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

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Date:	Thu, 19 Apr 2007 13:01:01 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	dada1@...mosbay.com
Cc:	shemminger@...ux-foundation.org, xemul@...ru,
	netdev@...r.kernel.org, bridge@...ts.osdl.org, devel@...nvz.org
Subject: Re: [BRIDGE] Unaligned access on IA64 when comparing ethernet
 addresses

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

> 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

I was prehaps thinking about strlen() where I know several
implementations work a word at a time even though it is
a byte-based operation:

--------------------
#define LO_MAGIC 0x01010101
#define HI_MAGIC 0x80808080
 ...
	 sethi	%hi(HI_MAGIC), %o4
 ...
	 or	%o4, %lo(HI_MAGIC), %o3
 ...
	 sethi	%hi(LO_MAGIC), %o4
 ...
	 or	%o4, %lo(LO_MAGIC), %o2
 ...
8:
	ld	[%o0], %o5
2:
	sub	%o5, %o2, %o4
	andcc	%o4, %o3, %g0
	be,pt	%icc, 8b
	 add	%o0, 4, %o0
--------------------

I figured some similar trick could be done with strcmp() and
memcmp().
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