bug#15880: Compute C declarations for DEFSYMs automatically.

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 09 Jan 2015 08:33:53 -0800
> From: Paul Eggert <eggert@cs.ucla.edu>
> CC: rgm@gnu.org, 15880@debbugs.gnu.org
> 
> Eli Zaretskii wrote:
> > Could we instead just keep the previous semantics of XIL and XLI,
> > please?  They are used in a few places outside lisp.h, so now all of
> > those places need to be audited and most probably changed.  Why isn't
> > it better to change the implementation of just 2 macros instead?
> 
> XIL and XLI haven't changed, and have the same semantics as before: they convert 
> between Lisp_Object and EMACS_INT without altering the bits (i.e., they are 
> no-ops at the machine level).

That's not what their previous semantics was.  They converted between
a Lisp integer and EMACS_INT.  Now the object that comes out of them
is a Lisp symbol which is invalid: it has no Lisp_Symbol structure
allocated for it, so e.g. displaying it via the prin* functions will
likely segfault.

> I expect it's better to keep them no-ops, for efficiency in the
> kernel of the Emacs Lisp interpreter.

If we need efficient macros for the interpreter, we can have new
ones for that.  I'm saying let's leave XIL and XLI do what they did
previously: convert from a Lisp integer to an EMACS_INT and back.