[Rivet] Eigen2

[Hendrik Hoeth]

Thus spake Frank Siegert (frank.siegert at durham.ac.uk):

> Andy Buckley, Tuesday 03 November 2009:
> 
> > We can certainly install these headers... I'd forgotten that we
> > were previously installing the eigen headers. I'll add something
> > equivalent for eigen2.
> 
> Is that really something that should live in the Rivet source tree?

I vote against it. We've failed before with that kind of experiment
(bundling boost with Rivet), and I really don't see why we need a
multi-MByte replacement for some few-kByte headers that we had before.

> If we were to make it external: Which Rivet features depend on it? I
> have to admit that I wasn't aware of eigen being used in and part of
> Rivet before this change.

I didn't know that either, so I'm also interested in an answer to this
question. And I want to add another one: How much of an performance
improvement is it really to switch from eigen-1 to eigen-2? How much
time do we actually spend in matrix operations?

If upgrading to eigen2 indeed has a large impact, then I'd still add it
as external requirement rather than bloating the Rivet package with it.
If the improvement is not dramatic, I'd keep eigen rather than
introducing a new dependency.

> While talking about not finding headers needed to compile with
> Rivet...  shouldn't we add the -I/path/to/boost/include and
> -I/path/to/eigen2 to rivet-config --cppflags?

I guess that can't hurt ...

Cheers,

    Hendrik

-- 
"Two equal numbers are equivalent, even if they come from two
 different people one of which you don't like"
(MLM about the theoretical top cross-section, in a meeting
about theoretical uncertainties on the top mass)