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. 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# this gives a lot of things the CPR quality that is so good for optimization and means they can be unboxed in data constructors. a small problem is that the compiler now doesn't know that Int#'s pulled out of a Bool# can only take on the values 0# or 1#, which is useful for things like case alternative elimination. I am not sure how to best handle it, but right now I am thinking of supporting 'restricted unboxed integers' somehow... John -- John Meacham - ⑆repetae.net⑆john⑈