[GHC] #15304: Huge increase of compile time and memory use from 8.0.2 to 8.2.2 or 8.4.2

[GHC]

#15304: Huge increase of compile time and memory use from 8.0.2 to 8.2.2 or 8.4.2
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        Reporter:  NathanWaivio      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by NathanWaivio):

 I have now been able to analyze the differences between ghc-8.0.2 to
 ghc-8.2.2.

 Here is a table of the growth of the number of terms per iteration of the
 Simplifier:
 ||= Simplifier Iteration =|| 1 || 2 || 3 || 4 ||
 || ghc-8.0.2 || 118,835 || 138,330 || 172,291 || 516,767 ||
 || ghc-8.2.2 || 149,046 || 185,066 || 15,190,006 || 12,310,166 ||

 Apparently the largest difference occurs in Simplifier Iteration 3.

 What is going on with Simplifier Iteration 3?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15304#comment:3>
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