Thanks for the supplying the full SP on SVs :)

...but you can't make any assumptions about SVs allocated from different arenas.

Would it be accurate to say that reference values stringified within Perl

  • Are always quad-aligned?
  • Will never be closer than 12 bytes apart?
  • Even if two SV headers are within different arenas, and the arenas are adjacent, the last SV in the lower address arena will still be at least 12 bytes from the end and therefore at least 12 bytes from the first one in the higher addressed arena?
  • If we're on a 64-bit machine, the SV headers are always more than 12 bytes, so using 12 as my divisor is safe?

    (Do they becomes 16 or 24 bytes on 64-bit systems?)

  • A future release of Perl 5 is unlikely to reduce the size of an SV header?

    Any thoughts on getting a rough value for the lowest and highest address that can be a reference at any given moment in time?

    For the lowest, I've been looking through the source trying discover when the various system globals come into being. The thought being that maybe say $^X or $^O would be likely to be allocated pretty early and addresses below that are unlikely to come up in user datastructures, leastwise not as self references? Suggestions for the best choice?

    For the top end, I've been thinking along the lines that any datastructure I am going to traverse already exists, so if I could force the allocation of a new SV, it's address would form an upper bound for my circular reference testing.

    The fly in that ointment is that if I simple declare a new variable, I am quite likely to re-use an old one that has gone out of scope.

    Do you know of any way that I could force the allocation of a completely new SV (header)?

    I thought about allocating a largish array, big enough that it would force a new allocation from the OS and then use the address of the highest as my upper bound. I also thought that if the largish array was immediately undef'd and I then pre-allocated my bit-vector, it might get to re-use the same space.

    From what you are saying, and what little I understand about the way Perl allocates memory, the space allocated for a long string is unlikely to re-use space allocated for a large array because they would be allocated from different pools?

    Examine what is said, not who speaks.
    Silence betokens consent.
    Love the truth but pardon error.

    In reply to Re^2: Circular reference testing. by BrowserUk
    in thread Circular reference testing. by BrowserUk

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