What does "tag space" mean?

What does "tag space" mean?

[Xue Fuqiao]

In (info "(elisp) Object Internals"):

  Since the tag space is limited, all other types are the subtypes of
  either `Lisp_Vectorlike' or `Lisp_Misc'.

What does "tag space" mean here?  I had searched the manual and web, but I
didn't find anything useful.  Can anybody help?  Thanks.

-- 
Best regards, Xue Fuqiao.
http://www.emacswiki.org/emacs/XueFuqiao

Re: What does "tag space" mean?

[Aurélien Aptel]

On Fri, Mar 1, 2013 at 1:39 PM, Xue Fuqiao <xfq.free@gmail.com> wrote:
> What does "tag space" mean here?  I had searched the manual and web, but I
> didn't find anything useful.  Can anybody help?  Thanks.

Lisp_Objet use 3 bits to store types ie. to "tag" a pointer. Thus, tag
space can hold 2^3 values.

I've started to write a small guide to the C code on emacswiki [1].
Any contribution welcome :)

http://www.emacswiki.org/emacs/HackerGuide

Re: What does "tag space" mean?

[Xue Fuqiao]

On Fri, 1 Mar 2013 14:21:45 +0100
Aurélien Aptel <aurelien.aptel+emacs@gmail.com> wrote:

> On Fri, Mar 1, 2013 at 1:39 PM, Xue Fuqiao <xfq.free@gmail.com> wrote:
> > What does "tag space" mean here?

> Lisp_Object use 3 bits to store types ie. to "tag" a pointer. Thus, tag
> space can hold 2^3 values.

Ah, I see, thanks.

> I've started to write a small guide to the C code on emacswiki [1].
> Any contribution welcome :)
> 
> http://www.emacswiki.org/emacs/HackerGuide

Looks great.  I'm not familiar with C code in Emacs, and I'm wondering how to
read the source these days.  Thanks for your help.

-- 
Best regards, Xue Fuqiao.
http://www.emacswiki.org/emacs/XueFuqiao

Re: What does "tag space" mean?

[Eli Zaretskii]

> Date: Fri, 1 Mar 2013 14:21:45 +0100
> From: Aurélien Aptel <aurelien.aptel+emacs@gmail.com>
> Cc: help-gnu-emacs@gnu.org
> 
> I've started to write a small guide to the C code on emacswiki [1].
> Any contribution welcome :)

Comments:

 . Unless you intend to cover non-Posix platforms, I suggest to remove
   from the "Build configuration" everything that is not applicable to
   Posix systems, like config.bat.

 . I suggest to show the relevant pieces of C code when you describe
   something like Lisp_Object or XVECTORP.  I think it will make the
   document much more self-contained, as anyone who reads it will need
   to consult the C code anyway.

 . Don't point to specific source line numbers, they change very
   frequently as part of development.  "Around line NNNN" is much
   better, and sometimes it is better to omit that entirely.

 . The part about 256MB limitation of buffers on 32-bit systems is
   incorrect: the max size is 512MB.  Your bit calculations are in
   error.  See VAL_MAX.

 . I question the usefulness of listing all of the Xfoo macros.  It
   should be enough to explain that Xfoo(x) returns a C object
   corresponding to a Lisp object of type foo.  Same goes for the fooP
   macros.

The document doesn't yet go too far to become interesting, so the
above are just small nits.

Thanks.

Re: What does "tag space" mean?

[Thien-Thi Nguyen]

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() Xue Fuqiao <xfq.free@gmail.com>
() Fri, 1 Mar 2013 20:39:52 +0800

   What does "tag space" mean here?

Personally, i think of "space" as the contents of an empty container,
which is similar to the "set of possible elements of a container" (more
classical meaning), which is similar to "sets with some added structure"
(<https://en.wikipedia.org/wiki/Space>).  Leave it to the mathematicians
to drive straight into the fog.  :-D
                                          
Here, the container is a bit field, and a "tag" is a particular pattern
in that bit field.  If all bits are orthogonal, the space for an N-bit
field numbers 2^N.  So that's where the "structure" comes in; In some
tag systems, all bits are "equal" (either 1 or 0), but some bits are
"more equal than others".  This usually manifests as "if bit A is 1,
then the value of bit B doesn't matter (don't care)".  Such a structure
reduces the space:

- orthogonal -- count: 4
  A B
  0 0
  0 1
  1 0
  1 1

- non-orthogonal -- count: 3
  A B
  0 0
  0 1
  1 X

The tags stand for "types" and so when there is no more "space" in the
primary "space", one has to resort to "sub-types", represented by way of
a secondary (additional) bit field, w/ its own structure (perhaps).

Now, if the primary and secondary bit fields abut, you might think of
them to be simply THE bit field (w/o distinction, but definitely WITH
structure).  That's a matter of taste.  (If they do not abut, well then,
there is no choice, unless you squint a little and elide the invervening
bits.  Personally, i imagine it like a diamond-back snake slithering
past, each diamond on its back adjacent to another, even when looped...)

Oops, sorry about that.  It's very easy for me to "space out" on this
topic.  See src/lisp.h, starting w/ comment "Select the tagging scheme",
for details.

-- 
Thien-Thi Nguyen
GPG key: 4C807502

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