RFC: reference counting for value
Daniel Jacobowitz
drow@false.org
Tue Jul 7 01:49:00 GMT 2009
On Tue, Jun 23, 2009 at 02:26:31PM -0600, Tom Tromey wrote:
> The "increment reference" function is called "release_value" for
> historical reasons (I can change this if anybody cares). Most code
> does not require any change, as one always had to pair release_value
> and value_free calls anyhow.
IMO it would be nice to rename it (separately).
I am trying to resurrect an old patch of Vladimir's, which gives
bitfield values a parent pointer. We fetch the parent once, instead
of once per bitfield. That raised an interesting question:
> + /* The reference count. A value that is still on the `all_values'
> + list will have a reference count of 0. A call to `release_value'
> + will increment the reference count (and remove the value from the
> + list, the first time). A call to `value_free' will decrement the
> + reference count, and will free the value when there are no more
> + references. */
> + int refcount;
> +
> /* Register number if the value is from a register. */
> short regnum;
>
If we release_value the parent every time we create a child, and
value_free it every time we collect a child, the parent will be freed
as soon as its last child is. This is a change in the value behavior,
because otherwise it would hang around until value_free_to_mark or
free_all_values.
Is this going to bite us? We could, instead, record release_value
references separately from parent references and leave the value on
the chain. But if it doesn't matter, I'd rather not.
--
Daniel Jacobowitz
CodeSourcery
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