[eigen] enabling OpenMP in only a subset of a library

[Jim Bosch <talljimbo@xxxxxxxxx>]

We have a large software framework that uses Eigen in a lot of places 
(many libraries linked together; often these are linked into Python 
modules). We're almost always using Eigen in single-threaded processes 
that can only use a single core (because any other cores will be busy 
with other similar processes), and we're quite happy just using Eigen 
without any threading or OpenMP options.

However, we also have a larger problem for which we'd like to turn on 
OpenMP support in Eigen, and utilize multiple cores in a single process 
- but we want to be able to link the OpenMP-enabled code against the 
rest of the framework that uses Eigen without any kind of threading.

As I understand it, that's not possible to do naively, as it means we 
end up defining some inline functions (e.g. nbThreads) in different ways 
in different compilation units, and that's Undefined Behavior.

Is there any way to make this work without having to recompile all the 
non-OpenMP code with OpenMP support while calling setNbThreads(1) to 
disable it until we want it?  Or other recommendations on how to 
accomplish what we want to do here?

Thanks in advance!


Jim