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1 Byte Compilation

XEmacs 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 19.29, the compiled code does not run in earlier versions. Files compiled in versions before 19.29 may not work in 19.29 if they contain character constants with modifier bits, because the bits were renumbered in Emacs 19.29.

@xref{Compilation Errors}, for how to investigate errors occurring in byte compilation.


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1.1 Performance of Byte-Compiled Code

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.


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1.2 The Compilation Functions

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 @ref{Compiling Macros}.

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 (@pxref{Named Features}). To avoid loading the macro definition files when someone runs the compiled program, write eval-when-compile around the require calls (see section Evaluation During Compilation).

Function: byte-compile symbol

This function byte-compiles the function definition of symbol, replacing the previous definition with the compiled one. The function definition of symbol must be the actual code for the function; i.e., the compiler does not follow indirection to another symbol. 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)
     ⇒
#<byte-code (integer)
 "ÁU«‚Á‡ÂS!_‡"
 [integer 1 factorial]
 3 "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.

Command: compile-defun &optional arg

This command reads the defun containing point, compiles it, and evaluates the result. If you use this on a defun that is actually a function definition, the effect is to install a compiled version of that function.

If arg is non-nil, the result is inserted in the current buffer after the form; otherwise, it is printed in the minibuffer.

Command: byte-compile-file filename &optional load

This function compiles a file of Lisp code named filename into a file of byte-code. The output file’s name is made by appending ‘c’ to the end of filename.

If load is non-nil, the file is loaded after having been compiled.

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
Command: byte-recompile-directory directory &optional flag

This function recompiles every ‘.el’ file in directory that needs recompilation. A file needs recompilation if a ‘.elc’ file exists but is older than the ‘.el’ file.

When a ‘.el’ file has no corresponding ‘.elc’ file, then flag says what to do. If it is nil, these files are ignored. If it is non-nil, the user is asked whether to compile each such file.

The returned value of this command is unpredictable.

Function: batch-byte-compile

This function runs 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
Function: batch-byte-recompile-directory

This function is similar to batch-byte-compile but runs the command byte-recompile-directory on the files remaining on the command line.

Variable: byte-recompile-directory-ignore-errors-p

If non-nil, this specifies that byte-recompile-directory will continue compiling even when an error occurs in a file. This is normally nil, but is bound to t by batch-byte-recompile-directory.

Function: byte-code code-string data-vector max-stack

This function actually interprets byte-code. A byte-compiled function is actually defined with a body that calls 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.


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1.3 Documentation Strings and Compilation

Functions and variables loaded from a byte-compiled file access their documentation strings dynamically from the file whenever needed. This saves space within Emacs, and makes loading faster because the documentation strings themselves need not be processed while loading the file. Actual access to the documentation strings becomes slower as a result, but this normally is not enough to bother users.

Dynamic access to documentation strings does have drawbacks:

If your site installs Emacs following the usual procedures, these problems will never normally occur. Installing a new version uses a new directory with a different name; as long as the old version remains installed, its files will remain unmodified in the places where they are expected to be.

However, if you have built Emacs yourself and use it from the directory where you built it, you will experience this problem occasionally if you edit and recompile Lisp files. When it happens, you can cure the problem by reloading the file after recompiling it.

Byte-compiled files made with Emacs 19.29 will not load into older versions because the older versions don’t support this feature. You can turn off this feature by setting byte-compile-dynamic-docstrings to nil. Once this is done, you can compile files that will load into older Emacs versions. You can do this globally, or for one source file by specifying a file-local binding for the variable. Here’s one way to do that:

-*-byte-compile-dynamic-docstrings: nil;-*-
Variable: byte-compile-dynamic-docstrings

If this is non-nil, the byte compiler generates compiled files that are set up for dynamic loading of documentation strings.

The dynamic documentation string feature writes compiled files that use a special Lisp reader construct, ‘#@count’. This construct skips the next count characters. It also uses the ‘#$’ construct, which stands for “the name of this file, as a string.” It is best not to use these constructs in Lisp source files.


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1.4 Dynamic Loading of Individual Functions

When you compile a file, you can optionally enable the dynamic function loading feature (also known as lazy loading). With dynamic function loading, loading the file doesn’t fully read the function definitions in the file. Instead, each function definition contains a place-holder which refers to the file. The first time each function is called, it reads the full definition from the file, to replace the place-holder.

The advantage of dynamic function loading is that loading the file becomes much faster. This is a good thing for a file which contains many separate commands, provided that using one of them does not imply you will soon (or ever) use the rest. A specialized mode which provides many keyboard commands often has that usage pattern: a user may invoke the mode, but use only a few of the commands it provides.

The dynamic loading feature has certain disadvantages:

If you compile a new version of the file, the best thing to do is immediately load the new compiled file. That will prevent any future problems.

The byte compiler uses the dynamic function loading feature if the variable byte-compile-dynamic is non-nil at compilation time. Do not set this variable globally, since dynamic loading is desirable only for certain files. Instead, enable the feature for specific source files with file-local variable bindings, like this:

-*-byte-compile-dynamic: t;-*-
Variable: byte-compile-dynamic

If this is non-nil, the byte compiler generates compiled files that are set up for dynamic function loading.

Function: fetch-bytecode function

This immediately finishes loading the definition of function from its byte-compiled file, if it is not fully loaded already. The argument function may be a byte-code function object or a function name.


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1.5 Evaluation During Compilation

These features permit you to write code to be evaluated during compilation of a program.

Special Form: eval-and-compile body

This form marks body to be evaluated both when you compile the containing code and when you run it (whether compiled or not).

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.

Special Form: eval-when-compile body

This form marks body to be evaluated at compile time and not when the compiled program is loaded. The result of evaluation by the compiler becomes a constant which appears in the compiled program. When the program is interpreted, not compiled at all, body is evaluated normally.

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.


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1.6 Byte-Code Function Objects

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 begins with ‘#<byte-code’ and ends with ‘>’.

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:

arglist

The list of argument symbols.

byte-code

The string containing the byte-code instructions.

constants

The vector of Lisp objects referenced by the byte code. These include symbols used as function names and variable names.

stacksize

The maximum stack size this function needs.

docstring

The documentation string (if any); otherwise, nil. The value may be a number or a list, in case the documentation string is stored in a file. Use the function documentation to get the real documentation string (@pxref{Accessing Documentation}).

interactive

The interactive spec (if any). This can be a string or a Lisp expression. It is 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:

Function: make-byte-code &rest elements

This function constructs and returns a byte-code function object with elements as its elements.

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.


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1.7 Disassembled Byte-Code

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.

Command: disassemble object &optional stream

This function prints the disassembled code for object. If stream is supplied, then output goes there. Otherwise, the disassembled code is printed to the stream 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

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