Re: [eigen] alignment issues, conversion between aligned / unaligned data, unaligned Eigen::Rotation2D

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 2018-08-20 15:14, bubnikv wrote:
> Hello.
>
> We are developing a 3D printing software https://github.com/prusa3d/slic3r
> (fork of http://slic3r.org/)
>
> We are now trying to replace the homebrew point classes with the Eigen
> fixed size types. As the code base is quite large, we want to avoid
> alignment issues by declaring fixed vector types with the Eigen::DontAlign
> attribute. So far so good, with the exception of Eigen::Rotation2D, which
> is not templated with the alignment attribute, so it cannot be used against
> the fixed size matrices with Eigen::DontAlign. Is there any reason for
> that? Would you guys please extend the class with the alignment template
> attribute?

Rotation2D stores just a single Scalar (the angle), so there is no 
need/possibility to align it.
Alternatively, you could store a 2D rotation as a 2x2 matrix, or as a 
complex number (neither of them inherits from RotationBase, though).


> We are thinking of using https://github.com/libigl/libigl for some
> operations on triangle meshes, where the vectorized operations may or may
> not be beneficial. We may just disable alignment / vectorization for the
> whole application. We are not sure about the performance penalty, but
> looking at the evaluation of Eigen 2 from 2008
> http://eigen.tuxfamily.org/index.php?title=Benchmark-August2008
> the loss may be significant.

On modern CPUs the difference between aligned and not-aligned is far 
less or even non-existing (except for effects like crossing 
cache-lines). And with Eigen 3.3 it is possible to use vectorization on 
unaligned data (enabled by default):
http://eigen.tuxfamily.org/bz/show_bug.cgi?id=256#c10

So make meaningful benchmarks on the CPU you want to target to decide if 
you need to invest time into that.

> We may then think about converting between the aligned / non aligned
> values. Does Eigen support any conversion between aligned / unaligned
> types? I know about the .cast<>() operator, but it does not allow me to
> change the alignment attribute.

You can simply copy in either direction, or you can mix aligned and 
non-aligned data inside expressions. No need to cast here.



Christoph


> Thanks for your time,
> Vojtech

--
 Dr.-Ing. Christoph Hertzberg

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