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

[Michael Riesch <michael.riesch@xxxxxx>]

Hello all,

Thank you very much for your work on Eigen. We found it very useful for 
our simulation software mbsolve [1] (BTW maybe you would like to add it 
to the projects list that uses the Eigen library).

The code I am working on at the moment consists mostly of dense 
matrix-matrix and matrix-vector multiplications. I compiled the code 
with both Intel compiler 19 and gcc 6.3.0 and found that there is a 
strange performance difference. Unless I define

#EIGEN_STRONG_INLINE inline

the binary compiled by icc is ~13x slower. The gcc binary performance 
remains the same, as inline seems to be the standard setting of this 
macro for gcc.

Why can this behavior occur? Or, alternatively, which possible 
anti-pattern could be the cause of this performance difference?

Any hints are welcome. If you need more information, please let me know.

Thanks in advance and best regards,
Michael

[1] https://github.com/mriesch-tum/mbsolve