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bf50c64e86
"scripts" is a more descriptive name than "tools", which sounds too similar to "programs".
109 lines
3.6 KiB
C
109 lines
3.6 KiB
C
/*
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* gen_crc32_multipliers.c
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*
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* Copyright 2016 Eric Biggers
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*
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* Permission is hereby granted, free of charge, to any person
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* obtaining a copy of this software and associated documentation
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* files (the "Software"), to deal in the Software without
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* restriction, including without limitation the rights to use,
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* copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the
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* Software is furnished to do so, subject to the following
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* conditions:
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*
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* The above copyright notice and this permission notice shall be
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* included in all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
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* OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
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* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
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* HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
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* WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
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* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
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* OTHER DEALINGS IN THE SOFTWARE.
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*/
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#include <inttypes.h>
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#include <stdio.h>
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/* generator polynomial G(x) */
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#define CRCPOLY 0xEDB88320 /* G(x) without x^32 term */
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#define CRCPOLY_FULL (((uint64_t)CRCPOLY << 1) | 1) /* G(x) */
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/* Compute x^D mod G(x) */
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static uint32_t
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compute_multiplier(int D)
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{
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/* Start with x^0 mod G(x) */
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uint32_t remainder = 0x80000000;
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/* Each iteration, 'remainder' becomes x^i mod G(x) */
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for (int i = 1; i <= D; i++)
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remainder = (remainder >> 1) ^ ((remainder & 1) ? CRCPOLY : 0);
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/* Now 'remainder' is x^D mod G(x) */
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return remainder;
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}
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/* Compute floor(x^64 / G(x)) */
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static uint64_t
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compute_barrett_reduction_constant(void)
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{
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uint64_t quotient = 0;
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uint64_t dividend = 0x1;
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for (int i = 0; i < 64 - 32 + 1; i++) {
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if ((dividend >> i) & 1) {
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quotient |= (uint64_t)1 << i;
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dividend ^= CRCPOLY_FULL << i;
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}
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}
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return quotient;
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}
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/*
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* This program computes the constant multipliers needed for carryless
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* multiplication accelerated CRC-32. It assumes 128-bit vectors divided into
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* two 64-bit halves which are multiplied separately with different 32-bit
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* multipliers, producing two 95-bit products. For a given number of 128-bit
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* vectors per iteration, the program outputs a pair of multipliers, one for
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* each 64-bit half.
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*
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* Careful: all polynomials are "bit-reversed", meaning that the low-order bits
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* have the highest degree and the high-order bits have the lowest degree!
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*/
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int
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main(void)
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{
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printf("\t/* Constants precomputed by gen_crc32_multipliers.c. "
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"Do not edit! */\n");
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/* High and low multipliers for each needed vector count */
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for (int order = 2; order >= 0; order--) {
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int vecs_per_iteration = 1 << order;
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int right = (128 * vecs_per_iteration) + 95;
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printf("\tconst __v2di multipliers_%d = (__v2di)"
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"{ 0x%08"PRIX32", 0x%08"PRIX32" };\n",
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vecs_per_iteration,
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compute_multiplier(right - 64) /* higher degree half */,
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compute_multiplier(right - 128) /* lower degree half */);
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}
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/* Multiplier for final 96 => 64 bit fold */
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printf("\tconst __v2di final_multiplier = (__v2di){ 0x%08"PRIX32" };\n",
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compute_multiplier(63));
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/* 32-bit mask */
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printf("\tconst __m128i mask32 = (__m128i)(__v4si){ 0xFFFFFFFF };\n");
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/* Constants for final 64 => 32 bit reduction */
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printf("\tconst __v2di barrett_reduction_constants =\n"
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"\t\t\t(__v2di){ 0x%016"PRIX64", 0x%016"PRIX64" };\n",
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compute_barrett_reduction_constant(), CRCPOLY_FULL);
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return 0;
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}
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