[GHC] #8662: GHC does not inline cheap inner loop when used in two places

[GHC]

#8662: GHC does not inline cheap inner loop when used in two places
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        Reporter:  nh2                      |            Owner:
            Type:  bug                      |           Status:  new
        Priority:  normal                   |        Milestone:
       Component:  Compiler                 |          Version:  7.6.3
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  Runtime performance bug  |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Comment (by simonpj):

 GHC decides when to inline like this:
  * It computes the "size" of the function
  * At a call site, it takes the size of the function, subtracts a call-
 site "discount", and if the result is less than the "unfolding threshold"
 it does the inlining.
  * The discount is increased if both (a) an actual argument has some
 structure (eg is a constructor application) and (b) that argument is
 scrutinised by a case expression in the function body.
  * There is also a "result discount" if (a) the function call is consumed
 by a `case`, and (b) the function body returns a constructor application

 All this is computed in module `CoreUnfold`, so it is nicely separated
 from the rest of the compiler.

 Do by all means have a go at changing the computation of size or discount.
 Then measure (a) the change in code size and complication time vs (b) the
 change in allocation and/or run-time.  If you can reduce (b) without
 increasing (a) you are winning!

 But it's easy to mess up.  I have spent many happy hours fiddling with
 these heuristics.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8662#comment:3>
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