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/RuntimeLibcallUtil.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",
 115                                       RI->getIterator());
 116
 117   RI->eraseFromParent();
 118
 119   if (EraseIVIs) {
 120     if (SelIVI->use_empty())
 121       SelIVI->eraseFromParent();
 122     if (ExcIVI->use_empty())
 123       ExcIVI->eraseFromParent();
 124     if (SelLoad && SelLoad->use_empty())
 125       SelLoad->eraseFromParent();
 126   }
 127
 128   return ExnObj;
 129 }
 130
 131 size_t DwarfEHPrepare::pruneUnreachableResumes(
 132     SmallVectorImpl<ResumeInst *> &Resumes,
 133     SmallVectorImpl<LandingPadInst *> &CleanupLPads) {
 134   assert(DTU && "Should have DomTreeUpdater here.");
 135
 136   BitVector ResumeReachable(Resumes.size());
 137   size_t ResumeIndex = 0;
 138   for (auto *RI : Resumes) {
 139     for (auto *LP : CleanupLPads) {
 140       if (isPotentiallyReachable(LP, RI, nullptr, &DTU->getDomTree())) {
 141         ResumeReachable.set(ResumeIndex);
 142         break;
 143       }
 144     }
 145     ++ResumeIndex;
 146   }
 147
 148   // If everything is reachable, there is no change.
 149   if (ResumeReachable.all())
 150     return Resumes.size();
 151
 152   LLVMContext &Ctx = F.getContext();
 153
 154   // Otherwise, insert unreachable instructions and call simplifycfg.
 155   size_t ResumesLeft = 0;
 156   for (size_t I = 0, E = Resumes.size(); I < E; ++I) {
 157     ResumeInst *RI = Resumes[I];
 158     if (ResumeReachable[I]) {
 159       Resumes[ResumesLeft++] = RI;
 160     } else {
 161       BasicBlock *BB = RI->getParent();
 162       new UnreachableInst(Ctx, RI->getIterator());
 163       RI->eraseFromParent();
 164       simplifyCFG(BB, *TTI, DTU);
 165     }
 166   }
 167   Resumes.resize(ResumesLeft);
 168   return ResumesLeft;
 169 }
 170
 171 bool DwarfEHPrepare::InsertUnwindResumeCalls() {
 172   SmallVector<ResumeInst *, 16> Resumes;
 173   SmallVector<LandingPadInst *, 16> CleanupLPads;
 174   if (F.doesNotThrow())
 175     NumNoUnwind++;
 176   else
 177     NumUnwind++;
 178   for (BasicBlock &BB : F) {
 179     if (auto *RI = dyn_cast<ResumeInst>(BB.getTerminator()))
 180       Resumes.push_back(RI);
 181     if (auto *LP = BB.getLandingPadInst())
 182       if (LP->isCleanup())
 183         CleanupLPads.push_back(LP);
 184   }
 185
 186   NumCleanupLandingPadsRemaining += CleanupLPads.size();
 187
 188   if (Resumes.empty())
 189     return false;
 190
 191   // Check the personality, don't do anything if it's scope-based.
 192   EHPersonality Pers = classifyEHPersonality(F.getPersonalityFn());
 193   if (isScopedEHPersonality(Pers))
 194     return false;
 195
 196   LLVMContext &Ctx = F.getContext();
 197
 198   size_t ResumesLeft = Resumes.size();
 199   if (OptLevel != CodeGenOptLevel::None) {
 200     ResumesLeft = pruneUnreachableResumes(Resumes, CleanupLPads);
 201 #if LLVM_ENABLE_STATS
 202     unsigned NumRemainingLPs = 0;
 203     for (BasicBlock &BB : F) {
 204       if (auto *LP = BB.getLandingPadInst())
 205         if (LP->isCleanup())
 206           NumRemainingLPs++;
 207     }
 208     NumCleanupLandingPadsUnreachable += CleanupLPads.size() - NumRemainingLPs;
 209     NumCleanupLandingPadsRemaining -= CleanupLPads.size() - NumRemainingLPs;
 210 #endif
 211   }
 212
 213   if (ResumesLeft == 0)
 214     return true; // We pruned them all.
 215
 216   // RewindFunction - _Unwind_Resume or the target equivalent.
 217   FunctionCallee RewindFunction;
 218   CallingConv::ID RewindFunctionCallingConv;
 219   FunctionType *FTy;
 220   const char *RewindName;
 221   bool DoesRewindFunctionNeedExceptionObject;
 222
 223   if ((Pers == EHPersonality::GNU_CXX || Pers == EHPersonality::GNU_CXX_SjLj) &&
 224       TargetTriple.isTargetEHABICompatible()) {
 225     RewindName = TLI.getLibcallName(RTLIB::CXA_END_CLEANUP);
 226     FTy = FunctionType::get(Type::getVoidTy(Ctx), false);
 227     RewindFunctionCallingConv =
 228         TLI.getLibcallCallingConv(RTLIB::CXA_END_CLEANUP);
 229     DoesRewindFunctionNeedExceptionObject = false;
 230   } else {
 231     RewindName = TLI.getLibcallName(RTLIB::UNWIND_RESUME);
 232     FTy = FunctionType::get(Type::getVoidTy(Ctx), PointerType::getUnqual(Ctx),
 233                             false);
 234     RewindFunctionCallingConv = TLI.getLibcallCallingConv(RTLIB::UNWIND_RESUME);
 235     DoesRewindFunctionNeedExceptionObject = true;
 236   }
 237   RewindFunction = F.getParent()->getOrInsertFunction(RewindName, FTy);
 238
 239   // Create the basic block where the _Unwind_Resume call will live.
 240   if (ResumesLeft == 1) {
 241     // Instead of creating a new BB and PHI node, just append the call to
 242     // _Unwind_Resume to the end of the single resume block.
 243     ResumeInst *RI = Resumes.front();
 244     BasicBlock *UnwindBB = RI->getParent();
 245     Value *ExnObj = GetExceptionObject(RI);
 246     llvm::SmallVector<Value *, 1> RewindFunctionArgs;
 247     if (DoesRewindFunctionNeedExceptionObject)
 248       RewindFunctionArgs.push_back(ExnObj);
 249
 250     // Call the rewind function.
 251     CallInst *CI =
 252         CallInst::Create(RewindFunction, RewindFunctionArgs, "", UnwindBB);
 253     // The verifier requires that all calls of debug-info-bearing functions
 254     // from debug-info-bearing functions have a debug location (for inlining
 255     // purposes). Assign a dummy location to satisfy the constraint.
 256     Function *RewindFn = dyn_cast<Function>(RewindFunction.getCallee());
 257     if (RewindFn && RewindFn->getSubprogram())
 258       if (DISubprogram *SP = F.getSubprogram())
 259         CI->setDebugLoc(DILocation::get(SP->getContext(), 0, 0, SP));
 260     CI->setCallingConv(RewindFunctionCallingConv);
 261
 262     // We never expect _Unwind_Resume to return.
 263     CI->setDoesNotReturn();
 264     new UnreachableInst(Ctx, UnwindBB);
 265     return true;
 266   }
 267
 268   std::vector<DominatorTree::UpdateType> Updates;
 269   Updates.reserve(Resumes.size());
 270
 271   llvm::SmallVector<Value *, 1> RewindFunctionArgs;
 272
 273   BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &F);
 274   PHINode *PN = PHINode::Create(PointerType::getUnqual(Ctx), ResumesLeft,
 275                                 "exn.obj", UnwindBB);
 276
 277   // Extract the exception object from the ResumeInst and add it to the PHI node
 278   // that feeds the _Unwind_Resume call.
 279   for (ResumeInst *RI : Resumes) {
 280     BasicBlock *Parent = RI->getParent();
 281     BranchInst::Create(UnwindBB, Parent);
 282     Updates.push_back({DominatorTree::Insert, Parent, UnwindBB});
 283
 284     Value *ExnObj = GetExceptionObject(RI);
 285     PN->addIncoming(ExnObj, Parent);
 286
 287     ++NumResumesLowered;
 288   }
 289
 290   if (DoesRewindFunctionNeedExceptionObject)
 291     RewindFunctionArgs.push_back(PN);
 292
 293   // Call the function.
 294   CallInst *CI =
 295       CallInst::Create(RewindFunction, RewindFunctionArgs, "", UnwindBB);
 296   // The verifier requires that all calls of debug-info-bearing functions
 297   // from debug-info-bearing functions have a debug location (for inlining
 298   // purposes). Assign a dummy location to satisfy the constraint.
 299   Function *RewindFn = dyn_cast<Function>(RewindFunction.getCallee());
 300   if (RewindFn && RewindFn->getSubprogram())
 301     if (DISubprogram *SP = F.getSubprogram())
 302       CI->setDebugLoc(DILocation::get(SP->getContext(), 0, 0, SP));
 303   CI->setCallingConv(RewindFunctionCallingConv);
 304
 305   // We never expect _Unwind_Resume to return.
 306   CI->setDoesNotReturn();
 307   new UnreachableInst(Ctx, UnwindBB);
 308
 309   if (DTU)
 310     DTU->applyUpdates(Updates);
 311
 312   return true;
 313 }
 314
 315 bool DwarfEHPrepare::run() {
 316   bool Changed = InsertUnwindResumeCalls();
 317
 318   return Changed;
 319 }
 320
 321 static bool prepareDwarfEH(CodeGenOptLevel OptLevel, Function &F,
 322                            const TargetLowering &TLI, DominatorTree *DT,
 323                            const TargetTransformInfo *TTI,
 324                            const Triple &TargetTriple) {
 325   DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
 326
 327   return DwarfEHPrepare(OptLevel, F, TLI, DT ? &DTU : nullptr, TTI,
 328                         TargetTriple)
 329       .run();
 330 }
 331
 332 namespace {
 333
 334 class DwarfEHPrepareLegacyPass : public FunctionPass {
 335
 336   CodeGenOptLevel OptLevel;
 337
 338 public:
 339   static char ID; // Pass identification, replacement for typeid.
 340
 341   DwarfEHPrepareLegacyPass(CodeGenOptLevel OptLevel = CodeGenOptLevel::Default)
 342       : FunctionPass(ID), OptLevel(OptLevel) {}
 343
 344   bool runOnFunction(Function &F) override {
 345     const TargetMachine &TM =
 346         getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
 347     const TargetLowering &TLI = *TM.getSubtargetImpl(F)->getTargetLowering();
 348     DominatorTree *DT = nullptr;
 349     const TargetTransformInfo *TTI = nullptr;
 350     if (auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>())
 351       DT = &DTWP->getDomTree();
 352     if (OptLevel != CodeGenOptLevel::None) {
 353       if (!DT)
 354         DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
 355       TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
 356     }
 357     return prepareDwarfEH(OptLevel, F, TLI, DT, TTI, TM.getTargetTriple());
 358   }
 359
 360   void getAnalysisUsage(AnalysisUsage &AU) const override {
 361     AU.addRequired<TargetPassConfig>();
 362     AU.addRequired<TargetTransformInfoWrapperPass>();
 363     if (OptLevel != CodeGenOptLevel::None) {
 364       AU.addRequired<DominatorTreeWrapperPass>();
 365       AU.addRequired<TargetTransformInfoWrapperPass>();
 366     }
 367     AU.addPreserved<DominatorTreeWrapperPass>();
 368   }
 369
 370   StringRef getPassName() const override {
 371     return "Exception handling preparation";
 372   }
 373 };
 374
 375 } // end anonymous namespace
 376
 377 PreservedAnalyses DwarfEHPreparePass::run(Function &F,
 378                                           FunctionAnalysisManager &FAM) {
 379   const auto &TLI = *TM->getSubtargetImpl(F)->getTargetLowering();
 380   auto *DT = FAM.getCachedResult<DominatorTreeAnalysis>(F);
 381   const TargetTransformInfo *TTI = nullptr;
 382   auto OptLevel = TM->getOptLevel();
 383   if (OptLevel != CodeGenOptLevel::None) {
 384     if (!DT)
 385       DT = &FAM.getResult<DominatorTreeAnalysis>(F);
 386     TTI = &FAM.getResult<TargetIRAnalysis>(F);
 387   }
 388   bool Changed =
 389       prepareDwarfEH(OptLevel, F, TLI, DT, TTI, TM->getTargetTriple());
 390
 391   if (!Changed)
 392     return PreservedAnalyses::all();
 393   PreservedAnalyses PA;
 394   PA.preserve<DominatorTreeAnalysis>();
 395   return PA;
 396 }
 397
 398 char DwarfEHPrepareLegacyPass::ID = 0;
 399
 400 INITIALIZE_PASS_BEGIN(DwarfEHPrepareLegacyPass, DEBUG_TYPE,
 401                       "Prepare DWARF exceptions", false, false)
 402 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
 403 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
 404 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
 405 INITIALIZE_PASS_END(DwarfEHPrepareLegacyPass, DEBUG_TYPE,
 406                     "Prepare DWARF exceptions", false, false)
 407
 408 FunctionPass *llvm::createDwarfEHPass(CodeGenOptLevel OptLevel) {
 409   return new DwarfEHPrepareLegacyPass(OptLevel);
 410 }