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65 lines
2.3 KiB
C++
65 lines
2.3 KiB
C++
//===-- KnownBits.cpp - Stores known zeros/ones ---------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This file contains a class for representing known zeros and ones used by
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// computeKnownBits.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Support/KnownBits.h"
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using namespace llvm;
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KnownBits KnownBits::computeForAddSub(bool Add, bool NSW,
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const KnownBits &LHS, KnownBits RHS) {
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// Carry in a 1 for a subtract, rather than 0.
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bool CarryIn = false;
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if (!Add) {
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// Sum = LHS + ~RHS + 1
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std::swap(RHS.Zero, RHS.One);
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CarryIn = true;
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}
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APInt PossibleSumZero = ~LHS.Zero + ~RHS.Zero + CarryIn;
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APInt PossibleSumOne = LHS.One + RHS.One + CarryIn;
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// Compute known bits of the carry.
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APInt CarryKnownZero = ~(PossibleSumZero ^ LHS.Zero ^ RHS.Zero);
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APInt CarryKnownOne = PossibleSumOne ^ LHS.One ^ RHS.One;
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// Compute set of known bits (where all three relevant bits are known).
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APInt LHSKnownUnion = LHS.Zero | LHS.One;
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APInt RHSKnownUnion = RHS.Zero | RHS.One;
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APInt CarryKnownUnion = std::move(CarryKnownZero) | CarryKnownOne;
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APInt Known = std::move(LHSKnownUnion) & RHSKnownUnion & CarryKnownUnion;
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assert((PossibleSumZero & Known) == (PossibleSumOne & Known) &&
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"known bits of sum differ");
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// Compute known bits of the result.
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KnownBits KnownOut;
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KnownOut.Zero = ~std::move(PossibleSumZero) & Known;
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KnownOut.One = std::move(PossibleSumOne) & Known;
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// Are we still trying to solve for the sign bit?
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if (!Known.isSignBitSet()) {
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if (NSW) {
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// Adding two non-negative numbers, or subtracting a negative number from
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// a non-negative one, can't wrap into negative.
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if (LHS.isNonNegative() && RHS.isNonNegative())
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KnownOut.makeNonNegative();
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// Adding two negative numbers, or subtracting a non-negative number from
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// a negative one, can't wrap into non-negative.
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else if (LHS.isNegative() && RHS.isNegative())
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KnownOut.makeNegative();
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}
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}
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return KnownOut;
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}
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