[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:  (none)
            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 harendra):

 When I look at `-ddump-rewrite-rules` output the list fusion rules seem to
 get fired in the `-fno-pre-inlining` case but not in the `-O1` case.
 Specifically I do not see the "map" rule (and subsequent other rules
 triggered by it) in the `-O1` output.

 As long as we understand why it is happening and there is no easy way to
 fix it, I guess it is fine. But I hope this can be fixed so that we do not
 spend time wondering about and figuring out such things and manually
 tweaking options and pragmas to get to the right combination.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14208#comment:14>
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