[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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Changes (by hvr):

 * status:  new => patch


Comment:

 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.

 I'm wondering if it would be feasible and sensible to return a properly
 constructed `Integer` value from the Cmm wrappers (i.e. to fold
 `tupToInteger` into gmp-`wrappers.cmm`), but I haven't figured out yet how
 to implement that in Cmm.

 Moreover, I'm also still wondering how to properly allocate stack-space in
 "high-level" Cmm procedures (see [http://www.haskell.org/pipermail/ghc-
 devs/2014-January/003616.html posting on ghc-devs@]), as the current code
 seems to work properly only as long the Cmm procedures don't force the
 code generator to save registers to the stack.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8647#comment:6>
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