Subject: Re: denormal floats (was Re: [linux-audio-dev] XAP spec - early scribbles)
From: Dave Griffiths (dave@pawfal.org)
Date: Thu Feb 06 2003 - 17:46:10 EET
On Thu, 6 Feb 2003 15:28:00 +0000, Steve Harris wrote
> On Thu, Feb 06, 2003 at 03:59:31 +0100, Dave Griffiths wrote:
> > On Thu, 6 Feb 2003 14:07:06 +0000, Steve Harris wrote
> > > On Thu, Feb 06, 2003 at 12:18:47 +0100, Dave Griffiths wrote:
> > > > I had a vague attempt at doing something like this (after noticing that
> > > > filters filtering silence uses up a lot of cpu). Each sample buffer object
> > >
> > > OT: Thats probably because the zeros weren't 0.0, they were probably
> > > denormal numbers. If you squash them the filter will run at the same
> > > cost as when its processing any other data.
> >
> > ahah, I was hoping for an explanation :) any ideas on how to combat this, what
> > the squashing threshold should be?
>
> I use:
>
> #define FLUSH_TO_ZERO(fv) (((*(unsigned int*)&(fv))&0x7f800000)==0)?0.0f:(fv)
> I think it came from the music-dsp list.
>
> Its uses pointer casting, which is mildly evil, but C doesn't have a
> non-interprting cast like C++ has, I cant remeber the C++ syntax,
> but it should be easy to convert.
>
> It looks for the denormal pattern in the IEEE bitpattern and zeros
> the float if its there IIRC. You probably can't do the comparison
> with < as that would cause the denormal to be evaluated.
Is this processor specific? I used to get it loads on my PII desktop, but I
haven't noticed it as much my PIII machine (might just be because it's twice
the speed).
Cheers for the info guys, I haven't come across this before (day job=games
programming)
dave
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