GHC compiled program slower with -O1 than -O0

[Simon Marlow]

Neil Mitchell wrote:
> Hi Simon
> 
>> One thing to check is heap usage and GC time. Does GC time, or 
>> residency (both reported by -Sstderr)  go up a lot?  It's possible 
>> that an optimisation is changing space behaviour for the worse.
> 
> I've attached the logs for both -O0 (norm) and -O1 (opt), it appears
> that there are 4 times more garbage collections with -O1, so thats
> likely to be it.
> 
>> Does it still happen if you use profiling?  If so, the profile should 
>> narrow down the culprit a lot.  That might avoid the need to reproduce 
>> it elsewhere.
> 
> Profiling with -O1 is faster than -O1 and -O0, so turning on profiling
> fixes whatever is going wrong.
> 
> The darcs repo for the code is at:
> 
> http://www.cs.york.ac.uk/fp/darcs/catch/
> 
> This is the catch_1 directory. Unfortunately it will be a nightmare to 
> build...

There are two things that I know of that can cause this.  The first is the bug 
that affects calendar from nofib:

   http://hackage.haskell.org/trac/ghc/ticket/1168

and the second is full-laziness, which can give your program a space leak if you 
compile with -O.  You'll be able to tell if it's a space leak problem by looking 
at the "+RTS -sstderr" output and checking the GC time.

Cheers,
	Simon