[GHC] #12437: 20% regression in max_bytes_used for T1969

[GHC]

#12437: 20% regression in max_bytes_used for T1969
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        Reporter:  simonmar          |                Owner:  osa1
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by osa1):

 I also compared "Implement unboxed sums" with a13fda7 (current HEAD,
 "Clarify comment on makeCorePair"). `bytes_allocated` is even lower now:

 ("bytes allocated", "786255456")
 to
 ("bytes allocated", "786244136")

 Difference: -11,320

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12437#comment:4>
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