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/ADT/BitVector.h"
  15 #include "llvm/ADT/SmallVector.h"
  16 #include "llvm/ADT/Statistic.h"
  17 #include "llvm/Analysis/CFG.h"
  18 #include "llvm/Analysis/DomTreeUpdater.h"
  19 #include "llvm/Analysis/EHPersonalities.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/DerivedTypes.h"
  28 #include "llvm/IR/Dominators.h"
  29 #include "llvm/IR/Function.h"
  30 #include "llvm/IR/Instructions.h"
  31 #include "llvm/IR/Module.h"
  32 #include "llvm/IR/Type.h"
  33 #include "llvm/InitializePasses.h"
  34 #include "llvm/Pass.h"
  35 #include "llvm/Support/Casting.h"
  36 #include "llvm/Target/TargetMachine.h"
  37 #include "llvm/Transforms/Utils/Local.h"
  38 #include <cstddef>
  39
  40 using namespace llvm;
  41
  42 #define DEBUG_TYPE "dwarfehprepare"
  43
  44 STATISTIC(NumResumesLowered, "Number of resume calls lowered");
  45 STATISTIC(NumCleanupLandingPadsUnreachable,
  46           "Number of cleanup landing pads found unreachable");
  47 STATISTIC(NumCleanupLandingPadsRemaining,
  48           "Number of cleanup landing pads remaining");
  49 STATISTIC(NumNoUnwind, "Number of functions with nounwind");
  50 STATISTIC(NumUnwind, "Number of functions with unwind");
  51
  52 namespace {
  53
  54 class DwarfEHPrepare {
  55   CodeGenOpt::Level OptLevel;
  56
  57   // RewindFunction - _Unwind_Resume or the target equivalent.
  58   FunctionCallee &RewindFunction;
  59
  60   Function &F;
  61   const TargetLowering &TLI;
  62   DomTreeUpdater *DTU;
  63   const TargetTransformInfo *TTI;
  64
  65   /// Return the exception object from the value passed into
  66   /// the 'resume' instruction (typically an aggregate). Clean up any dead
  67   /// instructions, including the 'resume' instruction.
  68   Value *GetExceptionObject(ResumeInst *RI);
  69
  70   /// Replace resumes that are not reachable from a cleanup landing pad with
  71   /// unreachable and then simplify those blocks.
  72   size_t
  73   pruneUnreachableResumes(SmallVectorImpl<ResumeInst *> &Resumes,
  74                           SmallVectorImpl<LandingPadInst *> &CleanupLPads);
  75
  76   /// Convert the ResumeInsts that are still present
  77   /// into calls to the appropriate _Unwind_Resume function.
  78   bool InsertUnwindResumeCalls();
  79
  80 public:
  81   DwarfEHPrepare(CodeGenOpt::Level OptLevel_, FunctionCallee &RewindFunction_,
  82                  Function &F_, const TargetLowering &TLI_, DomTreeUpdater *DTU_,
  83                  const TargetTransformInfo *TTI_)
  84       : OptLevel(OptLevel_), RewindFunction(RewindFunction_), F(F_), TLI(TLI_),
  85         DTU(DTU_), TTI(TTI_) {}
  86
  87   bool run();
  88 };
  89
  90 } // namespace
  91
  92 Value *DwarfEHPrepare::GetExceptionObject(ResumeInst *RI) {
  93   Value *V = RI->getOperand(0);
  94   Value *ExnObj = nullptr;
  95   InsertValueInst *SelIVI = dyn_cast<InsertValueInst>(V);
  96   LoadInst *SelLoad = nullptr;
  97   InsertValueInst *ExcIVI = nullptr;
  98   bool EraseIVIs = false;
  99
 100   if (SelIVI) {
 101     if (SelIVI->getNumIndices() == 1 && *SelIVI->idx_begin() == 1) {
 102       ExcIVI = dyn_cast<InsertValueInst>(SelIVI->getOperand(0));
 103       if (ExcIVI && isa<UndefValue>(ExcIVI->getOperand(0)) &&
 104           ExcIVI->getNumIndices() == 1 && *ExcIVI->idx_begin() == 0) {
 105         ExnObj = ExcIVI->getOperand(1);
 106         SelLoad = dyn_cast<LoadInst>(SelIVI->getOperand(1));
 107         EraseIVIs = true;
 108       }
 109     }
 110   }
 111
 112   if (!ExnObj)
 113     ExnObj = ExtractValueInst::Create(RI->getOperand(0), 0, "exn.obj", RI);
 114
 115   RI->eraseFromParent();
 116
 117   if (EraseIVIs) {
 118     if (SelIVI->use_empty())
 119       SelIVI->eraseFromParent();
 120     if (ExcIVI->use_empty())
 121       ExcIVI->eraseFromParent();
 122     if (SelLoad && SelLoad->use_empty())
 123       SelLoad->eraseFromParent();
 124   }
 125
 126   return ExnObj;
 127 }
 128
 129 size_t DwarfEHPrepare::pruneUnreachableResumes(
 130     SmallVectorImpl<ResumeInst *> &Resumes,
 131     SmallVectorImpl<LandingPadInst *> &CleanupLPads) {
 132   assert(DTU && "Should have DomTreeUpdater here.");
 133
 134   BitVector ResumeReachable(Resumes.size());
 135   size_t ResumeIndex = 0;
 136   for (auto *RI : Resumes) {
 137     for (auto *LP : CleanupLPads) {
 138       if (isPotentiallyReachable(LP, RI, nullptr, &DTU->getDomTree())) {
 139         ResumeReachable.set(ResumeIndex);
 140         break;
 141       }
 142     }
 143     ++ResumeIndex;
 144   }
 145
 146   // If everything is reachable, there is no change.
 147   if (ResumeReachable.all())
 148     return Resumes.size();
 149
 150   LLVMContext &Ctx = F.getContext();
 151
 152   // Otherwise, insert unreachable instructions and call simplifycfg.
 153   size_t ResumesLeft = 0;
 154   for (size_t I = 0, E = Resumes.size(); I < E; ++I) {
 155     ResumeInst *RI = Resumes[I];
 156     if (ResumeReachable[I]) {
 157       Resumes[ResumesLeft++] = RI;
 158     } else {
 159       BasicBlock *BB = RI->getParent();
 160       new UnreachableInst(Ctx, RI);
 161       RI->eraseFromParent();
 162       simplifyCFG(BB, *TTI, DTU);
 163     }
 164   }
 165   Resumes.resize(ResumesLeft);
 166   return ResumesLeft;
 167 }
 168
 169 bool DwarfEHPrepare::InsertUnwindResumeCalls() {
 170   SmallVector<ResumeInst *, 16> Resumes;
 171   SmallVector<LandingPadInst *, 16> CleanupLPads;
 172   if (F.doesNotThrow())
 173     NumNoUnwind++;
 174   else
 175     NumUnwind++;
 176   for (BasicBlock &BB : F) {
 177     if (auto *RI = dyn_cast<ResumeInst>(BB.getTerminator()))
 178       Resumes.push_back(RI);
 179     if (auto *LP = BB.getLandingPadInst())
 180       if (LP->isCleanup())
 181         CleanupLPads.push_back(LP);
 182   }
 183
 184   NumCleanupLandingPadsRemaining += CleanupLPads.size();
 185
 186   if (Resumes.empty())
 187     return false;
 188
 189   // Check the personality, don't do anything if it's scope-based.
 190   EHPersonality Pers = classifyEHPersonality(F.getPersonalityFn());
 191   if (isScopedEHPersonality(Pers))
 192     return false;
 193
 194   LLVMContext &Ctx = F.getContext();
 195
 196   size_t ResumesLeft = Resumes.size();
 197   if (OptLevel != CodeGenOpt::None) {
 198     ResumesLeft = pruneUnreachableResumes(Resumes, CleanupLPads);
 199 #if LLVM_ENABLE_STATS
 200     unsigned NumRemainingLPs = 0;
 201     for (BasicBlock &BB : F) {
 202       if (auto *LP = BB.getLandingPadInst())
 203         if (LP->isCleanup())
 204           NumRemainingLPs++;
 205     }
 206     NumCleanupLandingPadsUnreachable += CleanupLPads.size() - NumRemainingLPs;
 207     NumCleanupLandingPadsRemaining -= CleanupLPads.size() - NumRemainingLPs;
 208 #endif
 209   }
 210
 211   if (ResumesLeft == 0)
 212     return true; // We pruned them all.
 213
 214   // Find the rewind function if we didn't already.
 215   if (!RewindFunction) {
 216     FunctionType *FTy =
 217         FunctionType::get(Type::getVoidTy(Ctx), Type::getInt8PtrTy(Ctx), false);
 218     const char *RewindName = TLI.getLibcallName(RTLIB::UNWIND_RESUME);
 219     RewindFunction = F.getParent()->getOrInsertFunction(RewindName, FTy);
 220   }
 221
 222   // Create the basic block where the _Unwind_Resume call will live.
 223   if (ResumesLeft == 1) {
 224     // Instead of creating a new BB and PHI node, just append the call to
 225     // _Unwind_Resume to the end of the single resume block.
 226     ResumeInst *RI = Resumes.front();
 227     BasicBlock *UnwindBB = RI->getParent();
 228     Value *ExnObj = GetExceptionObject(RI);
 229
 230     // Call the _Unwind_Resume function.
 231     CallInst *CI = CallInst::Create(RewindFunction, ExnObj, "", UnwindBB);
 232     CI->setCallingConv(TLI.getLibcallCallingConv(RTLIB::UNWIND_RESUME));
 233
 234     // We never expect _Unwind_Resume to return.
 235     CI->setDoesNotReturn();
 236     new UnreachableInst(Ctx, UnwindBB);
 237     return true;
 238   }
 239
 240   std::vector<DominatorTree::UpdateType> Updates;
 241   Updates.reserve(Resumes.size());
 242
 243   BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &F);
 244   PHINode *PN = PHINode::Create(Type::getInt8PtrTy(Ctx), ResumesLeft, "exn.obj",
 245                                 UnwindBB);
 246
 247   // Extract the exception object from the ResumeInst and add it to the PHI node
 248   // that feeds the _Unwind_Resume call.
 249   for (ResumeInst *RI : Resumes) {
 250     BasicBlock *Parent = RI->getParent();
 251     BranchInst::Create(UnwindBB, Parent);
 252     Updates.push_back({DominatorTree::Insert, Parent, UnwindBB});
 253
 254     Value *ExnObj = GetExceptionObject(RI);
 255     PN->addIncoming(ExnObj, Parent);
 256
 257     ++NumResumesLowered;
 258   }
 259
 260   // Call the function.
 261   CallInst *CI = CallInst::Create(RewindFunction, PN, "", UnwindBB);
 262   CI->setCallingConv(TLI.getLibcallCallingConv(RTLIB::UNWIND_RESUME));
 263
 264   // We never expect _Unwind_Resume to return.
 265   CI->setDoesNotReturn();
 266   new UnreachableInst(Ctx, UnwindBB);
 267
 268   if (DTU)
 269     DTU->applyUpdates(Updates);
 270
 271   return true;
 272 }
 273
 274 bool DwarfEHPrepare::run() {
 275   bool Changed = InsertUnwindResumeCalls();
 276
 277   return Changed;
 278 }
 279
 280 static bool prepareDwarfEH(CodeGenOpt::Level OptLevel,
 281                            FunctionCallee &RewindFunction, Function &F,
 282                            const TargetLowering &TLI, DominatorTree *DT,
 283                            const TargetTransformInfo *TTI) {
 284   DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
 285
 286   return DwarfEHPrepare(OptLevel, RewindFunction, F, TLI, DT ? &DTU : nullptr,
 287                         TTI)
 288       .run();
 289 }
 290
 291 namespace {
 292
 293 class DwarfEHPrepareLegacyPass : public FunctionPass {
 294   // RewindFunction - _Unwind_Resume or the target equivalent.
 295   FunctionCallee RewindFunction = nullptr;
 296
 297   CodeGenOpt::Level OptLevel;
 298
 299 public:
 300   static char ID; // Pass identification, replacement for typeid.
 301
 302   DwarfEHPrepareLegacyPass(CodeGenOpt::Level OptLevel = CodeGenOpt::Default)
 303       : FunctionPass(ID), OptLevel(OptLevel) {}
 304
 305   bool runOnFunction(Function &F) override {
 306     const TargetMachine &TM =
 307         getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
 308     const TargetLowering &TLI = *TM.getSubtargetImpl(F)->getTargetLowering();
 309     DominatorTree *DT = nullptr;
 310     const TargetTransformInfo *TTI = nullptr;
 311     if (auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>())
 312       DT = &DTWP->getDomTree();
 313     if (OptLevel != CodeGenOpt::None) {
 314       if (!DT)
 315         DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
 316       TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
 317     }
 318     return prepareDwarfEH(OptLevel, RewindFunction, F, TLI, DT, TTI);
 319   }
 320
 321   void getAnalysisUsage(AnalysisUsage &AU) const override {
 322     AU.addRequired<TargetPassConfig>();
 323     AU.addRequired<TargetTransformInfoWrapperPass>();
 324     if (OptLevel != CodeGenOpt::None) {
 325       AU.addRequired<DominatorTreeWrapperPass>();
 326       AU.addRequired<TargetTransformInfoWrapperPass>();
 327     }
 328     AU.addPreserved<DominatorTreeWrapperPass>();
 329   }
 330
 331   StringRef getPassName() const override {
 332     return "Exception handling preparation";
 333   }
 334 };
 335
 336 } // end anonymous namespace
 337
 338 char DwarfEHPrepareLegacyPass::ID = 0;
 339
 340 INITIALIZE_PASS_BEGIN(DwarfEHPrepareLegacyPass, DEBUG_TYPE,
 341                       "Prepare DWARF exceptions", false, false)
 342 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
 343 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
 344 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
 345 INITIALIZE_PASS_END(DwarfEHPrepareLegacyPass, DEBUG_TYPE,
 346                     "Prepare DWARF exceptions", false, false)
 347
 348 FunctionPass *llvm::createDwarfEHPass(CodeGenOpt::Level OptLevel) {
 349   return new DwarfEHPrepareLegacyPass(OptLevel);
 350 }