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

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Date:	Mon, 8 Aug 2011 13:11:14 +0200
From:	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>
To:	"George Spelvin" <linux@...izon.com>
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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