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81 lines
2.4 KiB
C
81 lines
2.4 KiB
C
/*
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Copyright (c) 2018-2019, tevador <tevador@gmail.com>
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All rights reserved.
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Redistribution and use in source and binary forms, with or without
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modification, are permitted provided that the following conditions are met:
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* Redistributions of source code must retain the above copyright
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notice, this list of conditions and the following disclaimer.
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* Redistributions in binary form must reproduce the above copyright
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notice, this list of conditions and the following disclaimer in the
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documentation and/or other materials provided with the distribution.
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* Neither the name of the copyright holder nor the
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names of its contributors may be used to endorse or promote products
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derived from this software without specific prior written permission.
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THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
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ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
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WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
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FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
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SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
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CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*/
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#include <assert.h>
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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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divisor must not be 0 or a power of 2
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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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assert(divisor != 0);
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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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#if !RANDOMX_HAVE_FAST_RECIPROCAL
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uint64_t randomx_reciprocal_fast(uint64_t divisor) {
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return randomx_reciprocal(divisor);
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}
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#endif
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