intro.texi

@node Introduction to Emacs Lisp Byte Code and LAP
@chapter Introduction to Emacs Lisp Byte Code and LAP

@menu
* Why is Emacs Lisp Bytecode Important and How is Emacs as a Program Different?::
* Emacs Lisp Bytecode and LAP::
* Emacs Lisp Virtual Machine::
* Wither Bytecode - Its Future::
@end menu

@SECTION{Why is Emacs Lisp Bytecode Important and How is Emacs as a Program Different?}

If we were to compare two similar complex programs in around 2018,
Firefox 53.0.3 and Emacs 25.3, we would see that the Firefox
tarball is 5 times bigger than the Emacs tarball. How are these made up,
and what languages are they comprised of?

@noindent
For Firefox whose core is written in C++ we have:

@verbatim
$ cloc --match-f='\.(js|c|cpp|html|py|css)$' firefox-53.0.3
   89156 text files.
   86240 unique files.
    1512 files ignored.

cloc v 1.60  T=244.20 s (353.2 files/s, 56012.8 lines/s)
-------------------------------------------------------------
Language                     files     comment           code
-------------------------------------------------------------
C++                           7267      418019        3057110
Javascript                   25855      532629        2859451
HTML                         45311      120520        2209067
C                             3482      400594        1664666
@end verbatim

@noindent
And for Emacs whose core is written in C we have:

@verbatim
$ cloc emacs-25.3.tar.xz
    3346 text files.
    3251 unique files.
    1130 files ignored.

cloc 1.60  T=13.85 s (160.1 files/s, 154670.7 lines/s)
-------------------------------------------------------------------
Language                      files     comment           code
--------------------------------------------------------------
Lisp                           1616      200820        1270511
C                               255       66169         256314
C/C++ Header                    176       11505          34891
@end verbatim

If we look at the relative ratio of C++ to Javascript code in
Firefox, and the ratio of C versus Lisp code in Emacs, we see that
 much more of Emacs is written in Lisp than Firefox is written
in Javascript. (And a lot of C code for Emacs looks like Lisp written using C syntax).

My take is that Emacs a
lot more orthogonal in its basic concepts and construction. Just as
Leibniz was amazed that such diversity could come out of such simple
rules of mathematics and physics, so it is remarkable that something
as complex as Emacs can come out of the relatively simple language,
Lisp.

@SECTION{Emacs Lisp Bytecode and LAP}

However pervasive Emacs Lisp is in the Emacs
ecosystem, Emacs Lisp is not and never has been a speedy language compared
to say, C, C++, Go, Rust or Java. And that's where LAP and bytecode
come in.

As stated in a commment in @code{byte-opt.el} added circa 1996:
@quotation
No matter how hard you try, you can't make a racehorse out of a pig.

You can, however, make a faster pig.
@author Eric Naggum
@end quotation

Emacs Lisp bytecode is the custom lower-level language used by the Emacs
bytecode interpreter. As with all bytecode, its instructions are
compact. For display purposes, there is a @code{disassemble} command
that unpacks the fields of the instruction. With this and the
constants vector, bytecode can be printed in an assembly language-like
format.

@cindex bytecode
I'll often use an Emacs Lisp bytecode instruction to refer to an assembly
representation of the instruction.

@cindex LAP
LAP stands for Lisp Assembly Program. It is an internal representation
of the bytecode instructions in a more symbolic form. It is used
behind the scenes to make bytecode more amenable to optimization,
since the instructions are in a structure which is easier to operate
on.

If we want to write the instruction sequence in this symbolic form
rather than give a byte-encoded form, we can do that using the
function @code{byte-compile-lapcode}.

@unnumberedsubsec Example showing use of @code{byte-compile-lapcode}
@findex byte-compile-lapcode
@findex make-byte-code
@verbatim
(defalias 'get-foo
  (make-byte code
    #x000                  ;; lexical parameter counts
    (byte-compile-lapcode
      '((byte-varref . 0)
        (byte-return)))    ;; instruction sequence
    [foo]                  ;; constants vector
    1))                    ;; max stack usage
@end verbatim


@uref{https://www.gnu.org/software/emacs/manual/html_node/elisp/Speed-of-Byte_002dCode.html,
Silly Loop Example} in the Emacs Lisp Manual gives a program to time
running in some code in the bytecode interpreter versus running the code in
the Lisp interpreter. When I ran this program, bytecode ran 2.5 times
faster@footnote{Code was compiled to use dynamic binding for variable
access,  as was probably the case in the Emacs Lisp manual. We should
note that byte-compiling with lexical binding for variable access
gives code that runs a bit faster than when dynamic binding is
used.}. The Emacs Lisp manual gets a speed improvement of about 3
times.

@SECTION{Emacs Lisp Virtual Machine}

@cindex Reverse Polish Notation
The Emacs Lisp bytecode interpreter, like many bytecode interpreters
such as Smalltalk, CPython, Forth, or PostScript, has an evaluation
stack and a code stack.  Emacs Lisp bytecode instructions use reverse Polish notation: operands appear prior to the
operator.  This is how many other bytecode interpreters work. It is
the opposite of the way Lisp works. To add the values of two
variables we might write @code{(+ a b)}. However in bytecode it is the
other way around: the operator or function comes last. So the
corresponding bytecode is:

@verbatim
0	varref	  a
1	varref	  b
2	plus
@end verbatim

As in most language-specific virtual machines, but in contrast to a
typical general-purpose virtual machine, the things that
are on the evaluation stack are the same objects that are found in the
system that they model. Here, these objects can include Emacs buffers,
or font faces, Lisp objects like hashes or vectors, or simply (30-bit)
Lisp integers. Compare this with, say, LLVM IR, or JVM instructions
where the underlying objects on the stack are registers which can act
as pointers, and the internal memory layout of objects is exposed.

Control flow in Lisp bytecode is similar to a conventional assembly
language: there are unconditional and conditional jumps. More complex
control structures are simply built out of these.

Although it may be obvious, one last thing to point out is that the
Emacs Lisp bytecode instruction set is custom to Emacs.  In addition
to primitives that we would expect for Lisp such @code{car} and
@code{cdr}, there are primitive bytecodes for more-complex Emacs
editor-specific concepts such as ``save-excursion''@footnote{The
semantic level difference between Emacs Lisp and its bytecode is not
great, so writing a decompiler for it more feasible than if the
bytecode language were of a general nature such as, say, LLVM IR.}.

The interpreter is largely backward compatible, but not forward
compatible (although eventually old Emacs Lisp bytecode instructions
do die). So old versions of Emacs cannot necessarily run new
bytecode. Each instruction is between 1 and 3 bytes. The first byte
is the opcode and the second and third bytes are either a single
operand or a single immediate value. Some operands are packed into
the opcode byte.

@SECTION{Wither Bytecode - Its Future}

Emacs's bytecode is pretty old---about as old as Emacs itself. And
although there have been some changes to it, there has always been lurking
in the background the question of whether it might be totally ditched,
either as a by-product of switching out the underlying Lisp
implementation for something else, or as a result of using JIT
technology.

Let's take these two situations where Emacs Lisp Bytecode might become
obsolete separately. Both ideas have been floating around for a
long time.

With respect to alternate programming-language implementations, there
have been many languages that been proposed and experimented with. The
big obstacle in totally replacing Emacs Lisp is in rewriting the huge
current Emacs Lisp code base. (The counts given in the last section
for Emacs 25.3 are 1.5K files and 100K lines of code.)

I think that if such an approach were to work, the language would have
to be available as an additional language until the current code base
was replaced. At present (circa 2018), alternate programming languages
haven't gained much of a foothold; they are not in the current Emacs
distribution or in any of its branches.

An obvious alternative language proposed is Common Lisp. Over time, an
Emacs Lisp package implementing Common Lisp has been providing more
and more Common Lisp functionality; names, however, are prefaced with
@code{cl-}.

The addition of features in Common Lisp has been somewhat
reflected in changes in the run-time systems, such as the
addition of lexical scoping.  And this approach partially solves the
large code-base migration problem. But it also reduces the need to
jump cold turkey from Emacs Lisp Bytecode to something else.

And what about the other possibility where Emacs incorporates JIT
technology? The motivation for this is to speed up Emacs. There is
widespread belief among the development community that there could be
big performance wins if this were done right. After all, it is not
uncommon for some people to live inside a single GNU Emacs session.

This idea of using a JIT to speed performance goes back over a decade,
at least back to 2006. Of the JITs that have been proposed, at least
four of them use Emacs Lisp Bytecode as the basis from which to JIT
from. I think that is because Emacs Lisp Bytecode is a reasonable
target to JIT: it is sufficiently low level, while also easy to hook a
JIT into.

Two alternatives to Emacs Lisp Bytecode which have sophisticated JIT
technology are LLVM IR and JVM IR. For each, the surrounding run-time
environment would have to be replicated. Another IR possibility might
be JavaScript IRs: specifically, the ones for V8 and Spidermonkey.

Pipcet's work that allows SpiderMonkey's garbage collector to be used in
Emacs, allows for a real possibility of using SpiderMonkey's JIT with
either JavaScript, Emacs Lisp bytecode, or Emacs Lisp bypassing Emacs
Lisp bytecode. That last route I think might be harder. JIT'ing from
Emacs Lisp bytecode to via SpiderMonkey (if it is possible) would
allow for dual Emacs Lisp and JavaScript scripting while the other
options don't.

Needless to say, such a lot of work remains in adding any sort of JIT
technology that I don't think Emacs Lisp Bytecode will be obsolete
anytime soon, should that JIT not work off of Emacs Lisp bytecode.