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16. Byte Compilation

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.

Compiling a Lisp file with the Emacs byte compiler always reads the file as multibyte text, even if Emacs was started with `--unibyte', unless the file specifies otherwise. This is so that compilation gives results compatible with running the same file without compilation. See section 15.3 Loading Non-ASCII Characters.

In general, any version of Emacs can run byte-compiled code produced by recent earlier versions of Emacs, but the reverse is not true. A major incompatible change was introduced in Emacs version 19.29, and files compiled with versions since that one will definitely not run in earlier versions unless you specify a special option. In addition, the modifier bits in keyboard characters were renumbered in Emacs 19.29; as a result, files compiled in versions before 19.29 will not work in subsequent versions if they contain character constants with modifier bits.

See section 18.4 Debugging Problems in Compilation, for how to investigate errors occurring in byte compilation.

16.1 Performance of Byte-Compiled Code  An example of speedup from byte compilation.
16.2 The Compilation Functions  Byte compilation functions.
16.3 Documentation Strings and Compilation  Dynamic loading of documentation strings.
16.4 Dynamic Loading of Individual Functions  Dynamic loading of individual functions.
16.5 Evaluation During Compilation  Code to be evaluated when you compile.
16.6 Byte-Code Function Objects  The data type used for byte-compiled functions.
16.7 Disassembled Byte-Code  Disassembling byte-code; how to read byte-code.

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16.1 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)) 
    (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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16.2 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.

The byte compiler produces error messages and warnings about each file in a buffer called `*Compile-Log*'. These report things in your program that suggest a problem but are not necessarily erroneous.

Be careful when writing macro calls in files that you may someday byte-compile. Macro calls are expanded when they are compiled, so the macros must already be defined for proper compilation. For more details, see 13.3 Macros and Byte Compilation. If a program does not work the same way when compiled as it does when interpreted, erroneous macro definitions are one likely cause (see section 13.6 Common Problems Using 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 (see section 15.6 Features). To avoid loading the macro definition files when someone runs the compiled program, write eval-when-compile around the require calls (see section 16.5 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)
  [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.

Command: compile-defun
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.

Command: byte-compile-file filename
This function compiles a file of Lisp code named filename into a file of byte-code. The output file's name is made by changing the `.el' suffix into `.elc'; if filename does not end in `.el', it adds `.elc' to the end of filename.

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 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, 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, but no output file will be generated for it, and the Emacs process will terminate with a nonzero status code.

% emacs -batch -f batch-byte-compile *.el

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 Emacs version 18, byte-code was always executed by way of a call to the function byte-code. Nowadays, byte-code is usually executed as part of a byte-code function object, and only rarely through an explicit call to byte-code.

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16.3 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 recent versions of Emacs (since 19.29) will not load into older versions because the older versions don't support this feature. You can turn off this feature at compile time by setting byte-compile-dynamic-docstrings to nil; then 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. One way to do that is by adding this string to the file's first line:

-*-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 usually best not to use these constructs in Lisp source files, since they are not designed to be clear to humans reading the file.

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16.4 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 user-callable functions, if using one of them does not imply you will probably also 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:

These problems will never happen in normal circumstances with installed Emacs files. But they are quite likely to happen with Lisp files that you are changing. The easiest way to prevent these problems is to reload the new compiled file immediately after each recompilation.

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. For example, you could do it by writing this text in the source file's first line:

-*-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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16.5 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. That method is preferable when body is large.

Special Form: eval-when-compile body
This form marks body to be evaluated at compile time but not when the compiled program is loaded. The result of evaluation by the compiler becomes a constant which appears in the compiled program. If you load the source file, rather than compiling it, body is evaluated normally.

Common Lisp Note: 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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16.6 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 is like that for a vector, with an additional `#' before the opening `['.

A byte-code function object must have at least four elements; there is no maximum number, but only the first six elements have any normal use. They are:

The list of argument symbols.

The string containing the byte-code instructions.

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

The maximum stack size this function needs.

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 (see section 24.2 Access to Documentation Strings).

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)
  [arg 1 forward-sexp]

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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16.7 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)) 
    (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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