for 循环中的 1 句话为什么写 8 次

[zhyoulun]

PHP 源码中 hash 的计算函数如下，想知道为什么 for 循环中把 1 句话写 8 次。

static inline ulong zend_inline_hash_func(const 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;
}

[billwsy]

@sujin190 另一方面 unrolling 有时会导致指令不能全部存入高级的 Cache ，从而带来性能的损失

[firebroo]

我能说我使用的时候就是直接扣了这个函数吗？ https://github.com/firebroo/UnixTools/blob/master/uniq/hashtable.c （逃

[gjc9620]

DUFF 装置吗？