[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 simonpj):

 * Yes, I think we should take the arity and CAFness from the STG code, not
 ''just after we generate it'', but rather ''just before we code-gen it'' ,
 which is the moment of truth.

 * Rather than use the `IfaceIdInfo` for these two fields, I think we could
 make them two fixed fields of each top-level Id binding in a `ModIface`.
 Every Id has CAF-ness and arity.

 * The final arity is really the ''representation'' arity, after
 unarisation of unboxed tuples etc.  I think we should probably treat that
 as a separate matter to the "arity" recorded by the Simplifier.   The
 latter really can be passed on by CoreTidy, exactly as now.  The former
 determines calling convention etc; it's a code-gen thing.  We currently
 fudge this issue: see `idRepArity`.

 * Fingerprints will indeed change; but they are in any case calculated
 separately; see `addFingerprints`.  So we probably want to defer that step
 too.

 * For now, I'd be inclined to simply hold onto the partly-complete
 `ModIface` until codegen; then add info from the just-before-codegen STG
 code; and construct a final `ModIface`.   If that gives rise to residency
 issues we can decide what to do then.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9718#comment:12>
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