lib/Support/KnownBits.cpp

//===-- KnownBits.cpp - Stores known zeros/ones ---------------------------===//
//
// 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 contains a class for representing known zeros and ones used by
// computeKnownBits.
//
//===----------------------------------------------------------------------===//

#include "llvm/Support/KnownBits.h"
#include <cassert>

using namespace llvm;

static KnownBits computeForAddCarry(
    const KnownBits &LHS, const KnownBits &RHS,
    bool CarryZero, bool CarryOne) {
  assert(!(CarryZero && CarryOne) &&
         "Carry can't be zero and one at the same time");

  APInt PossibleSumZero = LHS.getMaxValue() + RHS.getMaxValue() + !CarryZero;
  APInt PossibleSumOne = LHS.getMinValue() + RHS.getMinValue() + CarryOne;

  // Compute known bits of the carry.
  APInt CarryKnownZero = ~(PossibleSumZero ^ LHS.Zero ^ RHS.Zero);
  APInt CarryKnownOne = PossibleSumOne ^ LHS.One ^ RHS.One;

  // Compute set of known bits (where all three relevant bits are known).
  APInt LHSKnownUnion = LHS.Zero | LHS.One;
  APInt RHSKnownUnion = RHS.Zero | RHS.One;
  APInt CarryKnownUnion = std::move(CarryKnownZero) | CarryKnownOne;
  APInt Known = std::move(LHSKnownUnion) & RHSKnownUnion & CarryKnownUnion;

  assert((PossibleSumZero & Known) == (PossibleSumOne & Known) &&
         "known bits of sum differ");

  // Compute known bits of the result.
  KnownBits KnownOut;
  KnownOut.Zero = ~std::move(PossibleSumZero) & Known;
  KnownOut.One = std::move(PossibleSumOne) & Known;
  return KnownOut;
}

KnownBits KnownBits::computeForAddCarry(
    const KnownBits &LHS, const KnownBits &RHS, const KnownBits &Carry) {
  assert(Carry.getBitWidth() == 1 && "Carry must be 1-bit");
  return ::computeForAddCarry(
      LHS, RHS, Carry.Zero.getBoolValue(), Carry.One.getBoolValue());
}

KnownBits KnownBits::computeForAddSub(bool Add, bool NSW,
                                      const KnownBits &LHS, KnownBits RHS) {
  KnownBits KnownOut;
  if (Add) {
    // Sum = LHS + RHS + 0
    KnownOut = ::computeForAddCarry(
        LHS, RHS, /*CarryZero*/true, /*CarryOne*/false);
  } else {
    // Sum = LHS + ~RHS + 1
    std::swap(RHS.Zero, RHS.One);
    KnownOut = ::computeForAddCarry(
        LHS, RHS, /*CarryZero*/false, /*CarryOne*/true);
  }

  // Are we still trying to solve for the sign bit?
  if (!KnownOut.isNegative() && !KnownOut.isNonNegative()) {
    if (NSW) {
      // Adding two non-negative numbers, or subtracting a negative number from
      // a non-negative one, can't wrap into negative.
      if (LHS.isNonNegative() && RHS.isNonNegative())
        KnownOut.makeNonNegative();
      // Adding two negative numbers, or subtracting a non-negative number from
      // a negative one, can't wrap into non-negative.
      else if (LHS.isNegative() && RHS.isNegative())
        KnownOut.makeNegative();
    }
  }

  return KnownOut;
}

KnownBits KnownBits::makeGE(const APInt &Val) const {
  // Count the number of leading bit positions where our underlying value is
  // known to be less than or equal to Val.
  unsigned N = (Zero | Val).countLeadingOnes();

  // For each of those bit positions, if Val has a 1 in that bit then our
  // underlying value must also have a 1.
  APInt MaskedVal(Val);
  MaskedVal.clearLowBits(getBitWidth() - N);
  return KnownBits(Zero, One | MaskedVal);
}

KnownBits KnownBits::umax(const KnownBits &LHS, const KnownBits &RHS) {
  // If we can prove that LHS >= RHS then use LHS as the result. Likewise for
  // RHS. Ideally our caller would already have spotted these cases and
  // optimized away the umax operation, but we handle them here for
  // completeness.
  if (LHS.getMinValue().uge(RHS.getMaxValue()))
    return LHS;
  if (RHS.getMinValue().uge(LHS.getMaxValue()))
    return RHS;

  // If the result of the umax is LHS then it must be greater than or equal to
  // the minimum possible value of RHS. Likewise for RHS. Any known bits that
  // are common to these two values are also known in the result.
  KnownBits L = LHS.makeGE(RHS.getMinValue());
  KnownBits R = RHS.makeGE(LHS.getMinValue());
  return KnownBits(L.Zero & R.Zero, L.One & R.One);
}

KnownBits KnownBits::umin(const KnownBits &LHS, const KnownBits &RHS) {
  // Flip the range of values: [0, 0xFFFFFFFF] <-> [0xFFFFFFFF, 0]
  auto Flip = [](const KnownBits &Val) { return KnownBits(Val.One, Val.Zero); };
  return Flip(umax(Flip(LHS), Flip(RHS)));
}

KnownBits KnownBits::smax(const KnownBits &LHS, const KnownBits &RHS) {
  // Flip the range of values: [-0x80000000, 0x7FFFFFFF] <-> [0, 0xFFFFFFFF]
  auto Flip = [](const KnownBits &Val) {
    unsigned SignBitPosition = Val.getBitWidth() - 1;
    APInt Zero = Val.Zero;
    APInt One = Val.One;
    Zero.setBitVal(SignBitPosition, Val.One[SignBitPosition]);
    One.setBitVal(SignBitPosition, Val.Zero[SignBitPosition]);
    return KnownBits(Zero, One);
  };
  return Flip(umax(Flip(LHS), Flip(RHS)));
}

KnownBits KnownBits::smin(const KnownBits &LHS, const KnownBits &RHS) {
  // Flip the range of values: [-0x80000000, 0x7FFFFFFF] <-> [0xFFFFFFFF, 0]
  auto Flip = [](const KnownBits &Val) {
    unsigned SignBitPosition = Val.getBitWidth() - 1;
    APInt Zero = Val.One;
    APInt One = Val.Zero;
    Zero.setBitVal(SignBitPosition, Val.Zero[SignBitPosition]);
    One.setBitVal(SignBitPosition, Val.One[SignBitPosition]);
    return KnownBits(Zero, One);
  };
  return Flip(umax(Flip(LHS), Flip(RHS)));
}

KnownBits KnownBits::abs() const {
  // If the source's MSB is zero then we know the rest of the bits already.
  if (isNonNegative())
    return *this;

  // Assume we know nothing.
  KnownBits KnownAbs(getBitWidth());

  // We only know that the absolute values's MSB will be zero iff there is
  // a set bit that isn't the sign bit (otherwise it could be INT_MIN).
  APInt Val = One;
  Val.clearSignBit();
  if (!Val.isNullValue())
    KnownAbs.Zero.setSignBit();

  return KnownAbs;
}

KnownBits &KnownBits::operator&=(const KnownBits &RHS) {
  // Result bit is 0 if either operand bit is 0.
  Zero |= RHS.Zero;
  // Result bit is 1 if both operand bits are 1.
  One &= RHS.One;
  return *this;
}

KnownBits &KnownBits::operator|=(const KnownBits &RHS) {
  // Result bit is 0 if both operand bits are 0.
  Zero &= RHS.Zero;
  // Result bit is 1 if either operand bit is 1.
  One |= RHS.One;
  return *this;
}

KnownBits &KnownBits::operator^=(const KnownBits &RHS) {
  // Result bit is 0 if both operand bits are 0 or both are 1.
  APInt Z = (Zero & RHS.Zero) | (One & RHS.One);
  // Result bit is 1 if one operand bit is 0 and the other is 1.
  One = (Zero & RHS.One) | (One & RHS.Zero);
  Zero = std::move(Z);
  return *this;
}