RE: Which of the following PrimTyCons have a pointer-sized representations

Johan,

Well, I started to review your patch.  And then I re-discovered how horribly messy that code is; with independent decisions taken in the desugarer, MkId, and TcTyClsDcls, all of which must line up.

So I totally refactored everything which cost me a couple of days (because it has quite wide span).  I'm validating now.

I realise that "unbox-strict-fields" means "unbox it if it's strict", whereas your new "unbox-strict-primitive-fields" is the same but a bit less aggressive: 
   "unbox it if it's strict, AND the unboxed version
    is at most one word wide"
Where a Float or Double count as "one word".

So I changed it to "...AND the unboxed version is at most one field wide".  That is we get one field not 2 or 10.  So consider

	data T = MkT !S
  	data S = MkS Int

then my impl will unbox the S field of MkT because the result is only one field wide, namely an Int. 

It would be easy to also restrict to *primitive* types, but it seems mildly beneficial not to. 

However the flag is then a mis-nomer.

I'll commit shortly and you can look

Simon

| -----Original Message-----
| From: Johan Tibell [mailto:[email protected]]
| Sent: 07 December 2012 22:10
| To: Simon Peyton-Jones
| Cc: glasgow-haskell-users
| Subject: Re: Which of the following PrimTyCons have a pointer-sized
| representations
| 
| On Fri, Dec 7, 2012 at 10:48 AM, Johan Tibell <[email protected]>
| wrote:
| > On Fri, Dec 7, 2012 at 3:36 AM, Simon Peyton-Jones
| > <[email protected]> wrote:
| >> You can use TyCon.tyConPrimRep, followed by primRepSizeW
| >
| > Looking at primRepSizeW I see that the only PrimRep that is bigger
| > than one word is Doubles, Int64s, and Word64s on 32-bit platforms.
| > Manuel (I think wisely) suggested that we should make an exception for
| > these types and unpack them on 32-bit platforms if
| > -funbox-strict-primitive-fields is set, even thought technically they
| > will occupy more space than a pointer. The reasoning is that we want
| > to avoid surprising behavior when users move code between 32-bit and
| > 64-bit platforms, as e.g. unpacking vs not-unpacking Doubles can make
| > a large difference in certain tight loops.
| >
| > But this means that checking the size in can_unbox_prim is no longer
| > necessary, so I could remove that check. This does mean that if we
| > ever add a new PrimTyCon that has a size that's larger than a pointer,
| > the implementation of -funbox-strict-primitive-fields has to change.
| >
| > The alternative would be for me to add
| >
| > primRepSizeForUnboxW :: PrimRep -> Int
| > primRepSizeForUnboxW IntRep   = 1
| > primRepSizeForUnboxW WordRep  = 1
| > primRepSizeForUnboxW Int64Rep = 1    -- [Note: Primitive size
| exception]
| > primRepSizeForUnboxW Word64Rep= 1    -- [Note: Primitive size
| exception]
| > primRepSizeForUnboxW FloatRep = 1    -- NB. might not take a full word
| > primRepSizeForUnboxW DoubleRep= 1    -- [Note: Primitive size
| exception]
| > primRepSizeForUnboxW AddrRep  = 1
| > primRepSizeForUnboxW PtrRep   = 1
| > primRepSizeForUnboxW VoidRep  = 0
| >
| > And use that function in can_unbox_prim. That way we'd get a pattern
| > match warning if we ever add a new PrimRep (and thus need to evaluate
| > if PrimTyCons with that PrimRep should be unpacked by
| > -funbox-strict-primitive-fields).
| 
| I went with the latter approach as it seems safer.
| 
| -- Johan