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//===----- DivisonByConstantInfo.cpp - division by constant -*- C++ -*-----===//
//
// 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 implements support for optimizing divisions by a constant
///
//===----------------------------------------------------------------------===//
#include "llvm/Support/DivisionByConstantInfo.h"
using namespace llvm;
/// Calculate the magic numbers required to implement a signed integer division
/// by a constant as a sequence of multiplies, adds and shifts. Requires that
/// the divisor not be 0, 1, or -1. Taken from "Hacker's Delight", Henry S.
/// Warren, Jr., Chapter 10.
SignedDivisionByConstantInfo SignedDivisionByConstantInfo::get(const APInt &D) {
unsigned P;
APInt AD, ANC, Delta, Q1, R1, Q2, R2, T;
APInt SignedMin = APInt::getSignedMinValue(D.getBitWidth());
struct SignedDivisionByConstantInfo Retval;
AD = D.abs();
T = SignedMin + (D.lshr(D.getBitWidth() - 1));
ANC = T - 1 - T.urem(AD); // absolute value of NC
P = D.getBitWidth() - 1; // initialize P
Q1 = SignedMin.udiv(ANC); // initialize Q1 = 2P/abs(NC)
R1 = SignedMin - Q1 * ANC; // initialize R1 = rem(2P,abs(NC))
Q2 = SignedMin.udiv(AD); // initialize Q2 = 2P/abs(D)
R2 = SignedMin - Q2 * AD; // initialize R2 = rem(2P,abs(D))
do {
P = P + 1;
Q1 = Q1 << 1; // update Q1 = 2P/abs(NC)
R1 = R1 << 1; // update R1 = rem(2P/abs(NC))
if (R1.uge(ANC)) { // must be unsigned comparison
Q1 = Q1 + 1;
R1 = R1 - ANC;
}
Q2 = Q2 << 1; // update Q2 = 2P/abs(D)
R2 = R2 << 1; // update R2 = rem(2P/abs(D))
if (R2.uge(AD)) { // must be unsigned comparison
Q2 = Q2 + 1;
R2 = R2 - AD;
}
Delta = AD - R2;
} while (Q1.ult(Delta) || (Q1 == Delta && R1 == 0));
Retval.Magic = Q2 + 1;
if (D.isNegative())
Retval.Magic = -Retval.Magic; // resulting magic number
Retval.ShiftAmount = P - D.getBitWidth(); // resulting shift
return Retval;
}
/// Calculate the magic numbers required to implement an unsigned integer
/// division by a constant as a sequence of multiplies, adds and shifts.
/// Requires that the divisor not be 0. Taken from "Hacker's Delight", Henry
/// S. Warren, Jr., chapter 10.
/// LeadingZeros can be used to simplify the calculation if the upper bits
/// of the divided value are known zero.
UnsignedDivisonByConstantInfo
UnsignedDivisonByConstantInfo::get(const APInt &D, unsigned LeadingZeros) {
unsigned P;
APInt NC, Delta, Q1, R1, Q2, R2;
struct UnsignedDivisonByConstantInfo Retval;
Retval.IsAdd = 0; // initialize "add" indicator
APInt AllOnes = APInt::getAllOnes(D.getBitWidth()).lshr(LeadingZeros);
APInt SignedMin = APInt::getSignedMinValue(D.getBitWidth());
APInt SignedMax = APInt::getSignedMaxValue(D.getBitWidth());
NC = AllOnes - (AllOnes - D).urem(D);
P = D.getBitWidth() - 1; // initialize P
Q1 = SignedMin.udiv(NC); // initialize Q1 = 2P/NC
R1 = SignedMin - Q1 * NC; // initialize R1 = rem(2P,NC)
Q2 = SignedMax.udiv(D); // initialize Q2 = (2P-1)/D
R2 = SignedMax - Q2 * D; // initialize R2 = rem((2P-1),D)
do {
P = P + 1;
if (R1.uge(NC - R1)) {
Q1 = Q1 + Q1 + 1; // update Q1
R1 = R1 + R1 - NC; // update R1
} else {
Q1 = Q1 + Q1; // update Q1
R1 = R1 + R1; // update R1
}
if ((R2 + 1).uge(D - R2)) {
if (Q2.uge(SignedMax))
Retval.IsAdd = 1;
Q2 = Q2 + Q2 + 1; // update Q2
R2 = R2 + R2 + 1 - D; // update R2
} else {
if (Q2.uge(SignedMin))
Retval.IsAdd = 1;
Q2 = Q2 + Q2; // update Q2
R2 = R2 + R2 + 1; // update R2
}
Delta = D - 1 - R2;
} while (P < D.getBitWidth() * 2 &&
(Q1.ult(Delta) || (Q1 == Delta && R1 == 0)));
Retval.Magic = Q2 + 1; // resulting magic number
Retval.ShiftAmount = P - D.getBitWidth(); // resulting shift
return Retval;
}
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