[GHC] #13253: Exponential compilation time with RWST & ReaderT stack with `-02`

[GHC]

#13253: Exponential compilation time with RWST & ReaderT stack with `-02`
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        Reporter:  phadej            |                Owner:  dfeuer
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Questions

 * Are those `perf/compiler` improvements happening because (a) GHC is
 generating less code of (b) GHC's code is running faster?

 * Is compilation generally faster, or are those two cases exceptional?
 (The testsuite only reports changes when they break through a threshold.)

 * Where are the gains in `mate` and the losses in `gameteb`?   (Use
 `-ticky` to see.)  I'd like to be sure that the loss in `gameteb` isn't
 for some silly reason that could readily be fixed.

 Fundamentally this change looks good, but I don't want to commit it and
 forget it because I'd like to understand the reasons a bit better.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13253#comment:25>
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