[GHC] #9781: Make list monad operations fuse

[GHC]

#9781: Make list monad operations fuse
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              Reporter:  dfeuer      |            Owner:  dfeuer
                  Type:  task        |           Status:  closed
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Core        |          Version:  7.9
  Libraries                          |         Keywords:
            Resolution:  fixed       |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  Runtime     |  Related Tickets:
  performance bug                    |
             Test Case:              |
              Blocking:              |
Differential Revisions:  Phab:D455   |
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Comment (by dfeuer):

 Replying to [comment:5 nomeata]:
 > This showed some minor performance improvements, and one huge one, as
 you noticed already: cryptarithm2’s allocation went down by more than 50%!
 >
 > (I just spent a few minutes looking for list monad operations in that
 benchmark until I noticed that your commit also had an unrelated(?) change
 to mapM.)

 The change also improves cryptarithm2 without the `mapM` change, but not
 as much. So there must have been something else.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9781#comment:6>
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