[GHC] #13536: Program which terminates instantly in GHC 8.0.2 runs for minutes with 8.2.1

[GHC]

#13536: Program which terminates instantly in GHC 8.0.2 runs for minutes with 8.2.1
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 Before I forget, there's another opportunity here because of the special
 form of `sat_s5Dp` after CSE, namely optimizing it to
 {{{
 sat_s5Dp = eta_s5Di
 }}}
 We already do this at the Core level and I imagine it wouldn't be hard to
 do here also. I don't know whether this pattern arises frequently in
 practice. But consider for example
 {{{#!hs
 newtype T a = MkT a
 f (x, y) = (MkT x, y)
 }}}
 (Without the `MkT` the Core simplifier can turn this into `f z@(_, _) =
 z`, which turns into much simpler code.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13536#comment:15>
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