[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:  bgamari, osa1
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       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:  #15630            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Hypothesis:
 * `SpecConstr` is specialising a function `f1`
 * In `f`'s RHS there is a local function `f2`
 * In each specialised copy of `f1` we create two specialised copies of
 `f2`
 * Alas `f2` has a nested function `f3` -- and it too specialises...
 and so on.

 Result: the specialised code is exponential in the nesting depth.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13253#comment:43>
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