[linux-audio-dev] Traps in floating point code

[Pete Bessman]

At Wed, 30 Jun 2004 16:37:59 +0100 (BST),
Mike Rawes wrote:
> 
> --- Pete Bessman <ninjadroid at gazuga.net> wrote: 
> > At Mon, 28 Jun 2004 19:56:50 +1000,
> > Erik de Castro Lopo wrote:
> > > 
> > >     double fractional = 0.0, increment = 0.1;
> > >     int integer = 0;
> > > 
> > >     for (;;)
> > >     {   
> > >         /* Bunch of other code. */
> > > 
> > > 	fractional += increment ;
> > >         integer += lrint (floor (fractional));
> > >         fractional -= floor (fractional);
> > >         }
> > 
> > I don't understand this lrint (floor ()) stuff.  floor() returns a
> > truncated integer value, so what's the point of calling lrint() on
> > that?
> 
> The lrint converts the float value to an int (a lot) faster than a
> simple cast.

Ah.  Now I get to find all the places in my code that can benefit from
that.  If you assign a float value into an int variable, is that an
implicit cast?

> The floor is to get around the rounding rule used by the FPU when the float
> value is exactly halfway. It rounds to even numbers, at least it does on an
> AthlonXP:

Good to know, thanks for the tip.  Although, just using lrintf(val)
would be wrong anyway because it would round up if fmod(val, 1.0) >
0.5, right?

-Pete

www.gazuga.net