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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) |
Hello,
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”.
> 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>?
Thanks!
Ludo’.
PS: It would be really cool if this blog post were part of the “Data
Representation” node of the manual. :-)