Re: [eigen] Performance difference icc <-> gcc, EIGEN_STRONG_INLINE

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

Hi!

On 14/03/2019 05.08, Michael Riesch wrote:
> > We could easily remove EIGEN_COMP_ICC from the cases where 
> > EIGEN_STRONG_INLINE is a __force_inline:
> > https://bitbucket.org/eigen/eigen/src/default/Eigen/src/Core/util/Macros.h#Macros.h-755 
> >
> >
> > But I don't know the history of this, i.e., if it was necessary for 
> > older ICC versions. Or if it is necessary at other places.
> Same here, hence I wanted to ask whether defining EIGEN_STRONG_INLINE 
> has some disadvantages. According to the Intel profiling tools, my 
> function that contains all the calculations actually calls the Eigen 
> functions (e.g., Eigen::Matrix<...>::Matrix<...> and 
> Eigen::Matrix<...>::operator=). I expected that those will be inlined. 
> Is this something to worry about?

I think what you finally need to do is to look at the assembly generated 
from (the performance critical parts of) your program.
Defining `EIGEN_STRONG_INLINE` as `inline` should be save regarding 
functionality, but I can't give any promises on the performance impact 
with ICC, especially with different compile flags.

> Maybe I should add that I do not use the -inline-forceinline flag for 
> the Intel compiler since this lead to incredibly long compilation times.

Interesting. It could be that (if inactive) this flag implies that 
`__forceinline` is essentially evaluated to nothing.
Perhaps for ICC we should do by default:
  #define EIGEN_STRONG_INLINE __forceinline inline


> [...]
> I think "Science" is the best fit, since mbsolve [1] "is an open-source 
> solver tool for the Maxwell-Bloch equations, which are used to model 
> light-matter interaction in nonlinear optics."

Done!

Cheers,
Christoph


> Thanks a lot for any comments!
>
> Regards,
> Michael
>
> [1] https://github.com/mriesch-tum/mbsolve

--
 Dr.-Ing. Christoph Hertzberg

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