John Meacham wrote:
On Fri, Oct 20, 2006 at 10:38:39AM +0100, Simon Marlow wrote:
I'm not sure that -funbox-strict-fields always improves performance, even if you only do it on Ints for example. If you end up pulling out those fields and passing the Int to a lazy function, the Int will be re-boxed each time, leading to more allocation. This is the reason that -funbox-strict-fields isn't on by defualt, and why I recommend using {-# UNPACK #-} pragmas.
the happy medium I found in jhc was to always unbox any fields whose representation was smaller or equal to a pointer. It seems to work well.
Good idea.
another worthwhile optimization that benefits from this is unboxing all enums.
so data Bool = False | True
desugars into
data Bool = Bool# Int#
False = Bool# 0# True = Bool# 1#
Right, this occurred to me too. Alternatively we could have the strictness analyser represent a strict enumeration by Int# (I believe there's a ticket for this). I think when we discussed this for GHC the conclusion was that the latter was probably easier to implement, because we'd have to re-architect more of GHC to handle a data type with a representation that had a different number of constructors from the source data type (currently the difference between source and representation data types is only handled on a per-constructor basis). Still, I prefer the Bool# solution because it seems to expose more to the simplifier. Cheers, Simon