[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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Comment (by deech):

 I've put together a [https://github.com/deech/Trac12506 reduced example]
 that has the same issues. It's much easier to work with. In putting this
 together I noticed there is a vast difference in compilation time and
 memory between GHC 8.02 and GHC 8.22; the former takes 45 seconds and uses
 1.4GB RAM to compile, the latter, 10 seconds and 200 MB so that is a great
 improvement.

 However while compilation speed is much improved between 8.0.2 and 8.2.2
 that still only brings it up to par with GHC 7.10 which is GHC version
 against which I reported the issue in the first place so the problem still
 exists.

 Hopefully this reduced example helps clarify the underlying issue.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12506#comment:16>
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