GHC compiled program slower with -O1 than -O0

[Simon Peyton-Jones]

| I've written a Haskell program which goes slower with -O1 (19 seconds)
| than -O0 (12 seconds). Is this considered to be a bug? How interested
| are you in tracking this down?

Yes, very interested.  Should not happen.

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.

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.

Simon