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12776 :: Crash in ARC and ORC for large sequences of objects containing refs - Works with refc

• joppez (orginal) [2025-04-08T00:13:52+02:00] view original

  Maybe I should add this as a bug directly at Github, but I think I have found a bug in ARC/ORC, but please check that I'm not doing something fishy.

  Consider this program:

  
  const
      num_of_objects = 1_000_000
  
  type
    node = object
      name: string
      child: nodeRef
    nodeRef = ref node
  
  proc allocAndDump() =
    var s = newSeqOfCap[nodeRef](num_of_objects)
    for i in 1..num_of_objects:
      s &= nodeRef(name: "foo", child: nil)
    
    var prev: nodeRef = nil
    for o in s.mitems:
      o.child = prev
      prev = o
  
  allocAndDump()
  Run

  For small values of num_of_objects it works like it should, but for larger values like 1_000_000 it crashes in lib/system/orc.nim:503 (unregisterCycle(s)), when compiled with --mm:orc and in lib/system/arc.nim:213 (proc nimDecRefIsLast) when compiled with --mm:arc. With refc it works without problems.

  Am I doing something wrong?

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• Araq (orginal) [2025-04-08T00:30:09+02:00] view original

  That's a stack overflow caused by Nim's recursive destructors.

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• joppez (orginal) [2025-04-08T00:42:08+02:00] view original

  Thanks. I see. So the workaround is to increase the size of the stack or to not do this at all? :) Adding the acyclic pragmas does not help, since it still has to traverse all the objects, I guess?

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• joppez (orginal) [2025-04-08T01:06:05+02:00] view original

  Increased the size of the stack and now it works. Thanks for the help.

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• Clonk (orginal) [2025-04-09T00:51:37+02:00] view original

  Does Nim have some tail call optimisation available to optimise recursive call like these ?

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• Araq (orginal) [2025-04-09T12:03:30+02:00] view original

  No and in general it's not a tail call anyway. But in general "simple enough" destructors can be generated not to use recursion. Swift has this transformation but Nim currently lacks it.

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