RandomWOW/src/reciprocal.c

60 lines
No EOL
1.4 KiB
C

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