Re: [Qemu-devel] [PATCH 12/15] openpic: IRQ_check: search the queue a word at a time

[Scott Wood <scottwood@freescale.com>]

On 01/03/2013 12:53:13 PM, Alexander Graf wrote:
> 
> On 22.12.2012, at 03:15, Scott Wood wrote:
> 
> > Search the queue more efficiently by first looking for a non-zero  
> word,
> > and then using the common bit-searching function to find the bit  
> within
> > the word.  It would be even nicer if bitops_ffsl() could be hooked  
> up
> > to the compiler intrinsic so that bit-searching instructions could  
> be
> > used, but that's another matter.
> >
> > Signed-off-by: Scott Wood <scottwood@freescale.com>
> 
> What we really want is a bitmap wide ffs() bipops helper function  
> that returns the first set bit in a bitmap and can optimize the hell  
> out of that operation inside of itself. I don't think this belongs to  
> the OpenPIC code.

Well, we do have find_next_bit() in bitops.c, but it looks  
comparitively complicated in order to be generic and simply return a  
value rather than perform an action on each bit set.  I suspect that  
the code in this patch would be faster, and avoids the need for me to  
follow all the twists and turns of find_next_bit() to figure out  
whether the undocumented interface is actually exactly what I guess it  
to be (e.g. what does it return when no bit is found?).

-Scott