[GHC] #4962: Dead code fed to CorePrep because RULEs keep it alive spuriously

[GHC]

#4962: Dead code fed to CorePrep because RULEs keep it alive spuriously
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    Reporter:  batterseapower    |        Owner:                         
        Type:  bug               |       Status:  new                    
    Priority:  normal            |    Milestone:                         
   Component:  Compiler          |      Version:  7.0.1                  
    Keywords:                    |     Testcase:                         
   Blockedby:                    |   Difficulty:                         
          Os:  Unknown/Multiple  |     Blocking:                         
Architecture:  Unknown/Multiple  |      Failure:  Runtime performance bug
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Comment(by simonpj):

 OK, here's a refined plan
  * In `CorePrep.cloneBndr`, drop all unfoldings or rules
  * In `CorePrep.deFloatTop`, which is the bit right at the end of
 `CorePrep`, do `occurAnalyseExpr` on the RHS of each top-level binding.

 The second step will just drop ''local'' dead let-bindings which are the
 ones you want to drop.  For that, it's fine for the top level Ids to be
 `GlobalId`s.

 I think you are right that local bindings don't currently retain their
 RUES in `CoreTidy`, but I think that's ok.  It would only matter in a
 function that was going to be inlined.  But if it's small enough to be
 inlined, it's not going to be big enough to have interesting
 specialisations on local let-bindings.

 The reason we keep INLINE unfoldings is that Roman does write INLINE
 pragmas whose RHS has a local binding with an INLINE pragma.  If he wrote
 local bindings with RULEs I'd have to retract the previous para -- but we
 have no way to write RULES for local defns (yet)!

 Do you feel like trying this out?

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4962#comment:6>
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