Re: [eigen] Decrease in performances

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 25.06.2012 16:36, Maxime REIS wrote:
> Hi,
>
> Everything is fine up to the fix of 'Bug 363' :
> - 4281:25ba289d5292 runs fast
> - 4282:a07628375b86 doesn't compile
> - 4283:f76f77b7d086 is sloooooooow
> The bug fix apparently slowed down the whole thing, and the perfs were
> improved again in c29d777b278c (some 40 or so commits after the decrease),
> but not to the point they were before the bug fix.
> I guess it's the price to pay to have a robust library. From that point on,
> all subsequent commits don't seem to affect overall performances (at least
> from my observations)
>
> I don't know if it deserves further investigation...

It seems the performance loss is due to the integer division which 
checks for overflow.
I guess it can easily be replaced by 128bit integer multiplication. I 
suggested that back then, but I guess it was ignored due to portability 
concerns:
http://listengine.tuxfamily.org/lists.tuxfamily.org/eigen/2011/10/msg00041.html

Christoph



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Dipl.-Inf. Christoph Hertzberg
Cartesium 0.049
Universität Bremen
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