[GHC] #15019: Fix performance regressions from #14737

[GHC]

#15019: Fix performance regressions from #14737
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        Reporter:  tdammers          |                Owner:  tdammers
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14737            |  Differential Rev(s):  phab:D4568
       Wiki Page:                    |
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Comment (by simonpj):

 OK, got it.  The culprit is this:
 {{{
         addCoerce co1 (CastIt co2 cont)
           = addCoerce (mkTransCo co1 co2) cont
 }}}
 It's really very common for `co1` and `co2` to cancel each other out, but
 `mkTransCo` doesn't spot that.  Next thing to try:

 {{{
         addCoerce co cont
           | isReflCo co = return cont

         addCoerce co1 (CastIt co2 cont)
           | isReflexiveCo co' = return cont
           | otherwise         = {-#SCC "addCoerce-simple-recursion" #-}
                                 addCoerce co' cont
           where
             co' = mkTransCo co1 co2
 }}}
 And get rid of the `isReflexiveCo` at the end altogether.

 The initial `isReflCo` is optional -- just to catch the case where the
 `simplCorecion` returns Refl.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15019#comment:9>
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