Re: [RFC, PATCH] shrink struct vectorlike_header #2

[Paul Eggert]

On 11/08/2012 07:08 AM, Dmitry Antipov wrote:
> +#define NEXT_IN_FREE_LIST(v) ((struct Lisp_Vectorlike_Free *) v)->next

This solution is nicer than what we have, but it still
has the problem of casting pointers, which would let
the C compiler do optimizations that we don't want to
allow.

Another possibility would be something like this:

  static struct Lisp_Vector *
  next_in_free_list (struct Lisp_Vector *v)
  {
    intptr_t i = XIL (v->contents[0]);
    return (struct Lisp_Vector *) i;
  }
  static void
  set_next_in_free_list (struct Lisp_Vector *v, struct Lisp_Vector *next)
  {
    v->contents[0] = XIL ((intptr_t) (next));
  }

That is, define a setter as well as a getter, and use the setter
when modifying the next field.  This would be simpler and
arguably cleaner than the union and would be just as safe, since we
know that Lisp_Object is at least as wide as intptr_t.