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Re: struct displacements
From: |
Andy Wingo |
Subject: |
Re: struct displacements |
Date: |
Mon, 25 Jan 2010 15:55:27 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/23.0.92 (gnu/linux) |
Hi Ludo' :)
On Mon 25 Jan 2010 14:44, address@hidden (Ludovic Courtès) writes:
> Andy Wingo <address@hidden> writes:
>
>> - GC_REGISTER_DISPLACEMENT (2*sizeof(scm_t_bits)); /* for the self data
>> pointer */
>> - GC_REGISTER_DISPLACEMENT (2*sizeof(scm_t_bits)
>> - + scm_tc3_struct); /* for the vtable data
>> pointer */
>> + /* The first word of a struct is equal to `SCM_STRUCT_DATA (vtable) +
>> + scm_tc3_struct', and `SCM_STRUCT_DATA (vtable)' is 2 words after
>> VTABLE by
>> + default. */
>> + GC_REGISTER_DISPLACEMENT (2 * sizeof (scm_t_bits) + scm_tc3_struct);
>>
>> I'm pretty sure the first case is still needed, for instance
>> reallocation. It is, as the comment indicates, "for the self data
>> pointer" -- the second word of the object, which normally points to the
>> third word of the object, but for reallocated instances points to the
>> third word of *another* object.
>
> As an excuse, the comment was a bit too terse for me to understand, but
> I get your point now. However, it’s still unclear to me how an ‘s’
> field can point to something else than “self”.
Hm, I think you're right; the wording was not only terse but misleading.
It is "for the data pointer, which might point to another object's
data", or something like that. Nothing to do with `s' fields.
>> See my article on redefining classes for more info.
>
> By ‘self’ here, did you mean what’s known as ‘data’ on the diagrams at
> <http://wingolog.org/archives/2009/11/09/class-redefinition-in-guile>?
Yes indeed.
> PS: It would be really cool if this blog post were part of the “Data
> Representation” node of the manual. :-)
Heh, yeah. That whole chapter needs revision...
Andy
--
http://wingolog.org/