[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):

 Solution 1 (brutal): keep track of nesting depth, and simply stop
 specialising at some depth.

 Solution 2 (nicer): I think the actual code in this example looks like
 {{{
 join j1 x
   = join j2 y
       = join j3 z = blah3
         in blah2
     in blah1
 in blah0
 }}}
 (only nested more deeply).

 I've been working (slowly) on a join-point-floating patch that would do a
 kind of local lambda lifting to give
 {{{
 join j3 x y z = blah3 in
 join j2 x y = blah2 in
 join j1 x = blah1 in
 blah0
 }}}
 This would specialise a lot better, I think.

 Tobias will try (1); I will get back to (2).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13253#comment:44>
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