Re: Converting unboxed sum types in StgCmm

Some of the SSE types are too big for that even on 64 bit, I think. Like DoubleX8#. On Thu, Sep 10, 2015 at 1:16 AM, Johan Tibell wrote:

I wonder if rewriting any aliased pointer field as Any in Stg and any non-pointer field as Word# would work. I suspect that not all non-pointer fields (e.g. Double# on 32-bit) can be represented as Word#.

On Wed, Sep 9, 2015 at 3:22 PM, Johan Tibell wrote:

...

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 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 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

_______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs