[Haskell-cafe] Re: MD5 performance optimizations, and GHC -via-C producing segfaulting binary

Isaac Dupree isaacdupree at charter.net
Thu May 22 19:14:32 EDT 2008

Simon Peyton-Jones wrote:
> | [1] I'm not sure if this is true... if it has to rebox the Int, you get
> | another copy floating around, not the original, right?
> Correct.  If you have
>         data T = MkT {-# UNPACK #-} !Int
> then given
>         case x of { MkT y -> h y }
> then GHC must re-box the 'y' before passing it to h.  That's why unpacking strict fields isn't an unambiguous win.  Perhaps it should be the default, though, which can be switched off, rather than the reverse.

perhaps there are certain types that the compiler almost always manages 
to pass around unboxed -- Ints, for example?  (Or at least that the 
extra copies aren't likely to persist for a long time, wasting some 
memory. Especially not if they're always unboxed in strict fields -- 
except for strict polymorphic fields, unfortunately.) And other types 
that aren't usually passed around unboxed?  If this is clear, then maybe 
it's useful to do it by default for those types.  Perhaps there should 
be a {-# NOUNPACK #-} pragma just in case someone wants to make sure a 
strict field isn't represented unboxed (not even if you give 


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