[GHC] #9281: Rewrite `integer-gmp` to use only non-allocating GMP functions

[GHC]

#9281: Rewrite `integer-gmp` to use only non-allocating GMP functions
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              Reporter:  hvr         |             Owner:  hvr
                  Type:  task        |            Status:  patch
              Priority:  normal      |         Milestone:  7.10.1
             Component:  libraries   |           Version:
  (other)                            |          Keywords:  integer-gmp
            Resolution:              |  Operating System:  Unknown/Multiple
Differential Revisions:  Phab:D82    |   Type of failure:  None/Unknown
          Architecture:              |         Test Case:
  Unknown/Multiple                   |          Blocking:
            Difficulty:  Unknown     |
            Blocked By:              |
       Related Tickets:  #8647       |
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Comment (by hvr):

 fwiw, if you use `prim`-FFI imports instead of `ccall` , `... -> State# s
 -> State# s` as well as `... -> State# s -> (# s, Word# #)` already works.
 Is this maybe just a matter of replicating what's already done for
 `prim`-style FFI/Cmm calls?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9281#comment:8>
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