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

[GHC]

#8647: Reduce allocations in `integer-gmp`
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        Reporter:  hvr                      |            Owner:
            Type:  task                     |           Status:  new
        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):

 The two commits above implement the 2nd of the two proposed optimizations
 in the ticket description, the avoid-reallocations-optimization is a bit
 more difficult to get implemented properly (but I'm still working on it).

 Btw, this optimization helps also in cases, when several small-ints are
 added/multiplied to an accumulator as in e.g.:
 {{{#!hs
 sum (toInteger (maxBound :: Int) : [1..10000])
 -- or
 product [1..100]
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8647#comment:5>
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