lib/Support/KnownBits.cpp

//===-- KnownBits.cpp - Stores known zeros/ones ---------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains a class for representing known zeros and ones used by
// computeKnownBits.
//
//===----------------------------------------------------------------------===//

#include "llvm/Support/KnownBits.h"

using namespace llvm;

KnownBits KnownBits::computeForAddSub(bool Add, bool NSW,
                                      const KnownBits &LHS, KnownBits RHS) {
  // Carry in a 1 for a subtract, rather than 0.
  bool CarryIn = false;
  if (!Add) {
    // Sum = LHS + ~RHS + 1
    std::swap(RHS.Zero, RHS.One);
    CarryIn = true;
  }

  APInt PossibleSumZero = ~LHS.Zero + ~RHS.Zero + CarryIn;
  APInt PossibleSumOne = LHS.One + RHS.One + CarryIn;

  // 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;

  // Are we still trying to solve for the sign bit?
  if (!Known.isSignBitSet()) {
    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;
}