[GHC] #14375: Implement with# primop

[GHC]

#14375: Implement with# primop
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               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:  #14346            |  Differential Rev(s):  ​Phab:D4110
       Wiki Page:                    |
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Comment (by simonmar):

 But if we had

 {{{
 join j s = e
 in maskAsyncExceptions# j s
 }}}

 How can we compile this? `maskAsyncExceptions#` entails pushing a stack
 frame, but jumping to the join point entails truncating the stack back to
 the `join`. We can't do both! Am I missing something?

 I suggest that we treat `with#` in the same way as `runST` for the
 purposes of fixing the current bug. I think it'll be simpler to do it this
 way than what we have in Phab:D4110, because we don't need any support in
 the RTS at all.

 I agree it would be nicer to find a unified way of handing all these
 primops that take IO continuations, and especially it'd be great to avoid
 the allocation for `catch#`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14375#comment:6>
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