[eigen] Re: small sums: vectorization not worth it

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Note: i think the old heuristic was wrong anyway.

Maybe take this occasion to introduce a EIGEN_COST_OF_PREDUX (since
this cost depends greatly on the simd platform) ?
And then do a natural heuristic rather than a quick hack like we used to have?


2009/1/16 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
> Hi Gael *cough* List,
> ei_predux is costly because it consists of >1 SIMD instruction.
> So until recently we had sum() only vectorize if the size was big enough.
> However this was recently changed.
> Attached is a benchmark that runs 2.5x slower with SSE (2 or 3) than
> without. It's just Vector2d::sum().
> So, revert to old behavior?
> Moreover: matrix product innerVectorization also uses a ei_predux. Same here?
> Cheers,
> Benoit

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