[GHC] #5302: Unused arguments in join points

[GHC]

#5302: Unused arguments in join points
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        Reporter:  reinerp           |                Owner:  simonpj
            Type:  bug               |               Status:  new
        Priority:  low               |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  7.0.3
      Resolution:                    |             Keywords:  JoinPoints
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 simonpj):

 Still reproducible in HEAD.

 * The reason the late demand-analysis doesn't catch it is because we don't
 do w/w.  (Reason: see `Note [Final Demand Analyser run]` in `DmdAnal`.)

 * The absent argument is actually a case binder, passed as a result of
 `Note [Case binders and join points]` in `Simplify`.  But in this case it
 turns out that not only is the case-binder argument eventually unused, but
 (even worse) every `juump` to that join point looks like `jump j x (I#
 x)`.  So we box it, and then immediately discard it.  Urk.

 Just recording breadcrumbs for now. My thought: never pass the case binder
 to a join point; instead re-box it.  It's much the same trade-off as in
 other places where we worry about re-boxing.  And it would be a lot
 simpler; in particular, we can stop having unfoldings in lambda-binders,
 which is pretty dodgy anyway.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5302#comment:15>
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