这是 PHP 底层的 hash 代码，请大神指示为什么要这样写

swith 为什么不用 default, for 循环为什么步长用 8

static inline ulong zend_inline_hash_func(char *arKey, uint nKeyLength)
{
    register ulong hash = 5381;
 
    /* variant with the hash unrolled eight times */
    for (; nKeyLength >= 8; nKeyLength -= 8) {
        hash = ((hash << 5) + hash) + *arKey++;
        hash = ((hash << 5) + hash) + *arKey++;
        hash = ((hash << 5) + hash) + *arKey++;
        hash = ((hash << 5) + hash) + *arKey++;
        hash = ((hash << 5) + hash) + *arKey++;
        hash = ((hash << 5) + hash) + *arKey++;
        hash = ((hash << 5) + hash) + *arKey++;
        hash = ((hash << 5) + hash) + *arKey++;
    }
    switch (nKeyLength) {
        case 7: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
        case 6: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
        case 5: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
        case 4: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
        case 3: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
        case 2: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
        case 1: hash = ((hash << 5) + hash) + *arKey++; break;
        case 0: break;
EMPTY_SWITCH_DEFAULT_CASE()
    }
    return hash;
}

http://www.laruence.com/2009/07/23/994.html