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://github.com/ghc/ghc/blob/master/compiler/simplStg/UnariseStg.hs 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://github.com/ghc/ghc/blob/master/compiler/codeGen/StgCmm.hs 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