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

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Date:	Mon, 8 Aug 2011 09:15:13 +0200
From:	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>
To:	"Bob Pearson" <rpearson@...temfabricworks.com>
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

"Bob Pearson" <rpearson@...temfabricworks.com> wrote on 2011/08/05 19:27:26:
>
> > >
> > > >
> > > > >
> > > > > Modify all 'i' loops from for (i = 0; i < foo; i++) { ... } to for
> (i =
> > > foo
> > > > > - 1; i >= 0; i--) { ... }
> > > >
> > > > That should be (i = foo; i ; --i) { ... }
> > >
> > > Shouldn't make much difference, branch on zero bit or branch on sign
> bit.
> > > But at the end of the day didn't help on Nehalem.
>
> I figured out why "for (i = 0; i < len; i++) {...}" is faster than "for (;
> len; len--) {...}" on my system.
> The current code is
>
> for (; Ien; len--) {
>    load *++p
>    ...
> }
>
> Which turns into (in fake assembly)
>
> top:
>    dec len
>    inc p
>    load p
>    ...
>    test len
>    branch neq top
>
> But when I replace that with
>
> for(i = 0; i < len; i++) {
>    load *++p
>    ...
> }
>
> Gcc turns it into
>
> top:
>    load p[i]
>    i++
>    ...
>    compare i, len
>    branch lt top
>
> which is fewer instructions and i++ is well scheduled. Incrementing the
> pointer has been moved out of the loop.

I see. Lets leave the pre vs. post inc. for now. That is something
that can be sorted separately.

 Jocke

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