[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):

 Humph.  I was thinking that if a function has a particular strictness
 signature we can't change it.  Certainly we can't change the strictness
 signature for an imported function.

 BUT we CAN change the strictness signatures of local functions and
 actually those are the ones that matter, because their sharing properties
 may worsen, as above.

 Indeed we already have the idea of running the demand analyser just before
 code generation: `-flate-dmd-anal`.  (Main motivation: discover used-once
 thunks that weren't previously used-once.)

 Does using `-flate-dmd-anal` fix the problem?  It may not becuase there is
 a simplifer run afterwards; but cleraly this bug only shows up rarely
 anyway, so running it late might actually fix it

 Thinking about it
  * ''Until'' code generation, the used-once info is ignored; only one-
 shot-call info is used
  * ''During'' code generation, the used-once info is used; and the one-
 shot-call info is ignored.

 So perhaps
  * We should not even ''generate'' used-once info in the main run of the
 demand analyser, since it is so easily invalidated.
  * In the run of the simplifier after late demand analysis we should be
 careful not to eliminate the dangerous lets.

 I tried the test program with `-O`; but it only prints `"Evaluated"` once.
 So, strangely, I can't reproduce the bug.  What exact commands did you
 use?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11731#comment:8>
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