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