Re: [eigen] generic unrollers

[Benoît Jacob <jacob@xxxxxxxxxxxxxxx>]

Hi,

attached are my own measurements. Intel Core 1 (32bit), g++ 4.3.0, compiled 
with "-O3 -DNDEBUG", that is without vectorization, to make things more fun 
since you already measured with vectorization. 

As you can see, here, the overall winner is "<=" but not by a big margin. It 
looks like we have yet to find the definitive formula...

Cheers,
Benoit

On Monday 16 June 2008 00:07:22 Gael Guennebaud wrote:
> >> , for the a+b and 2*a cases I'll
> >> write an exhaustive benchmark... If there is no obvious reason to eval
> >> a+b for a 2x2 product then it might be better to not eval since this
> >> allows the user to perform fine tuning for his specific case that is not
> >> possible if we do (abusive?) evaluation.
> >
> > It's great if you do a benchmark, I don't see any other way of moving
> > forward!
>
> here you go (see attached files). So M,N,K denotes the size of the
> matrix product:
>
> MxN  = MxK  *   KxN.
>
> I benchmarked  both (a+b)*c and (2*a)*c,  with 4 different conditions:
> the current one with "<", the same with "<=", never evaluate, and
> evaluate if N>1 (e.i. if a coeff is read at least twice). I compiled
> with gcc-4.2, -O3 -DNDEBUG using float and vectorization enabled.
>
> So for this benchmark it is quite clear that, as expected, "<=" works
> much better than the current "<".  But surprisingly, N>1, which
> implies the evaluation of (2*a) with N==2 works even slightly better !
>  This is probably because the compiler can cache the temporaries into
> the registers (I have a 64bits CPU, so 16 SSE registers). In that case
> counting for the extra loads and stores is wrong. So we could try this
> one:
>
>     r*SC <= (r-1) * RC
>
> which basically means let's forget the extra store and evaluate even
> if it does not look really better (equality).  In practice this should
> give better results (at least for gcc-4.2 with a lot of floating point
> registers).
>
> Gael.

Attachment: EigenCostModel_benoit.ods
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