Re: Question regarding Lisp_Object representation of symbols

[Po Lu <luangruo@yahoo.com>]

Gerd Möllmann <gerd.moellmann@gmail.com> writes:

> - Does someone remember the reasons why this has been done?

To speed up comparisons between objects and Qnil, I think.

> - Is there something that relies on this? Maybe something that
>   relies on 0x0 being a valid Lisp_Object?

Yes, in the many places where memset/memclear is invoked to initialize
Lisp_Object fields or variables, for instance.  It's supposed to be the
case that these instances are marked with NIL_IS_ZERO for easy locating
if the representation of Qnil is ever to be changed, but that convention
is not observed in practice, or is actively circumvented, as by callers
of memclear, which defeat the purpose of that marker by calling this one
of its bearers to initialize a variety of structures that may or may not
comprise only Lisp_Objects, greatly inflating the number of callers that
must be examined before such a change in object representation.