Demonstrate that taking an lvalue substr ref causes the referenced substring to be copied, And that this is a bug.

Related to ref to read-only alias ... why?? Can't replicate with substr.

Related to ref to read-only alias ... why??

I don't think so.

This demonstrates the problem (bug):

perl -E"$x=chr(0); $x x= 100e6; <>; $r = \substr $x, 50e6; <>"
[download]

Watch the process memory with ProcessManager:

1. When it pauses the first time memory usage is ~98MB. Hit enter
2. Now the memory usage shoots up to ~145MB.

   All that's changes is that it has taken a substr ref to the second half of the string.

I suspect it is a result of this "fix".

But I believe that fix is wrong, and that the original bug is spurious. (At least in part.)

If you do this:

my $ref;
{
    my $string = "123";
    $ref = \$string;
}

## Here $string persists.
[download]

No one is surprised that (the memory allocated to) $string persists beyond the block.

Why should this be any different:

my $ref;
{
    my $string = "123";
    $ref = \substr $string, 1, 1;
}

## This was the (IMO false) "bug" scenario.
[download]

In this case -- when the string goes out of scope -- I could make arguments for 3 possible outcomes:

1. The memory allocated to $string persists:

   $ref continues to refer (directly) to the memory allocate to $string.

   Those parts of $string outside of $ref are inaccessible and irrecoverable until $ref is GC's.

2. The memory allocated to $string is GC'd; $ref becomes undef.

   Probably difficult to orchestrate.

3. The memory allocated to $string persists until $ref is used the next time:

   At which point the extraneous (pre and/or post fix) bits of $string are GC'd leaving $ref pointing at just that which it references.

   This would require retaining a pointer back to 'parent' string with the lvalue ref, and when the lvalue magic fires it checks the refcount of its parent.

   If that refcount is 1 -- meaning it holds the only reference to it, it frees off the extraneous parts of the parent.

Of the three, I'd prefer C, but would find A completely in keeping with Perl's philosophy.

Duplicating the referenced memory of every lvalue ref and the having to copy it back each time it is modified -- just to cover off a really obscure scenario that is at worst, only mildly anomalous -- is crazy.

---

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In the absence of evidence, opinion is indistinguishable from prejudice.

The start of some sanity?