[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
-------------------------------------+-------------------------------------
        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):

 I added a much simpler example on the "simplified" branch in the same
 repo. I can paste it here as well:

 Main.hs
 {{{#!hs
 import List
 ...

 len :: IO Int
 len = do
     xs <- toList $ (foldr (<>) mempty $ map (\x -> Yield x Stop)
 [1..100000 :: Int])
     return (length xs)
 }}}

 List.hs
 {{{#!hs
 module List where

 import Control.Monad  (liftM)

 data List a = Stop | Yield a (List a)

 instance Monoid (List a) where
     mempty = Stop
     mappend x y =
         case x of
             Stop -> y
             Yield a r -> Yield a (mappend r y)

 toList :: Monad m => List a -> m [a]
 toList m =
     case m of
         Stop -> return []
         Yield a r -> liftM (a :) (toList r)
 }}}

 It essentially generates a custom list in the main module and calls
 `toList` function from another module to covert it into a Haskell list.
 The perf difference is not as dramatic as the previous example but it is
 significant.

 All in the same module:
 {{{
 -O0              : 14ms
 -O1              : 8ms
 -fno-pre-inlining: 4ms
 }}}

 Different modules:
 {{{
 -O0              : 8ms
 -O1              : 14ms
 -fno-pre-inlining: 8ms
 INLINE toList    : 4ms
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14208#comment:13>
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