[GHC] #14208: Performance with O0 is much better than the default or with -O2, runghc performs the best

[GHC]

#14208: Performance with O0 is much better than the default or with -O2, runghc
performs the best
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        Reporter:  harendra          |                Owner:  osa1
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by mpickering):

 I noticed two things from a brief look.

 1. `fmap` is not SATed. Which means the static `f` won't be eliminated.
 2. A bad(?) loopbreaker is chosen in `Prelude.foldrM`, `yield` rather than
 `go`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14208#comment:35>
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