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

[GHC]

#12506: Compile time regression in GHC 8.
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        Reporter:  deech             |                Owner:  bgamari
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       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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Description changed by deech:

@@ -20,0 +20,1 @@
+   - `cabal install c2hs`
@@ -21,1 +22,0 @@
-   - `cabal install c2hs`

New description:

 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 is about 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 c2hs`
   - `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#comment:7>
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