You can see that the STG code just reflects the original Haskell source code and that the faulty register initialization has not yet been dropped down to within the loop(s), so the problem is not here. The problem is in
#12808: For primitive (Addr#) operations, Loop Invariant Code Flow not lifted outside the loop... -------------------------------------+------------------------------------- Reporter: GordonBGood | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): the generation of the first CMM Aha! Could you possibly make the tiniest possible example that illustrates precisely this point. You can motivate its importance by this thread, but in thinking about how to fix it, it's MUCH easier to grok a small example. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12808#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler