[GHC] #15754: Move free variable computation to after STG passes

[GHC]

#15754: Move free variable computation to after STG passes
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        Reporter:  bgamari           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9718             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by sgraf):

 LLL will run as late as possible, ideally immediately before going to Cmm.
 It's pretty easy for LLL to maintain FV information accurately, I'd say,
 so generating it once (or as needed) would still be feasible.

 I'm still not sure if it's a good idea to store free variable information
 ''directly in STG syntax'' if we only guarantee the implied invariants
 immediately after StgFV is run. Sort of the same problem which is solved
 by Trees to grow in the frontend.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15754#comment:7>
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