GHC lazy eval optimization bug
haskell at list.mightyreason.com
Sun Feb 3 15:34:41 EST 2008
Thanks Don. That is the issue. I just added this comment/request to trac #1168:
> This just came up on the mailing list again. The program was slow unless
> (-fno-state-hack) was used.
> Is it possible to have GHC emit a warning (as part of -Wall) to let the use
> know that GHC is applying a potentially risky simplification?
I had tried -Wall before you told me about -fno-state-hack. If -Wall had warned
me about the hack then I could have understood the problem without a frequently
asked question to the list.
Don Stewart wrote:
>> I can confirm this performance bug with GHC version 6.6.1 on OS X PCC (G4)
>> It performs correctly, lazily keeping 'primes' for all 4 repetitions, when
>> using "ghc -Onot" but the the doSlow fails to retain 'primes' when
>> compiling with "ghc -O" and "ghc -O2"
> Sounds like the known issue of the simplifier eta-expanding `State#`
> lambdas on purpose, which is good for IO, but risks re-computation. The
> 'calendar' nofib program exhibits this, iirc.
> Try compiling with:
> -- Don
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