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This is for D60460. computeForAddSub() essentially already supports carries because it has to deal with subtractions. This revision extracts a lower-level computeForAddCarry() function, which allows computing the known bits for add (carry known zero), sub (carry known one) and addcarry (carry unknown). As we don't seem to have any yet, I've added a unit test file for KnownBits and exhaustive tests for the new computeForAddCarry() functionality, as well the existing computeForAddSub() function. Differential Revision: https://reviews.llvm.org/D60522 llvm-svn: 358297
84 lines
3.0 KiB
C++
84 lines
3.0 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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static KnownBits computeForAddCarry(
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const KnownBits &LHS, const KnownBits &RHS,
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bool CarryZero, bool CarryOne) {
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assert(!(CarryZero && CarryOne) &&
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"Carry can't be zero and one at the same time");
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APInt PossibleSumZero = ~LHS.Zero + ~RHS.Zero + !CarryZero;
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APInt PossibleSumOne = LHS.One + RHS.One + CarryOne;
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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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return KnownOut;
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}
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KnownBits KnownBits::computeForAddCarry(
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const KnownBits &LHS, const KnownBits &RHS, const KnownBits &Carry) {
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assert(Carry.getBitWidth() == 1 && "Carry must be 1-bit");
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return ::computeForAddCarry(
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LHS, RHS, Carry.Zero.getBoolValue(), Carry.One.getBoolValue());
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}
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KnownBits KnownBits::computeForAddSub(bool Add, bool NSW,
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const KnownBits &LHS, KnownBits RHS) {
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KnownBits KnownOut;
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if (Add) {
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// Sum = LHS + RHS + 0
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KnownOut = ::computeForAddCarry(
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LHS, RHS, /*CarryZero*/true, /*CarryOne*/false);
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} else {
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// Sum = LHS + ~RHS + 1
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std::swap(RHS.Zero, RHS.One);
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KnownOut = ::computeForAddCarry(
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LHS, RHS, /*CarryZero*/false, /*CarryOne*/true);
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}
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// Are we still trying to solve for the sign bit?
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if (!KnownOut.isNegative() && !KnownOut.isNonNegative()) {
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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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