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      1 //===-- DwarfEHPrepare - Prepare exception handling for code generation ---===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 // This pass mulches exception handling code into a form adapted to code
     11 // generation. Required if using dwarf exception handling.
     12 //
     13 //===----------------------------------------------------------------------===//
     14 
     15 #include "llvm/CodeGen/Passes.h"
     16 #include "llvm/ADT/BitVector.h"
     17 #include "llvm/ADT/Statistic.h"
     18 #include "llvm/Analysis/CFG.h"
     19 #include "llvm/Analysis/EHPersonalities.h"
     20 #include "llvm/Analysis/TargetTransformInfo.h"
     21 #include "llvm/IR/Dominators.h"
     22 #include "llvm/IR/Function.h"
     23 #include "llvm/IR/Instructions.h"
     24 #include "llvm/IR/Module.h"
     25 #include "llvm/Pass.h"
     26 #include "llvm/Target/TargetLowering.h"
     27 #include "llvm/Target/TargetSubtargetInfo.h"
     28 #include "llvm/Transforms/Utils/Local.h"
     29 using namespace llvm;
     30 
     31 #define DEBUG_TYPE "dwarfehprepare"
     32 
     33 STATISTIC(NumResumesLowered, "Number of resume calls lowered");
     34 
     35 namespace {
     36   class DwarfEHPrepare : public FunctionPass {
     37     const TargetMachine *TM;
     38 
     39     // RewindFunction - _Unwind_Resume or the target equivalent.
     40     Constant *RewindFunction;
     41 
     42     DominatorTree *DT;
     43     const TargetLowering *TLI;
     44 
     45     bool InsertUnwindResumeCalls(Function &Fn);
     46     Value *GetExceptionObject(ResumeInst *RI);
     47     size_t
     48     pruneUnreachableResumes(Function &Fn,
     49                             SmallVectorImpl<ResumeInst *> &Resumes,
     50                             SmallVectorImpl<LandingPadInst *> &CleanupLPads);
     51 
     52   public:
     53     static char ID; // Pass identification, replacement for typeid.
     54 
     55     // INITIALIZE_TM_PASS requires a default constructor, but it isn't used in
     56     // practice.
     57     DwarfEHPrepare()
     58         : FunctionPass(ID), TM(nullptr), RewindFunction(nullptr), DT(nullptr),
     59           TLI(nullptr) {}
     60 
     61     DwarfEHPrepare(const TargetMachine *TM)
     62         : FunctionPass(ID), TM(TM), RewindFunction(nullptr), DT(nullptr),
     63           TLI(nullptr) {}
     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     const char *getPassName() const override {
     75       return "Exception handling preparation";
     76     }
     77   };
     78 } // end anonymous namespace
     79 
     80 char DwarfEHPrepare::ID = 0;
     81 INITIALIZE_TM_PASS_BEGIN(DwarfEHPrepare, "dwarfehprepare",
     82                          "Prepare DWARF exceptions", false, false)
     83 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
     84 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
     85 INITIALIZE_TM_PASS_END(DwarfEHPrepare, "dwarfehprepare",
     86                        "Prepare DWARF exceptions", false, false)
     87 
     88 FunctionPass *llvm::createDwarfEHPass(const TargetMachine *TM) {
     89   return new DwarfEHPrepare(TM);
     90 }
     91 
     92 void DwarfEHPrepare::getAnalysisUsage(AnalysisUsage &AU) const {
     93   AU.addRequired<TargetTransformInfoWrapperPass>();
     94   AU.addRequired<DominatorTreeWrapperPass>();
     95 }
     96 
     97 /// GetExceptionObject - Return the exception object from the value passed into
     98 /// the 'resume' instruction (typically an aggregate). Clean up any dead
     99 /// instructions, including the 'resume' instruction.
    100 Value *DwarfEHPrepare::GetExceptionObject(ResumeInst *RI) {
    101   Value *V = RI->getOperand(0);
    102   Value *ExnObj = nullptr;
    103   InsertValueInst *SelIVI = dyn_cast<InsertValueInst>(V);
    104   LoadInst *SelLoad = nullptr;
    105   InsertValueInst *ExcIVI = nullptr;
    106   bool EraseIVIs = false;
    107 
    108   if (SelIVI) {
    109     if (SelIVI->getNumIndices() == 1 && *SelIVI->idx_begin() == 1) {
    110       ExcIVI = dyn_cast<InsertValueInst>(SelIVI->getOperand(0));
    111       if (ExcIVI && isa<UndefValue>(ExcIVI->getOperand(0)) &&
    112           ExcIVI->getNumIndices() == 1 && *ExcIVI->idx_begin() == 0) {
    113         ExnObj = ExcIVI->getOperand(1);
    114         SelLoad = dyn_cast<LoadInst>(SelIVI->getOperand(1));
    115         EraseIVIs = true;
    116       }
    117     }
    118   }
    119 
    120   if (!ExnObj)
    121     ExnObj = ExtractValueInst::Create(RI->getOperand(0), 0, "exn.obj", RI);
    122 
    123   RI->eraseFromParent();
    124 
    125   if (EraseIVIs) {
    126     if (SelIVI->use_empty())
    127       SelIVI->eraseFromParent();
    128     if (ExcIVI->use_empty())
    129       ExcIVI->eraseFromParent();
    130     if (SelLoad && SelLoad->use_empty())
    131       SelLoad->eraseFromParent();
    132   }
    133 
    134   return ExnObj;
    135 }
    136 
    137 /// Replace resumes that are not reachable from a cleanup landing pad with
    138 /// unreachable and then simplify those blocks.
    139 size_t DwarfEHPrepare::pruneUnreachableResumes(
    140     Function &Fn, SmallVectorImpl<ResumeInst *> &Resumes,
    141     SmallVectorImpl<LandingPadInst *> &CleanupLPads) {
    142   BitVector ResumeReachable(Resumes.size());
    143   size_t ResumeIndex = 0;
    144   for (auto *RI : Resumes) {
    145     for (auto *LP : CleanupLPads) {
    146       if (isPotentiallyReachable(LP, RI, DT)) {
    147         ResumeReachable.set(ResumeIndex);
    148         break;
    149       }
    150     }
    151     ++ResumeIndex;
    152   }
    153 
    154   // If everything is reachable, there is no change.
    155   if (ResumeReachable.all())
    156     return Resumes.size();
    157 
    158   const TargetTransformInfo &TTI =
    159       getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn);
    160   LLVMContext &Ctx = Fn.getContext();
    161 
    162   // Otherwise, insert unreachable instructions and call simplifycfg.
    163   size_t ResumesLeft = 0;
    164   for (size_t I = 0, E = Resumes.size(); I < E; ++I) {
    165     ResumeInst *RI = Resumes[I];
    166     if (ResumeReachable[I]) {
    167       Resumes[ResumesLeft++] = RI;
    168     } else {
    169       BasicBlock *BB = RI->getParent();
    170       new UnreachableInst(Ctx, RI);
    171       RI->eraseFromParent();
    172       SimplifyCFG(BB, TTI, 1);
    173     }
    174   }
    175   Resumes.resize(ResumesLeft);
    176   return ResumesLeft;
    177 }
    178 
    179 /// InsertUnwindResumeCalls - Convert the ResumeInsts that are still present
    180 /// into calls to the appropriate _Unwind_Resume function.
    181 bool DwarfEHPrepare::InsertUnwindResumeCalls(Function &Fn) {
    182   SmallVector<ResumeInst*, 16> Resumes;
    183   SmallVector<LandingPadInst*, 16> CleanupLPads;
    184   for (BasicBlock &BB : Fn) {
    185     if (auto *RI = dyn_cast<ResumeInst>(BB.getTerminator()))
    186       Resumes.push_back(RI);
    187     if (auto *LP = BB.getLandingPadInst())
    188       if (LP->isCleanup())
    189         CleanupLPads.push_back(LP);
    190   }
    191 
    192   if (Resumes.empty())
    193     return false;
    194 
    195   // Check the personality, don't do anything if it's funclet-based.
    196   EHPersonality Pers = classifyEHPersonality(Fn.getPersonalityFn());
    197   if (isFuncletEHPersonality(Pers))
    198     return false;
    199 
    200   LLVMContext &Ctx = Fn.getContext();
    201 
    202   size_t ResumesLeft = pruneUnreachableResumes(Fn, Resumes, CleanupLPads);
    203   if (ResumesLeft == 0)
    204     return true; // We pruned them all.
    205 
    206   // Find the rewind function if we didn't already.
    207   if (!RewindFunction) {
    208     FunctionType *FTy = FunctionType::get(Type::getVoidTy(Ctx),
    209                                           Type::getInt8PtrTy(Ctx), false);
    210     const char *RewindName = TLI->getLibcallName(RTLIB::UNWIND_RESUME);
    211     RewindFunction = Fn.getParent()->getOrInsertFunction(RewindName, FTy);
    212   }
    213 
    214   // Create the basic block where the _Unwind_Resume call will live.
    215   if (ResumesLeft == 1) {
    216     // Instead of creating a new BB and PHI node, just append the call to
    217     // _Unwind_Resume to the end of the single resume block.
    218     ResumeInst *RI = Resumes.front();
    219     BasicBlock *UnwindBB = RI->getParent();
    220     Value *ExnObj = GetExceptionObject(RI);
    221 
    222     // Call the _Unwind_Resume function.
    223     CallInst *CI = CallInst::Create(RewindFunction, ExnObj, "", UnwindBB);
    224     CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
    225 
    226     // We never expect _Unwind_Resume to return.
    227     new UnreachableInst(Ctx, UnwindBB);
    228     return true;
    229   }
    230 
    231   BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &Fn);
    232   PHINode *PN = PHINode::Create(Type::getInt8PtrTy(Ctx), ResumesLeft,
    233                                 "exn.obj", UnwindBB);
    234 
    235   // Extract the exception object from the ResumeInst and add it to the PHI node
    236   // that feeds the _Unwind_Resume call.
    237   for (ResumeInst *RI : Resumes) {
    238     BasicBlock *Parent = RI->getParent();
    239     BranchInst::Create(UnwindBB, Parent);
    240 
    241     Value *ExnObj = GetExceptionObject(RI);
    242     PN->addIncoming(ExnObj, Parent);
    243 
    244     ++NumResumesLowered;
    245   }
    246 
    247   // Call the function.
    248   CallInst *CI = CallInst::Create(RewindFunction, PN, "", UnwindBB);
    249   CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
    250 
    251   // We never expect _Unwind_Resume to return.
    252   new UnreachableInst(Ctx, UnwindBB);
    253   return true;
    254 }
    255 
    256 bool DwarfEHPrepare::runOnFunction(Function &Fn) {
    257   assert(TM && "DWARF EH preparation requires a target machine");
    258   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
    259   TLI = TM->getSubtargetImpl(Fn)->getTargetLowering();
    260   bool Changed = InsertUnwindResumeCalls(Fn);
    261   DT = nullptr;
    262   TLI = nullptr;
    263   return Changed;
    264 }
    265