[GHC] #15176: Superclass `Monad m =>` makes program run 100 times slower

[GHC]

#15176: Superclass `Monad m =>` makes program run 100 times slower
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        Reporter:  danilo2           |                Owner:  osa1
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.4.2
      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 osa1):

 So, I've built two versions of the package with `-dump-simpl -dump-to-file
 -dsuppress-uniques` (which took about an hour to build twice on my
 i7-8700K desktop). Unfortunately there are 71 modules in this package and
 none of the files are identical between two versions (there are at least
 very minor identifier changes) so this is still not easy to debug, but
 while checking some random files I found some relevant changes between two
 versions:

 This expression in the original version:

 {{{
 $dLayersFoldableBuilder__
   `cast` (Data.Graph.Fold.Layer.N:LayersFoldableBuilder__[0]
               <t>_N <layers>_N <m>_N
           :: (LayersFoldableBuilder__ t layers m :: Constraint)
              ~R# (SomePtr -> m (Fold.Result t) -> m (Fold.Result t) :: *))
 }}}

 becomes this after `Monad =>`:

 {{{
 buildLayersFold__
   @ t
   @ layers
   @ m
   $dLayersFoldableBuilder__
 }}}

 If my understanding is correct, in the original version the typeclass
 dictionary is represented as its method, but with `Monad m =>` it's not as
 it not has one more field for the `Monad m` dictionary. This means one
 layer of indirection in the `Monad m =>` version.

 There are lots of changes similar to this. Not sure if this by itself
 explains 20x increase in runtime though.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15176#comment:8>
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