Garbage collection

[Simon Peyton Jones]

Isn't it this simple: the Static Pointer Table must be a source of roots for the garbage collector.  Of course!  An item in the SPT may be looked up at any time.

Easy.  What am I missing?

I'm deeply suspicious of any solution that involves a call to performGC. That smells wrong.

Simon

|  -----Original Message-----
|  From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of
|  Facundo Domínguez
|  Sent: 18 November 2014 00:44
|  To: ghc-devs at haskell.org
|  Subject: Fwd: Garbage collection
|  
|  Hello,
|     While working in the StaticPointers language extension [1], we
|  found we have some unusual CAFs which can be accessed after some
|  periods of time where there is no reference to them.
|  
|     For instance, the following program when compiled contains no
|  reference to `g`. `g` is actually looked up at runtime in symbol
|  tables via the call to `deRefStaticPtr`.
|  
|  g :: String
|  g = "hello"
|  
|  main =
|    deRefStaticPtr (static g) >>= putStrLn
|  
|  Desugars to:
|  
|  g :: String
|  g = "hello"
|  
|  main =
|    deRefStaticPtr (StaticPtr (StaticName "" "Main" "g")) >>= putStrLn
|  
|  In principle, there is nothing stopping the garbage collector from
|  reclaiming the closure of `g` before it is dynamically looked up.
|  
|  We are considering using StablePtrs to preserve `g`. So the code
|  desugars instead to:
|  
|  g :: String
|  g = "hello"
|  
|  main =
|    deRefStaticPtr (let x = StaticPtr (StaticName "" "Main" "g")
|                           in unsafePerformIO $ newStablePtr g >> return
|  x
|                          ) >>= putStrLn
|  
|  This solution could be temporal though, until we implement the so
|  called static pointer table, which would keep the values alive.
|  
|  Would you have any comments about such a solution or maybe would you
|  advice some other alternative?
|  
|  Thanks,
|  Facundo
|  
|  [1] https://ghc.haskell.org/trac/ghc/wiki/StaticPointers
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