[GHC] #12506: Compile time regression in GHC 8.

[GHC]

#12506: Compile time regression in GHC 8.
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           Reporter:  deech          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This bug is entirely reproducible but unfortunately takes some setup. I
 don't have a minimal example.

 The regression is happening in FLTKHS (https://github.com/deech/fltkhs),
 the Haskell bindings to the FLTK(http://fltk.org) toolkit, and in
 particular to this GUI (https://github.com/deech/fltkhs/blob/master/images
 /tree-complex-windows.png). In GHC 7.8, it taks 5-9 seconds to compile and
 link in 7.10.3, that goes up to 12-15 seconds and finally in GHC 8 it
 shoots up to 45 seconds. While these numbers come from my i7, 16GB RAM
 Linux laptop, the compile time regression is proportionally reproducible
 across machines and operating systems.

 Here are the steps to reproduce it
 * Install FLTK
   Please follow the instructions here (https://github.com/deech/fltkhs
 #linux-and-bsd) but only the first set which about for installing FLTK
 1.3.
 * Clone the repo with the offending application: `git clone
 https://github.com/deech/fltkhs-fluid-demos`
 * Build the application and reproduce the problem:
   - `cabal install fltkhs`
   - At the root of the repo do `cabal build fltkhs-fluid-tree`

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12506>
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