[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):

 It was hard to find in the manual the difference between O0 and the
 default.  The manual says "O0 is the default", which seems to be incorrect
 (NEED TO BE FIXED). So I had to turn to the GHC code. Alright, so O0 seems
 to ignore or omit "interface-pragmas" whatever the heck they are, from
 DynFlags.hs:

 {{{#!hs
     , ([0],     Opt_IgnoreInterfacePragmas)
     , ([0],     Opt_OmitInterfacePragmas)
 }}}

 When I used "-fignore-interface-pragmas" I got the same improvement in
 performance. The GHC manual documents this flag but says nothing about
 what this really means (NEED TO BE FIXED). I can only guess. Since this
 has to do something with the interface, this also explains why the
 performance is good when the whole code is in the same module and bad when
 the code is split into two modules.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14208#comment:5>
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