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

From: Joakim Tjernlund
Date: Mon Aug 08 2011 - 03:11:44 EST

"Bob Pearson" <rpearson@xxxxxxxxxxxxxxxxxxxxx> wrote on 2011/08/05 17:51:00:
> > > 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.

I see. There is one thing that strikes me as a bit odd. Why do 8 byte alignment
when you do 32 bit accesses? Surely the current 4 byte alignment will do?


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