[GHC] #13429: Optimizer produces Core with an infinite <<loop>>

[GHC]

#13429: Optimizer produces Core with an infinite <<loop>>
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        Reporter:  lehins            |                Owner:  simonpj
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Runtime crash     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13750            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"2b74bd9d8b4c6b20f3e8d9ada12e7db645cc3c19/ghc"
 2b74bd9d/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="2b74bd9d8b4c6b20f3e8d9ada12e7db645cc3c19"
 Stop the specialiser generating loopy code

 This patch fixes a bad bug in the specialiser, which showed up as
 Trac #13429.  When specialising an imported DFun, the specialiser could
 generate a recusive loop where none existed in the original program.

 It's all rather tricky, and I've documented it at some length in
    Note [Avoiding loops]

 We'd encoutered exactly this before (Trac #3591) but I had failed
 to realise that the very same thing could happen for /imported/
 DFuns.

 I did quite a bit of refactoring.

 The compiler seems to get a tiny bit faster on
    deriving/perf/T10858
 but almost all the gain had occurred before now; this
 patch just pushed it over the line.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13429#comment:25>
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