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Re: Why is Elisp slow?

From: Ergus
Subject: Re: Why is Elisp slow?
Date: Tue, 7 May 2019 14:38:04 +0200
User-agent: NeoMutt/20180716

On Tue, May 07, 2019 at 08:51:27PM +0900, ????????? wrote:

2019. 5. 7. ?????? 7:49, Ergus <address@hidden> ??????:

Hi all:

After reading the whole SBCL manual and the internals I have some
comments maybe useful if we decide to go in that direction. (If some of
the core developers starts with this I will be very pleased to work on
that, but I am too new in emacs development to start it myself alone.)

1) + Portability does not seems to be a big issue, they already support
our architectures (and some extra) and they are putting a lot of effort
on that. There is work to do, but in general, they already have what we
need from Common Lisp.

2) + They have documented how to port SBCL to new architectures, and some
of the optimizations they implement, so, if we have a compiler
specialist on board and finally our developers choose not to go for
SBCL, then maybe some of those optimization could be considered for our
Elisp compiler.

3) - There is not C binding for SBCL. To extend it, there is not a
public-documented API and their approach is wrapping the C functions
from Lisp (FFI). I wrote in their mailing list and they redirected me to
the CFFI page.

  + The only advantage (I see) in this approach is that as CFFI has
  been ported to almost all common list compilers/interpreters; if we
  go in that direction for our C code we could potentially use any of
  those and jump from one to another at configuration time. So we
  don't have this problem (of being stock with a "bad" compiler)

  - The overhead produced by wrapping in the high level instead of the
    low level in my experience (python, ruby, Julia) is usually much
    higher than having C bindings.
  - Writing wrappers in lisp for all our C functions exposed to Lisp I
    think is not an option if we don't create an automatic method. So
    if no one creates a plugin for sbcl to support Elisp, I really
    doubt the feasibility of using SBCL except if many of our current
    core developers agree on that and join effort for that. Else it
    will happen as with guile.

There is an automatic method of generating C FFI bindings from header files.
See <> 
What I was thinking about using CL to support Elisp is to define a new
namespace for symbols (which, in CL terms, is a so-called ???package???) named 
All Cl-needed functions/macros are defined in the ???cl??? package,
which means that you can call any cl function/macro by
prefixing ???cl:??? in the ???elisp??? package.

This is what I understand as a plugin for SBCL. Because we will need
some steroid for that :p
As CL and Elisp has many common things (such as the similarity of
many, many macros like ???defun??? or ???defvar???, etc??? or that it is a 
we can implement elisp entirely inside CL itself.
No need for developing a plugin specially for SBCL; any CL implementation will 
For functions defined in C, we can make or generate wrappers to get
passed-in-arguments and dynamically wrap values into a Lisp_Obj C struct
(AFAIU all lisp objects are represented as a struct of type info and some 
am I right?) and call the underlying function generated by cl-autowrap.

This is so huge that I don't know where to start looking :(. What I was
looking was a representation of the types in SBCL, but I couldn't find

AFAIK cl-autowrap is to wrap simple C functions with C types and
structs, but Elisp actually has it's own representation of lisp objects
and functions. And the exposed functions already have a unified function
format to store the pointer in the function's hash table. If we use a
wrapper on the hight level we will be packing and unpacking structs
(copies and more copies) constantly and wasting the advantage of having
a similar representation. Or we won't be allowed to pass Elisp object to
CL functions.

So the first to do should be to unify the Elisp types with the CL
representation as much as possible and check the differences we can't
solve implicitly to create an explicit conversion when needed. Because
with guile, after doing the hard work, we found that there was a problem
with strings representations (if I understood right what Stefan

We also need to find a method to create lisp objects from C to implement
DEFVAR_LISP, DEFSYM and similes in C if we don't want to reimplement big
portions of code (like the display engine for example). There is still
the problem with buffer local variable that we don't have a solution for
that yet either.

Then we can start checking the functions we have that are Elisp agnostic
and can be compiled with SBCL. We could also have another set of
functions we have implemented in C we will not need anymore and finally
there will be functions we need to wrap (like everything in editfns.c
for example)... But again... this is huge.

4) + With FFI as they implemented, it will be pretty easy to interface
and call C libraries and functions without needing a C wrapper to expose
it to lisp, so maybe an important part of our C code could be
substituted with Lisp (simpler and smaller) glue code. But also we will
be allowed to use C dynamic libraries directly without all the modules
complexity. (Somehow I feel that not everyone will like that if we
comment it in the developers mailing list :p ).

On Mon, May 06, 2019 at 02:47:12PM -0400, Stefan Monnier wrote:
[ IIUC of the 4 cases above, at most 2 run the same version, so we'd
need to make sure the same Emacs version can be compiled against all
of those versions.  No idea if it would impose a significant extra
burden or not, but it's something to be considered.  Also the fact
that the latest release doesn't work on all those platforms is rather
worrying.  ]

Hmm? I can't understand :-(
Why can't Emacs can include a specific version of SBCL's source (e.g. as
a git module) and compile them all together?

Exactly because the version that can run on ARM is not the same as the
one that can run under Windows, which is not the same as the one that
runs under AMD64.

Of course, maybe I'm confused by the table of available binaries (at, and in reality the latest version
works fine on all supported platforms.


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