[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 simonpj):

 I agreed comment:18 is odd, but first why does "demand information become
 something we should try to keep hold on"?  What, specifically, goes wrong?

 The reasoning here is as follows. Consider
 {{{
 let f = \xy. x+y
 in ...map (f expensive)....
 }}}
 Now, in `f`'s definition `x` and `y` are both marked
  * `OneOcc not-in-lambda` (syntactically occurring once not inside a
 lambda; added by the occurrence analyser)
  * Demanded (added by the demand analyser)
 These annotations assume that `f` is fully applied

 But when we inline `f` at this partial application site, we get
 {{{
 ...map (let x = expensive in \y. x+y)...
 }}}
 and now `x` jolly well shouldn't be marked as occurring not inside a
 lambda; nor should it be marked as strictly demanded.

 Hence the `zapLamIdInfo`.

 What I had forgotten (and has somehow never arisen) is that this ''also''
 happens at `f` definition site.

 Maybe it doesn't matter.  Back to the quesion at the beginning: why does
 it matter?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11731#comment:20>
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