[GHC] #8326: Place heap checks common in case alternatives before the case

[GHC]

#8326: Place heap checks common in case alternatives before the case
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              Reporter:  jstolarek   |            Owner:
                  Type:  task        |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.7
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:  #1498
             Test Case:              |
              Blocking:  8317        |
Differential Revisions:              |
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Comment (by dfeuer):

 Replying to [comment:7 jstolarek]:
 > Replying to [comment:3 dfeuer]:
 > > If I hand it `12#`, it will dutifully tell me `False`. What exactly
 have I learned from that?
 > That the argument you passed is not a valid tag for `True`.
 >
 > This ticket is not really about `isTrue#` or `isFalse#` - which are just
 tools a programmer might want or not want to use - but about fixing the
 heap checks and thus fixing #8317. If you feel that `isTrue#` and
 `isFalse#` don't offer you any benefit you can still use `tagToEnum#`.

 I'm not sure where the appropriate place is for this line of discussion,
 but it seems that in the wild (all over the library source), `isTrue#` is
 typically used as a function for converting from `Int#`, produced by a
 comparison operator, to `Bool`, rather than as a validity test for `True`.
 I have yet to see any explanation of why that is appropriate. Certainly,
 optimizing heap checks is an entirely different matter, and presumably a
 good idea regardless.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8326#comment:8>
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