text: //===-- Lint.cpp - Check for common errors in LLVM IR ---------------------===//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  //
  // This pass statically checks for common and easily-identified constructs
  // which produce undefined or likely unintended behavior in LLVM IR.
  //
  // It is not a guarantee of correctness, in two ways. First, it isn't
  // comprehensive. There are checks which could be done statically which are
  // not yet implemented. Some of these are indicated by TODO comments, but
  // those aren't comprehensive either. Second, many conditions cannot be
  // checked statically. This pass does no dynamic instrumentation, so it
  // can't check for all possible problems.
  //
  // Another limitation is that it assumes all code will be executed. A store
  // through a null pointer in a basic block which is never reached is harmless,
  // but this pass will warn about it anyway. This is the main reason why most
  // of these checks live here instead of in the Verifier pass.
  //
  // Optimization passes may make conditions that this pass checks for more or
  // less obvious. If an optimization pass appears to be introducing a warning,
  // it may be that the optimization pass is merely exposing an existing
  // condition in the code.
  //
  // This code may be run before instcombine. In many cases, instcombine checks
  // for the same kinds of things and turns instructions with undefined behavior
  // into unreachable (or equivalent). Because of this, this pass makes some
  // effort to look through bitcasts and so on.
  //
  //===----------------------------------------------------------------------===//
  
  #include "llvm/Analysis/Lint.h"
  #include "llvm/ADT/STLExtras.h"
  #include "llvm/ADT/SmallSet.h"
  #include "llvm/Analysis/AliasAnalysis.h"
  #include "llvm/Analysis/AssumptionCache.h"
  #include "llvm/Analysis/ConstantFolding.h"
  #include "llvm/Analysis/InstructionSimplify.h"
  #include "llvm/Analysis/Loads.h"
  #include "llvm/Analysis/Passes.h"
  #include "llvm/Analysis/TargetLibraryInfo.h"
  #include "llvm/Analysis/ValueTracking.h"
  #include "llvm/IR/CallSite.h"
  #include "llvm/IR/DataLayout.h"
  #include "llvm/IR/Dominators.h"
  #include "llvm/IR/Function.h"
  #include "llvm/IR/Module.h"
  #include "llvm/IR/InstVisitor.h"
  #include "llvm/IR/IntrinsicInst.h"
  #include "llvm/IR/LegacyPassManager.h"
  #include "llvm/Pass.h"
  #include "llvm/Support/Debug.h"
  #include "llvm/Support/raw_ostream.h"
  using namespace llvm;
  
  namespace {
    namespace MemRef {
      static const unsigned Read     = 1;
      static const unsigned Write    = 2;
      static const unsigned Callee   = 4;
      static const unsigned Branchee = 8;
    }
  
    class Lint : public FunctionPass, public InstVisitor<Lint> {
      friend class InstVisitor<Lint>;
  
      void visitFunction(Function &F);
  
      void visitCallSite(CallSite CS);
      void visitMemoryReference(Instruction &I, Value *Ptr,
                                uint64_t Size, unsigned Align,
                                Type *Ty, unsigned Flags);
      void visitEHBeginCatch(IntrinsicInst *II);
      void visitEHEndCatch(IntrinsicInst *II);
  
      void visitCallInst(CallInst &I);
      void visitInvokeInst(InvokeInst &I);
      void visitReturnInst(ReturnInst &I);
      void visitLoadInst(LoadInst &I);
      void visitStoreInst(StoreInst &I);
      void visitXor(BinaryOperator &I);
      void visitSub(BinaryOperator &I);
      void visitLShr(BinaryOperator &I);
      void visitAShr(BinaryOperator &I);
      void visitShl(BinaryOperator &I);
      void visitSDiv(BinaryOperator &I);
      void visitUDiv(BinaryOperator &I);
      void visitSRem(BinaryOperator &I);
      void visitURem(BinaryOperator &I);
      void visitAllocaInst(AllocaInst &I);
      void visitVAArgInst(VAArgInst &I);
      void visitIndirectBrInst(IndirectBrInst &I);
      void visitExtractElementInst(ExtractElementInst &I);
      void visitInsertElementInst(InsertElementInst &I);
      void visitUnreachableInst(UnreachableInst &I);
  
      Value *findValue(Value *V, bool OffsetOk) const;
      Value *findValueImpl(Value *V, bool OffsetOk,
                           SmallPtrSetImpl<Value *> &Visited) const;
  
    public:
      Module *Mod;
      const DataLayout *DL;
      AliasAnalysis *AA;
      AssumptionCache *AC;
      DominatorTree *DT;
      TargetLibraryInfo *TLI;
  
      std::string Messages;
      raw_string_ostream MessagesStr;
  
      static char ID; // Pass identification, replacement for typeid
      Lint() : FunctionPass(ID), MessagesStr(Messages) {
        initializeLintPass(*PassRegistry::getPassRegistry());
      }
  
      bool runOnFunction(Function &F) override;
  
      void getAnalysisUsage(AnalysisUsage &AU) const override {
        AU.setPreservesAll();
        AU.addRequired<AAResultsWrapperPass>();
        AU.addRequired<AssumptionCacheTracker>();
        AU.addRequired<TargetLibraryInfoWrapperPass>();
        AU.addRequired<DominatorTreeWrapperPass>();
      }
      void print(raw_ostream &O, const Module *M) const override {}
  
      void WriteValues(ArrayRef<const Value *> Vs) {
        for (const Value *V : Vs) {
          if (!V)
            continue;
          if (isa<Instruction>(V)) {
            MessagesStr << *V << '\n';
          } else {
            V->printAsOperand(MessagesStr, true, Mod);
            MessagesStr << '\n';
          }
        }
      }
  
      /// \brief A check failed, so printout out the condition and the message.
      ///
      /// This provides a nice place to put a breakpoint if you want to see why
      /// something is not correct.
      void CheckFailed(const Twine &Message) { MessagesStr << Message << '\n'; }
  
      /// \brief A check failed (with values to print).
      ///
      /// This calls the Message-only version so that the above is easier to set
      /// a breakpoint on.
      template <typename T1, typename... Ts>
      void CheckFailed(const Twine &Message, const T1 &V1, const Ts &...Vs) {
        CheckFailed(Message);
        WriteValues({V1, Vs...});
      }
    };
  }
  
  char Lint::ID = 0;
  INITIALIZE_PASS_BEGIN(Lint, "lint", "Statically lint-checks LLVM IR",
                        false, true)
  INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
  INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
  INITIALIZE_PASS_END(Lint, "lint", "Statically lint-checks LLVM IR",
                      false, true)
  
  // Assert - We know that cond should be true, if not print an error message.
  #define Assert(C, ...) \
      do { if (!(C)) { CheckFailed(__VA_ARGS__); return; } } while (0)
  
  // Lint::run - This is the main Analysis entry point for a
  // function.
  //
  bool Lint::runOnFunction(Function &F) {
    Mod = F.getParent();
    DL = &F.getParent()->getDataLayout();
    AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
    AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
    DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
    TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
    visit(F);
    dbgs() << MessagesStr.str();
    Messages.clear();
    return false;
  }
  
  void Lint::visitFunction(Function &F) {
    // This isn't undefined behavior, it's just a little unusual, and it's a
    // fairly common mistake to neglect to name a function.
    Assert(F.hasName() || F.hasLocalLinkage(),
           "Unusual: Unnamed function with non-local linkage", &F);
  
    // TODO: Check for irreducible control flow.
  }
  
  void Lint::visitCallSite(CallSite CS) {
    Instruction &I = *CS.getInstruction();
    Value *Callee = CS.getCalledValue();
  
    visitMemoryReference(I, Callee, MemoryLocation::UnknownSize, 0, nullptr,
                         MemRef::Callee);
  
    if (Function *F = dyn_cast<Function>(findValue(Callee,
                                                   /*OffsetOk=*/false))) {
      Assert(CS.getCallingConv() == F->getCallingConv(),
             "Undefined behavior: Caller and callee calling convention differ",
             &I);
  
      FunctionType *FT = F->getFunctionType();
      unsigned NumActualArgs = CS.arg_size();
  
      Assert(FT->isVarArg() ? FT->getNumParams() <= NumActualArgs
                            : FT->getNumParams() == NumActualArgs,
             "Undefined behavior: Call argument count mismatches callee "
             "argument count",
             &I);
  
      Assert(FT->getReturnType() == I.getType(),
             "Undefined behavior: Call return type mismatches "
             "callee return type",
             &I);
  
      // Check argument types (in case the callee was casted) and attributes.
      // TODO: Verify that caller and callee attributes are compatible.
      Function::arg_iterator PI = F->arg_begin(), PE = F->arg_end();
      CallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
      for (; AI != AE; ++AI) {
        Value *Actual = *AI;
        if (PI != PE) {
          Argument *Formal = &*PI++;
          Assert(Formal->getType() == Actual->getType(),
                 "Undefined behavior: Call argument type mismatches "
                 "callee parameter type",
                 &I);
  
          // Check that noalias arguments don't alias other arguments. This is
          // not fully precise because we don't know the sizes of the dereferenced
          // memory regions.
          if (Formal->hasNoAliasAttr() && Actual->getType()->isPointerTy())
            for (CallSite::arg_iterator BI = CS.arg_begin(); BI != AE; ++BI)
              if (AI != BI && (*BI)->getType()->isPointerTy()) {
                AliasResult Result = AA->alias(*AI, *BI);
                Assert(Result != MustAlias && Result != PartialAlias,
                       "Unusual: noalias argument aliases another argument", &I);
              }
  
          // Check that an sret argument points to valid memory.
          if (Formal->hasStructRetAttr() && Actual->getType()->isPointerTy()) {
            Type *Ty =
              cast<PointerType>(Formal->getType())->getElementType();
            visitMemoryReference(I, Actual, DL->getTypeStoreSize(Ty),
                                 DL->getABITypeAlignment(Ty), Ty,
                                 MemRef::Read | MemRef::Write);
          }
        }
      }
    }
  
    if (CS.isCall() && cast<CallInst>(CS.getInstruction())->isTailCall())
      for (CallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
           AI != AE; ++AI) {
        Value *Obj = findValue(*AI, /*OffsetOk=*/true);
        Assert(!isa<AllocaInst>(Obj),
               "Undefined behavior: Call with \"tail\" keyword references "
               "alloca",
               &I);
      }
  
  
    if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(&I))
      switch (II->getIntrinsicID()) {
      default: break;
  
      // TODO: Check more intrinsics
  
      case Intrinsic::memcpy: {
        MemCpyInst *MCI = cast<MemCpyInst>(&I);
        // TODO: If the size is known, use it.
        visitMemoryReference(I, MCI->getDest(), MemoryLocation::UnknownSize,
                             MCI->getAlignment(), nullptr, MemRef::Write);
        visitMemoryReference(I, MCI->getSource(), MemoryLocation::UnknownSize,
                             MCI->getAlignment(), nullptr, MemRef::Read);
  
        // Check that the memcpy arguments don't overlap. The AliasAnalysis API
        // isn't expressive enough for what we really want to do. Known partial
        // overlap is not distinguished from the case where nothing is known.
        uint64_t Size = 0;
        if (const ConstantInt *Len =
                dyn_cast<ConstantInt>(findValue(MCI->getLength(),
                                                /*OffsetOk=*/false)))
          if (Len->getValue().isIntN(32))
            Size = Len->getValue().getZExtValue();
        Assert(AA->alias(MCI->getSource(), Size, MCI->getDest(), Size) !=
                   MustAlias,
               "Undefined behavior: memcpy source and destination overlap", &I);
        break;
      }
      case Intrinsic::memmove: {
        MemMoveInst *MMI = cast<MemMoveInst>(&I);
        // TODO: If the size is known, use it.
        visitMemoryReference(I, MMI->getDest(), MemoryLocation::UnknownSize,
                             MMI->getAlignment(), nullptr, MemRef::Write);
        visitMemoryReference(I, MMI->getSource(), MemoryLocation::UnknownSize,
                             MMI->getAlignment(), nullptr, MemRef::Read);
        break;
      }
      case Intrinsic::memset: {
        MemSetInst *MSI = cast<MemSetInst>(&I);
        // TODO: If the size is known, use it.
        visitMemoryReference(I, MSI->getDest(), MemoryLocation::UnknownSize,
                             MSI->getAlignment(), nullptr, MemRef::Write);
        break;
      }
  
      case Intrinsic::vastart:
        Assert(I.getParent()->getParent()->isVarArg(),
               "Undefined behavior: va_start called in a non-varargs function",
               &I);
  
        visitMemoryReference(I, CS.getArgument(0), MemoryLocation::UnknownSize, 0,
                             nullptr, MemRef::Read | MemRef::Write);
        break;
      case Intrinsic::vacopy:
        visitMemoryReference(I, CS.getArgument(0), MemoryLocation::UnknownSize, 0,
                             nullptr, MemRef::Write);
        visitMemoryReference(I, CS.getArgument(1), MemoryLocation::UnknownSize, 0,
                             nullptr, MemRef::Read);
        break;
      case Intrinsic::vaend:
        visitMemoryReference(I, CS.getArgument(0), MemoryLocation::UnknownSize, 0,
                             nullptr, MemRef::Read | MemRef::Write);
        break;
  
      case Intrinsic::stackrestore:
        // Stackrestore doesn't read or write memory, but it sets the
        // stack pointer, which the compiler may read from or write to
        // at any time, so check it for both readability and writeability.
        visitMemoryReference(I, CS.getArgument(0), MemoryLocation::UnknownSize, 0,
                             nullptr, MemRef::Read | MemRef::Write);
        break;
      }
  }
  
  void Lint::visitCallInst(CallInst &I) {
    return visitCallSite(&I);
  }
  
  void Lint::visitInvokeInst(InvokeInst &I) {
    return visitCallSite(&I);
  }
  
  void Lint::visitReturnInst(ReturnInst &I) {
    Function *F = I.getParent()->getParent();
    Assert(!F->doesNotReturn(),
           "Unusual: Return statement in function with noreturn attribute", &I);
  
    if (Value *V = I.getReturnValue()) {
      Value *Obj = findValue(V, /*OffsetOk=*/true);
      Assert(!isa<AllocaInst>(Obj), "Unusual: Returning alloca value", &I);
    }
  }
  
  // TODO: Check that the reference is in bounds.
  // TODO: Check readnone/readonly function attributes.
  void Lint::visitMemoryReference(Instruction &I,
                                  Value *Ptr, uint64_t Size, unsigned Align,
                                  Type *Ty, unsigned Flags) {
    // If no memory is being referenced, it doesn't matter if the pointer
    // is valid.
    if (Size == 0)
      return;
  
    Value *UnderlyingObject = findValue(Ptr, /*OffsetOk=*/true);
    Assert(!isa<ConstantPointerNull>(UnderlyingObject),
           "Undefined behavior: Null pointer dereference", &I);
    Assert(!isa<UndefValue>(UnderlyingObject),
           "Undefined behavior: Undef pointer dereference", &I);
    Assert(!isa<ConstantInt>(UnderlyingObject) ||
               !cast<ConstantInt>(UnderlyingObject)->isAllOnesValue(),
           "Unusual: All-ones pointer dereference", &I);
    Assert(!isa<ConstantInt>(UnderlyingObject) ||
               !cast<ConstantInt>(UnderlyingObject)->isOne(),
           "Unusual: Address one pointer dereference", &I);
  
    if (Flags & MemRef::Write) {
      if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(UnderlyingObject))
        Assert(!GV->isConstant(), "Undefined behavior: Write to read-only memory",
               &I);
      Assert(!isa<Function>(UnderlyingObject) &&
                 !isa<BlockAddress>(UnderlyingObject),
             "Undefined behavior: Write to text section", &I);
    }
    if (Flags & MemRef::Read) {
      Assert(!isa<Function>(UnderlyingObject), "Unusual: Load from function body",
             &I);
      Assert(!isa<BlockAddress>(UnderlyingObject),
             "Undefined behavior: Load from block address", &I);
    }
    if (Flags & MemRef::Callee) {
      Assert(!isa<BlockAddress>(UnderlyingObject),
             "Undefined behavior: Call to block address", &I);
    }
    if (Flags & MemRef::Branchee) {
      Assert(!isa<Constant>(UnderlyingObject) ||
                 isa<BlockAddress>(UnderlyingObject),
             "Undefined behavior: Branch to non-blockaddress", &I);
    }
  
    // Check for buffer overflows and misalignment.
    // Only handles memory references that read/write something simple like an
    // alloca instruction or a global variable.
    int64_t Offset = 0;
    if (Value *Base = GetPointerBaseWithConstantOffset(Ptr, Offset, *DL)) {
      // OK, so the access is to a constant offset from Ptr.  Check that Ptr is
      // something we can handle and if so extract the size of this base object
      // along with its alignment.
      uint64_t BaseSize = MemoryLocation::UnknownSize;
      unsigned BaseAlign = 0;
  
      if (AllocaInst *AI = dyn_cast<AllocaInst>(Base)) {
        Type *ATy = AI->getAllocatedType();
        if (!AI->isArrayAllocation() && ATy->isSized())
          BaseSize = DL->getTypeAllocSize(ATy);
        BaseAlign = AI->getAlignment();
        if (BaseAlign == 0 && ATy->isSized())
          BaseAlign = DL->getABITypeAlignment(ATy);
      } else if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) {
        // If the global may be defined differently in another compilation unit
        // then don't warn about funky memory accesses.
        if (GV->hasDefinitiveInitializer()) {
          Type *GTy = GV->getType()->getElementType();
          if (GTy->isSized())
            BaseSize = DL->getTypeAllocSize(GTy);
          BaseAlign = GV->getAlignment();
          if (BaseAlign == 0 && GTy->isSized())
            BaseAlign = DL->getABITypeAlignment(GTy);
        }
      }
  
      // Accesses from before the start or after the end of the object are not
      // defined.
      Assert(Size == MemoryLocation::UnknownSize ||
                 BaseSize == MemoryLocation::UnknownSize ||
                 (Offset >= 0 && Offset + Size <= BaseSize),
             "Undefined behavior: Buffer overflow", &I);
  
      // Accesses that say that the memory is more aligned than it is are not
      // defined.
      if (Align == 0 && Ty && Ty->isSized())
        Align = DL->getABITypeAlignment(Ty);
      Assert(!BaseAlign || Align <= MinAlign(BaseAlign, Offset),
             "Undefined behavior: Memory reference address is misaligned", &I);
    }
  }
  
  void Lint::visitLoadInst(LoadInst &I) {
    visitMemoryReference(I, I.getPointerOperand(),
                         DL->getTypeStoreSize(I.getType()), I.getAlignment(),
                         I.getType(), MemRef::Read);
  }
  
  void Lint::visitStoreInst(StoreInst &I) {
    visitMemoryReference(I, I.getPointerOperand(),
                         DL->getTypeStoreSize(I.getOperand(0)->getType()),
                         I.getAlignment(),
                         I.getOperand(0)->getType(), MemRef::Write);
  }
  
  void Lint::visitXor(BinaryOperator &I) {
    Assert(!isa<UndefValue>(I.getOperand(0)) || !isa<UndefValue>(I.getOperand(1)),
           "Undefined result: xor(undef, undef)", &I);
  }
  
  void Lint::visitSub(BinaryOperator &I) {
    Assert(!isa<UndefValue>(I.getOperand(0)) || !isa<UndefValue>(I.getOperand(1)),
           "Undefined result: sub(undef, undef)", &I);
  }
  
  void Lint::visitLShr(BinaryOperator &I) {
    if (ConstantInt *CI = dyn_cast<ConstantInt>(findValue(I.getOperand(1),
                                                          /*OffsetOk=*/false)))
      Assert(CI->getValue().ult(cast<IntegerType>(I.getType())->getBitWidth()),
             "Undefined result: Shift count out of range", &I);
  }
  
  void Lint::visitAShr(BinaryOperator &I) {
    if (ConstantInt *CI =
            dyn_cast<ConstantInt>(findValue(I.getOperand(1), /*OffsetOk=*/false)))
      Assert(CI->getValue().ult(cast<IntegerType>(I.getType())->getBitWidth()),
             "Undefined result: Shift count out of range", &I);
  }
  
  void Lint::visitShl(BinaryOperator &I) {
    if (ConstantInt *CI =
            dyn_cast<ConstantInt>(findValue(I.getOperand(1), /*OffsetOk=*/false)))
      Assert(CI->getValue().ult(cast<IntegerType>(I.getType())->getBitWidth()),
             "Undefined result: Shift count out of range", &I);
  }
  
  static bool isZero(Value *V, const DataLayout &DL, DominatorTree *DT,
                     AssumptionCache *AC) {
    // Assume undef could be zero.
    if (isa<UndefValue>(V))
      return true;
  
    VectorType *VecTy = dyn_cast<VectorType>(V->getType());
    if (!VecTy) {
      unsigned BitWidth = V->getType()->getIntegerBitWidth();
      APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
      computeKnownBits(V, KnownZero, KnownOne, DL, 0, AC,
                       dyn_cast<Instruction>(V), DT);
      return KnownZero.isAllOnesValue();
    }
  
    // Per-component check doesn't work with zeroinitializer
    Constant *C = dyn_cast<Constant>(V);
    if (!C)
      return false;
  
    if (C->isZeroValue())
      return true;
  
    // For a vector, KnownZero will only be true if all values are zero, so check
    // this per component
    unsigned BitWidth = VecTy->getElementType()->getIntegerBitWidth();
    for (unsigned I = 0, N = VecTy->getNumElements(); I != N; ++I) {
      Constant *Elem = C->getAggregateElement(I);
      if (isa<UndefValue>(Elem))
        return true;
  
      APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
      computeKnownBits(Elem, KnownZero, KnownOne, DL);
      if (KnownZero.isAllOnesValue())
        return true;
    }
  
    return false;
  }
  
  void Lint::visitSDiv(BinaryOperator &I) {
    Assert(!isZero(I.getOperand(1), I.getModule()->getDataLayout(), DT, AC),
           "Undefined behavior: Division by zero", &I);
  }
  
  void Lint::visitUDiv(BinaryOperator &I) {
    Assert(!isZero(I.getOperand(1), I.getModule()->getDataLayout(), DT, AC),
           "Undefined behavior: Division by zero", &I);
  }
  
  void Lint::visitSRem(BinaryOperator &I) {
    Assert(!isZero(I.getOperand(1), I.getModule()->getDataLayout(), DT, AC),
           "Undefined behavior: Division by zero", &I);
  }
  
  void Lint::visitURem(BinaryOperator &I) {
    Assert(!isZero(I.getOperand(1), I.getModule()->getDataLayout(), DT, AC),
           "Undefined behavior: Division by zero", &I);
  }
  
  void Lint::visitAllocaInst(AllocaInst &I) {
    if (isa<ConstantInt>(I.getArraySize()))
      // This isn't undefined behavior, it's just an obvious pessimization.
      Assert(&I.getParent()->getParent()->getEntryBlock() == I.getParent(),
             "Pessimization: Static alloca outside of entry block", &I);
  
    // TODO: Check for an unusual size (MSB set?)
  }
  
  void Lint::visitVAArgInst(VAArgInst &I) {
    visitMemoryReference(I, I.getOperand(0), MemoryLocation::UnknownSize, 0,
                         nullptr, MemRef::Read | MemRef::Write);
  }
  
  void Lint::visitIndirectBrInst(IndirectBrInst &I) {
    visitMemoryReference(I, I.getAddress(), MemoryLocation::UnknownSize, 0,
                         nullptr, MemRef::Branchee);
  
    Assert(I.getNumDestinations() != 0,
           "Undefined behavior: indirectbr with no destinations", &I);
  }
  
  void Lint::visitExtractElementInst(ExtractElementInst &I) {
    if (ConstantInt *CI = dyn_cast<ConstantInt>(findValue(I.getIndexOperand(),
                                                          /*OffsetOk=*/false)))
      Assert(CI->getValue().ult(I.getVectorOperandType()->getNumElements()),
             "Undefined result: extractelement index out of range", &I);
  }
  
  void Lint::visitInsertElementInst(InsertElementInst &I) {
    if (ConstantInt *CI = dyn_cast<ConstantInt>(findValue(I.getOperand(2),
                                                          /*OffsetOk=*/false)))
      Assert(CI->getValue().ult(I.getType()->getNumElements()),
             "Undefined result: insertelement index out of range", &I);
  }
  
  void Lint::visitUnreachableInst(UnreachableInst &I) {
    // This isn't undefined behavior, it's merely suspicious.
    Assert(&I == &I.getParent()->front() ||
               std::prev(I.getIterator())->mayHaveSideEffects(),
           "Unusual: unreachable immediately preceded by instruction without "
           "side effects",
           &I);
  }
  
  /// findValue - Look through bitcasts and simple memory reference patterns
  /// to identify an equivalent, but more informative, value.  If OffsetOk
  /// is true, look through getelementptrs with non-zero offsets too.
  ///
  /// Most analysis passes don't require this logic, because instcombine
  /// will simplify most of these kinds of things away. But it's a goal of
  /// this Lint pass to be useful even on non-optimized IR.
  Value *Lint::findValue(Value *V, bool OffsetOk) const {
    SmallPtrSet<Value *, 4> Visited;
    return findValueImpl(V, OffsetOk, Visited);
  }
  
  /// findValueImpl - Implementation helper for findValue.
  Value *Lint::findValueImpl(Value *V, bool OffsetOk,
                             SmallPtrSetImpl<Value *> &Visited) const {
    // Detect self-referential values.
    if (!Visited.insert(V).second)
      return UndefValue::get(V->getType());
  
    // TODO: Look through sext or zext cast, when the result is known to
    // be interpreted as signed or unsigned, respectively.
    // TODO: Look through eliminable cast pairs.
    // TODO: Look through calls with unique return values.
    // TODO: Look through vector insert/extract/shuffle.
    V = OffsetOk ? GetUnderlyingObject(V, *DL) : V->stripPointerCasts();
    if (LoadInst *L = dyn_cast<LoadInst>(V)) {
      BasicBlock::iterator BBI = L->getIterator();
      BasicBlock *BB = L->getParent();
      SmallPtrSet<BasicBlock *, 4> VisitedBlocks;
      for (;;) {
        if (!VisitedBlocks.insert(BB).second)
          break;
        if (Value *U =
            FindAvailableLoadedValue(L->getPointerOperand(),
                                     BB, BBI, DefMaxInstsToScan, AA))
          return findValueImpl(U, OffsetOk, Visited);
        if (BBI != BB->begin()) break;
        BB = BB->getUniquePredecessor();
        if (!BB) break;
        BBI = BB->end();
      }
    } else if (PHINode *PN = dyn_cast<PHINode>(V)) {
      if (Value *W = PN->hasConstantValue())
        if (W != V)
          return findValueImpl(W, OffsetOk, Visited);
    } else if (CastInst *CI = dyn_cast<CastInst>(V)) {
      if (CI->isNoopCast(*DL))
        return findValueImpl(CI->getOperand(0), OffsetOk, Visited);
    } else if (ExtractValueInst *Ex = dyn_cast<ExtractValueInst>(V)) {
      if (Value *W = FindInsertedValue(Ex->getAggregateOperand(),
                                       Ex->getIndices()))
        if (W != V)
          return findValueImpl(W, OffsetOk, Visited);
    } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
      // Same as above, but for ConstantExpr instead of Instruction.
      if (Instruction::isCast(CE->getOpcode())) {
        if (CastInst::isNoopCast(Instruction::CastOps(CE->getOpcode()),
                                 CE->getOperand(0)->getType(), CE->getType(),
                                 DL->getIntPtrType(V->getType())))
          return findValueImpl(CE->getOperand(0), OffsetOk, Visited);
      } else if (CE->getOpcode() == Instruction::ExtractValue) {
        ArrayRef<unsigned> Indices = CE->getIndices();
        if (Value *W = FindInsertedValue(CE->getOperand(0), Indices))
          if (W != V)
            return findValueImpl(W, OffsetOk, Visited);
      }
    }
  
    // As a last resort, try SimplifyInstruction or constant folding.
    if (Instruction *Inst = dyn_cast<Instruction>(V)) {
      if (Value *W = SimplifyInstruction(Inst, *DL, TLI, DT, AC))
        return findValueImpl(W, OffsetOk, Visited);
    } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
      if (Value *W = ConstantFoldConstantExpression(CE, *DL, TLI))
        if (W != V)
          return findValueImpl(W, OffsetOk, Visited);
    }
  
    return V;
  }
  
  //===----------------------------------------------------------------------===//
  //  Implement the public interfaces to this file...
  //===----------------------------------------------------------------------===//
  
  FunctionPass *llvm::createLintPass() {
    return new Lint();
  }
  
  /// lintFunction - Check a function for errors, printing messages on stderr.
  ///
  void llvm::lintFunction(const Function &f) {
    Function &F = const_cast<Function&>(f);
    assert(!F.isDeclaration() && "Cannot lint external functions");
  
    legacy::FunctionPassManager FPM(F.getParent());
    Lint *V = new Lint();
    FPM.add(V);
    FPM.run(F);
  }
  
  /// lintModule - Check a module for errors, printing messages on stderr.
  ///
  void llvm::lintModule(const Module &M) {
    legacy::PassManager PM;
    Lint *V = new Lint();
    PM.add(V);
    PM.run(const_cast<Module&>(M));
  }
  