[GHC] #8742: Reuse scavenge_small_bitmap

[GHC]

#8742: Reuse scavenge_small_bitmap
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        Reporter:  Tarrasch          |            Owner:
            Type:  task              |           Status:  patch
        Priority:  lowest            |        Milestone:  7.10.1
       Component:  Runtime System    |          Version:  7.6.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by Tarrasch):

 Hello Simon,

 I'll be more than happy to change the uses of `size` to use type
 `StgWord`, tough I start realizing that such patch can become quite big if
 I go outside of `Scav.c`. For example, in `Compact.c`, there is a usage of
 `closure_sizeW_` which have a return type of `nat`. My best judgement says
 that I should change it to return of type `StgWord`, but since it's such a
 big and used function it might affect other parts of the code.

 Am I in the right direction? Regardless I think that this effort to unify
 the types is beneficial, because at many of these in-lined call sites
 there are implicit type conversions which cost cycles performance. ''We''
 know that the `size` will always fit in 32 bits, but gcc doesn't.

 So, would a patch including a change of `nat closure_sizeW_` to `StgWord
 closure_sizeW_` be a good idea?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8742#comment:12>
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