[GHC] #13944: Introduce synchronized FFI

[GHC]

#13944: Introduce synchronized FFI
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        Reporter:  winter            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by winter):

 OK, now i'm pretty sure what `synchronized` would do, it will behave just
 like a `safe` FFI call with one difference: a task which comes across a
 `synchronized` call will give off its capacity, but will not hand off it
 to other tasks unless that's a GC task. This means `synchronized` calls
 will still block whole capacity, but it will not spawn new threads, nor
 blocking GC.

 I'm not sure if this make sense, it seems the overhead of safe FFI calls
 come from both the capacity transfer and creating new OS threads. But i'm
 not sure which is more costly.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13944#comment:5>
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