CmpInstAnalysis.cpp

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//===- CmpInstAnalysis.cpp - Utils to help fold compares ---------------===//
//
// 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 file holds routines to help analyse compare instructions
// and fold them into constants or other compare instructions
//
//===----------------------------------------------------------------------===//
 
#include "llvm/Analysis/CmpInstAnalysis.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/PatternMatch.h"
 
using namespace llvm;
 
unsigned llvm::getICmpCode(CmpInst::Predicate Pred) {
  switch (Pred) {
      // False -> 0
    case ICmpInst::ICMP_UGT: return 1;  // 001
    case ICmpInst::ICMP_SGT: return 1;  // 001
    case ICmpInst::ICMP_EQ:  return 2;  // 010
    case ICmpInst::ICMP_UGE: return 3;  // 011
    case ICmpInst::ICMP_SGE: return 3;  // 011
    case ICmpInst::ICMP_ULT: return 4;  // 100
    case ICmpInst::ICMP_SLT: return 4;  // 100
    case ICmpInst::ICMP_NE:  return 5;  // 101
    case ICmpInst::ICMP_ULE: return 6;  // 110
    case ICmpInst::ICMP_SLE: return 6;  // 110
      // True -> 7
    default:
      llvm_unreachable("Invalid ICmp predicate!");
  }
}
 
Constant *llvm::getPredForICmpCode(unsigned Code, bool Sign, Type *OpTy,
                                   CmpInst::Predicate &Pred) {
  switch (Code) {
    default: llvm_unreachable("Illegal ICmp code!");
    case 0: // False.
      return ConstantInt::get(CmpInst::makeCmpResultType(OpTy), 0);
    case 1: Pred = Sign ? ICmpInst::ICMP_SGT : ICmpInst::ICMP_UGT; break;
    case 2: Pred = ICmpInst::ICMP_EQ; break;
    case 3: Pred = Sign ? ICmpInst::ICMP_SGE : ICmpInst::ICMP_UGE; break;
    case 4: Pred = Sign ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT; break;
    case 5: Pred = ICmpInst::ICMP_NE; break;
    case 6: Pred = Sign ? ICmpInst::ICMP_SLE : ICmpInst::ICMP_ULE; break;
    case 7: // True.
      return ConstantInt::get(CmpInst::makeCmpResultType(OpTy), 1);
  }
  return nullptr;
}
 
bool llvm::predicatesFoldable(ICmpInst::Predicate P1, ICmpInst::Predicate P2) {
  return (CmpInst::isSigned(P1) == CmpInst::isSigned(P2)) ||
         (CmpInst::isSigned(P1) && ICmpInst::isEquality(P2)) ||
         (CmpInst::isSigned(P2) && ICmpInst::isEquality(P1));
}
 
Constant *llvm::getPredForFCmpCode(unsigned Code, Type *OpTy,
                                   CmpInst::Predicate &Pred) {
  Pred = static_cast<FCmpInst::Predicate>(Code);
  assert(FCmpInst::FCMP_FALSE <= Pred && Pred <= FCmpInst::FCMP_TRUE &&
         "Unexpected FCmp predicate!");
  if (Pred == FCmpInst::FCMP_FALSE)
    return ConstantInt::get(CmpInst::makeCmpResultType(OpTy), 0);
  if (Pred == FCmpInst::FCMP_TRUE)
    return ConstantInt::get(CmpInst::makeCmpResultType(OpTy), 1);
  return nullptr;
}
 
std::optional<DecomposedBitTest>
llvm::decomposeBitTestICmp(Value *LHS, Value *RHS, CmpInst::Predicate Pred,
                           bool LookThroughTrunc, bool AllowNonZeroC,
                           bool DecomposeAnd) {
  using namespace PatternMatch;
 
  const APInt *OrigC;
  if ((ICmpInst::isEquality(Pred) && !DecomposeAnd) ||
      !match(RHS, m_APIntAllowPoison(OrigC)))
    return std::nullopt;
 
  bool Inverted = false;
  if (ICmpInst::isGT(Pred) || ICmpInst::isGE(Pred)) {
    Inverted = true;
    Pred = ICmpInst::getInversePredicate(Pred);
  }
 
  APInt C = *OrigC;
  if (ICmpInst::isLE(Pred)) {
    if (ICmpInst::isSigned(Pred) ? C.isMaxSignedValue() : C.isMaxValue())
      return std::nullopt;
    ++C;
    Pred = ICmpInst::getStrictPredicate(Pred);
  }
 
  DecomposedBitTest Result;
  switch (Pred) {
  default:
    llvm_unreachable("Unexpected predicate");
  case ICmpInst::ICMP_SLT: {
    // X < 0 is equivalent to (X & SignMask) != 0.
    if (C.isZero()) {
      Result.Mask = APInt::getSignMask(C.getBitWidth());
      Result.C = APInt::getZero(C.getBitWidth());
      Result.Pred = ICmpInst::ICMP_NE;
      break;
    }
 
    APInt FlippedSign = C ^ APInt::getSignMask(C.getBitWidth());
    if (FlippedSign.isPowerOf2()) {
      // X s< 10000100 is equivalent to (X & 11111100 == 10000000)
      Result.Mask = -FlippedSign;
      Result.C = APInt::getSignMask(C.getBitWidth());
      Result.Pred = ICmpInst::ICMP_EQ;
      break;
    }
 
    if (FlippedSign.isNegatedPowerOf2()) {
      // X s< 01111100 is equivalent to (X & 11111100 != 01111100)
      Result.Mask = FlippedSign;
      Result.C = C;
      Result.Pred = ICmpInst::ICMP_NE;
      break;
    }
 
    return std::nullopt;
  }
  case ICmpInst::ICMP_ULT: {
    // X <u 2^n is equivalent to (X & ~(2^n-1)) == 0.
    if (C.isPowerOf2()) {
      Result.Mask = -C;
      Result.C = APInt::getZero(C.getBitWidth());
      Result.Pred = ICmpInst::ICMP_EQ;
      break;
    }
 
    // X u< 11111100 is equivalent to (X & 11111100 != 11111100)
    if (C.isNegatedPowerOf2()) {
      Result.Mask = C;
      Result.C = C;
      Result.Pred = ICmpInst::ICMP_NE;
      break;
    }
 
    return std::nullopt;
  }
  case ICmpInst::ICMP_EQ:
  case ICmpInst::ICMP_NE: {
    assert(DecomposeAnd);
    const APInt *AndC;
    Value *AndVal;
    if (match(LHS, m_And(m_Value(AndVal), m_APIntAllowPoison(AndC)))) {
      LHS = AndVal;
      Result.Mask = *AndC;
      Result.C = C;
      Result.Pred = Pred;
      break;
    }
 
    // Try to convert (trunc X) eq/ne C into (X & Mask) eq/ne C
    if (LookThroughTrunc && isa<TruncInst>(LHS)) {
      Result.Pred = Pred;
      Result.Mask = APInt::getAllOnes(C.getBitWidth());
      Result.C = C;
      break;
    }
 
    return std::nullopt;
  }
  }
 
  if (!AllowNonZeroC && !Result.C.isZero())
    return std::nullopt;
 
  if (Inverted)
    Result.Pred = ICmpInst::getInversePredicate(Result.Pred);
 
  Value *X;
  if (LookThroughTrunc && match(LHS, m_Trunc(m_Value(X)))) {
    Result.X = X;
    Result.Mask = Result.Mask.zext(X->getType()->getScalarSizeInBits());
    Result.C = Result.C.zext(X->getType()->getScalarSizeInBits());
  } else {
    Result.X = LHS;
  }
 
  return Result;
}
 
std::optional<DecomposedBitTest> llvm::decomposeBitTest(Value *Cond,
                                                        bool LookThroughTrunc,
                                                        bool AllowNonZeroC,
                                                        bool DecomposeAnd) {
  using namespace PatternMatch;
  if (auto *ICmp = dyn_cast<ICmpInst>(Cond)) {
    // Don't allow pointers. Splat vectors are fine.
    if (!ICmp->getOperand(0)->getType()->isIntOrIntVectorTy())
      return std::nullopt;
    return decomposeBitTestICmp(ICmp->getOperand(0), ICmp->getOperand(1),
                                ICmp->getPredicate(), LookThroughTrunc,
                                AllowNonZeroC, DecomposeAnd);
  }
  Value *X;
  if (Cond->getType()->isIntOrIntVectorTy(1) &&
      (match(Cond, m_Trunc(m_Value(X))) ||
       match(Cond, m_Not(m_Trunc(m_Value(X)))))) {
    DecomposedBitTest Result;
    Result.X = X;
    unsigned BitWidth = X->getType()->getScalarSizeInBits();
    Result.Mask = APInt(BitWidth, 1);
    Result.C = APInt::getZero(BitWidth);
    Result.Pred = isa<TruncInst>(Cond) ? ICmpInst::ICMP_NE : ICmpInst::ICMP_EQ;
 
    return Result;
  }
 
  return std::nullopt;
}