From owner-freebsd-hackers@FreeBSD.ORG  Mon Jun 13 18:31:11 2005
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Date: Mon, 13 Jun 2005 18:27:43 +0200
From: Joerg Sonnenberger <joerg@britannica.bec.de>
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Subject: Re: Obvious bug in /sys/i386/include/bus.h (was: bus_at386.h)
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On Mon, Jun 13, 2005 at 05:58:24PM +0200, Hans Petter Selasky wrote:
> static void
> filter(struct fifo *f)
> {
>   do {
>      if(!f->len)
>      {
>          if(f->m) ...;
> 
>          f->m = get_mbuf();
>          if(!f->m) break;
> 
>          f->len = m->m_len;
>          f->ptr = m->m_data;
>      }
> 
>      /* f->Z_chip is the maximum transfer length */
> 
>      io_len = min(f->len, f->Z_chip);

	if (io_len == 0)
		continue;

> 
>      bus_space_write_multi_1(t,h,xxx,f->ptr,io_len);
> 
>      f->len -= io_len;
>      f->Z_chip -= io_len;
>      f->ptr += io_len;
> 
>   } while(Z_chip);
> }
> 
[...]

> Adding that extra check for zero transfer length is not going to affect 
> performance at all. If one does that using "C", the compiler can optimize 
> away that "if(count) ..." when "count" is a constant. Besides the i386 
> machine instructions "ins" and "outs" already exhibit that behaviour.

The compiler can only optimize it away if it is known to be a constant.
But thinking again about it, it should be documented at least whether
a count of 0 is allowed or not. I think it makes more sense to not allow
it, but others (you included) might disagree.

Joerg