netdev - Re: [BRIDGE] Unaligned access on IA64 when comparing ethernet addresses

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Message-ID: <20070419111857.62241675@dxpl.pdx.osdl.net>
Date:	Thu, 19 Apr 2007 11:18:57 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	David Miller <davem@...emloft.net>, xemul@...ru,
	netdev@...r.kernel.org, bridge@...ts.osdl.org, devel@...nvz.org
Subject: Re: [BRIDGE] Unaligned access on IA64 when comparing ethernet
 addresses

On Thu, 19 Apr 2007 16:14:23 +0200
Eric Dumazet <dada1@...mosbay.com> wrote:

> On Wed, 18 Apr 2007 13:04:22 -0700 (PDT)
> David Miller <davem@...emloft.net> wrote:
> 
> > 
> > Although I don't think gcc does anything fancy since we don't
> > use memcmp().  It's a tradeoff, we'd like to use unsigned long
> > comparisons when both objects are aligned correctly but we also
> > don't want it to use any more than one potentially mispredicted
> > branch.
> 
> Again, memcmp() *cannot* be optimized, because its semantic is to compare bytes.
> 
> memcpy() can take into account alignement if known at compile time, not memcmp()
> 
> http://lists.openwall.net/netdev/2007/03/13/31

It can if we order bytes in the bridge id properly.  See ktime_t for example.
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