[GHC] #12274: GHC panic: simplifier ticks exhausted

[GHC]

#12274: GHC panic: simplifier ticks exhausted
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        Reporter:  mrkkrp            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Linux             |         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 simonpj):

 GHC 7.10 makes specialised versions of imported overloaded functions, at
 least if it can see its inlining.

 In the example

 * `mymap` is overloaded, and is small, so its inlining is visible to
 module `Test`

 * `mymap` is called at type `Integer` in `Test` so GHC makes specialised
 copy, and adds a local RULE to rewrite `mymap Integer` to `$smymap`.

 * The `fmap` rule fires twice, once in the RHS of `$smymap` and once in
 its (small) unfolding.

 * But when GHC encounters the call to `mymap` is just inlines it anyway,
 before thinking about rules.  That exposes another call to `fmap` so the
 rule fires again.

 Because `mymap` inlines, all the specalisation is in vain.  How to avoid
 that?  Maybe one round of simplification so that outright inlining
 precedes any attempt to specialise.

 Whether this is the original problem, I'm not sure.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12274#comment:4>
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