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Re: RFC: reference counting for value
- From: Daniel Jacobowitz <drow at false dot org>
- To: Tom Tromey <tromey at redhat dot com>
- Cc: gdb-patches at sourceware dot org
- Date: Mon, 6 Jul 2009 21:49:14 -0400
- Subject: Re: RFC: reference counting for value
- References: <m3tz26k87c.fsf@fleche.redhat.com>
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