(in-place) mutable unboxable Ints (was: [GHC] #8158: Replace IO manager's IntMap with a mutable hash table)

[Herbert Valerio Riedel]

Hello Simon,

On 2013-08-24 at 08:54:20 +0200, GHC wrote:
[...]
> Comment (by simonmar):
>
>  @bos: GHC has a `FastMutInt` type for this purpose, which is a 1-element
>  mutable unboxed array.  I'd be interested to know whether performance
>  differs between this and `ForeignPtr`.
>
>  source:ghc/compiler/utils/FastMutInt.lhs

One thing I have been wondering about:

...even with FastMutInt, which is defined as

  data FastMutInt = FastMutInt (MutableByteArray# RealWorld)

If I have a hypothetical data structure,

  data IoStats = IoStats { sBytesRecv    :: {-# UNPACK -#} !FastMutInt
                         , sBytesSent    :: {-# UNPACK -#} !FastMutInt
                         , sPacketsRecv  :: {-# UNPACK -#} !FastMutInt
                         , sPacketsSent  :: {-# UNPACK -#} !FastMutInt
                         }

Then (if have the numbers right) each instance of IoStats takes 1 word
for the IoStats info-ptr, plus 1 word for each unpacked FastMutInt
(which is basically reduces to pointer to a MutableByteArray#), and 3
words for each MutableByteArray# as those result in separate heap
objects; that is, a total of 17 words needs to be allocated to keep
track of 4 counters. The overhead could be improved somewhat, if I used
a 4-word-long MutableByteArray (which would still be a separate heap
object and thus result in a single 3-word overhead), but the data
declaration would be less expressive IMHO.

Would it be possible (w/ reasonable effort) to have an unpackable
version of FastMutInt which only has a total storage cost of 1 word
(when unpacked), so that IoStats would have a total storage cost of 5
words (and no indirections)? 

Or even just a new unboxed primitive MutInt# type in the style of Int#,
so that FastMutInt would be defined similiar to 'Int' as

  data FastMutInt = FMI# MutInt#

?

PS: The FastMutInt API doesn't seem to provide atomic
    decrements/increments, what would be needed to provide that?

Cheers,
  hvr