Forcing apps to collect GC stats?

[Bryan O'Sullivan]

I spent a bit of time over the weekend trying to figure out how to force
the RTS to collect GC statistics, but was unable to do so.

I'm currently working on enriching criterion's ability to gather data,
among which I'd like to see GC statistics. If I try to obtain GC stats
using criterion when I'm not running the benchmark app with +RTS -T, I get
an exception.

Is there a way to allow criterion to forcibly enable stats collection? My
efforts to do so have gotten me nowhere. It would be unfortunate if I had
to tell users of criterion that they should always run with +RTS -T or add
a -rtsopts clause, as they'll simply forget.

And while I'm asking, why does GHC not simply collect GC stats by default?
Collecting them seems to have zero cost, from what I can see?

Thanks!
Bryan.
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