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/EHPersonalities.h"
  19 #include "llvm/Analysis/TargetTransformInfo.h"
  20 #include "llvm/Transforms/Utils/Local.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/Pass.h"
  34 #include "llvm/Support/Casting.h"
  35 #include "llvm/Target/TargetMachine.h"
  36 #include <cstddef>
  37
  38 using namespace llvm;
  39
  40 #define DEBUG_TYPE "dwarfehprepare"
  41
  42 STATISTIC(NumResumesLowered, "Number of resume calls lowered");
  43
  44 namespace {
  45
  46   class DwarfEHPrepare : public FunctionPass {
  47     // RewindFunction - _Unwind_Resume or the target equivalent.
  48     FunctionCallee RewindFunction = nullptr;
  49
  50     DominatorTree *DT = nullptr;
  51     const TargetLowering *TLI = nullptr;
  52
  53     bool InsertUnwindResumeCalls(Function &Fn);
  54     Value *GetExceptionObject(ResumeInst *RI);
  55     size_t
  56     pruneUnreachableResumes(Function &Fn,
  57                             SmallVectorImpl<ResumeInst *> &Resumes,
  58                             SmallVectorImpl<LandingPadInst *> &CleanupLPads);
  59
  60   public:
  61     static char ID; // Pass identification, replacement for typeid.
  62
  63     DwarfEHPrepare() : FunctionPass(ID) {}
  64
  65     bool runOnFunction(Function &Fn) override;
  66
  67     bool doFinalization(Module &M) override {
  68       RewindFunction = nullptr;
  69       return false;
  70     }
  71
  72     void getAnalysisUsage(AnalysisUsage &AU) const override;
  73
  74     StringRef getPassName() const override {
  75       return "Exception handling preparation";
  76     }
  77   };
  78
  79 } // end anonymous namespace
  80
  81 char DwarfEHPrepare::ID = 0;
  82
  83 INITIALIZE_PASS_BEGIN(DwarfEHPrepare, DEBUG_TYPE,
  84                       "Prepare DWARF exceptions", false, false)
  85 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
  86 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
  87 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
  88 INITIALIZE_PASS_END(DwarfEHPrepare, DEBUG_TYPE,
  89                     "Prepare DWARF exceptions", false, false)
  90
  91 FunctionPass *llvm::createDwarfEHPass() { return new DwarfEHPrepare(); }
  92
  93 void DwarfEHPrepare::getAnalysisUsage(AnalysisUsage &AU) const {
  94   AU.addRequired<TargetPassConfig>();
  95   AU.addRequired<TargetTransformInfoWrapperPass>();
  96   AU.addRequired<DominatorTreeWrapperPass>();
  97 }
  98
  99 /// GetExceptionObject - Return the exception object from the value passed into
 100 /// the 'resume' instruction (typically an aggregate). Clean up any dead
 101 /// instructions, including the 'resume' instruction.
 102 Value *DwarfEHPrepare::GetExceptionObject(ResumeInst *RI) {
 103   Value *V = RI->getOperand(0);
 104   Value *ExnObj = nullptr;
 105   InsertValueInst *SelIVI = dyn_cast<InsertValueInst>(V);
 106   LoadInst *SelLoad = nullptr;
 107   InsertValueInst *ExcIVI = nullptr;
 108   bool EraseIVIs = false;
 109
 110   if (SelIVI) {
 111     if (SelIVI->getNumIndices() == 1 && *SelIVI->idx_begin() == 1) {
 112       ExcIVI = dyn_cast<InsertValueInst>(SelIVI->getOperand(0));
 113       if (ExcIVI && isa<UndefValue>(ExcIVI->getOperand(0)) &&
 114           ExcIVI->getNumIndices() == 1 && *ExcIVI->idx_begin() == 0) {
 115         ExnObj = ExcIVI->getOperand(1);
 116         SelLoad = dyn_cast<LoadInst>(SelIVI->getOperand(1));
 117         EraseIVIs = true;
 118       }
 119     }
 120   }
 121
 122   if (!ExnObj)
 123     ExnObj = ExtractValueInst::Create(RI->getOperand(0), 0, "exn.obj", RI);
 124
 125   RI->eraseFromParent();
 126
 127   if (EraseIVIs) {
 128     if (SelIVI->use_empty())
 129       SelIVI->eraseFromParent();
 130     if (ExcIVI->use_empty())
 131       ExcIVI->eraseFromParent();
 132     if (SelLoad && SelLoad->use_empty())
 133       SelLoad->eraseFromParent();
 134   }
 135
 136   return ExnObj;
 137 }
 138
 139 /// Replace resumes that are not reachable from a cleanup landing pad with
 140 /// unreachable and then simplify those blocks.
 141 size_t DwarfEHPrepare::pruneUnreachableResumes(
 142     Function &Fn, SmallVectorImpl<ResumeInst *> &Resumes,
 143     SmallVectorImpl<LandingPadInst *> &CleanupLPads) {
 144   BitVector ResumeReachable(Resumes.size());
 145   size_t ResumeIndex = 0;
 146   for (auto *RI : Resumes) {
 147     for (auto *LP : CleanupLPads) {
 148       if (isPotentiallyReachable(LP, RI, nullptr, DT)) {
 149         ResumeReachable.set(ResumeIndex);
 150         break;
 151       }
 152     }
 153     ++ResumeIndex;
 154   }
 155
 156   // If everything is reachable, there is no change.
 157   if (ResumeReachable.all())
 158     return Resumes.size();
 159
 160   const TargetTransformInfo &TTI =
 161       getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn);
 162   LLVMContext &Ctx = Fn.getContext();
 163
 164   // Otherwise, insert unreachable instructions and call simplifycfg.
 165   size_t ResumesLeft = 0;
 166   for (size_t I = 0, E = Resumes.size(); I < E; ++I) {
 167     ResumeInst *RI = Resumes[I];
 168     if (ResumeReachable[I]) {
 169       Resumes[ResumesLeft++] = RI;
 170     } else {
 171       BasicBlock *BB = RI->getParent();
 172       new UnreachableInst(Ctx, RI);
 173       RI->eraseFromParent();
 174       simplifyCFG(BB, TTI);
 175     }
 176   }
 177   Resumes.resize(ResumesLeft);
 178   return ResumesLeft;
 179 }
 180
 181 /// InsertUnwindResumeCalls - Convert the ResumeInsts that are still present
 182 /// into calls to the appropriate _Unwind_Resume function.
 183 bool DwarfEHPrepare::InsertUnwindResumeCalls(Function &Fn) {
 184   SmallVector<ResumeInst*, 16> Resumes;
 185   SmallVector<LandingPadInst*, 16> CleanupLPads;
 186   for (BasicBlock &BB : Fn) {
 187     if (auto *RI = dyn_cast<ResumeInst>(BB.getTerminator()))
 188       Resumes.push_back(RI);
 189     if (auto *LP = BB.getLandingPadInst())
 190       if (LP->isCleanup())
 191         CleanupLPads.push_back(LP);
 192   }
 193
 194   if (Resumes.empty())
 195     return false;
 196
 197   // Check the personality, don't do anything if it's scope-based.
 198   EHPersonality Pers = classifyEHPersonality(Fn.getPersonalityFn());
 199   if (isScopedEHPersonality(Pers))
 200     return false;
 201
 202   LLVMContext &Ctx = Fn.getContext();
 203
 204   size_t ResumesLeft = pruneUnreachableResumes(Fn, Resumes, CleanupLPads);
 205   if (ResumesLeft == 0)
 206     return true; // We pruned them all.
 207
 208   // Find the rewind function if we didn't already.
 209   if (!RewindFunction) {
 210     FunctionType *FTy = FunctionType::get(Type::getVoidTy(Ctx),
 211                                           Type::getInt8PtrTy(Ctx), false);
 212     const char *RewindName = TLI->getLibcallName(RTLIB::UNWIND_RESUME);
 213     RewindFunction = Fn.getParent()->getOrInsertFunction(RewindName, FTy);
 214   }
 215
 216   // Create the basic block where the _Unwind_Resume call will live.
 217   if (ResumesLeft == 1) {
 218     // Instead of creating a new BB and PHI node, just append the call to
 219     // _Unwind_Resume to the end of the single resume block.
 220     ResumeInst *RI = Resumes.front();
 221     BasicBlock *UnwindBB = RI->getParent();
 222     Value *ExnObj = GetExceptionObject(RI);
 223
 224     // Call the _Unwind_Resume function.
 225     CallInst *CI = CallInst::Create(RewindFunction, ExnObj, "", UnwindBB);
 226     CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
 227
 228     // We never expect _Unwind_Resume to return.
 229     new UnreachableInst(Ctx, UnwindBB);
 230     return true;
 231   }
 232
 233   BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &Fn);
 234   PHINode *PN = PHINode::Create(Type::getInt8PtrTy(Ctx), ResumesLeft,
 235                                 "exn.obj", UnwindBB);
 236
 237   // Extract the exception object from the ResumeInst and add it to the PHI node
 238   // that feeds the _Unwind_Resume call.
 239   for (ResumeInst *RI : Resumes) {
 240     BasicBlock *Parent = RI->getParent();
 241     BranchInst::Create(UnwindBB, Parent);
 242
 243     Value *ExnObj = GetExceptionObject(RI);
 244     PN->addIncoming(ExnObj, Parent);
 245
 246     ++NumResumesLowered;
 247   }
 248
 249   // Call the function.
 250   CallInst *CI = CallInst::Create(RewindFunction, PN, "", UnwindBB);
 251   CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
 252
 253   // We never expect _Unwind_Resume to return.
 254   new UnreachableInst(Ctx, UnwindBB);
 255   return true;
 256 }
 257
 258 bool DwarfEHPrepare::runOnFunction(Function &Fn) {
 259   const TargetMachine &TM =
 260       getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
 261   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
 262   TLI = TM.getSubtargetImpl(Fn)->getTargetLowering();
 263   bool Changed = InsertUnwindResumeCalls(Fn);
 264   DT = nullptr;
 265   TLI = nullptr;
 266   return Changed;
 267 }