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.tib...@gmail.com]
*Sent:* 09 September 2015 23:22
*To:* Simon Peyton Jones; Simon Marlow; ghc-devs@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
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