Re: struct displacements

[Andy Wingo]

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
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