[GHC] #12961: Duplicate exported functions?

[GHC]

#12961: Duplicate exported functions?
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           Reporter:  nomeata        |             Owner:
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 In my paper on Call Arity, I wrote
 > Unfortunately it also means that adding a top-level function to the
 export list of the module can prevent Call Arity from eta-expanding it and
 other functions in the module.
 and a reviewer asked
 > Why don’t you duplicate and specialize the function?

 And that is an interesting question, which I don’t want to pursue at the
 moment in detail, but note down.

 Obviously, if the the function can be inlined in all uses in the current
 module, we essentially do that already. But if not (for example, because
 it is recursive), then this could indeed improve the performance of the
 in-module uses.

 The question is: Is it worth it?

 Questions on the way:

 Do we have more analyses like Call Arity that gather information form the
 use sites, and that need to make pessimistic assumptions for exported
 functions, or is it the only one?

 In terms of final code size: Will CSE reliably undo the duplication of
 possible large swathes of code if both copies of the function were
 compiled identically?

 Maybe instead of duplicating all exported functions opportunistically,
 Call Arity could analyze them twice and only if it gets useful data from
 the use-sites cause them to be duplicated.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12961>
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