[GHC] #7662: Improve GC of mutable objects

[GHC]

#7662: Improve GC of mutable objects
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        Reporter:  ezyang            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  ⊥
       Component:  Runtime System    |              Version:  7.7
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dobenour):

 MLton does manage to unpack refs in records though.  I think that it can
 be done in general IF you either:

 - support interior pointers in the GC (which GHC probably doesn't).
 - represent mutable refs as "fat pointers", consisting of both a pointer
 to the object being mutated and a pointer to the cell within the object.
 In this case
 {{{#!hs
 data SomeType = SomeType { x :: {-# UNPACK #-} !(IORef Int)
                          , y :: {-# UNPACK #-} !Char
                          }
 getRef :: SomeType -> IORef Int
 getRef = x
 }}}
 `x (v :: SomeType)` would produce an object that contains a pointer to `v`
 (for the GC) and a pointer to field `x` of `v` (used to mutate `x`).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7662#comment:9>
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