[GHC] #12791: Superclass methods could be more aggressively specialised.

[GHC]

#12791: Superclass methods could be more aggressively specialised.
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        Reporter:  mpickering        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ryantrinkle):

 Hmm, that makes a lot of sense.  I think we probably would run into that
 sometimes, although I'm not entirely sure how often.  We  may be able to
 factor out these superclasses - in the cases I know of, they're for
 convenience, not out of necessity.

 There's a lot I don't understand here, but I do suspect that in our real-
 world cases, extremely aggressive inlining (like I'm hoping to get
 working, see https://mail.haskell.org/pipermail/ghc-
 devs/2016-October/013142.html) may result in enough inlining that this
 issue doesn't matter.  Ideally, I'd like both the subclass and the
 superclass to be inlined.  I'm just worried that sometimes the subclass
 won't be inlinable, and if that also prevents the superclass from being
 inlined, that'll make the optimization very brittle.

 With regard to having the wrong dictionary being passed in, I suppose
 changing this behavior would lean very heavily on the canonicity of
 instances - and perhaps would interfere with uses of incoherent instances
 and such?  I'm fairly certain it wouldn't be a problem in any of our code,
 but I wouldn't want to cause problems for others.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12791#comment:6>
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