 1> RewindFunctionArgs;
 246     if (DoesRewindFunctionNeedExceptionObject)
 247       RewindFunctionArgs.push_back(ExnObj);
 248
 249     // Call the rewind function.
 250     CallInst *CI =
 251         CallInst::Create(RewindFunction, RewindFunctionArgs, "", UnwindBB);
 252     // The verifier requires that all calls of debug-info-bearing functions
 253     // from debug-info-bearing functions have a debug location (for inlining
 254     // purposes). Assign a dummy location to satisfy the constraint.
 255     Function *RewindFn = dyn_cast<Function>(RewindFunction.getCallee());
 256     if (RewindFn && RewindFn->getSubprogram())
 257       if (DISubprogram *SP = F.getSubprogram())
 258         CI->setDebugLoc(DILocation::get(SP->getContext(), 0, 0, SP));
 259     CI->setCallingConv(RewindFunctionCallingConv);
 260
 261     // We never expect _Unwind_Resume to return.
 262     CI->setDoesNotReturn();
 263     new UnreachableInst(Ctx, UnwindBB);
 264     return true;
 265   }
 266
 267   std::vector<DominatorTree::UpdateType> Updates;
 268   Updates.reserve(Resumes.size());
 269
 270   llvm::SmallVector<Value *, 1> RewindFunctionArgs;
 271
 272   BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &F);
 273   PHINode *PN = PHINode::Create(PointerType::getUnqual(Ctx), ResumesLeft,
 274                                 "exn.obj", UnwindBB);
 275
 276   // Extract the exception object from the ResumeInst and add it to the PHI node
 277   // that feeds the _Unwind_Resume call.
 278   for (ResumeInst *RI : Resumes) {
 279     BasicBlock *Parent = RI->getParent();
 280     BranchInst::Create(UnwindBB, Parent);
 281     Updates.push_back({DominatorTree::Insert, Parent, UnwindBB});
 282
 283     Value *ExnObj = GetExceptionObject(RI);
 284     PN->addIncoming(ExnObj, Parent);
 285
 286     ++NumResumesLowered;
 287   }
 288
 289   if (DoesRewindFunctionNeedExceptionObject)
 290     RewindFunctionArgs.push_back(PN);
 291
 292   // Call the function.
 293   CallInst *CI =
 294       CallInst::Create(RewindFunction, RewindFunctionArgs, "", UnwindBB);
 295   CI->setCallingConv(RewindFunctionCallingConv);
 296
 297   // We never expect _Unwind_Resume to return.
 298   CI->setDoesNotReturn();
 299   new UnreachableInst(Ctx, UnwindBB);
 300
 301   if (DTU)
 302     DTU->applyUpdates(Updates);
 303
 304   return true;
 305 }
 306
 307 bool DwarfEHPrepare::run() {
 308   bool Changed = InsertUnwindResumeCalls();
 309
 310   return Changed;
 311 }
 312
 313 static bool prepareDwarfEH(CodeGenOptLevel OptLevel, Function &F,
 314                            const TargetLowering &TLI, DominatorTree *DT,
 315                            const TargetTransformInfo *TTI,
 316                            const Triple &TargetTriple) {
 317   DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
 318
 319   return DwarfEHPrepare(OptLevel, F, TLI, DT ? &DTU : nullptr, TTI,
 320                         TargetTriple)
 321       .run();
 322 }
 323
 324 namespace {
 325
 326 class DwarfEHPrepareLegacyPass : public FunctionPass {
 327
 328   CodeGenOptLevel OptLevel;
 329
 330 public:
 331   static char ID; // Pass identification, replacement for typeid.
 332
 333   DwarfEHPrepareLegacyPass(CodeGenOptLevel OptLevel = CodeGenOptLevel::Default)
 334       : FunctionPass(ID), OptLevel(OptLevel) {}
 335
 336   bool runOnFunction(Function &F) override {
 337     const TargetMachine &TM =
 338         getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
 339     const TargetLowering &TLI = *TM.getSubtargetImpl(F)->getTargetLowering();
 340     DominatorTree *DT = nullptr;
 341     const TargetTransformInfo *TTI = nullptr;
 342     if (auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>())
 343       DT = &DTWP->getDomTree();
 344     if (OptLevel != CodeGenOptLevel::None) {
 345       if (!DT)
 346         DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
 347       TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
 348     }
 349     return prepareDwarfEH(OptLevel, F, TLI, DT, TTI, TM.getTargetTriple());
 350   }
 351
 352   void getAnalysisUsage(AnalysisUsage &AU) const override {
 353     AU.addRequired<TargetPassConfig>();
 354     AU.addRequired<TargetTransformInfoWrapperPass>();
 355     if (OptLevel != CodeGenOptLevel::None) {
 356       AU.addRequired<DominatorTreeWrapperPass>();
 357       AU.addRequired<TargetTransformInfoWrapperPass>();
 358     }
 359     AU.addPreserved<DominatorTreeWrapperPass>();
 360   }
 361
 362   StringRef getPassName() const override {
 363     return "Exception handling preparation";
 364   }
 365 };
 366
 367 } // end anonymous namespace
 368
 369 PreservedAnalyses DwarfEHPreparePass::run(Function &F,
 370                                           FunctionAnalysisManager &FAM) {
 371   const auto &TLI = *TM->getSubtargetImpl(F)->getTargetLowering();
 372   auto *DT = FAM.getCachedResult<DominatorTreeAnalysis>(F);
 373   const TargetTransformInfo *TTI = nullptr;
 374   auto OptLevel = TM->getOptLevel();
 375   if (OptLevel != CodeGenOptLevel::None) {
 376     if (!DT)
 377       DT = &FAM.getResult<DominatorTreeAnalysis>(F);
 378     TTI = &FAM.getResult<TargetIRAnalysis>(F);
 379   }
 380   bool Changed =
 381       prepareDwarfEH(OptLevel, F, TLI, DT, TTI, TM->getTargetTriple());
 382
 383   if (!Changed)
 384     return PreservedAnalyses::all();
 385   PreservedAnalyses PA;
 386   PA.preserve<DominatorTreeAnalysis>();
 387   return PA;
 388 }
 389
 390 char DwarfEHPrepareLegacyPass::ID = 0;
 391
 392 INITIALIZE_PASS_BEGIN(DwarfEHPrepareLegacyPass, DEBUG_TYPE,
 393                       "Prepare DWARF exceptions", false, false)
 394 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
 395 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
 396 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
 397 INITIALIZE_PASS_END(DwarfEHPrepareLegacyPass, DEBUG_TYPE,
 398                     "Prepare DWARF exceptions", false, false)
 399
 400 FunctionPass *llvm::createDwarfEHPass(CodeGenOptLevel OptLevel) {
 401   return new DwarfEHPrepareLegacyPass(OptLevel);
 402 }