Computing the final representation type of a TyCon (Was: Unpack primitive types by default in data)

[Johan Tibell]

Hi,

I've created an initial implementation that seems to work. I'd
appreciate it if someone could review the code (it's short!) to tell
me if it's sane, can be improved, etc:

https://github.com/tibbe/ghc/commit/6b44024173eae3029b7b43f7cc9fc7d9d801c367

On Thu, Nov 29, 2012 at 12:27 AM, Johan Tibell <johan.tibell at gmail.com> wrote:
> I've decided to try to implement the proposal included in the end of
> this message. To do so I need to write a function
>
>     hasPointerSizedRepr :: TyCon -> Bool
>
> This function would check that that the TyCon is either
>
>  * a newtype, which representation type has a pointer-sized representation, or
>  * an algebraic data type, with one field that has a pointer-sized
> representation.
>
> I'm kinda lost in all the data types that GHC defines to represent
> types. I've gotten no further than
>
>     hasPointerSizedRepr :: TyCon -> Bool
>     hasPointerSizedRepr tc@(AlgTyCon {}) = case algTcRhs tc of
>                                              DataTyCon{ data_cons = [data_con] }
>                                                          -> ...
>                                              NewTyCon { data_con = [data_con] }
>                                                          -> ...
>                                              _           -> False
>     hasPointerSizedRepr _                = False
>
> I could use some pointers (no pun intended!) at this point. The
> function ought to return True for all the following types:
>
>     data A = A Int#
>     newtype B = B A
>     data C = C !B
>     data D = D !C
>     data E = E !()
>     data F = F !D
>
> One part that confuses me is figuring out the representation type of a
> data constructor after unpacking. For example, the function should not
> return true if called on G in this example:
>
>     data G = G !H
>     data H = H {-# UNPACK #-} !I
>     data I = I !Int !Int
>
> because if we unpacked H into G's constructor it would take up two
> words, due to I being unpacked.
>
> Does DataCon contain the unpacked representation of the data
> constructor or only the before-optimizations representation?