include the new -flate-dmd-anal in -O2?

[Edward Z. Yang]

Perf builds of GHC also use -O2 for ghc-stage2, so check out what happens
to GHC itself with late demand analysis.

Edward

Excerpts from Nicolas Frisby's message of Fri Aug 30 10:28:24 -0700 2013:
> TO: Performance czars and devs
> 
> I pushed a patch yesterday enabling a second demand analysis at the end of
> the core2core simplification pipeline. The flag is -flate-dmd-anal, and it
> is off by default.
> 
> My question:
> 
>     What's the protocol for deciding if -O2 should imply it?
> 
> See http://ghc.haskell.org/trac/ghc/wiki/LateDmd for context.
> 
> In particular, this section includes highlights of some nofib runs I did.
> 
>   http://ghc.haskell.org/trac/ghc/wiki/LateDmd#Newperformancenumbers
> 
> For some tests, it decreases allocation by 10% to 20%. But on the platforms
> I have tried, it causes a couple repeatable slowdowns, up to 10%. I've
> investigated a bit, but haven't found any clear explanations. I'm worried
> that it's caching effects, eg.
> 
> Any suggestions on how I should proceed with my investigation?
> 
> Also: I'd appreciate if any developer would generously run some benchmarks
> on various platforms they might have and add them to the same section in
> the wiki page.
> 
>   http://ghc.haskell.org/trac/ghc/wiki/LateDmd#Newperformancenumbers
> 
> NB That it is unfortunately key to build the libraries twice: once with
> -flate-dmd-anal in GhcLibHcOpts and once without. I have not determined how
> to do this robustly without a distclean — please let me know if you have a
> better method.
> 
> So I've used
> 
> # one of the following
> #GhcLibHcOpts    = -O2  # both with and without -flate-dmd-anal
> GhcLibHcOpts    = -O2 -flate-dmd-anal
> SplitObjs          = NO
> DYNAMIC_BY_DEFAULT   = NO
> DYNAMIC_GHC_PROGRAMS = NO
> 
> The last three aren't necessary, but please record what you use, if you are
> so generous as to run it :).
> 
> Thanks.