[GHC] #11735: Optimize coercionKind

[GHC]

#11735: Optimize coercionKind
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 >  I would love to have some hard evidence that we actually do have a
 problem.

 Well, without caching we know that getting the role of `NthCo i1 (NthCo i2
 (NthCo i3 ...))` will take time at least quadratic in the nesting depth.
 Caching cheaply avoids nasty corner case. And not much code is involved.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11735#comment:24>
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