text: //===-- Lint.cpp - Check for common errors in LLVM IR ---------------------===//
  //
  // 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 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/APInt.h"
  #include "llvm/ADT/ArrayRef.h"
  #include "llvm/ADT/SmallPtrSet.h"
  #include "llvm/ADT/Twine.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/MemoryLocation.h"
  #include "llvm/Analysis/TargetLibraryInfo.h"
  #include "llvm/Analysis/ValueTracking.h"
  #include "llvm/IR/Argument.h"
  #include "llvm/IR/BasicBlock.h"
  #include "llvm/IR/Constant.h"
  #include "llvm/IR/Constants.h"
  #include "llvm/IR/DataLayout.h"
  #include "llvm/IR/DerivedTypes.h"
  #include "llvm/IR/Dominators.h"
  #include "llvm/IR/Function.h"
  #include "llvm/IR/GlobalVariable.h"
  #include "llvm/IR/InstVisitor.h"
  #include "llvm/IR/InstrTypes.h"
  #include "llvm/IR/Instruction.h"
  #include "llvm/IR/Instructions.h"
  #include "llvm/IR/IntrinsicInst.h"
  #include "llvm/IR/LegacyPassManager.h"
  #include "llvm/IR/Module.h"
  #include "llvm/IR/PassManager.h"
  #include "llvm/IR/Type.h"
  #include "llvm/IR/Value.h"
  #include "llvm/InitializePasses.h"
  #include "llvm/Pass.h"
  #include "llvm/Support/Casting.h"
  #include "llvm/Support/KnownBits.h"
  #include "llvm/Support/raw_ostream.h"
  #include <cassert>
  #include <cstdint>
  #include <iterator>
  #include <string>
  
  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;
  } // end namespace MemRef
  
  class Lint : public InstVisitor<Lint> {
    friend class InstVisitor<Lint>;
  
    void visitFunction(Function &F);
  
    void visitCallBase(CallBase &CB);
    void visitMemoryReference(Instruction &I, const MemoryLocation &Loc,
                              MaybeAlign Alignment, Type *Ty, unsigned Flags);
    void visitEHBeginCatch(IntrinsicInst *II);
    void visitEHEndCatch(IntrinsicInst *II);
  
    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;
  
    Lint(Module *Mod, const DataLayout *DL, AliasAnalysis *AA,
         AssumptionCache *AC, DominatorTree *DT, TargetLibraryInfo *TLI)
        : Mod(Mod), DL(DL), AA(AA), AC(AC), DT(DT), TLI(TLI),
          MessagesStr(Messages) {}
  
    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';
        }
      }
    }
  
    /// 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'; }
  
    /// 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...});
    }
  };
  } // end anonymous namespace
  
  // Check - We know that cond should be true, if not print an error message.
  #define Check(C, ...)                                                          \
    do {                                                                         \
      if (!(C)) {                                                                \
        CheckFailed(__VA_ARGS__);                                                \
        return;                                                                  \
      }                                                                          \
    } while (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.
    Check(F.hasName() || F.hasLocalLinkage(),
          "Unusual: Unnamed function with non-local linkage", &F);
  
    // TODO: Check for irreducible control flow.
  }
  
  void Lint::visitCallBase(CallBase &I) {
    Value *Callee = I.getCalledOperand();
  
    visitMemoryReference(I, MemoryLocation::getAfter(Callee), None, nullptr,
                         MemRef::Callee);
  
    if (Function *F = dyn_cast<Function>(findValue(Callee,
                                                   /*OffsetOk=*/false))) {
      Check(I.getCallingConv() == F->getCallingConv(),
            "Undefined behavior: Caller and callee calling convention differ",
            &I);
  
      FunctionType *FT = F->getFunctionType();
      unsigned NumActualArgs = I.arg_size();
  
      Check(FT->isVarArg() ? FT->getNumParams() <= NumActualArgs
                           : FT->getNumParams() == NumActualArgs,
            "Undefined behavior: Call argument count mismatches callee "
            "argument count",
            &I);
  
      Check(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();
      auto AI = I.arg_begin(), AE = I.arg_end();
      for (; AI != AE; ++AI) {
        Value *Actual = *AI;
        if (PI != PE) {
          Argument *Formal = &*PI++;
          Check(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()) {
            AttributeList PAL = I.getAttributes();
            unsigned ArgNo = 0;
            for (auto BI = I.arg_begin(); BI != AE; ++BI, ++ArgNo) {
              // Skip ByVal arguments since they will be memcpy'd to the callee's
              // stack so we're not really passing the pointer anyway.
              if (PAL.hasParamAttr(ArgNo, Attribute::ByVal))
                continue;
              // If both arguments are readonly, they have no dependence.
              if (Formal->onlyReadsMemory() && I.onlyReadsMemory(ArgNo))
                continue;
              if (AI != BI && (*BI)->getType()->isPointerTy()) {
                AliasResult Result = AA->alias(*AI, *BI);
                Check(Result != AliasResult::MustAlias &&
                          Result != AliasResult::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 = Formal->getParamStructRetType();
            MemoryLocation Loc(
                Actual, LocationSize::precise(DL->getTypeStoreSize(Ty)));
            visitMemoryReference(I, Loc, DL->getABITypeAlign(Ty), Ty,
                                 MemRef::Read | MemRef::Write);
          }
        }
      }
    }
  
    if (const auto *CI = dyn_cast<CallInst>(&I)) {
      if (CI->isTailCall()) {
        const AttributeList &PAL = CI->getAttributes();
        unsigned ArgNo = 0;
        for (Value *Arg : I.args()) {
          // Skip ByVal arguments since they will be memcpy'd to the callee's
          // stack anyway.
          if (PAL.hasParamAttr(ArgNo++, Attribute::ByVal))
            continue;
          Value *Obj = findValue(Arg, /*OffsetOk=*/true);
          Check(!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);
        visitMemoryReference(I, MemoryLocation::getForDest(MCI),
                             MCI->getDestAlign(), nullptr, MemRef::Write);
        visitMemoryReference(I, MemoryLocation::getForSource(MCI),
                             MCI->getSourceAlign(), 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.
        auto Size = LocationSize::afterPointer();
        if (const ConstantInt *Len =
                dyn_cast<ConstantInt>(findValue(MCI->getLength(),
                                                /*OffsetOk=*/false)))
          if (Len->getValue().isIntN(32))
            Size = LocationSize::precise(Len->getValue().getZExtValue());
        Check(AA->alias(MCI->getSource(), Size, MCI->getDest(), Size) !=
                  AliasResult::MustAlias,
              "Undefined behavior: memcpy source and destination overlap", &I);
        break;
      }
      case Intrinsic::memcpy_inline: {
        MemCpyInlineInst *MCII = cast<MemCpyInlineInst>(&I);
        const uint64_t Size = MCII->getLength()->getValue().getLimitedValue();
        visitMemoryReference(I, MemoryLocation::getForDest(MCII),
                             MCII->getDestAlign(), nullptr, MemRef::Write);
        visitMemoryReference(I, MemoryLocation::getForSource(MCII),
                             MCII->getSourceAlign(), 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.
        const LocationSize LS = LocationSize::precise(Size);
        Check(AA->alias(MCII->getSource(), LS, MCII->getDest(), LS) !=
                  AliasResult::MustAlias,
              "Undefined behavior: memcpy source and destination overlap", &I);
        break;
      }
      case Intrinsic::memmove: {
        MemMoveInst *MMI = cast<MemMoveInst>(&I);
        visitMemoryReference(I, MemoryLocation::getForDest(MMI),
                             MMI->getDestAlign(), nullptr, MemRef::Write);
        visitMemoryReference(I, MemoryLocation::getForSource(MMI),
                             MMI->getSourceAlign(), nullptr, MemRef::Read);
        break;
      }
      case Intrinsic::memset: {
        MemSetInst *MSI = cast<MemSetInst>(&I);
        visitMemoryReference(I, MemoryLocation::getForDest(MSI),
                             MSI->getDestAlign(), nullptr, MemRef::Write);
        break;
      }
  
      case Intrinsic::vastart:
        Check(I.getParent()->getParent()->isVarArg(),
              "Undefined behavior: va_start called in a non-varargs function",
              &I);
  
        visitMemoryReference(I, MemoryLocation::getForArgument(&I, 0, TLI), None,
                             nullptr, MemRef::Read | MemRef::Write);
        break;
      case Intrinsic::vacopy:
        visitMemoryReference(I, MemoryLocation::getForArgument(&I, 0, TLI), None,
                             nullptr, MemRef::Write);
        visitMemoryReference(I, MemoryLocation::getForArgument(&I, 1, TLI), None,
                             nullptr, MemRef::Read);
        break;
      case Intrinsic::vaend:
        visitMemoryReference(I, MemoryLocation::getForArgument(&I, 0, TLI), None,
                             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, MemoryLocation::getForArgument(&I, 0, TLI), None,
                             nullptr, MemRef::Read | MemRef::Write);
        break;
      case Intrinsic::get_active_lane_mask:
        if (auto *TripCount = dyn_cast<ConstantInt>(I.getArgOperand(1)))
          Check(!TripCount->isZero(),
                "get_active_lane_mask: operand #2 "
                "must be greater than 0",
                &I);
        break;
      }
  }
  
  void Lint::visitReturnInst(ReturnInst &I) {
    Function *F = I.getParent()->getParent();
    Check(!F->doesNotReturn(),
          "Unusual: Return statement in function with noreturn attribute", &I);
  
    if (Value *V = I.getReturnValue()) {
      Value *Obj = findValue(V, /*OffsetOk=*/true);
      Check(!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, const MemoryLocation &Loc,
                                  MaybeAlign Align, Type *Ty, unsigned Flags) {
    // If no memory is being referenced, it doesn't matter if the pointer
    // is valid.
    if (Loc.Size.isZero())
      return;
  
    Value *Ptr = const_cast<Value *>(Loc.Ptr);
    Value *UnderlyingObject = findValue(Ptr, /*OffsetOk=*/true);
    Check(!isa<ConstantPointerNull>(UnderlyingObject),
          "Undefined behavior: Null pointer dereference", &I);
    Check(!isa<UndefValue>(UnderlyingObject),
          "Undefined behavior: Undef pointer dereference", &I);
    Check(!isa<ConstantInt>(UnderlyingObject) ||
              !cast<ConstantInt>(UnderlyingObject)->isMinusOne(),
          "Unusual: All-ones pointer dereference", &I);
    Check(!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))
        Check(!GV->isConstant(), "Undefined behavior: Write to read-only memory",
              &I);
      Check(!isa<Function>(UnderlyingObject) &&
                !isa<BlockAddress>(UnderlyingObject),
            "Undefined behavior: Write to text section", &I);
    }
    if (Flags & MemRef::Read) {
      Check(!isa<Function>(UnderlyingObject), "Unusual: Load from function body",
            &I);
      Check(!isa<BlockAddress>(UnderlyingObject),
            "Undefined behavior: Load from block address", &I);
    }
    if (Flags & MemRef::Callee) {
      Check(!isa<BlockAddress>(UnderlyingObject),
            "Undefined behavior: Call to block address", &I);
    }
    if (Flags & MemRef::Branchee) {
      Check(!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;
      MaybeAlign BaseAlign;
  
      if (AllocaInst *AI = dyn_cast<AllocaInst>(Base)) {
        Type *ATy = AI->getAllocatedType();
        if (!AI->isArrayAllocation() && ATy->isSized())
          BaseSize = DL->getTypeAllocSize(ATy);
        BaseAlign = AI->getAlign();
      } 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->getValueType();
          if (GTy->isSized())
            BaseSize = DL->getTypeAllocSize(GTy);
          BaseAlign = GV->getAlign();
          if (!BaseAlign && GTy->isSized())
            BaseAlign = DL->getABITypeAlign(GTy);
        }
      }
  
      // Accesses from before the start or after the end of the object are not
      // defined.
      Check(!Loc.Size.hasValue() || BaseSize == MemoryLocation::UnknownSize ||
                (Offset >= 0 && Offset + Loc.Size.getValue() <= BaseSize),
            "Undefined behavior: Buffer overflow", &I);
  
      // Accesses that say that the memory is more aligned than it is are not
      // defined.
      if (!Align && Ty && Ty->isSized())
        Align = DL->getABITypeAlign(Ty);
      if (BaseAlign && Align)
        Check(*Align <= commonAlignment(*BaseAlign, Offset),
              "Undefined behavior: Memory reference address is misaligned", &I);
    }
  }
  
  void Lint::visitLoadInst(LoadInst &I) {
    visitMemoryReference(I, MemoryLocation::get(&I), I.getAlign(), I.getType(),
                         MemRef::Read);
  }
  
  void Lint::visitStoreInst(StoreInst &I) {
    visitMemoryReference(I, MemoryLocation::get(&I), I.getAlign(),
                         I.getOperand(0)->getType(), MemRef::Write);
  }
  
  void Lint::visitXor(BinaryOperator &I) {
    Check(!isa<UndefValue>(I.getOperand(0)) || !isa<UndefValue>(I.getOperand(1)),
          "Undefined result: xor(undef, undef)", &I);
  }
  
  void Lint::visitSub(BinaryOperator &I) {
    Check(!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)))
      Check(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)))
      Check(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)))
      Check(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) {
      KnownBits Known =
          computeKnownBits(V, DL, 0, AC, dyn_cast<Instruction>(V), DT);
      return Known.isZero();
    }
  
    // 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
    for (unsigned I = 0, N = cast<FixedVectorType>(VecTy)->getNumElements();
         I != N; ++I) {
      Constant *Elem = C->getAggregateElement(I);
      if (isa<UndefValue>(Elem))
        return true;
  
      KnownBits Known = computeKnownBits(Elem, DL);
      if (Known.isZero())
        return true;
    }
  
    return false;
  }
  
  void Lint::visitSDiv(BinaryOperator &I) {
    Check(!isZero(I.getOperand(1), I.getModule()->getDataLayout(), DT, AC),
          "Undefined behavior: Division by zero", &I);
  }
  
  void Lint::visitUDiv(BinaryOperator &I) {
    Check(!isZero(I.getOperand(1), I.getModule()->getDataLayout(), DT, AC),
          "Undefined behavior: Division by zero", &I);
  }
  
  void Lint::visitSRem(BinaryOperator &I) {
    Check(!isZero(I.getOperand(1), I.getModule()->getDataLayout(), DT, AC),
          "Undefined behavior: Division by zero", &I);
  }
  
  void Lint::visitURem(BinaryOperator &I) {
    Check(!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.
      Check(&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, MemoryLocation::get(&I), None, nullptr,
                         MemRef::Read | MemRef::Write);
  }
  
  void Lint::visitIndirectBrInst(IndirectBrInst &I) {
    visitMemoryReference(I, MemoryLocation::getAfter(I.getAddress()), None,
                         nullptr, MemRef::Branchee);
  
    Check(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)))
      Check(
          CI->getValue().ult(
              cast<FixedVectorType>(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)))
      Check(CI->getValue().ult(
                cast<FixedVectorType>(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.
    Check(&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) : 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, 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())
        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))
          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 (auto *C = dyn_cast<Constant>(V)) {
      Value *W = ConstantFoldConstant(C, *DL, TLI);
      if (W != V)
        return findValueImpl(W, OffsetOk, Visited);
    }
  
    return V;
  }
  
  PreservedAnalyses LintPass::run(Function &F, FunctionAnalysisManager &AM) {
    auto *Mod = F.getParent();
    auto *DL = &F.getParent()->getDataLayout();
    auto *AA = &AM.getResult<AAManager>(F);
    auto *AC = &AM.getResult<AssumptionAnalysis>(F);
    auto *DT = &AM.getResult<DominatorTreeAnalysis>(F);
    auto *TLI = &AM.getResult<TargetLibraryAnalysis>(F);
    Lint L(Mod, DL, AA, AC, DT, TLI);
    L.visit(F);
    dbgs() << L.MessagesStr.str();
    return PreservedAnalyses::all();
  }
  
  namespace {
  class LintLegacyPass : public FunctionPass {
  public:
    static char ID; // Pass identification, replacement for typeid
    LintLegacyPass() : FunctionPass(ID) {
      initializeLintLegacyPassPass(*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 {}
  };
  } // namespace
  
  char LintLegacyPass::ID = 0;
  INITIALIZE_PASS_BEGIN(LintLegacyPass, "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(LintLegacyPass, "lint", "Statically lint-checks LLVM IR",
                      false, true)
  
  bool LintLegacyPass::runOnFunction(Function &F) {
    auto *Mod = F.getParent();
    auto *DL = &F.getParent()->getDataLayout();
    auto *AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
    auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
    auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
    auto *TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
    Lint L(Mod, DL, AA, AC, DT, TLI);
    L.visit(F);
    dbgs() << L.MessagesStr.str();
    return false;
  }
  
  //===----------------------------------------------------------------------===//
  //  Implement the public interfaces to this file...
  //===----------------------------------------------------------------------===//
  
  FunctionPass *llvm::createLintLegacyPassPass() { return new LintLegacyPass(); }
  
  /// 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());
    auto *V = new LintLegacyPass();
    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;
    auto *V = new LintLegacyPass();
    PM.add(V);
    PM.run(const_cast<Module &>(M));
  }
  