linux-kernel - RE: [PATCH] add slice by 8 algorithm to crc32.c

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Date:	Fri, 5 Aug 2011 10:51:00 -0500
From:	"Bob Pearson" <rpearson@...temfabricworks.com>
To:	"'Joakim Tjernlund'" <joakim.tjernlund@...nsmode.se>
Cc:	"'Andrew Morton'" <akpm@...ux-foundation.org>,
	"'frank zago'" <fzago@...temfabricworks.com>,
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] add slice by 8 algorithm to crc32.c

> > version. While I haven't done the experiment you suggest there is
> something
> > to the point that the second q computation in the new version can be
> moved
> > ahead of the table lookups from the first q computation . My guess is
that
> > the unrolled version will be significantly slower.
> 
> Ah, didn't see that. Don't understand how this works though.
> Why do you do two 32 bit loads instead of one 64 bit load?
> 
> >

The two expression trees can be computed in parallel and combined with the
final xor. If the compiler/instruction scheduler are smart enough and can
process enough instructions per cycle they overlap well and you get some
speedup. I did try a 64 bit load on Nehalem but got about 2 cycles per byte
which is a little worse than doing two loads and better than the 32 bit
version. I'm not really sure why.

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