On Wed, 2010-05-05 at 21:24 +1000, Roman Leshchinskiy wrote: > Whenever I do cabal sdist on one of my projects, I get this warning: > > Distribution quality warnings: > 'ghc-options: -O2' is rarely needed. Check that it is giving a real benefit > and not just imposing longer compile times on your users. > > This finally got me curious and I did a nofib run to compare -O to > -O2. The results are below (this is with the current HEAD). > > Is there a real-world example of -O2 causing significantly longer > compile times without providing a real benefit? If not, would it > perhaps make sense for Cabal to use -O2 by default or even for GHC to > make the two flags equivalent?
It should be -O1 for default/balanced optimisations and -O2 for things involving a bigger tradeoff in terms of code size or compile time. so any optimisations in -O2 that GHC HQ believe are a no-brainer for the majority of packages should be moved into -O1. It's fine for people writing performance sensitive code to use -O2 in their packages. It's just not something we need to encourage for random packages. Before we added that warning, many package authors were not really thinking and just chucking in -O2 "because 2 is bigger than 1 so it must be better right?". There certainly used to be packages that took longer to compile, generated more code, and ran slower when using -O2. That was some time ago of course. Duncan _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
