[eigen] Low performance in DEBUG mode

[Mathias Goldau <math@xxxxxxxxxxxxxxxxxxxxxxxxx>]

Hi,

today I encountered a really strange thing: I compiled my unittest which need
in Release mode just about 3 sec, in Debug mode. It uses just some matrix
vector multiplication and the inverse computation. However in Debug mode it
needs about 3:32 minutes!!
A colleague of mine told me this could have to do with expression templates,
and as I've seen in the "We need more Gaels"-thread that Eigen uses a lot of
expression templates, I want to ask: Is this assumption true?, is there a way
around to compile my unit tests also in debug mode, but with a bit more
performance in the end?
Even setting our cmake build type to: RelWithDebugInfo resulting effective in
those CXX flags: "-g -DDEBUG -O2" shows no improvement. I assume this depends
on the "-DDEBUG", am I right?

best regards
Mathias

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