[Bug target/84280] [6/7/8 Regression] Performance regression in g++-7 with Eigen for non-AVX2 CPUs

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84280

--- Comment #12 from Richard Biener <rguenth at gcc dot gnu.org> ---
Hmm, the preprocessed source(s) are hard to work with given the eigen headers
seem to have conditional code on the enabled ISAs.

>From a quick look it seems to be inlining related?  My past experience says
that compute kernels in C++ should have the flatten attribute attached to
them...

Did you try with FDO?  (-fprofile-generate, run, -fprofile-use)