On 10/05/2010 03:43, Roman Leshchinskiy wrote:
On 09/05/2010, at 07:50, Duncan Coutts wrote:
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.
Unless I'm mistaken, the only difference between -O1 and -O2 are SpecConstr and LiberateCase. These are quite heavily constrained by default (e.g., SpecConstr will not specialise big functions and will not generate more than 3 specialisations for smaller ones).
I'd like to keep a distinction between -O1 and -O2, but I agree with you that we could probably look at how the optimisations are currently distributed between the options. We could probably also add a -O3 that turns up various thresholds for people who don't mind generating larger code. -O1 is supposed to represent good value for compile-time. It gets all the low-hanging fruit. I use -O in my development builds, whereas it makes sense for Cabal to turn on -O2 by default and for us to use -O2 for the nightly builds and when building GHC distributions (we already do this). Cheers, Simon