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

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Date:	Mon, 8 Aug 2011 12:04:05 -0500
From:	"Bob Pearson" <rpearson@...temfabricworks.com>
To:	"'Joakim Tjernlund'" <joakim.tjernlund@...nsmode.se>,
	"'George Spelvin'" <linux@...izon.com>
Cc:	<akpm@...ux-foundation.org>, <fzago@...temfabricworks.com>,
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] add slice by 8 algorithm to crc32.c

Same for x86. The code for the pre and post cleanup loops are the same
there.

> -----Original Message-----
> From: Joakim Tjernlund [mailto:joakim.tjernlund@...nsmode.se]
> Sent: Monday, August 08, 2011 6:11 AM
> To: George Spelvin
> Cc: akpm@...ux-foundation.org; fzago@...temfabricworks.com; linux-
> kernel@...r.kernel.org; rpearson@...temfabricworks.com
> Subject: Re: [PATCH] add slice by 8 algorithm to crc32.c
> 
> "George Spelvin" <linux@...izon.com> wrote on 2011/08/08 12:52:01:
> >
> > > I prefer to keep the current code which (at the time) generated good
> code
> > > for at least ppc:
> > >    /* Align it */
> > >    if (unlikely((long)buf & 3 && len)) {
> > >       do {
> > >          DO_CRC(*buf++);
> > >       } while ((--len) && ((long)buf)&3);
> > >    }
> >
> > Ah, I was looking at fzago's initial patch; I hadn't realized you'd
> > tweaked it.  That's pretty much what I was talking about.
> >
> > Would
> >     if (unlikely((long)buf & 3) && len) {
> >
> > give the compiler better hints?  len != 0 is awfully
> > likely, actually.
> 
> Doesn't matter on ppc(gcc 4.4.4). The whole while loop is moved out of
line
> in both cases and the generated asm is identical.


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