这是 CRC-32C 的软件和硬件版本。软件版本经过优化，可以一次处理 8 个字节。硬件版本经过优化，可以crc32q在单个内核上有效地并行运行三个指令，因为该指令的吞吐量是一个周期，但延迟是三个周期。
crc32c.c:
/* crc32c.c -- compute CRC-32C using the Intel crc32 instruction
* Copyright (C) 2013, 2021 Mark Adler
* Version 1.2 5 Jun 2021 Mark Adler
*/
/*
This software is provided 'as-is', without any express or implied
warranty. In no event will the author be held liable for any damages
arising from the use of this software.
Permission is granted to anyone to use this software for any purpose,
including commercial applications, and to alter it and redistribute it
freely, subject to the following restrictions:
1. The origin of this software must not be misrepresented; you must not
claim that you wrote the original software. If you use this software
in a product, an acknowledgment in the product documentation would be
appreciated but is not required.
2. Altered source versions must be plainly marked as such, and must not be
misrepresented as being the original software.
3. This notice may not be removed or altered from any source distribution.
Mark Adler
madler@alumni.caltech.edu
*/
/* Version History:
1.0 10 Feb 2013 First version
1.1 31 May 2021 Correct register constraints on assembly instructions
Include pre-computed tables to avoid use of pthreads
Return zero for the CRC when buf is NULL, as initial value
1.2 5 Jun 2021 Make tables constant
*/
// Use hardware CRC instruction on Intel SSE 4.2 processors. This computes a
// CRC-32C, *not* the CRC-32 used by Ethernet and zip, gzip, etc. A software
// version is provided as a fall-back, as well as for speed comparisons.
#include <stddef.h>
#include <stdint.h>
// Tables for CRC word-wise calculation, definitions of LONG and SHORT, and CRC
// shifts by LONG and SHORT bytes.
#include "crc32c.h"
// Table-driven software version as a fall-back. This is about 15 times slower
// than using the hardware instructions. This assumes little-endian integers,
// as is the case on Intel processors that the assembler code here is for.
static uint32_t crc32c_sw(uint32_t crc, void const *buf, size_t len) {
if (buf == NULL)
return 0;
unsigned char const *data = buf;
while (len && ((uintptr_t)data & 7) != 0) {
crc = (crc >> 8) ^ crc32c_table[0][(crc ^ *data++) & 0xff];
len--;
}
size_t n = len >> 3;
for (size_t i = 0; i < n; i++) {
uint64_t word = crc ^ ((uint64_t const *)data)[i];
crc = crc32c_table[7][word & 0xff] ^
crc32c_table[6][(word >> 8) & 0xff] ^
crc32c_table[5][(word >> 16) & 0xff] ^
crc32c_table[4][(word >> 24) & 0xff] ^
crc32c_table[3][(word >> 32) & 0xff] ^
crc32c_table[2][(word >> 40) & 0xff] ^
crc32c_table[1][(word >> 48) & 0xff] ^
crc32c_table[0][word >> 56];
}
data += n << 3;
len &= 7;
while (len) {
len--;
crc = (crc >> 8) ^ crc32c_table[0][(crc ^ *data++) & 0xff];
}
return crc;
}
// Apply the zeros operator table to crc.
static uint32_t crc32c_shift(uint32_t const zeros[][256], uint32_t crc) {
return zeros[0][crc & 0xff] ^ zeros[1][(crc >> 8) & 0xff] ^
zeros[2][(crc >> 16) & 0xff] ^ zeros[3][crc >> 24];
}
// Compute CRC-32C using the Intel hardware instruction. Three crc32q
// instructions are run in parallel on a single core. This gives a
// factor-of-three speedup over a single crc32q instruction, since the
// throughput of that instruction is one cycle, but the latency is three
// cycles.
static uint32_t crc32c_hw(uint32_t crc, void const *buf, size_t len) {
if (buf == NULL)
return 0;
// Pre-process the crc.
uint64_t crc0 = crc ^ 0xffffffff;
// Compute the crc for up to seven leading bytes, bringing the data pointer
// to an eight-byte boundary.
unsigned char const *next = buf;
while (len && ((uintptr_t)next & 7) != 0) {
__asm__("crc32b\t" "(%1), %0"
: "+r"(crc0)
: "r"(next), "m"(*next));
next++;
len--;
}
// Compute the crc on sets of LONG*3 bytes, making use of three ALUs in
// parallel on a single core.
while (len >= LONG*3) {
uint64_t crc1 = 0;
uint64_t crc2 = 0;
unsigned char const *end = next + LONG;
do {
__asm__("crc32q\t" "(%3), %0\n\t"
"crc32q\t" LONGx1 "(%3), %1\n\t"
"crc32q\t" LONGx2 "(%3), %2"
: "+r"(crc0), "+r"(crc1), "+r"(crc2)
: "r"(next), "m"(*next));
next += 8;
} while (next < end);
crc0 = crc32c_shift(crc32c_long, crc0) ^ crc1;
crc0 = crc32c_shift(crc32c_long, crc0) ^ crc2;
next += LONG*2;
len -= LONG*3;
}
// Do the same thing, but now on SHORT*3 blocks for the remaining data less
// than a LONG*3 block.
while (len >= SHORT*3) {
uint64_t crc1 = 0;
uint64_t crc2 = 0;
unsigned char const *end = next + SHORT;
do {
__asm__("crc32q\t" "(%3), %0\n\t"
"crc32q\t" SHORTx1 "(%3), %1\n\t"
"crc32q\t" SHORTx2 "(%3), %2"
: "+r"(crc0), "+r"(crc1), "+r"(crc2)
: "r"(next), "m"(*next));
next += 8;
} while (next < end);
crc0 = crc32c_shift(crc32c_short, crc0) ^ crc1;
crc0 = crc32c_shift(crc32c_short, crc0) ^ crc2;
next += SHORT*2;
len -= SHORT*3;
}
// Compute the crc on the remaining eight-byte units less than a SHORT*3
// block.
unsigned char const *end = next + (len - (len & 7));
while (next < end) {
__asm__("crc32q\t" "(%1), %0"
: "+r"(crc0)
: "r"(next), "m"(*next));
next += 8;
}
len &= 7;
// Compute the crc for up to seven trailing bytes.
while (len) {
__asm__("crc32b\t" "(%1), %0"
: "+r"(crc0)
: "r"(next), "m"(*next));
next++;
len--;
}
// Return the crc, post-processed.
return ~(uint32_t)crc0;
}
// Check for SSE 4.2. SSE 4.2 was first supported in Nehalem processors
// introduced in November, 2008. This does not check for the existence of the
// cpuid instruction itself, which was introduced on the 486SL in 1992, so this
// will fail on earlier x86 processors. cpuid works on all Pentium and later
// processors.
#define SSE42(have) \
do { \
uint32_t eax, ecx; \
eax = 1; \
__asm__("cpuid" \
: "=c"(ecx) \
: "a"(eax) \
: "%ebx", "%edx"); \
(have) = (ecx >> 20) & 1; \
} while (0)
// Compute a CRC-32C. If the crc32 instruction is available, use the hardware
// version. Otherwise, use the software version.
uint32_t crc32c(uint32_t crc, void const *buf, size_t len) {
int sse42;
SSE42(sse42);
return sse42 ? crc32c_hw(crc, buf, len) : crc32c_sw(crc, buf, len);
}
生成 crc32c.h 的代码（stackoverflow 不允许我自己发布表格，因为答案中有 30,000 个字符的限制）：
// Generate crc32c.h for crc32c.c.
#include <stdio.h>
#include <stdint.h>
#define LONG 8192
#define SHORT 256
// Print a 2-D table of four-byte constants in hex.
static void print_table(uint32_t *tab, size_t rows, size_t cols, char *name) {
printf("static uint32_t const %s[][%zu] = {\n", name, cols);
size_t end = rows * cols;
size_t k = 0;
for (;;) {
fputs(" {", stdout);
size_t n = 0, j = 0;
for (;;) {
printf("0x%08x", tab[k + n]);
if (++n == cols)
break;
putchar(',');
if (++j == 6) {
fputs("\n ", stdout);
j = 0;
}
putchar(' ');
}
k += cols;
if (k == end)
break;
puts("},");
}
puts("}\n};");
}
/* CRC-32C (iSCSI) polynomial in reversed bit order. */
#define POLY 0x82f63b78
static void crc32c_word_table(void) {
uint32_t table[8][256];
// Generate byte-wise table.
for (unsigned n = 0; n < 256; n++) {
uint32_t crc = ~n;
for (unsigned k = 0; k < 8; k++)
crc = crc & 1 ? (crc >> 1) ^ POLY : crc >> 1;
table[0][n] = ~crc;
}
// Use byte-wise table to generate word-wise table.
for (unsigned n = 0; n < 256; n++) {
uint32_t crc = ~table[0][n];
for (unsigned k = 1; k < 8; k++) {
crc = table[0][crc & 0xff] ^ (crc >> 8);
table[k][n] = ~crc;
}
}
// Print table.
print_table(table[0], 8, 256, "crc32c_table");
}
// Return a(x) multiplied by b(x) modulo p(x), where p(x) is the CRC
// polynomial. For speed, this requires that a not be zero.
static uint32_t multmodp(uint32_t a, uint32_t b) {
uint32_t prod = 0;
for (;;) {
if (a & 0x80000000) {
prod ^= b;
if ((a & 0x7fffffff) == 0)
break;
}
a <<= 1;
b = b & 1 ? (b >> 1) ^ POLY : b >> 1;
}
return prod;
}
/* Take a length and build four lookup tables for applying the zeros operator
for that length, byte-by-byte, on the operand. */
static void crc32c_zero_table(size_t len, char *name) {
// Generate operator for len zeros.
uint32_t op = 0x80000000; // 1 (x^0)
uint32_t sq = op >> 4; // x^4
while (len) {
sq = multmodp(sq, sq); // x^2^(k+3), k == len bit position
if (len & 1)
op = multmodp(sq, op);
len >>= 1;
}
// Generate table to update each byte of a CRC using op.
uint32_t table[4][256];
for (unsigned n = 0; n < 256; n++) {
table[0][n] = multmodp(op, n);
table[1][n] = multmodp(op, n << 8);
table[2][n] = multmodp(op, n << 16);
table[3][n] = multmodp(op, n << 24);
}
// Print the table to stdout.
print_table(table[0], 4, 256, name);
}
int main(void) {
puts(
"// crc32c.h\n"
"// Tables and constants for crc32c.c software and hardware calculations.\n"
"\n"
"// Table for a 64-bits-at-a-time software CRC-32C calculation. This table\n"
"// has built into it the pre and post bit inversion of the CRC."
);
crc32c_word_table();
puts(
"\n// Block sizes for three-way parallel crc computation. LONG and SHORT\n"
"// must both be powers of two. The associated string constants must be set\n"
"// accordingly, for use in constructing the assembler instructions."
);
printf("#define LONG %d\n", LONG);
printf("#define LONGx1 \"%d\"\n", LONG);
printf("#define LONGx2 \"%d\"\n", 2 * LONG);
printf("#define SHORT %d\n", SHORT);
printf("#define SHORTx1 \"%d\"\n", SHORT);
printf("#define SHORTx2 \"%d\"\n", 2 * SHORT);
puts(
"\n// Table to shift a CRC-32C by LONG bytes."
);
crc32c_zero_table(8192, "crc32c_long");
puts(
"\n// Table to shift a CRC-32C by SHORT bytes."
);
crc32c_zero_table(256, "crc32c_short");
return 0;
}