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//===-- 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"
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;
}
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