[GHC] #8647: Reduce allocations in `integer-gmp`

[GHC]

#8647: Reduce allocations in `integer-gmp`
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        Reporter:  hvr                      |            Owner:
            Type:  task                     |           Status:  patch
        Priority:  normal                   |        Milestone:  7.8.1
       Component:  libraries (other)        |          Version:  7.6.3
      Resolution:                           |         Keywords:  integer-
Operating System:  Unknown/Multiple         |  gmp
 Type of failure:  Runtime performance bug  |     Architecture:  x86_64
       Test Case:                           |  (amd64)
        Blocking:                           |       Difficulty:  Unknown
                                            |       Blocked By:
                                            |  Related Tickets:  #8638
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Comment (by hvr):

 Replying to [comment:6 hvr]:
 > I attached the patch instead of pushing to Git, as I'm not satisfied yet
 with it. It's works stable here (passes `validate` & `nofib`), but I'm not
 sure about the `unsafeCoerce#`.
 >
 > In my first version of this patch, I used a 3-tuple `(# Int#,
 ByteArray#, Word# #)` as return value, but that wasted 1 return register
 per `Integer` result, and still had to return a NULL pointer for
 `ByteArray#` for the case it wasn't used.

 After a conversation with Duncan, I'm now convinced that the `(# Int#,
 ByteArray# #)` + `unsafeCoerce#` hack is really bad, as the GC could try
 to follow the `ByteArray#` pointer even though it was really just a
 `Word#` in disguise.

 Otoh, passing a 0-pointer for the 3-tuple variant has the same danger.

 So now I'm left wondering if I can somehow statically allocate a dummy
 `ByteArray#` I can return in Cmm when there's no need to allocate a proper
 `ByteArray#` in order to avoid confusing the GC.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8647#comment:7>
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