text: //===- DwarfEHPrepare - Prepare exception handling for code generation ----===//
  //
  // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
  // See https://llvm.org/LICENSE.txt for license information.
  // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
  //
  //===----------------------------------------------------------------------===//
  //
  // This pass mulches exception handling code into a form adapted to code
  // generation. Required if using dwarf exception handling.
  //
  //===----------------------------------------------------------------------===//
  
  #include "llvm/ADT/BitVector.h"
  #include "llvm/ADT/SmallVector.h"
  #include "llvm/ADT/Statistic.h"
  #include "llvm/Analysis/CFG.h"
  #include "llvm/Analysis/EHPersonalities.h"
  #include "llvm/Analysis/TargetTransformInfo.h"
  #include "llvm/CodeGen/RuntimeLibcalls.h"
  #include "llvm/CodeGen/TargetLowering.h"
  #include "llvm/CodeGen/TargetPassConfig.h"
  #include "llvm/CodeGen/TargetSubtargetInfo.h"
  #include "llvm/IR/BasicBlock.h"
  #include "llvm/IR/Constants.h"
  #include "llvm/IR/DerivedTypes.h"
  #include "llvm/IR/Dominators.h"
  #include "llvm/IR/Function.h"
  #include "llvm/IR/Instructions.h"
  #include "llvm/IR/Module.h"
  #include "llvm/IR/Type.h"
  #include "llvm/InitializePasses.h"
  #include "llvm/Pass.h"
  #include "llvm/Support/Casting.h"
  #include "llvm/Target/TargetMachine.h"
  #include "llvm/Transforms/Utils/Local.h"
  #include <cstddef>
  
  using namespace llvm;
  
  #define DEBUG_TYPE "dwarfehprepare"
  
  STATISTIC(NumResumesLowered, "Number of resume calls lowered");
  
  namespace {
  
    class DwarfEHPrepare : public FunctionPass {
      // RewindFunction - _Unwind_Resume or the target equivalent.
      FunctionCallee RewindFunction = nullptr;
  
      DominatorTree *DT = nullptr;
      const TargetLowering *TLI = nullptr;
  
      bool InsertUnwindResumeCalls(Function &Fn);
      Value *GetExceptionObject(ResumeInst *RI);
      size_t
      pruneUnreachableResumes(Function &Fn,
                              SmallVectorImpl<ResumeInst *> &Resumes,
                              SmallVectorImpl<LandingPadInst *> &CleanupLPads);
  
    public:
      static char ID; // Pass identification, replacement for typeid.
  
      DwarfEHPrepare() : FunctionPass(ID) {}
  
      bool runOnFunction(Function &Fn) override;
  
      bool doFinalization(Module &M) override {
        RewindFunction = nullptr;
        return false;
      }
  
      void getAnalysisUsage(AnalysisUsage &AU) const override;
  
      StringRef getPassName() const override {
        return "Exception handling preparation";
      }
    };
  
  } // end anonymous namespace
  
  char DwarfEHPrepare::ID = 0;
  
  INITIALIZE_PASS_BEGIN(DwarfEHPrepare, DEBUG_TYPE,
                        "Prepare DWARF exceptions", false, false)
  INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
  INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
  INITIALIZE_PASS_END(DwarfEHPrepare, DEBUG_TYPE,
                      "Prepare DWARF exceptions", false, false)
  
  FunctionPass *llvm::createDwarfEHPass() { return new DwarfEHPrepare(); }
  
  void DwarfEHPrepare::getAnalysisUsage(AnalysisUsage &AU) const {
    AU.addRequired<TargetPassConfig>();
    AU.addRequired<TargetTransformInfoWrapperPass>();
    AU.addRequired<DominatorTreeWrapperPass>();
  }
  
  /// GetExceptionObject - Return the exception object from the value passed into
  /// the 'resume' instruction (typically an aggregate). Clean up any dead
  /// instructions, including the 'resume' instruction.
  Value *DwarfEHPrepare::GetExceptionObject(ResumeInst *RI) {
    Value *V = RI->getOperand(0);
    Value *ExnObj = nullptr;
    InsertValueInst *SelIVI = dyn_cast<InsertValueInst>(V);
    LoadInst *SelLoad = nullptr;
    InsertValueInst *ExcIVI = nullptr;
    bool EraseIVIs = false;
  
    if (SelIVI) {
      if (SelIVI->getNumIndices() == 1 && *SelIVI->idx_begin() == 1) {
        ExcIVI = dyn_cast<InsertValueInst>(SelIVI->getOperand(0));
        if (ExcIVI && isa<UndefValue>(ExcIVI->getOperand(0)) &&
            ExcIVI->getNumIndices() == 1 && *ExcIVI->idx_begin() == 0) {
          ExnObj = ExcIVI->getOperand(1);
          SelLoad = dyn_cast<LoadInst>(SelIVI->getOperand(1));
          EraseIVIs = true;
        }
      }
    }
  
    if (!ExnObj)
      ExnObj = ExtractValueInst::Create(RI->getOperand(0), 0, "exn.obj", RI);
  
    RI->eraseFromParent();
  
    if (EraseIVIs) {
      if (SelIVI->use_empty())
        SelIVI->eraseFromParent();
      if (ExcIVI->use_empty())
        ExcIVI->eraseFromParent();
      if (SelLoad && SelLoad->use_empty())
        SelLoad->eraseFromParent();
    }
  
    return ExnObj;
  }
  
  /// Replace resumes that are not reachable from a cleanup landing pad with
  /// unreachable and then simplify those blocks.
  size_t DwarfEHPrepare::pruneUnreachableResumes(
      Function &Fn, SmallVectorImpl<ResumeInst *> &Resumes,
      SmallVectorImpl<LandingPadInst *> &CleanupLPads) {
    BitVector ResumeReachable(Resumes.size());
    size_t ResumeIndex = 0;
    for (auto *RI : Resumes) {
      for (auto *LP : CleanupLPads) {
        if (isPotentiallyReachable(LP, RI, nullptr, DT)) {
          ResumeReachable.set(ResumeIndex);
          break;
        }
      }
      ++ResumeIndex;
    }
  
    // If everything is reachable, there is no change.
    if (ResumeReachable.all())
      return Resumes.size();
  
    const TargetTransformInfo &TTI =
        getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn);
    LLVMContext &Ctx = Fn.getContext();
  
    // Otherwise, insert unreachable instructions and call simplifycfg.
    size_t ResumesLeft = 0;
    for (size_t I = 0, E = Resumes.size(); I < E; ++I) {
      ResumeInst *RI = Resumes[I];
      if (ResumeReachable[I]) {
        Resumes[ResumesLeft++] = RI;
      } else {
        BasicBlock *BB = RI->getParent();
        new UnreachableInst(Ctx, RI);
        RI->eraseFromParent();
        simplifyCFG(BB, TTI);
      }
    }
    Resumes.resize(ResumesLeft);
    return ResumesLeft;
  }
  
  /// InsertUnwindResumeCalls - Convert the ResumeInsts that are still present
  /// into calls to the appropriate _Unwind_Resume function.
  bool DwarfEHPrepare::InsertUnwindResumeCalls(Function &Fn) {
    SmallVector<ResumeInst*, 16> Resumes;
    SmallVector<LandingPadInst*, 16> CleanupLPads;
    for (BasicBlock &BB : Fn) {
      if (auto *RI = dyn_cast<ResumeInst>(BB.getTerminator()))
        Resumes.push_back(RI);
      if (auto *LP = BB.getLandingPadInst())
        if (LP->isCleanup())
          CleanupLPads.push_back(LP);
    }
  
    if (Resumes.empty())
      return false;
  
    // Check the personality, don't do anything if it's scope-based.
    EHPersonality Pers = classifyEHPersonality(Fn.getPersonalityFn());
    if (isScopedEHPersonality(Pers))
      return false;
  
    LLVMContext &Ctx = Fn.getContext();
  
    size_t ResumesLeft = pruneUnreachableResumes(Fn, Resumes, CleanupLPads);
    if (ResumesLeft == 0)
      return true; // We pruned them all.
  
    // Find the rewind function if we didn't already.
    if (!RewindFunction) {
      FunctionType *FTy = FunctionType::get(Type::getVoidTy(Ctx),
                                            Type::getInt8PtrTy(Ctx), false);
      const char *RewindName = TLI->getLibcallName(RTLIB::UNWIND_RESUME);
      RewindFunction = Fn.getParent()->getOrInsertFunction(RewindName, FTy);
    }
  
    // Create the basic block where the _Unwind_Resume call will live.
    if (ResumesLeft == 1) {
      // Instead of creating a new BB and PHI node, just append the call to
      // _Unwind_Resume to the end of the single resume block.
      ResumeInst *RI = Resumes.front();
      BasicBlock *UnwindBB = RI->getParent();
      Value *ExnObj = GetExceptionObject(RI);
  
      // Call the _Unwind_Resume function.
      CallInst *CI = CallInst::Create(RewindFunction, ExnObj, "", UnwindBB);
      CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
  
      // We never expect _Unwind_Resume to return.
      new UnreachableInst(Ctx, UnwindBB);
      return true;
    }
  
    BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &Fn);
    PHINode *PN = PHINode::Create(Type::getInt8PtrTy(Ctx), ResumesLeft,
                                  "exn.obj", UnwindBB);
  
    // Extract the exception object from the ResumeInst and add it to the PHI node
    // that feeds the _Unwind_Resume call.
    for (ResumeInst *RI : Resumes) {
      BasicBlock *Parent = RI->getParent();
      BranchInst::Create(UnwindBB, Parent);
  
      Value *ExnObj = GetExceptionObject(RI);
      PN->addIncoming(ExnObj, Parent);
  
      ++NumResumesLowered;
    }
  
    // Call the function.
    CallInst *CI = CallInst::Create(RewindFunction, PN, "", UnwindBB);
    CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
  
    // We never expect _Unwind_Resume to return.
    new UnreachableInst(Ctx, UnwindBB);
    return true;
  }
  
  bool DwarfEHPrepare::runOnFunction(Function &Fn) {
    const TargetMachine &TM =
        getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
    DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
    TLI = TM.getSubtargetImpl(Fn)->getTargetLowering();
    bool Changed = InsertUnwindResumeCalls(Fn);
    DT = nullptr;
    TLI = nullptr;
    return Changed;
  }
  