[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):

 Another question that I am seeking an answer for - is there a combination
 of options to make a multi-module program behave the same way as if all
 the code is in a single module, from the performance perspective? I
 expected that `-fexpose-all-unfoldings` will do that for me but it does
 not seem to be equivalent. I thought it is equivalent to making all
 functions INLINABLE but it does not seem to be doing that. Even when using
 this flag I need to mark a function INLINABLE to get it inlined. What
 exactly does this flag do?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14208#comment:15>
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