[GHC] #14461: Reuse free variable lists through nested closures

[GHC]

#14461: Reuse free variable lists through nested closures
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           Reporter:  tdammers       |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  7258
Differential Rev(s):                 |         Wiki Page:  NestedClosures
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 Consider [wiki:NestedClosures] and [ticket:7258]; essentially, deeply
 nested closures exhibit quadratic compiler performance due to the fact
 that when allocating registers, each nesting level will have the compiler
 unpack the entire parent closure and then re-pack the variables into the
 child closure.

 To solve this, check if the parent closure can be carried along wholesale,
 and pull variables from there so that the repackaging can be bypassed.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14461>
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