Transforms/Scalar/ConstraintElimination.cpp

//===-- ConstraintElimination.cpp - Eliminate conds using constraints. ----===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// Eliminate conditions based on constraints collected from dominating
// conditions.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar/ConstraintElimination.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ConstraintSystem.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GetElementPtrTypeIterator.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Verifier.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/DebugCounter.h"
#include "llvm/Support/KnownBits.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/ValueMapper.h"

#include <cmath>
#include <optional>
#include <string>

using namespace llvm;
using namespace PatternMatch;

#define DEBUG_TYPE "constraint-elimination"

STATISTIC(NumCondsRemoved, "Number of instructions removed");
DEBUG_COUNTER(EliminatedCounter, "conds-eliminated",
              "Controls which conditions are eliminated");

static cl::opt<unsigned>
    MaxRows("constraint-elimination-max-rows", cl::init(500), cl::Hidden,
            cl::desc("Maximum number of rows to keep in constraint system"));

static cl::opt<bool> DumpReproducers(
    "constraint-elimination-dump-reproducers", cl::init(false), cl::Hidden,
    cl::desc("Dump IR to reproduce successful transformations."));

static int64_t MaxConstraintValue = std::numeric_limits<int64_t>::max();
static int64_t MinSignedConstraintValue = std::numeric_limits<int64_t>::min();

// A helper to multiply 2 signed integers where overflowing is allowed.
static int64_t multiplyWithOverflow(int64_t A, int64_t B) {
  int64_t Result;
  MulOverflow(A, B, Result);
  return Result;
}

// A helper to add 2 signed integers where overflowing is allowed.
static int64_t addWithOverflow(int64_t A, int64_t B) {
  int64_t Result;
  AddOverflow(A, B, Result);
  return Result;
}

namespace {

class ConstraintInfo;

struct StackEntry {
  unsigned NumIn;
  unsigned NumOut;
  bool IsSigned = false;
  /// Variables that can be removed from the system once the stack entry gets
  /// removed.
  SmallVector<Value *, 2> ValuesToRelease;

  StackEntry(unsigned NumIn, unsigned NumOut, bool IsSigned,
             SmallVector<Value *, 2> ValuesToRelease)
      : NumIn(NumIn), NumOut(NumOut), IsSigned(IsSigned),
        ValuesToRelease(ValuesToRelease) {}
};

/// Struct to express a pre-condition of the form %Op0 Pred %Op1.
struct PreconditionTy {
  CmpInst::Predicate Pred;
  Value *Op0;
  Value *Op1;

  PreconditionTy(CmpInst::Predicate Pred, Value *Op0, Value *Op1)
      : Pred(Pred), Op0(Op0), Op1(Op1) {}
};

struct ConstraintTy {
  SmallVector<int64_t, 8> Coefficients;
  SmallVector<PreconditionTy, 2> Preconditions;

  SmallVector<SmallVector<int64_t, 8>> ExtraInfo;

  bool IsSigned = false;

  ConstraintTy() = default;

  ConstraintTy(SmallVector<int64_t, 8> Coefficients, bool IsSigned, bool IsEq)
      : Coefficients(Coefficients), IsSigned(IsSigned), IsEq(IsEq) {}

  unsigned size() const { return Coefficients.size(); }

  unsigned empty() const { return Coefficients.empty(); }

  /// Returns true if all preconditions for this list of constraints are
  /// satisfied given \p CS and the corresponding \p Value2Index mapping.
  bool isValid(const ConstraintInfo &Info) const;

  bool isEq() const { return IsEq; }

  /// Check if the current constraint is implied by the given ConstraintSystem.
  ///
  /// \return true or false if the constraint is proven to be respectively true,
  /// or false. When the constraint cannot be proven to be either true or false,
  /// std::nullopt is returned.
  std::optional<bool> isImpliedBy(const ConstraintSystem &CS) const;

private:
  bool IsEq = false;
};

/// Wrapper encapsulating separate constraint systems and corresponding value
/// mappings for both unsigned and signed information. Facts are added to and
/// conditions are checked against the corresponding system depending on the
/// signed-ness of their predicates. While the information is kept separate
/// based on signed-ness, certain conditions can be transferred between the two
/// systems.
class ConstraintInfo {

  ConstraintSystem UnsignedCS;
  ConstraintSystem SignedCS;

  const DataLayout &DL;

public:
  ConstraintInfo(const DataLayout &DL, ArrayRef<Value *> FunctionArgs)
      : UnsignedCS(FunctionArgs), SignedCS(FunctionArgs), DL(DL) {}

  DenseMap<Value *, unsigned> &getValue2Index(bool Signed) {
    return Signed ? SignedCS.getValue2Index() : UnsignedCS.getValue2Index();
  }
  const DenseMap<Value *, unsigned> &getValue2Index(bool Signed) const {
    return Signed ? SignedCS.getValue2Index() : UnsignedCS.getValue2Index();
  }

  ConstraintSystem &getCS(bool Signed) {
    return Signed ? SignedCS : UnsignedCS;
  }
  const ConstraintSystem &getCS(bool Signed) const {
    return Signed ? SignedCS : UnsignedCS;
  }

  void popLastConstraint(bool Signed) { getCS(Signed).popLastConstraint(); }
  void popLastNVariables(bool Signed, unsigned N) {
    getCS(Signed).popLastNVariables(N);
  }

  bool doesHold(CmpInst::Predicate Pred, Value *A, Value *B) const;

  void addFact(CmpInst::Predicate Pred, Value *A, Value *B, unsigned NumIn,
               unsigned NumOut, SmallVectorImpl<StackEntry> &DFSInStack);

  /// Turn a comparison of the form \p Op0 \p Pred \p Op1 into a vector of
  /// constraints, using indices from the corresponding constraint system.
  /// New variables that need to be added to the system are collected in
  /// \p NewVariables.
  ConstraintTy getConstraint(CmpInst::Predicate Pred, Value *Op0, Value *Op1,
                             SmallVectorImpl<Value *> &NewVariables) const;

  /// Turns a comparison of the form \p Op0 \p Pred \p Op1 into a vector of
  /// constraints using getConstraint. Returns an empty constraint if the result
  /// cannot be used to query the existing constraint system, e.g. because it
  /// would require adding new variables. Also tries to convert signed
  /// predicates to unsigned ones if possible to allow using the unsigned system
  /// which increases the effectiveness of the signed <-> unsigned transfer
  /// logic.
  ConstraintTy getConstraintForSolving(CmpInst::Predicate Pred, Value *Op0,
                                       Value *Op1) const;

  /// Try to add information from \p A \p Pred \p B to the unsigned/signed
  /// system if \p Pred is signed/unsigned.
  void transferToOtherSystem(CmpInst::Predicate Pred, Value *A, Value *B,
                             unsigned NumIn, unsigned NumOut,
                             SmallVectorImpl<StackEntry> &DFSInStack);
};

/// Represents a (Coefficient * Variable) entry after IR decomposition.
struct DecompEntry {
  int64_t Coefficient;
  Value *Variable;
  /// True if the variable is known positive in the current constraint.
  bool IsKnownNonNegative;

  DecompEntry(int64_t Coefficient, Value *Variable,
              bool IsKnownNonNegative = false)
      : Coefficient(Coefficient), Variable(Variable),
        IsKnownNonNegative(IsKnownNonNegative) {}
};

/// Represents an Offset + Coefficient1 * Variable1 + ... decomposition.
struct Decomposition {
  int64_t Offset = 0;
  SmallVector<DecompEntry, 3> Vars;

  Decomposition(int64_t Offset) : Offset(Offset) {}
  Decomposition(Value *V, bool IsKnownNonNegative = false) {
    Vars.emplace_back(1, V, IsKnownNonNegative);
  }
  Decomposition(int64_t Offset, ArrayRef<DecompEntry> Vars)
      : Offset(Offset), Vars(Vars) {}

  void add(int64_t OtherOffset) {
    Offset = addWithOverflow(Offset, OtherOffset);
  }

  void add(const Decomposition &Other) {
    add(Other.Offset);
    append_range(Vars, Other.Vars);
  }

  void mul(int64_t Factor) {
    Offset = multiplyWithOverflow(Offset, Factor);
    for (auto &Var : Vars)
      Var.Coefficient = multiplyWithOverflow(Var.Coefficient, Factor);
  }
};

} // namespace

static Decomposition decompose(Value *V,
                               SmallVectorImpl<PreconditionTy> &Preconditions,
                               bool IsSigned, const DataLayout &DL);

static bool canUseSExt(ConstantInt *CI) {
  const APInt &Val = CI->getValue();
  return Val.sgt(MinSignedConstraintValue) && Val.slt(MaxConstraintValue);
}

static Decomposition
decomposeGEP(GEPOperator &GEP, SmallVectorImpl<PreconditionTy> &Preconditions,
             bool IsSigned, const DataLayout &DL) {
  // Do not reason about pointers where the index size is larger than 64 bits,
  // as the coefficients used to encode constraints are 64 bit integers.
  if (DL.getIndexTypeSizeInBits(GEP.getPointerOperand()->getType()) > 64)
    return &GEP;

  if (!GEP.isInBounds())
    return &GEP;

  assert(!IsSigned && "The logic below only supports decomposition for "
                      "unsinged predicates at the moment.");
  Type *PtrTy = GEP.getType()->getScalarType();
  unsigned BitWidth = DL.getIndexTypeSizeInBits(PtrTy);
  MapVector<Value *, APInt> VariableOffsets;
  APInt ConstantOffset(BitWidth, 0);
  if (!GEP.collectOffset(DL, BitWidth, VariableOffsets, ConstantOffset))
    return &GEP;

  // Handle the (gep (gep ....), C) case by incrementing the constant
  // coefficient of the inner GEP, if C is a constant.
  auto *InnerGEP = dyn_cast<GEPOperator>(GEP.getPointerOperand());
  if (VariableOffsets.empty() && InnerGEP && InnerGEP->getNumOperands() == 2) {
    auto Result = decompose(InnerGEP, Preconditions, IsSigned, DL);
    Result.add(ConstantOffset.getSExtValue());

    if (ConstantOffset.isNegative()) {
      unsigned Scale = DL.getTypeAllocSize(InnerGEP->getResultElementType());
      int64_t ConstantOffsetI = ConstantOffset.getSExtValue();
      if (ConstantOffsetI % Scale != 0)
        return &GEP;
      // Add pre-condition ensuring the GEP is increasing monotonically and
      // can be de-composed.
      // Both sides are normalized by being divided by Scale.
      Preconditions.emplace_back(
          CmpInst::ICMP_SGE, InnerGEP->getOperand(1),
          ConstantInt::get(InnerGEP->getOperand(1)->getType(),
                           -1 * (ConstantOffsetI / Scale)));
    }
    return Result;
  }

  Decomposition Result(ConstantOffset.getSExtValue(),
                       DecompEntry(1, GEP.getPointerOperand()));
  for (auto [Index, Scale] : VariableOffsets) {
    auto IdxResult = decompose(Index, Preconditions, IsSigned, DL);
    IdxResult.mul(Scale.getSExtValue());
    Result.add(IdxResult);

    // If Op0 is signed non-negative, the GEP is increasing monotonically and
    // can be de-composed.
    if (!isKnownNonNegative(Index, DL, /*Depth=*/MaxAnalysisRecursionDepth - 1))
      Preconditions.emplace_back(CmpInst::ICMP_SGE, Index,
                                 ConstantInt::get(Index->getType(), 0));
  }
  return Result;
}

// Decomposes \p V into a constant offset + list of pairs { Coefficient,
// Variable } where Coefficient * Variable. The sum of the constant offset and
// pairs equals \p V.
static Decomposition decompose(Value *V,
                               SmallVectorImpl<PreconditionTy> &Preconditions,
                               bool IsSigned, const DataLayout &DL) {

  auto MergeResults = [&Preconditions, IsSigned, &DL](Value *A, Value *B,
                                                      bool IsSignedB) {
    auto ResA = decompose(A, Preconditions, IsSigned, DL);
    auto ResB = decompose(B, Preconditions, IsSignedB, DL);
    ResA.add(ResB);
    return ResA;
  };

  // Decompose \p V used with a signed predicate.
  if (IsSigned) {
    if (auto *CI = dyn_cast<ConstantInt>(V)) {
      if (canUseSExt(CI))
        return CI->getSExtValue();
    }
    Value *Op0;
    Value *Op1;
    if (match(V, m_NSWAdd(m_Value(Op0), m_Value(Op1))))
      return MergeResults(Op0, Op1, IsSigned);

    ConstantInt *CI;
    if (match(V, m_NSWMul(m_Value(Op0), m_ConstantInt(CI)))) {
      auto Result = decompose(Op0, Preconditions, IsSigned, DL);
      Result.mul(CI->getSExtValue());
      return Result;
    }

    return V;
  }

  if (auto *CI = dyn_cast<ConstantInt>(V)) {
    if (CI->uge(MaxConstraintValue))
      return V;
    return int64_t(CI->getZExtValue());
  }

  if (auto *GEP = dyn_cast<GEPOperator>(V))
    return decomposeGEP(*GEP, Preconditions, IsSigned, DL);

  Value *Op0;
  bool IsKnownNonNegative = false;
  if (match(V, m_ZExt(m_Value(Op0)))) {
    IsKnownNonNegative = true;
    V = Op0;
  }

  Value *Op1;
  ConstantInt *CI;
  if (match(V, m_NUWAdd(m_Value(Op0), m_Value(Op1)))) {
    return MergeResults(Op0, Op1, IsSigned);
  }
  if (match(V, m_NSWAdd(m_Value(Op0), m_Value(Op1)))) {
    if (!isKnownNonNegative(Op0, DL, /*Depth=*/MaxAnalysisRecursionDepth - 1))
      Preconditions.emplace_back(CmpInst::ICMP_SGE, Op0,
                                 ConstantInt::get(Op0->getType(), 0));
    if (!isKnownNonNegative(Op1, DL, /*Depth=*/MaxAnalysisRecursionDepth - 1))
      Preconditions.emplace_back(CmpInst::ICMP_SGE, Op1,
                                 ConstantInt::get(Op1->getType(), 0));

    return MergeResults(Op0, Op1, IsSigned);
  }

  if (match(V, m_Add(m_Value(Op0), m_ConstantInt(CI))) && CI->isNegative() &&
      canUseSExt(CI)) {
    Preconditions.emplace_back(
        CmpInst::ICMP_UGE, Op0,
        ConstantInt::get(Op0->getType(), CI->getSExtValue() * -1));
    return MergeResults(Op0, CI, true);
  }

  // Decompose or as an add if there are no common bits between the operands.
  if (match(V, m_Or(m_Value(Op0), m_ConstantInt(CI))) &&
      haveNoCommonBitsSet(Op0, CI, DL)) {
    return MergeResults(Op0, CI, IsSigned);
  }

  if (match(V, m_NUWShl(m_Value(Op1), m_ConstantInt(CI))) && canUseSExt(CI)) {
    if (CI->getSExtValue() < 0 || CI->getSExtValue() >= 64)
      return {V, IsKnownNonNegative};
    auto Result = decompose(Op1, Preconditions, IsSigned, DL);
    Result.mul(int64_t{1} << CI->getSExtValue());
    return Result;
  }

  if (match(V, m_NUWMul(m_Value(Op1), m_ConstantInt(CI))) && canUseSExt(CI) &&
      (!CI->isNegative())) {
    auto Result = decompose(Op1, Preconditions, IsSigned, DL);
    Result.mul(CI->getSExtValue());
    return Result;
  }

  if (match(V, m_NUWSub(m_Value(Op0), m_ConstantInt(CI))) && canUseSExt(CI))
    return {-1 * CI->getSExtValue(), {{1, Op0}}};
  if (match(V, m_NUWSub(m_Value(Op0), m_Value(Op1))))
    return {0, {{1, Op0}, {-1, Op1}}};

  return {V, IsKnownNonNegative};
}

ConstraintTy
ConstraintInfo::getConstraint(CmpInst::Predicate Pred, Value *Op0, Value *Op1,
                              SmallVectorImpl<Value *> &NewVariables) const {
  assert(NewVariables.empty() && "NewVariables must be empty when passed in");
  bool IsEq = false;
  // Try to convert Pred to one of ULE/SLT/SLE/SLT.
  switch (Pred) {
  case CmpInst::ICMP_UGT:
  case CmpInst::ICMP_UGE:
  case CmpInst::ICMP_SGT:
  case CmpInst::ICMP_SGE: {
    Pred = CmpInst::getSwappedPredicate(Pred);
    std::swap(Op0, Op1);
    break;
  }
  case CmpInst::ICMP_EQ:
    if (match(Op1, m_Zero())) {
      Pred = CmpInst::ICMP_ULE;
    } else {
      IsEq = true;
      Pred = CmpInst::ICMP_ULE;
    }
    break;
  case CmpInst::ICMP_NE:
    if (!match(Op1, m_Zero()))
      return {};
    Pred = CmpInst::getSwappedPredicate(CmpInst::ICMP_UGT);
    std::swap(Op0, Op1);
    break;
  default:
    break;
  }

  if (Pred != CmpInst::ICMP_ULE && Pred != CmpInst::ICMP_ULT &&
      Pred != CmpInst::ICMP_SLE && Pred != CmpInst::ICMP_SLT)
    return {};

  SmallVector<PreconditionTy, 4> Preconditions;
  bool IsSigned = CmpInst::isSigned(Pred);
  auto &Value2Index = getValue2Index(IsSigned);
  auto ADec = decompose(Op0->stripPointerCastsSameRepresentation(),
                        Preconditions, IsSigned, DL);
  auto BDec = decompose(Op1->stripPointerCastsSameRepresentation(),
                        Preconditions, IsSigned, DL);
  int64_t Offset1 = ADec.Offset;
  int64_t Offset2 = BDec.Offset;
  Offset1 *= -1;

  auto &VariablesA = ADec.Vars;
  auto &VariablesB = BDec.Vars;

  // First try to look up \p V in Value2Index and NewVariables. Otherwise add a
  // new entry to NewVariables.
  DenseMap<Value *, unsigned> NewIndexMap;
  auto GetOrAddIndex = [&Value2Index, &NewVariables,
                        &NewIndexMap](Value *V) -> unsigned {
    auto V2I = Value2Index.find(V);
    if (V2I != Value2Index.end())
      return V2I->second;
    auto Insert =
        NewIndexMap.insert({V, Value2Index.size() + NewVariables.size() + 1});
    if (Insert.second)
      NewVariables.push_back(V);
    return Insert.first->second;
  };

  // Make sure all variables have entries in Value2Index or NewVariables.
  for (const auto &KV : concat<DecompEntry>(VariablesA, VariablesB))
    GetOrAddIndex(KV.Variable);

  // Build result constraint, by first adding all coefficients from A and then
  // subtracting all coefficients from B.
  ConstraintTy Res(
      SmallVector<int64_t, 8>(Value2Index.size() + NewVariables.size() + 1, 0),
      IsSigned, IsEq);
  // Collect variables that are known to be positive in all uses in the
  // constraint.
  DenseMap<Value *, bool> KnownNonNegativeVariables;
  auto &R = Res.Coefficients;
  for (const auto &KV : VariablesA) {
    R[GetOrAddIndex(KV.Variable)] += KV.Coefficient;
    auto I =
        KnownNonNegativeVariables.insert({KV.Variable, KV.IsKnownNonNegative});
    I.first->second &= KV.IsKnownNonNegative;
  }

  for (const auto &KV : VariablesB) {
    if (SubOverflow(R[GetOrAddIndex(KV.Variable)], KV.Coefficient,
                    R[GetOrAddIndex(KV.Variable)]))
      return {};
    auto I =
        KnownNonNegativeVariables.insert({KV.Variable, KV.IsKnownNonNegative});
    I.first->second &= KV.IsKnownNonNegative;
  }

  int64_t OffsetSum;
  if (AddOverflow(Offset1, Offset2, OffsetSum))
    return {};
  if (Pred == (IsSigned ? CmpInst::ICMP_SLT : CmpInst::ICMP_ULT))
    if (AddOverflow(OffsetSum, int64_t(-1), OffsetSum))
      return {};
  R[0] = OffsetSum;
  Res.Preconditions = std::move(Preconditions);

  // Remove any (Coefficient, Variable) entry where the Coefficient is 0 for new
  // variables.
  while (!NewVariables.empty()) {
    int64_t Last = R.back();
    if (Last != 0)
      break;
    R.pop_back();
    Value *RemovedV = NewVariables.pop_back_val();
    NewIndexMap.erase(RemovedV);
  }

  // Add extra constraints for variables that are known positive.
  for (auto &KV : KnownNonNegativeVariables) {
    if (!KV.second ||
        (!Value2Index.contains(KV.first) && !NewIndexMap.contains(KV.first)))
      continue;
    SmallVector<int64_t, 8> C(Value2Index.size() + NewVariables.size() + 1, 0);
    C[GetOrAddIndex(KV.first)] = -1;
    Res.ExtraInfo.push_back(C);
  }
  return Res;
}

ConstraintTy ConstraintInfo::getConstraintForSolving(CmpInst::Predicate Pred,
                                                     Value *Op0,
                                                     Value *Op1) const {
  // If both operands are known to be non-negative, change signed predicates to
  // unsigned ones. This increases the reasoning effectiveness in combination
  // with the signed <-> unsigned transfer logic.
  if (CmpInst::isSigned(Pred) &&
      isKnownNonNegative(Op0, DL, /*Depth=*/MaxAnalysisRecursionDepth - 1) &&
      isKnownNonNegative(Op1, DL, /*Depth=*/MaxAnalysisRecursionDepth - 1))
    Pred = CmpInst::getUnsignedPredicate(Pred);

  SmallVector<Value *> NewVariables;
  ConstraintTy R = getConstraint(Pred, Op0, Op1, NewVariables);
  if (!NewVariables.empty())
    return {};
  return R;
}

bool ConstraintTy::isValid(const ConstraintInfo &Info) const {
  return Coefficients.size() > 0 &&
         all_of(Preconditions, [&Info](const PreconditionTy &C) {
           return Info.doesHold(C.Pred, C.Op0, C.Op1);
         });
}

std::optional<bool>
ConstraintTy::isImpliedBy(const ConstraintSystem &CS) const {
  bool IsConditionImplied = CS.isConditionImplied(Coefficients);

  if (IsEq) {
    auto NegatedOrEqual = ConstraintSystem::negateOrEqual(Coefficients);
    bool IsNegatedOrEqualImplied =
        !NegatedOrEqual.empty() && CS.isConditionImplied(NegatedOrEqual);

    // In order to check that `%a == %b` is true, we want to check that `%a >=
    // %b` and `%a <= %b` must hold.
    if (IsConditionImplied && IsNegatedOrEqualImplied)
      return true;

    auto Negated = ConstraintSystem::negate(Coefficients);
    bool IsNegatedImplied = !Negated.empty() && CS.isConditionImplied(Negated);

    auto StrictLessThan = ConstraintSystem::toStrictLessThan(Coefficients);
    bool IsStrictLessThanImplied =
        !StrictLessThan.empty() && CS.isConditionImplied(StrictLessThan);

    // In order to check that `%a == %b` is false, we want to check whether
    // either `%a > %b` or `%a < %b` holds.
    if (IsNegatedImplied || IsStrictLessThanImplied)
      return false;

    return std::nullopt;
  }

  if (IsConditionImplied)
    return true;

  auto Negated = ConstraintSystem::negate(Coefficients);
  auto IsNegatedImplied = !Negated.empty() && CS.isConditionImplied(Negated);
  if (IsNegatedImplied)
    return false;

  // Neither the condition nor its negated holds, did not prove anything.
  return std::nullopt;
}

bool ConstraintInfo::doesHold(CmpInst::Predicate Pred, Value *A,
                              Value *B) const {
  auto R = getConstraintForSolving(Pred, A, B);
  return R.Preconditions.empty() && !R.empty() &&
         getCS(R.IsSigned).isConditionImplied(R.Coefficients);
}

void ConstraintInfo::transferToOtherSystem(
    CmpInst::Predicate Pred, Value *A, Value *B, unsigned NumIn,
    unsigned NumOut, SmallVectorImpl<StackEntry> &DFSInStack) {
  // Check if we can combine facts from the signed and unsigned systems to
  // derive additional facts.
  if (!A->getType()->isIntegerTy())
    return;
  // FIXME: This currently depends on the order we add facts. Ideally we
  // would first add all known facts and only then try to add additional
  // facts.
  switch (Pred) {
  default:
    break;
  case CmpInst::ICMP_ULT:
    //  If B is a signed positive constant, A >=s 0 and A <s B.
    if (doesHold(CmpInst::ICMP_SGE, B, ConstantInt::get(B->getType(), 0))) {
      addFact(CmpInst::ICMP_SGE, A, ConstantInt::get(B->getType(), 0), NumIn,
              NumOut, DFSInStack);
      addFact(CmpInst::ICMP_SLT, A, B, NumIn, NumOut, DFSInStack);
    }
    break;
  case CmpInst::ICMP_SLT:
    if (doesHold(CmpInst::ICMP_SGE, A, ConstantInt::get(B->getType(), 0)))
      addFact(CmpInst::ICMP_ULT, A, B, NumIn, NumOut, DFSInStack);
    break;
  case CmpInst::ICMP_SGT: {
    if (doesHold(CmpInst::ICMP_SGE, B, ConstantInt::get(B->getType(), -1)))
      addFact(CmpInst::ICMP_UGE, A, ConstantInt::get(B->getType(), 0), NumIn,
              NumOut, DFSInStack);
    if (doesHold(CmpInst::ICMP_SGE, B, ConstantInt::get(B->getType(), 0)))
      addFact(CmpInst::ICMP_UGT, A, B, NumIn, NumOut, DFSInStack);

    break;
  }
  case CmpInst::ICMP_SGE:
    if (doesHold(CmpInst::ICMP_SGE, B, ConstantInt::get(B->getType(), 0))) {
      addFact(CmpInst::ICMP_UGE, A, B, NumIn, NumOut, DFSInStack);
    }
    break;
  }
}

static Instruction *getContextInstForUse(Use &U) {
  Instruction *UserI = cast<Instruction>(U.getUser());
  if (auto *Phi = dyn_cast<PHINode>(UserI))
    UserI = Phi->getIncomingBlock(U)->getTerminator();
  return UserI;
}

namespace {
/// Represents either
///  * a condition that holds on entry to a block (=conditional fact)
///  * an assume (=assume fact)
///  * a use of a compare instruction to simplify.
/// It also tracks the Dominator DFS in and out numbers for each entry.
struct FactOrCheck {
  union {
    Instruction *Inst;
    Use *U;
  };
  unsigned NumIn;
  unsigned NumOut;
  bool HasInst;
  bool Not;

  FactOrCheck(DomTreeNode *DTN, Instruction *Inst, bool Not)
      : Inst(Inst), NumIn(DTN->getDFSNumIn()), NumOut(DTN->getDFSNumOut()),
        HasInst(true), Not(Not) {}

  FactOrCheck(DomTreeNode *DTN, Use *U)
      : U(U), NumIn(DTN->getDFSNumIn()), NumOut(DTN->getDFSNumOut()),
        HasInst(false), Not(false) {}

  static FactOrCheck getFact(DomTreeNode *DTN, Instruction *Inst,
                             bool Not = false) {
    return FactOrCheck(DTN, Inst, Not);
  }

  static FactOrCheck getCheck(DomTreeNode *DTN, Use *U) {
    return FactOrCheck(DTN, U);
  }

  static FactOrCheck getCheck(DomTreeNode *DTN, CallInst *CI) {
    return FactOrCheck(DTN, CI, false);
  }

  bool isCheck() const {
    return !HasInst ||
           match(Inst, m_Intrinsic<Intrinsic::ssub_with_overflow>());
  }

  Instruction *getContextInst() const {
    if (HasInst)
      return Inst;
    return getContextInstForUse(*U);
  }
  Instruction *getInstructionToSimplify() const {
    assert(isCheck());
    if (HasInst)
      return Inst;
    // The use may have been simplified to a constant already.
    return dyn_cast<Instruction>(*U);
  }
  bool isConditionFact() const { return !isCheck() && isa<CmpInst>(Inst); }
};

/// Keep state required to build worklist.
struct State {
  DominatorTree &DT;
  SmallVector<FactOrCheck, 64> WorkList;

  State(DominatorTree &DT) : DT(DT) {}

  /// Process block \p BB and add known facts to work-list.
  void addInfoFor(BasicBlock &BB);

  /// Returns true if we can add a known condition from BB to its successor
  /// block Succ.
  bool canAddSuccessor(BasicBlock &BB, BasicBlock *Succ) const {
    return DT.dominates(BasicBlockEdge(&BB, Succ), Succ);
  }
};

} // namespace

#ifndef NDEBUG

static void dumpConstraint(ArrayRef<int64_t> C,
                           const DenseMap<Value *, unsigned> &Value2Index) {
  ConstraintSystem CS(Value2Index);
  CS.addVariableRowFill(C);
  CS.dump();
}
#endif

void State::addInfoFor(BasicBlock &BB) {
  // True as long as long as the current instruction is guaranteed to execute.
  bool GuaranteedToExecute = true;
  // Queue conditions and assumes.
  for (Instruction &I : BB) {
    if (auto Cmp = dyn_cast<ICmpInst>(&I)) {
      for (Use &U : Cmp->uses()) {
        auto *UserI = getContextInstForUse(U);
        auto *DTN = DT.getNode(UserI->getParent());
        if (!DTN)
          continue;
        WorkList.push_back(FactOrCheck::getCheck(DTN, &U));
      }
      continue;
    }

    if (match(&I, m_Intrinsic<Intrinsic::ssub_with_overflow>())) {
      WorkList.push_back(
          FactOrCheck::getCheck(DT.getNode(&BB), cast<CallInst>(&I)));
      continue;
    }

    Value *Cond;
    // For now, just handle assumes with a single compare as condition.
    if (match(&I, m_Intrinsic<Intrinsic::assume>(m_Value(Cond))) &&
        isa<ICmpInst>(Cond)) {
      if (GuaranteedToExecute) {
        // The assume is guaranteed to execute when BB is entered, hence Cond
        // holds on entry to BB.
        WorkList.emplace_back(FactOrCheck::getFact(DT.getNode(I.getParent()),
                                                   cast<Instruction>(Cond)));
      } else {
        WorkList.emplace_back(
            FactOrCheck::getFact(DT.getNode(I.getParent()), &I));
      }
    }
    GuaranteedToExecute &= isGuaranteedToTransferExecutionToSuccessor(&I);
  }

  auto *Br = dyn_cast<BranchInst>(BB.getTerminator());
  if (!Br || !Br->isConditional())
    return;

  Value *Cond = Br->getCondition();

  // If the condition is a chain of ORs/AND and the successor only has the
  // current block as predecessor, queue conditions for the successor.
  Value *Op0, *Op1;
  if (match(Cond, m_LogicalOr(m_Value(Op0), m_Value(Op1))) ||
      match(Cond, m_LogicalAnd(m_Value(Op0), m_Value(Op1)))) {
    bool IsOr = match(Cond, m_LogicalOr());
    bool IsAnd = match(Cond, m_LogicalAnd());
    // If there's a select that matches both AND and OR, we need to commit to
    // one of the options. Arbitrarily pick OR.
    if (IsOr && IsAnd)
      IsAnd = false;

    BasicBlock *Successor = Br->getSuccessor(IsOr ? 1 : 0);
    if (canAddSuccessor(BB, Successor)) {
      SmallVector<Value *> CondWorkList;
      SmallPtrSet<Value *, 8> SeenCond;
      auto QueueValue = [&CondWorkList, &SeenCond](Value *V) {
        if (SeenCond.insert(V).second)
          CondWorkList.push_back(V);
      };
      QueueValue(Op1);
      QueueValue(Op0);
      while (!CondWorkList.empty()) {
        Value *Cur = CondWorkList.pop_back_val();
        if (auto *Cmp = dyn_cast<ICmpInst>(Cur)) {
          WorkList.emplace_back(
              FactOrCheck::getFact(DT.getNode(Successor), Cmp, IsOr));
          continue;
        }
        if (IsOr && match(Cur, m_LogicalOr(m_Value(Op0), m_Value(Op1)))) {
          QueueValue(Op1);
          QueueValue(Op0);
          continue;
        }
        if (IsAnd && match(Cur, m_LogicalAnd(m_Value(Op0), m_Value(Op1)))) {
          QueueValue(Op1);
          QueueValue(Op0);
          continue;
        }
      }
    }
    return;
  }

  auto *CmpI = dyn_cast<ICmpInst>(Br->getCondition());
  if (!CmpI)
    return;
  if (canAddSuccessor(BB, Br->getSuccessor(0)))
    WorkList.emplace_back(
        FactOrCheck::getFact(DT.getNode(Br->getSuccessor(0)), CmpI));
  if (canAddSuccessor(BB, Br->getSuccessor(1)))
    WorkList.emplace_back(
        FactOrCheck::getFact(DT.getNode(Br->getSuccessor(1)), CmpI, true));
}

namespace {
/// Helper to keep track of a condition and if it should be treated as negated
/// for reproducer construction.
struct ReproducerEntry {
  CmpInst *Cond;
  bool IsNot;

  ReproducerEntry(CmpInst *Cond, bool IsNot) : Cond(Cond), IsNot(IsNot) {}
};
} // namespace

/// Helper function to generate a reproducer function for simplifying \p Cond.
/// The reproducer function contains a series of @llvm.assume calls, one for
/// each condition in \p Stack. For each condition, the operand instruction are
/// cloned until we reach operands that have an entry in \p Value2Index. Those
/// will then be added as function arguments. \p DT is used to order cloned
/// instructions. The reproducer function will get added to \p M, if it is
/// non-null. Otherwise no reproducer function is generated.
static void generateReproducer(CmpInst *Cond, Module *M,
                               ArrayRef<ReproducerEntry> Stack,
                               ConstraintInfo &Info, DominatorTree &DT) {
  if (!M)
    return;

  LLVMContext &Ctx = Cond->getContext();

  LLVM_DEBUG(dbgs() << "Creating reproducer for " << *Cond << "\n");

  ValueToValueMapTy Old2New;
  SmallVector<Value *> Args;
  SmallPtrSet<Value *, 8> Seen;
  // Traverse Cond and its operands recursively until we reach a value that's in
  // Value2Index or not an instruction, or not a operation that
  // ConstraintElimination can decompose. Such values will be considered as
  // external inputs to the reproducer, they are collected and added as function
  // arguments later.
  auto CollectArguments = [&](CmpInst *Cond) {
    if (!Cond)
      return;
    auto &Value2Index =
        Info.getValue2Index(CmpInst::isSigned(Cond->getPredicate()));
    SmallVector<Value *, 4> WorkList;
    WorkList.push_back(Cond);
    while (!WorkList.empty()) {
      Value *V = WorkList.pop_back_val();
      if (!Seen.insert(V).second)
        continue;
      if (Old2New.find(V) != Old2New.end())
        continue;
      if (isa<Constant>(V))
        continue;

      auto *I = dyn_cast<Instruction>(V);
      if (Value2Index.contains(V) || !I ||
          !isa<CmpInst, BinaryOperator, GEPOperator, CastInst>(V)) {
        Old2New[V] = V;
        Args.push_back(V);
        LLVM_DEBUG(dbgs() << "  found external input " << *V << "\n");
      } else {
        append_range(WorkList, I->operands());
      }
    }
  };

  for (auto &Entry : Stack)
    CollectArguments(Entry.Cond);
  CollectArguments(Cond);

  SmallVector<Type *> ParamTys;
  for (auto *P : Args)
    ParamTys.push_back(P->getType());

  FunctionType *FTy = FunctionType::get(Cond->getType(), ParamTys,
                                        /*isVarArg=*/false);
  Function *F = Function::Create(FTy, Function::ExternalLinkage,
                                 Cond->getModule()->getName() +
                                     Cond->getFunction()->getName() + "repro",
                                 M);
  // Add arguments to the reproducer function for each external value collected.
  for (unsigned I = 0; I < Args.size(); ++I) {
    F->getArg(I)->setName(Args[I]->getName());
    Old2New[Args[I]] = F->getArg(I);
  }

  BasicBlock *Entry = BasicBlock::Create(Ctx, "entry", F);
  IRBuilder<> Builder(Entry);
  Builder.CreateRet(Builder.getTrue());
  Builder.SetInsertPoint(Entry->getTerminator());

  // Clone instructions in \p Ops and their operands recursively until reaching
  // an value in Value2Index (external input to the reproducer). Update Old2New
  // mapping for the original and cloned instructions. Sort instructions to
  // clone by dominance, then insert the cloned instructions in the function.
  auto CloneInstructions = [&](ArrayRef<Value *> Ops, bool IsSigned) {
    SmallVector<Value *, 4> WorkList(Ops);
    SmallVector<Instruction *> ToClone;
    auto &Value2Index = Info.getValue2Index(IsSigned);
    while (!WorkList.empty()) {
      Value *V = WorkList.pop_back_val();
      if (Old2New.find(V) != Old2New.end())
        continue;

      auto *I = dyn_cast<Instruction>(V);
      if (!Value2Index.contains(V) && I) {
        Old2New[V] = nullptr;
        ToClone.push_back(I);
        append_range(WorkList, I->operands());
      }
    }

    sort(ToClone,
         [&DT](Instruction *A, Instruction *B) { return DT.dominates(A, B); });
    for (Instruction *I : ToClone) {
      Instruction *Cloned = I->clone();
      Old2New[I] = Cloned;
      Old2New[I]->setName(I->getName());
      Cloned->insertBefore(&*Builder.GetInsertPoint());
      Cloned->dropUnknownNonDebugMetadata();
      Cloned->setDebugLoc({});
    }
  };

  // Materialize the assumptions for the reproducer using the entries in Stack.
  // That is, first clone the operands of the condition recursively until we
  // reach an external input to the reproducer and add them to the reproducer
  // function. Then add an ICmp for the condition (with the inverse predicate if
  // the entry is negated) and an assert using the ICmp.
  for (auto &Entry : Stack) {
    if (!Entry.Cond)
      continue;

    LLVM_DEBUG(dbgs() << "  Materializing assumption " << *Entry.Cond << "\n");
    CmpInst::Predicate Pred = Entry.Cond->getPredicate();
    if (Entry.IsNot)
      Pred = CmpInst::getInversePredicate(Pred);

    CloneInstructions({Entry.Cond->getOperand(0), Entry.Cond->getOperand(1)},
                      CmpInst::isSigned(Entry.Cond->getPredicate()));

    auto *Cmp = Builder.CreateICmp(Pred, Entry.Cond->getOperand(0),
                                   Entry.Cond->getOperand(1));
    Builder.CreateAssumption(Cmp);
  }

  // Finally, clone the condition to reproduce and remap instruction operands in
  // the reproducer using Old2New.
  CloneInstructions(Cond, CmpInst::isSigned(Cond->getPredicate()));
  Entry->getTerminator()->setOperand(0, Cond);
  remapInstructionsInBlocks({Entry}, Old2New);

  assert(!verifyFunction(*F, &dbgs()));
}

static bool checkAndReplaceCondition(
    CmpInst *Cmp, ConstraintInfo &Info, unsigned NumIn, unsigned NumOut,
    Instruction *ContextInst, Module *ReproducerModule,
    ArrayRef<ReproducerEntry> ReproducerCondStack, DominatorTree &DT) {
  LLVM_DEBUG(dbgs() << "Checking " << *Cmp << "\n");

  CmpInst::Predicate Pred = Cmp->getPredicate();
  Value *A = Cmp->getOperand(0);
  Value *B = Cmp->getOperand(1);

  auto R = Info.getConstraintForSolving(Pred, A, B);
  if (R.empty() || !R.isValid(Info)){
    LLVM_DEBUG(dbgs() << "   failed to decompose condition\n");
    return false;
  }

  auto &CSToUse = Info.getCS(R.IsSigned);

  // If there was extra information collected during decomposition, apply
  // it now and remove it immediately once we are done with reasoning
  // about the constraint.
  for (auto &Row : R.ExtraInfo)
    CSToUse.addVariableRow(Row);
  auto InfoRestorer = make_scope_exit([&]() {
    for (unsigned I = 0; I < R.ExtraInfo.size(); ++I)
      CSToUse.popLastConstraint();
  });

  auto ReplaceCmpWithConstant = [&](CmpInst *Cmp, bool IsTrue) {
    if (!DebugCounter::shouldExecute(EliminatedCounter))
      return false;

    LLVM_DEBUG({
      if (IsTrue) {
        dbgs() << "Condition " << *Cmp;
      } else {
        auto InversePred = Cmp->getInversePredicate();
        dbgs() << "Condition " << CmpInst::getPredicateName(InversePred) << " "
               << *A << ", " << *B;
      }
      dbgs() << " implied by dominating constraints\n";
      CSToUse.dump();
    });

    generateReproducer(Cmp, ReproducerModule, ReproducerCondStack, Info, DT);
    Constant *ConstantC = ConstantInt::getBool(
        CmpInst::makeCmpResultType(Cmp->getType()), IsTrue);
    Cmp->replaceUsesWithIf(ConstantC, [&DT, NumIn, NumOut,
                                       ContextInst](Use &U) {
      auto *UserI = getContextInstForUse(U);
      auto *DTN = DT.getNode(UserI->getParent());
      if (!DTN || DTN->getDFSNumIn() < NumIn || DTN->getDFSNumOut() > NumOut)
        return false;
      if (UserI->getParent() == ContextInst->getParent() &&
          UserI->comesBefore(ContextInst))
        return false;

      // Conditions in an assume trivially simplify to true. Skip uses
      // in assume calls to not destroy the available information.
      auto *II = dyn_cast<IntrinsicInst>(U.getUser());
      return !II || II->getIntrinsicID() != Intrinsic::assume;
    });
    NumCondsRemoved++;
    return true;
  };

  if (auto ImpliedCondition = R.isImpliedBy(CSToUse))
    return ReplaceCmpWithConstant(Cmp, *ImpliedCondition);

  return false;
}

static void
removeEntryFromStack(const StackEntry &E, ConstraintInfo &Info,
                     Module *ReproducerModule,
                     SmallVectorImpl<ReproducerEntry> &ReproducerCondStack,
                     SmallVectorImpl<StackEntry> &DFSInStack) {
  Info.popLastConstraint(E.IsSigned);
  // Remove variables in the system that went out of scope.
  auto &Mapping = Info.getValue2Index(E.IsSigned);
  for (Value *V : E.ValuesToRelease)
    Mapping.erase(V);
  Info.popLastNVariables(E.IsSigned, E.ValuesToRelease.size());
  DFSInStack.pop_back();
  if (ReproducerModule)
    ReproducerCondStack.pop_back();
}

void ConstraintInfo::addFact(CmpInst::Predicate Pred, Value *A, Value *B,
                             unsigned NumIn, unsigned NumOut,
                             SmallVectorImpl<StackEntry> &DFSInStack) {
  // If the constraint has a pre-condition, skip the constraint if it does not
  // hold.
  SmallVector<Value *> NewVariables;
  auto R = getConstraint(Pred, A, B, NewVariables);
  if (!R.isValid(*this))
    return;

  LLVM_DEBUG(dbgs() << "Adding '" << Pred << " ";
             A->printAsOperand(dbgs(), false); dbgs() << ", ";
             B->printAsOperand(dbgs(), false); dbgs() << "'\n");
  bool Added = false;
  auto &CSToUse = getCS(R.IsSigned);
  if (R.Coefficients.empty())
    return;

  Added |= CSToUse.addVariableRowFill(R.Coefficients);

  // If R has been added to the system, add the new variables and queue it for
  // removal once it goes out-of-scope.
  if (Added) {
    SmallVector<Value *, 2> ValuesToRelease;
    auto &Value2Index = getValue2Index(R.IsSigned);
    for (Value *V : NewVariables) {
      Value2Index.insert({V, Value2Index.size() + 1});
      ValuesToRelease.push_back(V);
    }

    LLVM_DEBUG({
      dbgs() << "  constraint: ";
      dumpConstraint(R.Coefficients, getValue2Index(R.IsSigned));
      dbgs() << "\n";
    });

    DFSInStack.emplace_back(NumIn, NumOut, R.IsSigned,
                            std::move(ValuesToRelease));

    if (R.isEq()) {
      // Also add the inverted constraint for equality constraints.
      for (auto &Coeff : R.Coefficients)
        Coeff *= -1;
      CSToUse.addVariableRowFill(R.Coefficients);

      DFSInStack.emplace_back(NumIn, NumOut, R.IsSigned,
                              SmallVector<Value *, 2>());
    }
  }
}

static bool replaceSubOverflowUses(IntrinsicInst *II, Value *A, Value *B,
                                   SmallVectorImpl<Instruction *> &ToRemove) {
  bool Changed = false;
  IRBuilder<> Builder(II->getParent(), II->getIterator());
  Value *Sub = nullptr;
  for (User *U : make_early_inc_range(II->users())) {
    if (match(U, m_ExtractValue<0>(m_Value()))) {
      if (!Sub)
        Sub = Builder.CreateSub(A, B);
      U->replaceAllUsesWith(Sub);
      Changed = true;
    } else if (match(U, m_ExtractValue<1>(m_Value()))) {
      U->replaceAllUsesWith(Builder.getFalse());
      Changed = true;
    } else
      continue;

    if (U->use_empty()) {
      auto *I = cast<Instruction>(U);
      ToRemove.push_back(I);
      I->setOperand(0, PoisonValue::get(II->getType()));
      Changed = true;
    }
  }

  if (II->use_empty()) {
    II->eraseFromParent();
    Changed = true;
  }
  return Changed;
}

static bool
tryToSimplifyOverflowMath(IntrinsicInst *II, ConstraintInfo &Info,
                          SmallVectorImpl<Instruction *> &ToRemove) {
  auto DoesConditionHold = [](CmpInst::Predicate Pred, Value *A, Value *B,
                              ConstraintInfo &Info) {
    auto R = Info.getConstraintForSolving(Pred, A, B);
    if (R.size() < 2 || !R.isValid(Info))
      return false;

    auto &CSToUse = Info.getCS(R.IsSigned);
    return CSToUse.isConditionImplied(R.Coefficients);
  };

  bool Changed = false;
  if (II->getIntrinsicID() == Intrinsic::ssub_with_overflow) {
    // If A s>= B && B s>= 0, ssub.with.overflow(a, b) should not overflow and
    // can be simplified to a regular sub.
    Value *A = II->getArgOperand(0);
    Value *B = II->getArgOperand(1);
    if (!DoesConditionHold(CmpInst::ICMP_SGE, A, B, Info) ||
        !DoesConditionHold(CmpInst::ICMP_SGE, B,
                           ConstantInt::get(A->getType(), 0), Info))
      return false;
    Changed = replaceSubOverflowUses(II, A, B, ToRemove);
  }
  return Changed;
}

static bool eliminateConstraints(Function &F, DominatorTree &DT,
                                 OptimizationRemarkEmitter &ORE) {
  bool Changed = false;
  DT.updateDFSNumbers();
  SmallVector<Value *> FunctionArgs;
  for (Value &Arg : F.args())
    FunctionArgs.push_back(&Arg);
  ConstraintInfo Info(F.getParent()->getDataLayout(), FunctionArgs);
  State S(DT);
  std::unique_ptr<Module> ReproducerModule(
      DumpReproducers ? new Module(F.getName(), F.getContext()) : nullptr);

  // First, collect conditions implied by branches and blocks with their
  // Dominator DFS in and out numbers.
  for (BasicBlock &BB : F) {
    if (!DT.getNode(&BB))
      continue;
    S.addInfoFor(BB);
  }

  // Next, sort worklist by dominance, so that dominating conditions to check
  // and facts come before conditions and facts dominated by them. If a
  // condition to check and a fact have the same numbers, conditional facts come
  // first. Assume facts and checks are ordered according to their relative
  // order in the containing basic block. Also make sure conditions with
  // constant operands come before conditions without constant operands. This
  // increases the effectiveness of the current signed <-> unsigned fact
  // transfer logic.
  stable_sort(S.WorkList, [](const FactOrCheck &A, const FactOrCheck &B) {
    auto HasNoConstOp = [](const FactOrCheck &B) {
      return !isa<ConstantInt>(B.Inst->getOperand(0)) &&
             !isa<ConstantInt>(B.Inst->getOperand(1));
    };
    // If both entries have the same In numbers, conditional facts come first.
    // Otherwise use the relative order in the basic block.
    if (A.NumIn == B.NumIn) {
      if (A.isConditionFact() && B.isConditionFact()) {
        bool NoConstOpA = HasNoConstOp(A);
        bool NoConstOpB = HasNoConstOp(B);
        return NoConstOpA < NoConstOpB;
      }
      if (A.isConditionFact())
        return true;
      if (B.isConditionFact())
        return false;
      auto *InstA = A.getContextInst();
      auto *InstB = B.getContextInst();
      return InstA->comesBefore(InstB);
    }
    return A.NumIn < B.NumIn;
  });

  SmallVector<Instruction *> ToRemove;

  // Finally, process ordered worklist and eliminate implied conditions.
  SmallVector<StackEntry, 16> DFSInStack;
  SmallVector<ReproducerEntry> ReproducerCondStack;
  for (FactOrCheck &CB : S.WorkList) {
    // First, pop entries from the stack that are out-of-scope for CB. Remove
    // the corresponding entry from the constraint system.
    while (!DFSInStack.empty()) {
      auto &E = DFSInStack.back();
      LLVM_DEBUG(dbgs() << "Top of stack : " << E.NumIn << " " << E.NumOut
                        << "\n");
      LLVM_DEBUG(dbgs() << "CB: " << CB.NumIn << " " << CB.NumOut << "\n");
      assert(E.NumIn <= CB.NumIn);
      if (CB.NumOut <= E.NumOut)
        break;
      LLVM_DEBUG({
        dbgs() << "Removing ";
        dumpConstraint(Info.getCS(E.IsSigned).getLastConstraint(),
                       Info.getValue2Index(E.IsSigned));
        dbgs() << "\n";
      });
      removeEntryFromStack(E, Info, ReproducerModule.get(), ReproducerCondStack,
                           DFSInStack);
    }

    LLVM_DEBUG(dbgs() << "Processing ");

    // For a block, check if any CmpInsts become known based on the current set
    // of constraints.
    if (CB.isCheck()) {
      Instruction *Inst = CB.getInstructionToSimplify();
      if (!Inst)
        continue;
      LLVM_DEBUG(dbgs() << "condition to simplify: " << *Inst << "\n");
      if (auto *II = dyn_cast<WithOverflowInst>(Inst)) {
        Changed |= tryToSimplifyOverflowMath(II, Info, ToRemove);
      } else if (auto *Cmp = dyn_cast<ICmpInst>(Inst)) {
        Changed |= checkAndReplaceCondition(
            Cmp, Info, CB.NumIn, CB.NumOut, CB.getContextInst(),
            ReproducerModule.get(), ReproducerCondStack, S.DT);
      }
      continue;
    }

    LLVM_DEBUG(dbgs() << "fact to add to the system: " << *CB.Inst << "\n");
    ICmpInst::Predicate Pred;
    Value *A, *B;
    Value *Cmp = CB.Inst;
    match(Cmp, m_Intrinsic<Intrinsic::assume>(m_Value(Cmp)));
    if (match(Cmp, m_ICmp(Pred, m_Value(A), m_Value(B)))) {
      if (Info.getCS(CmpInst::isSigned(Pred)).size() > MaxRows) {
        LLVM_DEBUG(
            dbgs()
            << "Skip adding constraint because system has too many rows.\n");
        continue;
      }

      // Use the inverse predicate if required.
      if (CB.Not)
        Pred = CmpInst::getInversePredicate(Pred);

      Info.addFact(Pred, A, B, CB.NumIn, CB.NumOut, DFSInStack);
      if (ReproducerModule && DFSInStack.size() > ReproducerCondStack.size())
        ReproducerCondStack.emplace_back(cast<CmpInst>(Cmp), CB.Not);

      Info.transferToOtherSystem(Pred, A, B, CB.NumIn, CB.NumOut, DFSInStack);
      if (ReproducerModule && DFSInStack.size() > ReproducerCondStack.size()) {
        // Add dummy entries to ReproducerCondStack to keep it in sync with
        // DFSInStack.
        for (unsigned I = 0,
                      E = (DFSInStack.size() - ReproducerCondStack.size());
             I < E; ++I) {
          ReproducerCondStack.emplace_back(nullptr, false);
        }
      }
    }
  }

  if (ReproducerModule && !ReproducerModule->functions().empty()) {
    std::string S;
    raw_string_ostream StringS(S);
    ReproducerModule->print(StringS, nullptr);
    StringS.flush();
    OptimizationRemark Rem(DEBUG_TYPE, "Reproducer", &F);
    Rem << ore::NV("module") << S;
    ORE.emit(Rem);
  }

#ifndef NDEBUG
  unsigned SignedEntries =
      count_if(DFSInStack, [](const StackEntry &E) { return E.IsSigned; });
  assert(Info.getCS(false).size() == DFSInStack.size() - SignedEntries &&
         "updates to CS and DFSInStack are out of sync");
  assert(Info.getCS(true).size() == SignedEntries &&
         "updates to CS and DFSInStack are out of sync");
#endif

  for (Instruction *I : ToRemove)
    I->eraseFromParent();
  return Changed;
}

PreservedAnalyses ConstraintEliminationPass::run(Function &F,
                                                 FunctionAnalysisManager &AM) {
  auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
  auto &ORE = AM.getResult<OptimizationRemarkEmitterAnalysis>(F);
  if (!eliminateConstraints(F, DT, ORE))
    return PreservedAnalyses::all();

  PreservedAnalyses PA;
  PA.preserve<DominatorTreeAnalysis>();
  PA.preserveSet<CFGAnalyses>();
  return PA;
}