60 lines
1.4 KiB
C
60 lines
1.4 KiB
C
/*
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Copyright (c) 2019 tevador
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This file is part of RandomX.
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RandomX is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation, either version 3 of the License, or
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(at your option) any later version.
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RandomX is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with RandomX. If not, see<http://www.gnu.org/licenses/>.
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*/
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#include "reciprocal.h"
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/*
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Calculates rcp = 2**x / divisor for highest integer x such that rcp < 2**64.
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Equivalent x86 assembly (divisor in rcx):
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mov edx, 1
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mov r8, rcx
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xor eax, eax
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bsr rcx, rcx
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shl rdx, cl
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div r8
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ret
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*/
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uint64_t randomx_reciprocal(uint64_t divisor) {
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const uint64_t p2exp63 = 1ULL << 63;
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uint64_t quotient = p2exp63 / divisor, remainder = p2exp63 % divisor;
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unsigned bsr = 0; //highest set bit in divisor
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for (uint64_t bit = divisor; bit > 0; bit >>= 1)
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bsr++;
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for (unsigned shift = 0; shift < bsr; shift++) {
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if (remainder >= divisor - remainder) {
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quotient = quotient * 2 + 1;
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remainder = remainder * 2 - divisor;
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}
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else {
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quotient = quotient * 2;
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remainder = remainder * 2;
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}
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}
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return quotient;
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}
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