lib/Analysis/Lint.cpp

//===-- 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/BasicAliasAnalysis.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/ScopedNoAliasAA.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TypeBasedAliasAnalysis.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/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Value.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 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), std::nullopt,
                       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::memset_inline: {
      MemSetInlineInst *MSII = cast<MemSetInlineInst>(&I);
      visitMemoryReference(I, MemoryLocation::getForDest(MSII),
                           MSII->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),
                           std::nullopt, nullptr, MemRef::Read | MemRef::Write);
      break;
    case Intrinsic::vacopy:
      visitMemoryReference(I, MemoryLocation::getForArgument(&I, 0, TLI),
                           std::nullopt, nullptr, MemRef::Write);
      visitMemoryReference(I, MemoryLocation::getForArgument(&I, 1, TLI),
                           std::nullopt, nullptr, MemRef::Read);
      break;
    case Intrinsic::vaend:
      visitMemoryReference(I, MemoryLocation::getForArgument(&I, 0, TLI),
                           std::nullopt, 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),
                           std::nullopt, 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), std::nullopt, nullptr,
                       MemRef::Read | MemRef::Write);
}

void Lint::visitIndirectBrInst(IndirectBrInst &I) {
  visitMemoryReference(I, MemoryLocation::getAfter(I.getAddress()),
                       std::nullopt, 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);
    }
  }

  // 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();
}

//===----------------------------------------------------------------------===//
//  Implement the public interfaces to this file...
//===----------------------------------------------------------------------===//

/// 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");

  FunctionAnalysisManager FAM;
  FAM.registerPass([&] { return TargetLibraryAnalysis(); });
  FAM.registerPass([&] { return DominatorTreeAnalysis(); });
  FAM.registerPass([&] { return AssumptionAnalysis(); });
  FAM.registerPass([&] {
    AAManager AA;
    AA.registerFunctionAnalysis<BasicAA>();
    AA.registerFunctionAnalysis<ScopedNoAliasAA>();
    AA.registerFunctionAnalysis<TypeBasedAA>();
    return AA;
  });
  LintPass().run(F, FAM);
}

/// lintModule - Check a module for errors, printing messages on stderr.
///
void llvm::lintModule(const Module &M) {
  for (const Function &F : M) {
    if (!F.isDeclaration())
      lintFunction(F);
  }
}