GNU Emacs Lisp has a compiler that translates functions written in Lisp into a special representation called byte-code that can be executed more efficiently. The compiler replaces Lisp function definitions with byte-code. When a byte-code function is called, its definition is evaluated by the byte-code interpreter.
Because the byte-compiled code is evaluated by the byte-code interpreter, instead of being executed directly by the machine's hardware (as true compiled code is), byte-code is completely transportable from machine to machine without recompilation. It is not, however, as fast as true compiled code.
In general, any version of Emacs can run byte-compiled code produced by recent earlier versions of Emacs, but the reverse is not true. In particular, if you compile a program with Emacs 18, you can run the compiled code in Emacs 19, but not vice versa.
See section Debugging Problems in Compilation, for how to investigate errors occurring in byte compilation.
A byte-compiled function is not as efficient as a primitive function written in C, but runs much faster than the version written in Lisp. Here is an example:
(defun silly-loop (n)
"Return time before and after N iterations of a loop."
(let ((t1 (current-time-string)))
(while (> (setq n (1- n))
0))
(list t1 (current-time-string))))
=> silly-loop
(silly-loop 100000)
=> ("Fri Mar 18 17:25:57 1994"
"Fri Mar 18 17:26:28 1994") ; 31 seconds
(byte-compile 'silly-loop)
=> [Compiled code not shown]
(silly-loop 100000)
=> ("Fri Mar 18 17:26:52 1994"
"Fri Mar 18 17:26:58 1994") ; 6 seconds
In this example, the interpreted code required 31 seconds to run, whereas the byte-compiled code required 6 seconds. These results are representative, but actual results will vary greatly.
You can byte-compile an individual function or macro definition with
the byte-compile function. You can compile a whole file with
byte-compile-file, or several files with
byte-recompile-directory or batch-byte-compile.
When you run the byte compiler, you may get warnings in a buffer called `*Compile-Log*'. These report things in your program that suggest a problem but are not necessarily erroneous.
Be careful when byte-compiling code that uses macros. Macro calls are expanded when they are compiled, so the macros must already be defined for proper compilation. For more details, see section Macros and Byte Compilation.
Normally, compiling a file does not evaluate the file's contents or
load the file. But it does execute any require calls at
top level in the file. One way to ensure that necessary macro
definitions are available during compilation is to require the file that
defines them. See section Features.
byte-compile returns the new, compiled definition of
symbol.
If symbol's definition is a byte-code function object,
byte-compile does nothing and returns nil. Lisp records
only one function definition for any symbol, and if that is already
compiled, non-compiled code is not available anywhere. So there is no
way to "compile the same definition again."
(defun factorial (integer)
"Compute factorial of INTEGER."
(if (= 1 integer) 1
(* integer (factorial (1- integer)))))
=> factorial
(byte-compile 'factorial)
=>
#[(integer)
"^H\301U\203^H^@\301\207\302^H\303^HS!\"\207"
[integer 1 * factorial]
4 "Compute factorial of INTEGER."]
The result is a byte-code function object. The string it contains is the actual byte-code; each character in it is an instruction or an operand of an instruction. The vector contains all the constants, variable names and function names used by the function, except for certain primitives that are coded as special instructions.
Compilation works by reading the input file one form at a time. If it is a definition of a function or macro, the compiled function or macro definition is written out. Other forms are batched together, then each batch is compiled, and written so that its compiled code will be executed when the file is read. All comments are discarded when the input file is read.
This command returns t. When called interactively, it prompts
for the file name.
% ls -l push*
-rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el
(byte-compile-file "~/emacs/push.el")
=> t
% ls -l push*
-rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el
-rw-rw-rw- 1 lewis 638 Oct 8 20:25 push.elc
If a `.el' file exists, but there is no corresponding `.elc'
file, then flag says what to do. If it is nil, the file is
ignored. If it is non-nil, the user is asked whether to compile
the file.
The returned value of this command is unpredictable.
byte-compile-file on files specified on the
command line. This function must be used only in a batch execution of
Emacs, as it kills Emacs on completion. An error in one file does not
prevent processing of subsequent files. (The file that gets the error
will not, of course, produce any compiled code.)
% emacs -batch -f batch-byte-compile *.el
byte-code. Don't call
this function yourself. Only the byte compiler knows how to generate
valid calls to this function.
In newer Emacs versions (19 and up), byte-code is usually executed as
part of a byte-code function object, and only rarely due to an explicit
call to byte-code.
These features permit you to write code to be evaluated during compilation of a program.
You can get a similar result by putting body in a separate file
and referring to that file with require. Using require is
preferable if there is a substantial amount of code to be executed in
this way.
At top level, this is analogous to the Common Lisp idiom
(eval-when (compile eval) ...). Elsewhere, the Common Lisp
`#.' reader macro (but not when interpreting) is closer to what
eval-when-compile does.
Byte-compiled functions have a special data type: they are byte-code function objects.
Internally, a byte-code function object is much like a vector; however, the evaluator handles this data type specially when it appears as a function to be called. The printed representation for a byte-code function object is like that for a vector, with an additional `#' before the opening `['.
In Emacs version 18, there was no byte-code function object data type;
compiled functions used the function byte-code to run the byte
code.
A byte-code function object must have at least four elements; there is no maximum number, but only the first six elements are actually used. They are:
nil. For functions
preloaded before Emacs is dumped, this is usually an integer which is an
index into the `DOC' file; use documentation to convert this
into a string (see section Access to Documentation Strings).
nil for a function that isn't interactive.
Here's an example of a byte-code function object, in printed
representation. It is the definition of the command
backward-sexp.
#[(&optional arg) "^H\204^F^@\301^P\302^H[!\207" [arg 1 forward-sexp] 2 254435 "p"]
The primitive way to create a byte-code object is with
make-byte-code:
You should not try to come up with the elements for a byte-code function yourself, because if they are inconsistent, Emacs may crash when you call the function. Always leave it to the byte compiler to create these objects; it makes the elements consistent (we hope).
You can access the elements of a byte-code object using aref;
you can also use vconcat to create a vector with the same
elements.
People do not write byte-code; that job is left to the byte compiler. But we provide a disassembler to satisfy a cat-like curiosity. The disassembler converts the byte-compiled code into humanly readable form.
The byte-code interpreter is implemented as a simple stack machine. It pushes values onto a stack of its own, then pops them off to use them in calculations whose results are themselves pushed back on the stack. When a byte-code function returns, it pops a value off the stack and returns it as the value of the function.
In addition to the stack, byte-code functions can use, bind, and set ordinary Lisp variables, by transferring values between variables and the stack.
standard-output. The
argument object can be a function name or a lambda expression.
As a special exception, if this function is used interactively, it outputs to a buffer named `*Disassemble*'.
Here are two examples of using the disassemble function. We
have added explanatory comments to help you relate the byte-code to the
Lisp source; these do not appear in the output of disassemble.
These examples show unoptimized byte-code. Nowadays byte-code is
usually optimized, but we did not want to rewrite these examples, since
they still serve their purpose.
(defun factorial (integer)
"Compute factorial of an integer."
(if (= 1 integer) 1
(* integer (factorial (1- integer)))))
=> factorial
(factorial 4)
=> 24
(disassemble 'factorial)
-| byte-code for factorial:
doc: Compute factorial of an integer.
args: (integer)
0 constant 1 ; Push 1 onto stack.
1 varref integer ; Get value of integer
; from the environment
; and push the value
; onto the stack.
2 eqlsign ; Pop top two values off stack,
; compare them,
; and push result onto stack.
3 goto-if-nil 10 ; Pop and test top of stack;
; if nil, go to 10,
; else continue.
6 constant 1 ; Push 1 onto top of stack.
7 goto 17 ; Go to 17 (in this case, 1 will be
; returned by the function).
10 constant * ; Push symbol * onto stack.
11 varref integer ; Push value of integer onto stack.
12 constant factorial ; Push factorial onto stack.
13 varref integer ; Push value of integer onto stack.
14 sub1 ; Pop integer, decrement value,
; push new value onto stack.
; Stack now contains:
; - decremented value of integer
; - factorial
; - value of integer
; - *
15 call 1 ; Call function factorial using
; the first (i.e., the top) element
; of the stack as the argument;
; push returned value onto stack.
; Stack now contains:
; - result of recursive
; call to factorial
; - value of integer
; - *
16 call 2 ; Using the first two
; (i.e., the top two)
; elements of the stack
; as arguments,
; call the function *,
; pushing the result onto the stack.
17 return ; Return the top element
; of the stack.
=> nil
The silly-loop function is somewhat more complex:
(defun silly-loop (n)
"Return time before and after N iterations of a loop."
(let ((t1 (current-time-string)))
(while (> (setq n (1- n))
0))
(list t1 (current-time-string))))
=> silly-loop
(disassemble 'silly-loop)
-| byte-code for silly-loop:
doc: Return time before and after N iterations of a loop.
args: (n)
0 constant current-time-string ; Push
; current-time-string
; onto top of stack.
1 call 0 ; Call current-time-string
; with no argument,
; pushing result onto stack.
2 varbind t1 ; Pop stack and bind t1
; to popped value.
3 varref n ; Get value of n from
; the environment and push
; the value onto the stack.
4 sub1 ; Subtract 1 from top of stack.
5 dup ; Duplicate the top of the stack;
; i.e., copy the top of
; the stack and push the
; copy onto the stack.
6 varset n ; Pop the top of the stack,
; and bind n to the value.
; In effect, the sequence dup varset
; copies the top of the stack
; into the value of n
; without popping it.
7 constant 0 ; Push 0 onto stack.
8 gtr ; Pop top two values off stack,
; test if n is greater than 0
; and push result onto stack.
9 goto-if-nil-else-pop 17 ; Goto 17 if n <= 0
; (this exits the while loop).
; else pop top of stack
; and continue
12 constant nil ; Push nil onto stack
; (this is the body of the loop).
13 discard ; Discard result of the body
; of the loop (a while loop
; is always evaluated for
; its side effects).
14 goto 3 ; Jump back to beginning
; of while loop.
17 discard ; Discard result of while loop
; by popping top of stack.
; This result is the value nil that
; was not popped by the goto at 9.
18 varref t1 ; Push value of t1 onto stack.
19 constant current-time-string ; Push
; current-time-string
; onto top of stack.
20 call 0 ; Call current-time-string again.
21 list2 ; Pop top two elements off stack,
; create a list of them,
; and push list onto stack.
22 unbind 1 ; Unbind t1 in local environment.
23 return ; Return value of the top of stack.
=> nil