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

Simon Peyton-Jones simonpj at microsoft.com
Thu May 22 17:51:07 EDT 2008

| > I'm confused. GHC of course unboxes strict fields of primitive data types.
| >
| >     {-# OPTIONS -O2 -fvia-C -optc-O2 -funbox-strict-fields #-}
| ... but only when you give -funbox-strict-fields, as there, or UNPACK.
| The point is that it never loses sharing to unbox a strict Int field
| [1], so it should do that with -O, even without either of the above
| overrides.
| [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.


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