# adding isWHNF primop to 5.00.2 native code generator

Julian Seward (Intl Vendor) v-julsew@microsoft.com
Wed, 1 Aug 2001 03:09:49 -0700

| Sigbjorn gave me this suggestion, which is based on DataToTagOp.
|=20
|    \begin{code}
|    primCode [res] IsHNF [arg]
|       =3D let res'        =3D amodeToStix res
|             arg'        =3D amodeToStix arg
|             arg_info    =3D StInd PtrRep arg'
|             word_32     =3D StInd WordRep (StIndex PtrRep=20
| arg_info (StInt (-1)))
|             masked_le32 =3D StPrim SrlOp [word_32, StInt 16]
|             masked_be32 =3D StPrim AndOp [word_32, StInt 65535]
|    #ifdef WORDS_BIGENDIAN
|    #else
|    #endif
|             not_a_thunk =3D StPrim IntEqOp [ StPrim AndOp [ty_info,
StInt 0x10]
|                                          , StInt 0x0
|                                          ]
|                    -- ToDo: don't hardwire the value of=20
| _THUNK from InfoTables.h
|         in
|         returnUs (\ xs -> assign : xs)
|    \end{code}=20
|=20
|=20
| I get different results with the version using the C macro=20
| and the native code version. In particular I get the wrong=20
| (unexpected result from the native code version).
|=20
| The C macro version works like this (uses these macros):
|=20
|    #define closureFlags(c)         (closure_flags[get_itbl(c)->type])
|       -- in ghc/includes/InfoTables.h
|=20
|    #define closure_THUNK(c)        (  closureFlags(c) & _THU)
|       -- in ghc/includes/InfoTables.h
|=20
|       -- in ghc/includes/ClosureMacros.h
|=20
|    #define INFO_PTR_TO_STRUCT(info) ((StgInfoTable *)(info) - 1)
|       -- in ghc/includes/ClosureMacros.
|=20
| The (-1) scares me a little bit, in INFO_PTR_TO_STRUCT.
|=20
| The array closure_flags[] is just a look up table, indexed by=20
| the closure type. So the key part of the code above is:
|=20
|    get_itbl(c)->type
|=20
| In the native code version above, the lines below act like=20
| get_itbl(c):
|=20
|    arg_info    =3D StInd PtrRep arg'
|    word_32     =3D StInd WordRep (StIndex PtrRep arg_info (StInt =
(-1)))
|=20
| After this I am lost. It seems to be grabbing the top or=20
| bottom 16 bits of word_32 (depending on endianness) and then=20
| ANDing those bits with 0x10 (which is the bit mask for _THU),=20
| and checking for 0.
|=20
| My feeling is that I should be finding out the "type" field=20
| from the closure and then switching on its value.=20

What you have started with looks plausible.  If you can extract
enough info from the NCG, you can usually find the problem
without too much difficulty.

I suggest you make friends with -ddump-stix, so that you can
see directly the result of your extension to the primCode fn
above.

1.  Write a plausible primCode case -- as you've already done.
2.  Compile with -ddump-stix and see if it looks plausible
in context.
3.  Read the assembly code (-ddump-asm) and relate it to the
Stix.
4.  Try and relate (3) to the assembly code for the same
when compiled with -fvia-C.  This usually helps.

(3) and (4) are a lot easier if you do it on x86 than sparc
because x86 code is a lot less verbose.  You do need to be=20

| I don't understand the native code generator well enough. I=20
| am assuming that closures are laid out the same way as via-C,

Yes.  Code from the NCG and -fvia-C is 100% interoperable.
=20
| in particular that there is going to be some stuff in the=20
| info table before the "type" field (this looks variable=20
| depending on how things are built). Basically I'm looking for=20
| a way to do the equivalent of "->type". Maybe there is a=20
| better way to do it.=20

Suggestion: find some other primop which uses the ->type field
and which is implemented in the NCG (not all are).  That might
help.

Finally let me point you at ghc/utils/debugNCG, a small but
very useful program for debugging the NCG.  I would never have
got it working as well as it does without it.  This should
work -- but it might have slight bitrot -- it's very fragile
and makes lots of assumptions about gcc's output.  I haven't
used it for a good few months.

J