[GHC] #14337: typeRepKind can perform substantial amounts of allocation

[GHC]

#14337: typeRepKind can perform substantial amounts of allocation
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           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.4.1
          Component:  Core           |           Version:  8.2.1
  Libraries                          |
           Keywords:  Typeable       |  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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 I came up with a (rather contrived) test case to demonstrate that
 Phab:D4082 reduced big-O time complexity in pathological cases. But I
 expected it to increase space usage by a constant factor. What I found was
 very much the opposite: it dramatically reduced allocation. The reason for
 this is obvious in hindsight. Every time we call `typeRepKind`, we
 recalculate the kind entirely from scratch. That recalculation is only a
 potential ''time'' problem for `TrApp`, because we only need to walk down
 links, but it's also a ''space'' problem for `TrTyCon`, because we're
 building up a `TypeRep` from a `KindRep`.

 The solution, assuming we choose to keep `typeRepKind`, seems fairly
 clear: whether or not we choose to cache the kind in `TrApp`, we should
 almost certainly do so in `TrTyCon`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14337>
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