| Re: [BRIDGE] Unaligned access on IA64 when comparing ethernet addresses [message #12189 is a reply to message #12186] |
Thu, 19 April 2007 20:01   |
davem
Messages: 463
Registered: February 2006
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Senior Member |
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From: Eric Dumazet <dada1@cosmosbay.com>
Date: Thu, 19 Apr 2007 16:14:23 +0200
> On Wed, 18 Apr 2007 13:04:22 -0700 (PDT)
> David Miller <davem@davemloft.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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