[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:  new
        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):
       Wiki Page:                    |
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Comment (by simonpj):

 Good point!  For example, if the demand analyser saw
 {{{
 let y = factorial v in
 let x = y in
 x + x
 }}}
 I think it'd conclude that `x` was demanded many times, but `y` was
 demanded only once.  (Which is correct).  But if we substitute for `x`,
 and then use call-by-name for `y` we'll evaluate the `factorial` twice.

 Urk.

 It seems to affect bindings like
 {{{
 x = y    -- Or perhaps y |> gamma etc; exprIsTrivial anyway
 }}}
 where
  * The demand signature on `x` is '''not''' marked "used-once"
  * The demand signature on `y` '''is''' marked "used-once"

 Under these circumstances, the binding for `x` is serving a useful role,
 to memo-ise the computation of `y`.

 Perhaps we should simply refrain from inlining `x` under these
 circumstances, leaving the trivial let in place.  The fix would be in
 `postInlineUnonditionally`, `postInlineUnonditionally`, and
 `callSiteInline`.

 Would you like to try that?  I think it'd fix this bug.  But it would then
 be important to know how many trivial lets were thereby retained.  Perhaps
 not many.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11731#comment:5>
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