[GHC] #10744: Allow oneShot to work with unboxed types

[GHC]

#10744: Allow oneShot to work with unboxed types
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        Reporter:  akio              |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by simonpj):

 Replying to [comment:9 nomeata]:
 > Simon, I am worried that due to a wrong annotation here, a thunk is
 marked as single entry that is not really single entry, and I can imagine
 that bad things can happen then. But if you think that this cannot happen,
 then I’d be relieved, and akio can go forwared using `oneShot`.

 Yes, this is what happened in #10414 and #10218.  Suppose in my example in
 comment:8 that GHC does ''not'' float the `let x` thunk into the lambda
 for some reason. Then it might instead be marked single-entry.  So we get
 {{{
 f xs y = let x[single-entry] = expensive_fn y
          in map (oneShot (\v. x+v)) xs
 }}}
 The bad thing in #10414 and #10218 was that, with eager black-holing, `x`
 was blackholed, so that the second time it was entered we got `<<loop>>`.
 But the fix in ticket:10414#comment:29 switches off eager-blackholing for
 single-entry thunks, so it'll all be fine.

 So, because of that, I think we are good.

 Mind you, if we implement the sanity check in #10613, you might get
 failures reported due to bogus `oneShot` claims.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10744#comment:10>
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