[GHC] #7542: GHC doesn't optimize (strict) composition with id

[GHC]

#7542: GHC doesn't optimize (strict) composition with id
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        Reporter:  shachaf                  |            Owner:  simonpj
            Type:  bug                      |           Status:  new
        Priority:  normal                   |        Milestone:  7.8.1
       Component:  Compiler                 |          Version:  7.6.1
      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 ekmett):

 @nomeata:

 As I understand the situation, the fact that this optimization isn't
 happening without `-fpedantic-bottoms` means I'm basically stuck using the
 existing hacks in `lens`.

 The code in question is set up to `INLINE` in library into user code, and
 they very likely aren't compiling with that flag! (Unless I'm
 misunderstanding how and where that gets tracked and applid.)

 We have 3 cases to consider, the existing unsafeCoerce hack, the naive eta
 expansion we're getting without `-fpedantic-bottoms`, and the version
 without the eta expansion we can produce with `-fpedantic-bottoms`. It
 appears that the first and the third scenarios are now the same, but the
 difference in performance between the existing unsafeCoerce hack and the
 eta expanded code can be asymptotic, if this is closed as is, the fix does
 me no good, and I'll be left using the existing coercions to ensure a good
 user experience. =/

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7542#comment:12>
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