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

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Date:	Mon, 17 Nov 2008 16:01:27 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Eric Dumazet <dada1@...mosbay.com>
cc:	Ingo Molnar <mingo@...e.hu>, David Miller <davem@...emloft.net>,
	rjw@...k.pl, linux-kernel@...r.kernel.org,
	kernel-testers@...r.kernel.org, cl@...ux-foundation.org,
	efault@....de, a.p.zijlstra@...llo.nl,
	Stephen Hemminger <shemminger@...tta.com>
Subject: Re: eth_type_trans(): Re: [Bug #11308] tbench regression on each
 kernel release from 2.6.22 -&gt; 2.6.28

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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