Re: Question regarding Lisp_Object representation of symbols

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

Po Lu <luangruo@yahoo.com> writes:

> 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.

Thank you very much, that was really helpful!