[GHC] #8598: IO hack in demand analyzer gets in the way of CPR

[GHC]

#8598: IO hack in demand analyzer gets in the way of CPR
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        Reporter:  nomeata           |            Owner:
            Type:  task              |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.6.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:  T8598             |       Blocked By:
        Blocking:                    |  Related Tickets:
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Changes (by nomeata):

 * testcase:   => T8598


Comment:

 My extended approach suggested above has issues. For example, what should
 be the `DmdResult` of a function parameter be? Lets consider {{{f x y = x
 `seq` y `seq` ...}}} and look at the `DmdResult` of `x`.

 Theoretically, we’d want to be conservative, and hence `DmdResult {
 mayExit = True, mayDiverge = True, cpr = NoCPR }`. But that would loose
 far too much strictness information, in particular we’d be lazy in `y`,
 which is undesirable.


 OTOH, if we assume `DmdResult { mayExit = False, mayDiverge = True, cpr =
 NoCPR }` for `x` it would be wrong if we pass something that can cleanly
 exit to `f`. `unsafePerformIO exitSuccess`  would be such a thing – but we
 do not really care about that, right?

 If we take the stance that clean exists should only really be happening
 when evaluating `IO` (and any other kind exiting is allowed to be
 reordered with other evaluations), then a still correct, but more precise
 analysis that we have right now is to use the elaborated lattice described
 in comment:5 and move the ''IO hack'' from case expressions to arguments:
 Use `DmdResult { mayExit = True, mayDiverge = True, cpr = NoCPR }` for
 arguments of `IO` type, and `DmdResult { mayExit = False, mayDiverge =
 True, cpr = NoCPR }` for all others.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8598#comment:8>
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