advice needed for multi-threading patch

[Tom Tromey]

I was inspired by Giuseppe Scrivano's work from last year:

http://lists.gnu.org/archive/html/emacs-devel/2008-11/msg01067.html

... so I started working on my own patch to implement multi-threading in
Emacs.  I've tried to follow Stefan's advice as given in that thread.
In particular I've (initially) implemented cooperative multi-threading.

Some background for my particular question:

My implementation of dynamic binding uses a structure like so:

    struct Lisp_ThreadLocal
      {
        enum Lisp_Misc_Type type : 16;  /* = Lisp_Misc_ThreadLocal */
        unsigned gcmarkbit : 1;
        int spacer : 15;
        Lisp_Object global;
        Lisp_Object thread_alist;
      };

... the idea being that a let-bound variable will be on 'thread_alist'
(keyed by the thread), and other threads will see the value in 'global'.
These objects are found in symbol value slots.

I managed to handle the V* global variables by writing some elisp that
changed every declaration of a variable mentioned in a DEFVAR_LISP from
Vfoo to impl_Vfoo and also emitted a new header with a lot of:

    #define Vfoo *find_variable_location (&impl_Vfoo)

This was pretty simple and non-intrusive, in the sense that it is
totally automated, so I can easily reapply it as Emacs changes.

... which brings me to my problem.  I'd also like to apply a similar
treatment to buffer-local variables.  However, those do not have
convenient accessor macros, and before starting the laborious task of
wrapping all buffer field accesses, I thought I'd ask for advice.  Is
this a sane way to proceed?  Should I do something different?  Could I
get such a patch in before the rest of this work, just to make my local
divergence smaller?

I'll also note that the above approach does not work for DEFVAR_INT.  I
have a plan for those but I fear it is somewhat expensive.  If you have
an idea...

Tom