[GHC] #11731: Simplifier: Inlining trivial let can lose sharing

[GHC]

#11731: Simplifier: Inlining trivial let can lose sharing
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        Reporter:  nomeata           |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2064
       Wiki Page:                    |
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Comment (by nomeata):

 Over at Phab you write

 > I still think this is the wrong approach.

 but now I’m confused. Where did you say what’s wrong with this approach?
 This approach is my attempt to implement your suggestion “Perhaps we
 should simply refrain from inlining x under these circumstances” from
 comment:5!

 From this suggestion, we need to detect “these circumstances”, which
 requires us to detect that “The demand signature on y is marked "used-
 once"”. And this implies that looking at demand information. And this
 implies that we need to keep hold to it.

 Personally, I quite agree that the annotation should *not* be something we
 need to hold on to, although I don’t have a convincing solution, only the
 rough ideas above that boil down to systematically distinguishing between
 variables that guarantee to have sharing behavior and those where this is
 not guaranteed.

 Anyways, thanks for the explanation about `zapLamIdInfo`. It makes perfect
 sense when inlining a function into an expression. But do you agree that
 it should not be done to `f`’s definition?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11731#comment:21>
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