[GHC] #14152: Float exit paths out of recursive functions

[GHC]

#14152: Float exit paths out of recursive functions
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        Reporter:  nomeata           |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  JoinPoints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14137 #10918     |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by nomeata):

 Thanks for the guidance, that nailed it:
  * The code `isJoinId bndr, isOneOcc (idOccInfo bndr), occ_in_lam
 (idOccInfo bndr)` reliably detects an exit join point now, it seems. (I
 guess this deserves its own function `isExitJoinPoint`)
  * When I create the join point, I add the correct `OccInfo`, so that this
 is there even before OccurAnal runs the next time.
  * In `simplLetUnfolding`, when I detect an exit join point, I do not
 creating an unfolding. This prevents unfolding in `completeCall` and
 `postInlineUnconditionally`.
  * In `preInlineUnconditionally` I check for exit join points explicityly.

 Maybe it would be cleaner to leave the unfolding alone and simply check
 for `isExitJoinPoint` in all three places… but no, the simplifier seems to
 throw away the `idOccInfo`. So better stick to  not creating an unfolding.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14152#comment:12>
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