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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/ADT/BitVector.h"
     16 #include "llvm/ADT/SmallVector.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/Transforms/Utils/Local.h"
     22 #include "llvm/CodeGen/RuntimeLibcalls.h"
     23 #include "llvm/CodeGen/TargetLowering.h"
     24 #include "llvm/CodeGen/TargetPassConfig.h"
     25 #include "llvm/CodeGen/TargetSubtargetInfo.h"
     26 #include "llvm/IR/BasicBlock.h"
     27 #include "llvm/IR/Constants.h"
     28 #include "llvm/IR/DerivedTypes.h"
     29 #include "llvm/IR/Dominators.h"
     30 #include "llvm/IR/Function.h"
     31 #include "llvm/IR/Instructions.h"
     32 #include "llvm/IR/Module.h"
     33 #include "llvm/IR/Type.h"
     34 #include "llvm/Pass.h"
     35 #include "llvm/Support/Casting.h"
     36 #include "llvm/Target/TargetMachine.h"
     37 #include <cstddef>
     38 
     39 using namespace llvm;
     40 
     41 #define DEBUG_TYPE "dwarfehprepare"
     42 
     43 STATISTIC(NumResumesLowered, "Number of resume calls lowered");
     44 
     45 namespace {
     46 
     47   class DwarfEHPrepare : public FunctionPass {
     48     // RewindFunction - _Unwind_Resume or the target equivalent.
     49     Constant *RewindFunction = nullptr;
     50 
     51     DominatorTree *DT = nullptr;
     52     const TargetLowering *TLI = nullptr;
     53 
     54     bool InsertUnwindResumeCalls(Function &Fn);
     55     Value *GetExceptionObject(ResumeInst *RI);
     56     size_t
     57     pruneUnreachableResumes(Function &Fn,
     58                             SmallVectorImpl<ResumeInst *> &Resumes,
     59                             SmallVectorImpl<LandingPadInst *> &CleanupLPads);
     60 
     61   public:
     62     static char ID; // Pass identification, replacement for typeid.
     63 
     64     DwarfEHPrepare() : FunctionPass(ID) {}
     65 
     66     bool runOnFunction(Function &Fn) override;
     67 
     68     bool doFinalization(Module &M) override {
     69       RewindFunction = nullptr;
     70       return false;
     71     }
     72 
     73     void getAnalysisUsage(AnalysisUsage &AU) const override;
     74 
     75     StringRef getPassName() const override {
     76       return "Exception handling preparation";
     77     }
     78   };
     79 
     80 } // end anonymous namespace
     81 
     82 char DwarfEHPrepare::ID = 0;
     83 
     84 INITIALIZE_PASS_BEGIN(DwarfEHPrepare, DEBUG_TYPE,
     85                       "Prepare DWARF exceptions", false, false)
     86 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
     87 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
     88 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
     89 INITIALIZE_PASS_END(DwarfEHPrepare, DEBUG_TYPE,
     90                     "Prepare DWARF exceptions", false, false)
     91 
     92 FunctionPass *llvm::createDwarfEHPass() { return new DwarfEHPrepare(); }
     93 
     94 void DwarfEHPrepare::getAnalysisUsage(AnalysisUsage &AU) const {
     95   AU.addRequired<TargetPassConfig>();
     96   AU.addRequired<TargetTransformInfoWrapperPass>();
     97   AU.addRequired<DominatorTreeWrapperPass>();
     98 }
     99 
    100 /// GetExceptionObject - Return the exception object from the value passed into
    101 /// the 'resume' instruction (typically an aggregate). Clean up any dead
    102 /// instructions, including the 'resume' instruction.
    103 Value *DwarfEHPrepare::GetExceptionObject(ResumeInst *RI) {
    104   Value *V = RI->getOperand(0);
    105   Value *ExnObj = nullptr;
    106   InsertValueInst *SelIVI = dyn_cast<InsertValueInst>(V);
    107   LoadInst *SelLoad = nullptr;
    108   InsertValueInst *ExcIVI = nullptr;
    109   bool EraseIVIs = false;
    110 
    111   if (SelIVI) {
    112     if (SelIVI->getNumIndices() == 1 && *SelIVI->idx_begin() == 1) {
    113       ExcIVI = dyn_cast<InsertValueInst>(SelIVI->getOperand(0));
    114       if (ExcIVI && isa<UndefValue>(ExcIVI->getOperand(0)) &&
    115           ExcIVI->getNumIndices() == 1 && *ExcIVI->idx_begin() == 0) {
    116         ExnObj = ExcIVI->getOperand(1);
    117         SelLoad = dyn_cast<LoadInst>(SelIVI->getOperand(1));
    118         EraseIVIs = true;
    119       }
    120     }
    121   }
    122 
    123   if (!ExnObj)
    124     ExnObj = ExtractValueInst::Create(RI->getOperand(0), 0, "exn.obj", RI);
    125 
    126   RI->eraseFromParent();
    127 
    128   if (EraseIVIs) {
    129     if (SelIVI->use_empty())
    130       SelIVI->eraseFromParent();
    131     if (ExcIVI->use_empty())
    132       ExcIVI->eraseFromParent();
    133     if (SelLoad && SelLoad->use_empty())
    134       SelLoad->eraseFromParent();
    135   }
    136 
    137   return ExnObj;
    138 }
    139 
    140 /// Replace resumes that are not reachable from a cleanup landing pad with
    141 /// unreachable and then simplify those blocks.
    142 size_t DwarfEHPrepare::pruneUnreachableResumes(
    143     Function &Fn, SmallVectorImpl<ResumeInst *> &Resumes,
    144     SmallVectorImpl<LandingPadInst *> &CleanupLPads) {
    145   BitVector ResumeReachable(Resumes.size());
    146   size_t ResumeIndex = 0;
    147   for (auto *RI : Resumes) {
    148     for (auto *LP : CleanupLPads) {
    149       if (isPotentiallyReachable(LP, RI, DT)) {
    150         ResumeReachable.set(ResumeIndex);
    151         break;
    152       }
    153     }
    154     ++ResumeIndex;
    155   }
    156 
    157   // If everything is reachable, there is no change.
    158   if (ResumeReachable.all())
    159     return Resumes.size();
    160 
    161   const TargetTransformInfo &TTI =
    162       getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn);
    163   LLVMContext &Ctx = Fn.getContext();
    164 
    165   // Otherwise, insert unreachable instructions and call simplifycfg.
    166   size_t ResumesLeft = 0;
    167   for (size_t I = 0, E = Resumes.size(); I < E; ++I) {
    168     ResumeInst *RI = Resumes[I];
    169     if (ResumeReachable[I]) {
    170       Resumes[ResumesLeft++] = RI;
    171     } else {
    172       BasicBlock *BB = RI->getParent();
    173       new UnreachableInst(Ctx, RI);
    174       RI->eraseFromParent();
    175       simplifyCFG(BB, TTI);
    176     }
    177   }
    178   Resumes.resize(ResumesLeft);
    179   return ResumesLeft;
    180 }
    181 
    182 /// InsertUnwindResumeCalls - Convert the ResumeInsts that are still present
    183 /// into calls to the appropriate _Unwind_Resume function.
    184 bool DwarfEHPrepare::InsertUnwindResumeCalls(Function &Fn) {
    185   SmallVector<ResumeInst*, 16> Resumes;
    186   SmallVector<LandingPadInst*, 16> CleanupLPads;
    187   for (BasicBlock &BB : Fn) {
    188     if (auto *RI = dyn_cast<ResumeInst>(BB.getTerminator()))
    189       Resumes.push_back(RI);
    190     if (auto *LP = BB.getLandingPadInst())
    191       if (LP->isCleanup())
    192         CleanupLPads.push_back(LP);
    193   }
    194 
    195   if (Resumes.empty())
    196     return false;
    197 
    198   // Check the personality, don't do anything if it's scope-based.
    199   EHPersonality Pers = classifyEHPersonality(Fn.getPersonalityFn());
    200   if (isScopedEHPersonality(Pers))
    201     return false;
    202 
    203   LLVMContext &Ctx = Fn.getContext();
    204 
    205   size_t ResumesLeft = pruneUnreachableResumes(Fn, Resumes, CleanupLPads);
    206   if (ResumesLeft == 0)
    207     return true; // We pruned them all.
    208 
    209   // Find the rewind function if we didn't already.
    210   if (!RewindFunction) {
    211     FunctionType *FTy = FunctionType::get(Type::getVoidTy(Ctx),
    212                                           Type::getInt8PtrTy(Ctx), false);
    213     const char *RewindName = TLI->getLibcallName(RTLIB::UNWIND_RESUME);
    214     RewindFunction = Fn.getParent()->getOrInsertFunction(RewindName, FTy);
    215   }
    216 
    217   // Create the basic block where the _Unwind_Resume call will live.
    218   if (ResumesLeft == 1) {
    219     // Instead of creating a new BB and PHI node, just append the call to
    220     // _Unwind_Resume to the end of the single resume block.
    221     ResumeInst *RI = Resumes.front();
    222     BasicBlock *UnwindBB = RI->getParent();
    223     Value *ExnObj = GetExceptionObject(RI);
    224 
    225     // Call the _Unwind_Resume function.
    226     CallInst *CI = CallInst::Create(RewindFunction, ExnObj, "", UnwindBB);
    227     CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
    228 
    229     // We never expect _Unwind_Resume to return.
    230     new UnreachableInst(Ctx, UnwindBB);
    231     return true;
    232   }
    233 
    234   BasicBlock *UnwindBB = BasicBlock::Create(Ctx, "unwind_resume", &Fn);
    235   PHINode *PN = PHINode::Create(Type::getInt8PtrTy(Ctx), ResumesLeft,
    236                                 "exn.obj", UnwindBB);
    237 
    238   // Extract the exception object from the ResumeInst and add it to the PHI node
    239   // that feeds the _Unwind_Resume call.
    240   for (ResumeInst *RI : Resumes) {
    241     BasicBlock *Parent = RI->getParent();
    242     BranchInst::Create(UnwindBB, Parent);
    243 
    244     Value *ExnObj = GetExceptionObject(RI);
    245     PN->addIncoming(ExnObj, Parent);
    246 
    247     ++NumResumesLowered;
    248   }
    249 
    250   // Call the function.
    251   CallInst *CI = CallInst::Create(RewindFunction, PN, "", UnwindBB);
    252   CI->setCallingConv(TLI->getLibcallCallingConv(RTLIB::UNWIND_RESUME));
    253 
    254   // We never expect _Unwind_Resume to return.
    255   new UnreachableInst(Ctx, UnwindBB);
    256   return true;
    257 }
    258 
    259 bool DwarfEHPrepare::runOnFunction(Function &Fn) {
    260   const TargetMachine &TM =
    261       getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
    262   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
    263   TLI = TM.getSubtargetImpl(Fn)->getTargetLowering();
    264   bool Changed = InsertUnwindResumeCalls(Fn);
    265   DT = nullptr;
    266   TLI = nullptr;
    267   return Changed;
    268 }
    269