[vorbis-dev] Optimization and doubles vs. floats

[Segher Boessenkool]

> 
> >The mdct will be ok using floats. Don't know if all the rest of vorbis likes
> >floats, though.
> 
>   Well, that is sort of what I'm wondering.  I don't want to have to take  
> doubles as input to the MDCT, convert them to floats, do the operations,  
> and convert back to doubles.  It sounds like I can just always use float,  

You would only have to convert from double to float once; after the
mdct, floats are ok as well (decoder side, of course).

> though (at least according to Taral).
> 
> >Which algo are you planning to use for the mdct? Oh, and Altivec is 128 bit,
> >right?
> 
>   I'll probably start with what Takehiro Tominaga mailed out (haven't  
> looked at it yet) or whatever ends up on the head of the repository.  Yeah,  
> Altivec is 128 bit, so you can issue 4 floating point ops (including  

Those 4 ops have to be the same, and the data must be in the right order
in your registers (or I don't know Altivec well enough; that's quite
realistic).

Because of this, the choice of mdct algo gets different.
Even a plain matrix mult can be fast, you know...

> mult/add) per cycle, assuming you don't run out of memory bandwidth,  
> completion slots, etc.

Segher

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