DwarfEHPrepare.cpp

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//===-- DwarfEHPrepare - Prepare exception handling for code generation ---===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass mulches exception handling code into a form adapted to code
// generation. Required if using dwarf exception handling.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/Passes.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/Pass.h"
#include "llvm/Target/TargetLowering.h"
#include "llvm/Target/TargetSubtargetInfo.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
using namespace llvm;

#define DEBUG_TYPE "dwarfehprepare"

STATISTIC(NumResumesLowered, "Number of resume calls lowered");

namespace {
  class DwarfEHPrepare : public FunctionPass {
    const TargetMachine *TM;

    // RewindFunction - _Unwind_Resume or the target equivalent.
    Constant *RewindFunction;

    bool InsertUnwindResumeCalls(Function &Fn);
    Value *GetExceptionObject(ResumeInst *RI);

  public:
    static char ID; // Pass identification, replacement for typeid.
    DwarfEHPrepare(const TargetMachine *TM)
        : FunctionPass(ID), TM(TM), RewindFunction(nullptr) {
      initializeDominatorTreeWrapperPassPass(*PassRegistry::getPassRegistry());
    }

    bool runOnFunction(Function &Fn) override;

    bool doFinalization(Module &M) override {
      RewindFunction = nullptr;
      return false;
    }

    void getAnalysisUsage(AnalysisUsage &AU) const override { }

    const char *getPassName() const override {
      return "Exception handling preparation";
    }
  };
} // end anonymous namespace

char DwarfEHPrepare::ID = 0;

FunctionPass *llvm::createDwarfEHPass(const TargetMachine *TM) {
  return new DwarfEHPrepare(TM);
}

/// 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->getNumUses() == 0)
      SelIVI->eraseFromParent();
    if (ExcIVI->getNumUses() == 0)
      ExcIVI->eraseFromParent();
    if (SelLoad && SelLoad->getNumUses() == 0)
      SelLoad->eraseFromParent();
  }

  return ExnObj;
}

/// 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;
  for (Function::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I) {
    TerminatorInst *TI = I->getTerminator();
    if (ResumeInst *RI = dyn_cast<ResumeInst>(TI))
      Resumes.push_back(RI);
  }

  if (Resumes.empty())
    return false;

  // Find the rewind function if we didn't already.
  const TargetLowering *TLI = TM->getSubtargetImpl()->getTargetLowering();
  if (!RewindFunction) {
    LLVMContext &Ctx = Resumes[0]->getContext();
    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.
  LLVMContext &Ctx = Fn.getContext();
  unsigned ResumesSize = Resumes.size();

  if (ResumesSize == 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), ResumesSize,
                                "exn.obj", UnwindBB);

  // Extract the exception object from the ResumeInst and add it to the PHI node
  // that feeds the _Unwind_Resume call.
  for (SmallVectorImpl<ResumeInst*>::iterator
         I = Resumes.begin(), E = Resumes.end(); I != E; ++I) {
    ResumeInst *RI = *I;
    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) {
  bool Changed = InsertUnwindResumeCalls(Fn);
  return Changed;
}