Hello!

I have done a little testing on my own. My results is that memcpy is
many times faster even with aligned data.

I am testing in an ordinary console program. I am including the code below.
If I'm doing something wrong, please tell me so.

As you can see I am not using the same datadeclarations as the kernel
but I'm testing the algorithm here not the data. By testing various
data and types of data I can make sure the algorithm behaves correctly
in all situations.
The datamember 'd' in flowi is not part of the comparison, but by
changing it into an 'unsigned int' it becomes part of the comparison.



const int NUM_REP = 0x7FFFFFFF;

typedef unsigned int flow_compare_t;

struct flowi {
	unsigned int a,b,c;
	unsigned char d;
};


/* I hear what you're saying, use memcmp.  But memcmp cannot make
 * important assumptions that we can here, such as alignment and
 * constant size.
 */
static int flow_key_compare(struct flowi *key1, struct flowi *key2)
{
	flow_compare_t *k1, *k1_lim, *k2;
	const int n_elem = sizeof(struct flowi) / sizeof(flow_compare_t);

	k1 = (flow_compare_t *) key1;
	k1_lim = k1 + n_elem;

	k2 = (flow_compare_t *) key2;

	do {
		if (*k1++ != *k2++)
			return 1;
	} while (k1 < k1_lim);

	return 0;
}



static int flow_key_compare2(struct flowi *key1, struct flowi *key2)
{
	return memcmp(key1, key2, (sizeof(struct flowi) /
sizeof(flow_compare_t)) * sizeof(flow_compare_t));
}






int main()
{

	struct flowi key1 = {0,1,2,3};
	struct flowi key2 = {0,1,2,0};
	char str[300];
	int i;

	/* put data in aligned addresses */
	struct flowi *k1 = (struct flowi *)((int)(&str[100]) & 0xFFFFFFF0);
	struct flowi *k2 = (struct flowi *)((int)(&str[200]) & 0xFFFFFFF0);
	memcpy(k1, &key1, sizeof(struct flowi));
	memcpy(k2, &key2, sizeof(struct flowi));

	/* Compare data */
	printf("compare1..\n");
	for (i = 0; i < NUM_REP; i++)
		flow_key_compare(k1, k2);

	printf("compare2..\n");
	for (i = 0; i < NUM_REP; i++)
		flow_key_compare2(k1, k2);

	printf((flow_key_compare(k1,k2)==(flow_key_compare2(k1,k2)?1:0))?"ok\n":"error\n");

	return 0;
}











2007/1/1, Daniel Marjamäki [off-list ref]:

Hello!

So you mean that in this particular case it's faster with a handcoded
comparison than memcmp? Because both key1 and key2 are located at
word-aligned addresses?
That's fascinating.

Best regards,
Daniel

2006/12/31, David Miller [off-list ref]:

quoted

From: "Daniel_Marjamäki" <redacted>
Date: Sun, 31 Dec 2006 17:37:05 +0100

quoted

From: Daniel Marjamäki
This has been tested by me.
Signed-off-by: Daniel Marjamäki <redacted>

Please do not do this.

memcmp() cannot assume the alignment of the source and
destination buffers and thus will run more slowly than
that open-coded comparison.

That code was done like that on purpose because it is
one of the most critical paths in the networking flow
cache lookup which runs for every IPSEC packet going
throught the system.