Re: [eigen] help speeding up an expression?

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 02.08.2013 23:25, Dick Lyon wrote:
>    z += 0.175;
>    z = (z < 0).select(0.0, (z*z*z) / (z*z*z + z*z + 0.1));

I assume the bottleneck is the division here. I guess we could improve 
Eigen to use SSE's fast inverse with refinement for float divisions if 
FAST_MATH is enabled.

There might be further small improvements, like re-using the z*z and 
z*z*z part (but this should be done on the fly, without a big temporary 
array). And a very small improvement: (z<0).select(0,x) could be 
replaced e.g. by something like ((z>=0.0) & x).

If you really want to fix that bottleneck, I recommend to implement a 
custom unary expression with hand-optimized SIMD code.

> Can this be sped up by clever use of pre-allocated temp arrays?

I doubt that will help a lot.

> Is select fast?  Or are there better ways to threshold things?

Currently it's not vectorized, so you lose some time there as well (but 
most likely it will be optimized in future Eigen versions).

> I'm on an Intel platform, but ultimately also targeting Arm NEON, in case
> that matters.

I don't have any ideas about NEON's SIMD capacity.


Regards,
Christoph



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Dipl.-Inf., Dipl.-Math. Christoph Hertzberg
Cartesium 0.049
Universität Bremen
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