漫话Redis源码之六十九

反转字节，挺常见的操作。与内存有关的操作，常常会涉及到位运算，很快速。

/* Reverse the bytes in a 64-bit word. */
static inline uint64_t rev8(uint64_t a) {
#if defined(__GNUC__) || defined(__clang__)
    return __builtin_bswap64(a);
#else
    uint64_t m;
 
    m = UINT64_C(0xff00ff00ff00ff);
    a = ((a >> 8) & m) | (a & m) << 8;
    m = UINT64_C(0xffff0000ffff);
    a = ((a >> 16) & m) | (a & m) << 16;
    return a >> 32 | a << 32;
#endif
}
 
/* This function is called once to initialize the CRC table for use on a
   big-endian architecture. */
void crcspeed64big_init(crcfn64 fn, uint64_t big_table[8][256]) {
    /* Create the little endian table then reverse all the entries. */
    crcspeed64little_init(fn, big_table);
    for (int k = 0; k < 8; k++) {
        for (int n = 0; n < 256; n++) {
            big_table[k][n] = rev8(big_table[k][n]);
        }
    }
}
 
void crcspeed16big_init(crcfn16 fn, uint16_t big_table[8][256]) {
    /* Create the little endian table then reverse all the entries. */
    crcspeed16little_init(fn, big_table);
    for (int k = 0; k < 8; k++) {
        for (int n = 0; n < 256; n++) {
            big_table[k][n] = rev8(big_table[k][n]);
        }
    }
}
 
/* Calculate a non-inverted CRC multiple bytes at a time on a little-endian
 * architecture. If you need inverted CRC, invert *before* calling and invert
 * *after* calling.
 * 64 bit crc = process 8 bytes at once;
 */
uint64_t crcspeed64little(uint64_t little_table[8][256], uint64_t crc,
                          void *buf, size_t len) {
    unsigned char *next = buf;
 
    /* process individual bytes until we reach an 8-byte aligned pointer */
    while (len && ((uintptr_t)next & 7) != 0) {
        crc = little_table[0][(crc ^ *next++) & 0xff] ^ (crc >> 8);
        len--;
    }
 
    /* fast middle processing, 8 bytes (aligned!) per loop */
    while (len >= 8) {
        crc ^= *(uint64_t *)next;
        crc = little_table[7][crc & 0xff] ^
              little_table[6][(crc >> 8) & 0xff] ^
              little_table[5][(crc >> 16) & 0xff] ^
              little_table[4][(crc >> 24) & 0xff] ^
              little_table[3][(crc >> 32) & 0xff] ^
              little_table[2][(crc >> 40) & 0xff] ^
              little_table[1][(crc >> 48) & 0xff] ^
              little_table[0][crc >> 56];
        next += 8;
        len -= 8;
    }
 
    /* process remaining bytes (can't be larger than 8) */
    while (len) {
        crc = little_table[0][(crc ^ *next++) & 0xff] ^ (crc >> 8);
        len--;
    }
 
    return crc;
}
 
uint16_t crcspeed16little(uint16_t little_table[8][256], uint16_t crc,
                          void *buf, size_t len) {
    unsigned char *next = buf;
 
    /* process individual bytes until we reach an 8-byte aligned pointer */
    while (len && ((uintptr_t)next & 7) != 0) {
        crc = little_table[0][((crc >> 8) ^ *next++) & 0xff] ^ (crc << 8);
        len--;
    }
 
    /* fast middle processing, 8 bytes (aligned!) per loop */
    while (len >= 8) {
        uint64_t n = *(uint64_t *)next;
        crc = little_table[7][(n & 0xff) ^ ((crc >> 8) & 0xff)] ^
              little_table[6][((n >> 8) & 0xff) ^ (crc & 0xff)] ^
              little_table[5][(n >> 16) & 0xff] ^
              little_table[4][(n >> 24) & 0xff] ^
              little_table[3][(n >> 32) & 0xff] ^
              little_table[2][(n >> 40) & 0xff] ^
              little_table[1][(n >> 48) & 0xff] ^
              little_table[0][n >> 56];
        next += 8;
        len -= 8;
    }
 
    /* process remaining bytes (can't be larger than 8) */
    while (len) {
        crc = little_table[0][((crc >> 8) ^ *next++) & 0xff] ^ (crc << 8);
        len--;
    }
 
    return crc;
}
 
/* Calculate a non-inverted CRC eight bytes at a time on a big-endian
 * architecture.
 */
uint64_t crcspeed64big(uint64_t big_table[8][256], uint64_t crc, void *buf,
                       size_t len) {
    unsigned char *next = buf;
 
    crc = rev8(crc);
    while (len && ((uintptr_t)next & 7) != 0) {
        crc = big_table[0][(crc >> 56) ^ *next++] ^ (crc << 8);
        len--;
    }
 
    while (len >= 8) {
        crc ^= *(uint64_t *)next;
        crc = big_table[0][crc & 0xff] ^
              big_table[1][(crc >> 8) & 0xff] ^
              big_table[2][(crc >> 16) & 0xff] ^
              big_table[3][(crc >> 24) & 0xff] ^
              big_table[4][(crc >> 32) & 0xff] ^
              big_table[5][(crc >> 40) & 0xff] ^
              big_table[6][(crc >> 48) & 0xff] ^
              big_table[7][crc >> 56];
        next += 8;
        len -= 8;
    }
 
    while (len) {
        crc = big_table[0][(crc >> 56) ^ *next++] ^ (crc << 8);
        len--;
    }
 
    return rev8(crc);
}
 
/* WARNING: Completely untested on big endian architecture.  Possibly broken. */
uint16_t crcspeed16big(uint16_t big_table[8][256], uint16_t crc_in, void *buf,
                       size_t len) {
    unsigned char *next = buf;
    uint64_t crc = crc_in;
 
    crc = rev8(crc);
    while (len && ((uintptr_t)next & 7) != 0) {
        crc = big_table[0][((crc >> (56 - 8)) ^ *next++) & 0xff] ^ (crc >> 8);
        len--;
    }
 
    while (len >= 8) {
        uint64_t n = *(uint64_t *)next;
        crc = big_table[0][(n & 0xff) ^ ((crc >> (56 - 8)) & 0xff)] ^
              big_table[1][((n >> 8) & 0xff) ^ (crc & 0xff)] ^
              big_table[2][(n >> 16) & 0xff] ^
              big_table[3][(n >> 24) & 0xff] ^
              big_table[4][(n >> 32) & 0xff] ^
              big_table[5][(n >> 40) & 0xff] ^
              big_table[6][(n >> 48) & 0xff] ^
              big_table[7][n >> 56];
        next += 8;
        len -= 8;
    }
 
    while (len) {
        crc = big_table[0][((crc >> (56 - 8)) ^ *next++) & 0xff] ^ (crc >> 8);
        len--;
    }
 
    return rev8(crc);
}
 
/* Return the CRC of buf[0..len-1] with initial crc, processing eight bytes
   at a time using passed-in lookup table.
   This selects one of two routines depending on the endianess of
   the architecture. */
uint64_t crcspeed64native(uint64_t table[8][256], uint64_t crc, void *buf,
                          size_t len) {
    uint64_t n = 1;
 
    return *(char *)&n ? crcspeed64little(table, crc, buf, len)
                       : crcspeed64big(table, crc, buf, len);
}
 
uint16_t crcspeed16native(uint16_t table[8][256], uint16_t crc, void *buf,
                          size_t len) {
    uint64_t n = 1;
 
    return *(char *)&n ? crcspeed16little(table, crc, buf, len)
                       : crcspeed16big(table, crc, buf, len);
}
 
/* Initialize CRC lookup table in architecture-dependent manner. */
void crcspeed64native_init(crcfn64 fn, uint64_t table[8][256]) {
    uint64_t n = 1;
 
    *(char *)&n ? crcspeed64little_init(fn, table)
                : crcspeed64big_init(fn, table);
}
 
void crcspeed16native_init(crcfn16 fn, uint16_t table[8][256]) {
    uint64_t n = 1;
 
    *(char *)&n ? crcspeed16little_init(fn, table)
                : crcspeed16big_init(fn, table);
}