[GHC] #9718: Avoid TidyPgm predicting what CorePrep will do

[GHC]

#9718: Avoid TidyPgm predicting what CorePrep will do
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.3
      Resolution:                    |             Keywords:  CodeGen, CAFs
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by osa1):

 Another thing we need to figure out is how to update use sites of binders
 when
 we update CAFness info. For example, in the example in #15113:

 {{{
 lvl2_r3y3 :: [Char]
 [GblId]
 lvl2_r3y3 = unpackCString# lvl1_r3y2

 -- RHS size: {terms: 7, types: 6, coercions: 2, joins: 0/0}
 patError :: forall a. Addr# -> a
 [GblId, Arity=1, Str=<B,U>x, Unf=OtherCon []]
 patError
   = \ (@ a_a2kh) (s_a1Pi :: Addr#) ->
       raise#
         @ SomeException
         @ 'LiftedRep
         @ a_a2kh
         (Control.Exception.Base.$fExceptionPatternMatchFail_$ctoException
            ((untangle s_a1Pi lvl2_r3y3)
             `cast` (Sym (Control.Exception.Base.N:PatternMatchFail[0])
                     :: (String :: *) ~R# (PatternMatchFail :: *))))
 }}}

 We want to be able to make `lvl2_r3y3` re-entrant. When we do this
 `patError`
 won't have any CAF refs, but to actually realize this we need to update
 references to `lvl2_r3y3` to update the id info.

 Alternatively I think we could pass an environment of updated ids to
 `topStgBindHasCafRefs` and friends.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9718#comment:13>
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