[GHC] #12545: Compilation time/space regression in GHC 8.0/8.1 (search in type-level lists and -O)

[GHC]

#12545: Compilation time/space regression in GHC 8.0/8.1 (search in type-level
lists and -O)
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        Reporter:                    |                Owner:  (none)
  mikhail.vorozhtsov                 |
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  Inlining
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 simonpj):

 See [https://mail.haskell.org/pipermail/ghc-devs/2017-May/014277.html this
 ghc-devs thread], and [https://mail.haskell.org/pipermail/ghc-
 devs/2017-June/014278.html this one].

 There seem to be several issues:

 * Why does the small change identified by Ryan have such a huge effect?
 We need insight.

 * The change that Richard made to `CoreOpt.simple_opt_expr` actually fixed
 an outright bug. But it's ''still'' wrong now.  See Trac #13798.

 * The fuss that is caused by this `map/coerce` rule is out of all
 proportion to its importance.   We should think of a better way.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12545#comment:12>
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