Re: [PATCH] PPC64 iSeries: cleanup viopath

[Hollis Blanchard]

On Mar 15, 2005, at 9:53 AM, Stephen Rothwell wrote:

> On Tue, 15 Mar 2005 08:32:27 -0600 Hollis Blanchard 
> <hollis@xxxxxxxxxxxxxx> wrote:
> > On Mar 14, 2005, at 9:34 PM, Stephen Rothwell wrote:
> > > Since you brought this file to my attention, I figured I might as 
> > > well
> > > do
> > > some simple cleanups.  This patch does:
> > > 	- single bit int bitfields are a bit suspect and Anndrew pointed
> > > 	  out recently that they are probably slower to access than ints
> >
> > > --- linus/arch/ppc64/kernel/viopath.c	2005-03-13 04:07:42.000000000
> > > +1100
> > > +++ linus-cleanup.1/arch/ppc64/kernel/viopath.c	2005-03-15
> > > 14:02:48.000000000 +1100
> > > @@ -56,8 +57,8 @@
> > >   * But this allows for other support in the future.
> > >   */
> > >  static struct viopathStatus {
> > > -	int isOpen:1;		/* Did we open the path?            */
> > > -	int isActive:1;		/* Do we have a mon msg outstanding */
> > > +	int isOpen;		/* Did we open the path?            */
> > > +	int isActive;		/* Do we have a mon msg outstanding */
> > >  	int users[VIO_MAX_SUBTYPES];
> > >  	HvLpInstanceId mSourceInst;
> > >  	HvLpInstanceId mTargetInst;
> >
> > Why not use a byte instead of a full int (reordering the members for
> > alignment)?
>
> Because "classical" boleans are ints.
>
> Because I don't know the relative speed of accessing single byte 
> variables.

I didn't see the original observation that bitfields are slow. If the 
argument was that loading a bitfield requires a load then mask, then 
you'll be happy to find that PPC has word, halfword, and byte load 
instructions. So loading a byte (unsigned, as Brad pointed out) should 
be just as fast as loading a word.

> It really makes little difference, I was just trying to get rid of the
> silly signed single bit bitfields ...

I understand. I was half being nitpicky, and half wondering if there 
was an actual reason I was missing.

-Hollis

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/