unsigned long ZEXPORT crc32_z()

in zlib/crc32.c [575:665]

• 69 lines of code
• 10 McCabe index (conditional complexity)

unsigned long ZEXPORT crc32_z(unsigned long crc, const unsigned char FAR *buf,
                              z_size_t len) {
    z_crc_t val;
    z_word_t crc1, crc2;
    const z_word_t *word;
    z_word_t val0, val1, val2;
    z_size_t last, last2, i;
    z_size_t num;

    /* Return initial CRC, if requested. */
    if (buf == Z_NULL) return 0;

#ifdef DYNAMIC_CRC_TABLE
    once(&made, make_crc_table);
#endif /* DYNAMIC_CRC_TABLE */

    /* Pre-condition the CRC */
    crc = (~crc) & 0xffffffff;

    /* Compute the CRC up to a word boundary. */
    while (len && ((z_size_t)buf & 7) != 0) {
        len--;
        val = *buf++;
        __asm__ volatile("crc32b %w0, %w0, %w1" : "+r"(crc) : "r"(val));
    }

    /* Prepare to compute the CRC on full 64-bit words word[0..num-1]. */
    word = (z_word_t const *)buf;
    num = len >> 3;
    len &= 7;

    /* Do three interleaved CRCs to realize the throughput of one crc32x
       instruction per cycle. Each CRC is calculated on Z_BATCH words. The
       three CRCs are combined into a single CRC after each set of batches. */
    while (num >= 3 * Z_BATCH) {
        crc1 = 0;
        crc2 = 0;
        for (i = 0; i < Z_BATCH; i++) {
            val0 = word[i];
            val1 = word[i + Z_BATCH];
            val2 = word[i + 2 * Z_BATCH];
            __asm__ volatile("crc32x %w0, %w0, %x1" : "+r"(crc) : "r"(val0));
            __asm__ volatile("crc32x %w0, %w0, %x1" : "+r"(crc1) : "r"(val1));
            __asm__ volatile("crc32x %w0, %w0, %x1" : "+r"(crc2) : "r"(val2));
        }
        word += 3 * Z_BATCH;
        num -= 3 * Z_BATCH;
        crc = multmodp(Z_BATCH_ZEROS, crc) ^ crc1;
        crc = multmodp(Z_BATCH_ZEROS, crc) ^ crc2;
    }

    /* Do one last smaller batch with the remaining words, if there are enough
       to pay for the combination of CRCs. */
    last = num / 3;
    if (last >= Z_BATCH_MIN) {
        last2 = last << 1;
        crc1 = 0;
        crc2 = 0;
        for (i = 0; i < last; i++) {
            val0 = word[i];
            val1 = word[i + last];
            val2 = word[i + last2];
            __asm__ volatile("crc32x %w0, %w0, %x1" : "+r"(crc) : "r"(val0));
            __asm__ volatile("crc32x %w0, %w0, %x1" : "+r"(crc1) : "r"(val1));
            __asm__ volatile("crc32x %w0, %w0, %x1" : "+r"(crc2) : "r"(val2));
        }
        word += 3 * last;
        num -= 3 * last;
        val = x2nmodp(last, 6);
        crc = multmodp(val, crc) ^ crc1;
        crc = multmodp(val, crc) ^ crc2;
    }

    /* Compute the CRC on any remaining words. */
    for (i = 0; i < num; i++) {
        val0 = word[i];
        __asm__ volatile("crc32x %w0, %w0, %x1" : "+r"(crc) : "r"(val0));
    }
    word += num;

    /* Complete the CRC on any remaining bytes. */
    buf = (const unsigned char FAR *)word;
    while (len) {
        len--;
        val = *buf++;
        __asm__ volatile("crc32b %w0, %w0, %w1" : "+r"(crc) : "r"(val));
    }

    /* Return the CRC, post-conditioned. */
    return crc ^ 0xffffffff;
}