Converting unboxed sum types in StgCmm

Simon Peyton Jones simonpj at
Thu Sep 10 09:37:43 UTC 2015

The problem is that stg is too strongly typed

It’s not really typed, or at least only in a very half-hearted way.  To be concrete I think you can just use Any for any Pointer arg.   All STG needs to know, really, is which things are pointers.  Detailed type info like “are you a Char or a Bool” is strictly jam; indeed never used I think.  (I could be wrong but I’m pretty sure I’m not wrong in a fundamental way.


From: Johan Tibell [mailto:johan.tibell at]
Sent: 09 September 2015 23:22
To: Simon Peyton Jones; Simon Marlow; ghc-devs at
Subject: Converting unboxed sum types in StgCmm


The original idea for implementing the backend part of the unboxed sums proposal was to convert from the core representation to the actual data representation (i.e. a tag followed by some pointer and non-pointer fields) in the unarise stg-to-stg<> pass.

I have now realized that this won't work. The problem is that stg is too strongly typed. When we "desugar" sum types we need to convert functions receiving a value e.g. from

    f :: (# Bool | Char #) -> ...


    f :: NonPointer {-# tag#-} -> Pointer {-# Bool or Char #-} -> ...

Since stg is still typed with normal Haskell types (e.g. Bool, Char, etc), this is not possible, as we cannot represent an argument which has two different types.

It seems to me that we will have to do the conversion in the stg-to-cmm<> pass, which is quite a bit more involved. For example, StgCmmEnv.idToReg function will have to change from

    idToReg :: DynFlags -> NonVoid Id -> LocalReg


    idToReg :: DynFlags -> NonVoid Id -> [LocalReg]

to accommodate the fact that we might need more than one register to store a binder.

Any ideas for a better solution?

-- Johan

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