[GHC] #13900: Core lint in BuildFlavour=perf-llvm

[GHC]

#13900: Core lint in BuildFlavour=perf-llvm
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        Reporter:  bgamari           |                Owner:  bgamari
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 Simon thinks it's most likely that we have trouble with `case` of `join`:

 {{{#!hs
 case (join j x= ....  in ....) of ...
 ===>
 join j x = case ... of ...
 in case ... of ...
 }}}

 This goes wrong if `j` has a `RULE` attached. We either need to apply the
 transformation in the `RULE` one way or another (Simon thinks he remembers
 there being a reason not to) or drop the `RULE`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13900#comment:13>
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