Re: Shrinking the C core

[Ihor Radchenko <yantar92@posteo.net>]

"Alfred M. Szmidt" <ams@gnu.org> writes:

>    Then, what does GCC do? AFAIK, GCC JIT takes the Elisp byte code,
>    transforms it into JIT pseudocode, and optimizes the actual code flow.
>
> What does GCC do _WHERE_?  What backend? What language? You're
> speaking in such broad terms that it makes it impossible to continue
> this discussion.  I don't know how the native compilation works, but
> no matter what you feed to GCC it cannot do magic and any optimization
> should be done on what the Emacs compiler does.

Native compilation provides the necessary information about Elisp to GCC.
Otherwise, native compilation would be useless.
You may check out the details in
https://zenodo.org/record/3736363 and
https://toobnix.org/w/1f997b3c-00dc-4f7d-b2ce-74538c194fa7

>    For example, when I write
>
>    (when (> x y) (when (> x y) x))
>
>    I expect GCC JIT to throw away the duplicate comparison.
>
> Why do you expect that?  Why do you think it is duplicate?  Where are
> the guarantees that > or WHEN don't have side-effects?  Do you know
> the exact type of X and Y so you can skip a cascade of type checks to
> pick the right comparison operator?  Can you use fixnum comparison of
> a specific bit width? Do you need to use bignum comparison?  

At least some of these questions are answered by the code on Emacs side.
Native compiler transforms the Elisp byte code, using its knowledge
about function purity, types, and maybe other things, into LIMP that can
be fed to GCC JIT. Then, GCC JIT uses the provided info to do actual
optimization.

> That is the type of information SBCL knows about, or allows the user
> to specify.  Emacs does not have that today, and that incures one set
> of overhead.  There are plenty more...

AFAIK, users cannot specify type info manually, but types are tracked
when transforming Elisp byte code into LIMP representation.

The only problem (AFAIU) is that GCC JIT cannot reach inside subr level,
so all these information does not benefit Emacs functions implemented in
C.

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>