On Nov 5, 2007, at 16:21 , Alex Young wrote:
C:\Users\Alex\Documents\HaskellLearning\MonteCarlo>ghc BetterPi.hs
C:\Users\Alex\Documents\HaskellLearning\MonteCarlo>main.exe 1000000 Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it.
But:
C:\Users\Alex\Documents\HaskellLearning\MonteCarlo>ghc -O2 BetterPi.hs
C:\Users\Alex\Documents\HaskellLearning\MonteCarlo>main.exe 1000000 3.140636
This is a little confusing. Is there a simple explanation for this behaviour, or is it just a matter of "always use -O2 unless there's a reason not to?"
Basically, one of the optimizations enabled by -O2 causes ghc to notice that it doesn't need to collect a bunch of thunks on the stack, but instead can use them as they're generated. ("fusion" --- in this case probably some build/fold fusion) I would indeed say that in most cases you want to use -O2 just to get the smarter behavior, unless you're trying to learn how to write efficient code to start with; but on the other hand, it's nice to be able to write *readable* code and have the compiler figure out how to make it efficient. (Much the same goes for C, by the way; I can write code to the bare metal, or write comprehensible code and let cc work out how to make it fast. Unless I (a) absolutely need the performance and (b) know the optimizer's not smart enough to do it for me, I'll go for readable instead of e.g. Duff's Device.) -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH