Re: [eigen] Matrix multiplication seems to be exceptionally slow in one specific case

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 16.06.2014 17:02, Marc Glisse wrote:
> On Mon, 16 Jun 2014, Christian Seiler wrote:
>
> > There has been a proposal for 'operator auto' in C++ because of this,
> > but there doesn't seem to be enough traction for it.
>
> The latest related paper is:
>
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4035.pdf
>
> and the committee will discuss it this week. If you have any comments,
> please feel free to post them on:
>
> https://groups.google.com/a/isocpp.org/forum/#!forum/std-proposals
>
> There is some resistance to this kind of change, so any extra
> information (use cases, requirements, suggestions, etc) helps, not least
> because it shows interest in the proposal.

While I like the idea in general, I'm not totally sure about that 
either. This can bring very strange performance differences when 
switching between different C++ versions.
In some cases it might even degrade performance, e.g.:

  auto AB = A*B;          // shall this evaluate into a temporary?
  MatrixXd result = AB+C; // actually, no explicit temporary AB required
                          // direct evaluation would have been better

What would also be confusing is that passing a product to
  template<class Matrix> void foo(const Matrix&);
will not do automatic 'auto'-conversion, but this would:
  auto X = expression;    // auto-conversion
  foo(X);                 // converted X
  foo(expression);        // no conversion

I think the best thing we can do on our side (and we can do that 
independently of changes in the standard) is to warn users about 
pitfalls with the auto-keyword. E.g., we could create a doxygen-page for 
that and link it prominently (from all starting guides, etc).

I posted a bug for that:
http://eigen.tuxfamily.org/bz/show_bug.cgi?id=825

Christoph


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