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Re: struct displacements

From: Ludovic Courtès
Subject: Re: struct displacements
Date: Mon, 25 Jan 2010 14:44:53 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux)


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 
>     +     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”.

> See my article on redefining classes for more info.

By ‘self’ here, did you mean what’s known as ‘data’ on the diagrams at



PS: It would be really cool if this blog post were part of the “Data
    Representation” node of the manual.  :-)

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