[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:  patch
        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 #15867      |  Differential Rev(s):  Phab:D5324
  #15901                             |
       Wiki Page:                    |
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Comment (by Sebastian Graf <sebastian.graf@…>):

 In [changeset:"47bbc709cb221e32310c6e28eb2f33acf78488c7/ghc" 47bbc70/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="47bbc709cb221e32310c6e28eb2f33acf78488c7"
 Don't track free variables in STG syntax by default

 Summary:
 Currently, `CoreToStg` annotates `StgRhsClosure`s with their set of non-
 global
 free variables.  This free variable information is only needed in the
 final
 code generation step (i.e. `StgCmm.codeGen`), which leads to
 transformations
 such as `StgCse` and `StgUnarise` having to maintain this information.

 This is tiresome and unnecessary, so this patch introduces a trees-to-
 grow-like
 approach that only introduces the free variable set into the syntax tree
 in the
 code gen pass, along with a free variable analysis on STG terms to
 generate
 that information.

 Fixes #15754.

 Reviewers: simonpj, osa1, bgamari, simonmar

 Reviewed By: osa1

 Subscribers: rwbarton, carter

 GHC Trac Issues: #15754

 Differential Revision: https://phabricator.haskell.org/D5324
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15754#comment:14>
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