[linux-audio-dev] swh plugins and fixing undenormalize

[Tim Goetze]

[Tim Blechmann]
>On Fri, 25 Jun 2004 17:38:24 -0500
>Jan Depner <eviltwin69 at cableone.net> wrote:
>
>> On Fri, 2004-06-25 at 13:49, Tim Blechmann wrote:
>> > > I have a denormal fix without a branch but you probably don't want
>> > > to see it ;-)
>> > >  It's pretty simple, just OR the bits of the exponent together
>> > >  which gives either
>> > > 0 (denormal) or 1, typecast that to float, and then multiply the
>> > > original float by that (0.0 or 1.0).  Voila, no branch, but it is
>> > > messy looking ;-)

indeed sounds more like a fun proposal; nonetheless i'm wondering how
many cycles 'ORing the exponent bits together' would take. with an
8-bit exponent and no assumption about its value made, 8 binary
'shift', 7 'or' and 1 'and' statement if i'm not badly mistaken. and
if i'm not, a branch will probably hurt less.

(on a related note, is it true that the P4 units have a weakness at
binary shift operations?)

>> 	The definition of denormal is that the exponent is 0 so you will
>> 	never
>> multiply a denormal by 1, only by 0.  I'm not sure whether that would
>> be a denormal operation or not.  It depends on the compiler.

i'd expect the FPU to shortcut a multiplication by zero no matter if
the other operand is denormal or not, though i'm not too sure.

>hm ... someone should write a test for all these algorithms ... i'm
>curious, how different compilers / different algorithms actually affect
>the speed of the code ...

i think that the best denormal-avoiding approach depends on the
DSP algorithm in question and the extents of your demand for
computational accuracy and S/N ratio. lots of tradeoffs are possible.

tim