Re: eth_type_trans(): Re: [Bug #11308] tbench regression on eachkernel release from 2.6.22 -> 2.6.28

[Linus Torvalds]

On Tue, 18 Nov 2008, Eric Dumazet wrote:
> > *
> > * Compare two ethernet addresses, returns 0 if equal
> > */
> > static inline unsigned compare_ether_addr(const u8 *addr1, const u8 *addr2)
> > {
> >        const u16 *a = (const u16 *) addr1;
> >        const u16 *b = (const u16 *) addr2;
> > 
> >        BUILD_BUG_ON(ETH_ALEN != 6);
> >        return ((a[0] ^ b[0]) | (a[1] ^ b[1]) | (a[2] ^ b[2])) != 0;

Btw, at least on some Intel CPU's, it would be faster to do this as a 
32-bit xor and a 16-bit xor. And if we can know that there is always 2 
bytes at the end (because of how the thing was allocated), it's faster 
still to do it as a 64-bit xor and a mask.

And that's true even if the addresses are only 2-byte aligned.

The code that gcc generates for "memcmp()" for a constant-size small data 
thing is sadly crap. It always generates a "rep cmpsb", even if the size 
is something really trivial like 4 bytes, and even if you compare for 
exact equality rather than a smaller/greater-than. Gaah.

		Linus
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