[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 simonpj):

 > That's probably what Simon alludes to in comment:6 by suggesting a type
 index to solve this.

 Yes. Even without the full glory of TTG we could have something like this:
 {{{
 data GenStgRhs pass
 = StgRhsClosure
         CostCentreStack         -- CCS to be attached (default is
 CurrentCCS)
         StgBinderInfo           -- Info about how this binder is used (see
 below)
         (XRhsClosure pass)      -- non-global free vars; a list, rather
 than
                                 -- a set, because order is important
         !UpdateFlag             -- ReEntrant | Updatable | SingleEntry
         [StgBndr pass]          -- arguments; if empty, then not a
 function;
                                 -- as above, order is important.
         (GenStgExpr pass)   -- body

 type instance XRhsClosure pass where
   XRhsCLosure JustBeforeCodeGen = [Id]
   XRhsClosure _                 = ()
 }}}
 Now `GenStgRhs JustBeforeCodeGen` has free-var info, but `GenStgRhs
 OtherPass` does not.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15754#comment:8>
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