[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 batterseapower):

 Roman: yes, sorry - let's say that go is used in the body of the let.

 Simon: right. However, I'm not so sure that CoreTidy is actually
 preserving rules on local lets right now.. certainly nothing in
 tidyLetBndr seems to be using setIdSpecialisation or similar on the
 outgoing Id. I noticed this when writing the patch above and was actually
 wondering if was intentional...

 I'm not sure if your proposed plan will work exactly as described because
 at any point after CoreTidy (including in the output of CorePrep) all top
 level binders are global ids and hence not reported as free variables.
 This causes the occurrence analyser to rewrite e.g.:

 {{{
 Rec {
 f = ... f ... -- Note that f is a globalid!
 }
 }}}

 To:

 {{{
 f = ... f ... -- Note that the Rec wrapper has vanished
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4962#comment:5>
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