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

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 2018-08-20 18:32, bubnikv wrote:
> [...]
> Just for reference, I am using Visual Studio 2013 compiler, 64bit platform,
> and Eigen 3.3.5.
> [...]
> The issue I have with Rotation2D is the following:
>
>          Eigen::Matrix<double, 2, 1, Eigen::DontAlign> v, w;
>          Eigen::Rotation2Dd rot(3.);
>          // Following fails to compile:
>          v = rot * w;
>          // Explicit temporary helps:
>          v = rot * Eigen::Matrix<double, 2, 1>(w);

This should work (testing with Eigen 3.3.4 and MSVC2015):
https://godbolt.org/z/EFv5ok

Maybe something else causes an error or it is a defect in MSVC2013. 
Could you post the compile error you get (and a complete minimal source 
you tried to compile + compilation flags)?
Maybe we find a workaround for that.


Christoph



> So thanks for the hint about I understand that there is always a way to
> rotate the vector. I am just pointing out, that as a user of a finely
> polished library, I would expect the Rotation2D to just work with both
> aligned and non-aligned types.
>
> Gruesse nach Bremen,
> Vojtech
>
>
> On Mon, Aug 20, 2018 at 3:37 PM, Christoph Hertzberg <
> chtz@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> > On 2018-08-20 15:14, bubnikv wrote:
> >
> > > Hello.
> > >
> > > We are developing a 3D printing software https://github.com/prusa3d/sli
> > > c3r
> > > (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
> >
> >   Besuchsadresse der Nebengeschäftsstelle:
> >   DFKI GmbH
> >   Robotics Innovation Center
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> >   28359 Bremen, Germany
> >
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> >   Robotics Innovation Center
> >   Robert-Hooke-Straße 1
> >   28359 Bremen, Germany
> >
> >   Tel.:     +49 421 178 45-4021
> >   Zentrale: +49 421 178 45-0
> >   E-Mail:   christoph.hertzberg@xxxxxxx
> >
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--
 Dr.-Ing. Christoph Hertzberg

 Besuchsadresse der Nebengeschäftsstelle:
 DFKI GmbH
 Robotics Innovation Center
 Robert-Hooke-Straße 5
 28359 Bremen, Germany

 Postadresse der Hauptgeschäftsstelle Standort Bremen:
 DFKI GmbH
 Robotics Innovation Center
 Robert-Hooke-Straße 1
 28359 Bremen, Germany

 Tel.:     +49 421 178 45-4021
 Zentrale: +49 421 178 45-0
 E-Mail:   christoph.hertzberg@xxxxxxx

 Weitere Informationen: http://www.dfki.de/robotik
 -----------------------------------------------------------------------
 Deutsches Forschungszentrum fuer Kuenstliche Intelligenz GmbH
 Firmensitz: Trippstadter Straße 122, D-67663 Kaiserslautern
 Geschaeftsfuehrung: Prof. Dr. Dr. h.c. mult. Wolfgang Wahlster
 (Vorsitzender) Dr. Walter Olthoff
 Vorsitzender des Aufsichtsrats: Prof. Dr. h.c. Hans A. Aukes
 Amtsgericht Kaiserslautern, HRB 2313
 Sitz der Gesellschaft: Kaiserslautern (HRB 2313)
 USt-Id.Nr.:    DE 148646973
 Steuernummer:  19/672/50006
 -----------------------------------------------------------------------