[GHC] #7300: Allow CAFs kept reachable by FFI to be forcibly made unreachable for GC

[GHC]

#7300: Allow CAFs kept reachable by FFI to be forcibly made unreachable for GC
-------------------------------------+--------------------------------------
 Reporter:  absence                  |          Owner:                  
     Type:  feature request          |         Status:  new             
 Priority:  normal                   |      Component:  Compiler (FFI)  
  Version:  7.4.1                    |       Keywords:  unsafe caf gc   
       Os:  Unknown/Multiple         |   Architecture:  Unknown/Multiple
  Failure:  Runtime performance bug  |       Testcase:                  
Blockedby:                           |       Blocking:                  
  Related:                           |  
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 CAFs used by a foreign exported function are kept reachable the entire
 session because GHC can't know when the function will be called from C. If
 such a CAF is an evolving expression, like an FRP network, a space leak
 occurs because (I'm guessing) the thunks that build up during iteration go
 all the way back to the initial CAF, and the GC can't start collecting
 because it considers the CAF reachable. According to JaffaCake on the
 #haskell IRC channel, the runtime is capable of sovling this problem, it
 just needs a function that tells it to consider the specific CAF
 unreachable. It is then the responsibility of the user to not call the
 foreign exported function after the CAF is forced unreachable.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7300>
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