help needed for adding isWHNF primop to 5.00.2

[Bernard James POPE]

Hi GHC people,

I would like to add a primitive to GHC 5.00.2 of the form:

   isWHNF :: a -> Bool

I was able to do (something like) this a while ago in 4.06, but have got
stuck trying to do the same with 5.00.2.

Here's what I have tried:

   After reading ghc/compiler/prelude/primops.txt:

   - added 

        primop IsHNF "isHNF#" GenPrimOp
        a -> Int#
        with
        strictness = { \ arity -> StrictnessInfo [wwLazy] False }

     to ghc/compiler/prelude/primops.txt

   - added isHNFzh to ghc/lib/std/PrelGHC.hi-boot

   - added

        #define isHNFzh(r,a) r=(! closure_THUNK((StgClosure *)a))
  
     to ghc/includes/PrimOps.h

     {- 
        although I think this should be the same as:  ???
        #define isHNFzh(r,a) r=(closure_HNF((StgClosure *) a)) 
     -}
   
   - I think I need to do something in:

        ghc/compiler/nativeGen/StixPrim.lhs

     but I have no idea what, I looked at the code in there and got scared,
     so I left it alone, although I suspect that was a bad idea.

   - I then did: make boot; make all

   - everything compiled ok, but I get some serious problems when I try
     to use my new primitive:

     \begin{code}

        module Main where
        import GlaExts
        import PrelGHC

        main = print $ g

        g :: Bool
        g = fromUnboxedIntAsBoolean (isHNF# ())

        fromUnboxedIntAsBoolean :: Int# -> Bool
        fromUnboxedIntAsBoolean x
           = case x of
              1# -> True
              _  -> False

     \end{code}

     When I compile this code with my newly built compiler using:
  
        ghc -ddump-types -fglasgow-exts -package lang 

     I get:

        ==================== Interface ====================
        TYPE SIGNATURES
        Main.fromUnboxedIntAsBoolean :: PrelGHC.Int# -> PrelBase.Bool
        Main.g :: PrelBase.Bool
        Main.main :: PrelIOBase.IO ()
        {-# Generic type constructor details
        #-}


        ghc-5.00.2: panic! (the `impossible' happened, GHC version 5.00.2):
            getRegister(x86,unary primop)
        (Prim isHNFzh PrelBase.Z0T{-70-}_closure)

        Please report it as a compiler bug to glasgow-haskell-bugs@haskell.org,
        or http://sourceforge.net/projects/ghc/.

    
The file ghc/docs/rts/rts.tex talks of a predicate "isWHNF", however, I cannot
seem to find it anywhere, and suspect that it does not exist anymore.

If anyone could point me in the right direction to solving my problem I would
be very grateful.

Regards,
Bernie.

PS
Unfortunately I clobbered my version for 4.06 with isWHNF added, and now I 
can't get it to work anymore.