Converting unboxed sum types in StgCmm

[Simon Marlow]

On 10/09/2015 10:37, Simon Peyton Jones wrote:
> 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.

Yes, the only thing the code generator needs to do with types is convert 
them to PrimReps (see idPrimRep), and all GC pointer types have the same 
PrimRep (PtrRep).

Cheers
Simon



>
> SImon
>
> *From:*Johan Tibell [mailto:johan.tibell at gmail.com]
> *Sent:* 09 September 2015 23:22
> *To:* Simon Peyton Jones; Simon Marlow; ghc-devs at haskell.org
> *Subject:* Converting unboxed sum types in StgCmm
>
> Hi!
>
> 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
> <https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fgithub.com%2fghc%2fghc%2fblob%2fmaster%2fcompiler%2fsimplStg%2fUnariseStg.hs&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7cca7beffb01494517d75108d2b9652973%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=U%2bFUNsL87iEemajTnAW9SxD9N5b4%2bG8QB1q19%2fX%2bBI4%3d>
> 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 #) -> ...
>
> to
>
>      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
> <https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fgithub.com%2fghc%2fghc%2fblob%2fmaster%2fcompiler%2fcodeGen%2fStgCmm.hs&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7cca7beffb01494517d75108d2b9652973%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=aXKZ78eGNKbJ6eZkxZgyJHgsAXpgOBjg3Zvqj%2bq7pk0%3d>
> pass, which is quite a bit more involved. For example, StgCmmEnv.idToReg
> function will have to change from
>
>      idToReg :: DynFlags -> NonVoid Id -> LocalReg
>
> to
>
>      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
>