emacs/doc/lispref/compile.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990--1994, 2001--2024 Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node Byte Compilation
@chapter Byte Compilation
@cindex byte compilation
@cindex byte-code
@cindex compilation (Emacs Lisp)

  Emacs Lisp has a @dfn{compiler} that translates functions written
in Lisp into a special representation called @dfn{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 @dfn{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.

@vindex no-byte-compile
  If you do not want a Lisp file to be compiled, ever, put a file-local
variable binding for @code{no-byte-compile} into it, like this:

@example
;; -*-no-byte-compile: t; -*-
@end example

@menu
* Speed of Byte-Code::          An example of speedup from byte compilation.
* Compilation Functions::       Byte compilation functions.
* Docs and Compilation::        Dynamic loading of documentation strings.
* Eval During Compile::         Code to be evaluated when you compile.
* Compiler Errors::             Handling compiler error messages.
* Closure Objects::             The data type used for byte-compiled functions.
* Disassembly::                 Disassembling byte-code; how to read byte-code.
@end menu

@node Speed of Byte-Code
@section 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:

@example
@group
(defun silly-loop (n)
  "Return the time, in seconds, to run N iterations of a loop."
  (let ((t1 (float-time)))
    (while (> (setq n (1- n)) 0))
    (- (float-time) t1)))
@result{} silly-loop
@end group

@group
(silly-loop 50000000)
@result{} 5.200886011123657
@end group

@group
(byte-compile 'silly-loop)
@result{} @r{[Compiled code not shown]}
@end group

@group
(silly-loop 50000000)
@result{} 0.6239290237426758
@end group
@end example

  In this example, the interpreted code required more than 5 seconds to run,
whereas the byte-compiled code required less than 1 second.  These
results are representative, but actual results may vary.

@node Compilation Functions
@section Byte-Compilation Functions
@cindex compilation functions

  You can byte-compile an individual function or macro definition with
the @code{byte-compile} function.  You can compile a whole file with
@code{byte-compile-file}, or several files with
@code{byte-recompile-directory} or @code{batch-byte-compile}.

@vindex byte-compile-debug
  Sometimes, the byte compiler produces warning and/or error messages
(@pxref{Compiler Errors}, for details).  These messages are normally
recorded in a buffer called @file{*Compile-Log*}, which uses
Compilation mode.  @xref{Compilation Mode,,,emacs, The GNU Emacs
Manual}.  However, if the variable @code{byte-compile-debug} is
non-@code{nil}, error messages will be signaled as Lisp errors instead
(@pxref{Errors}).

@cindex macro compilation
  Be careful when writing macro calls in files that you intend to
byte-compile.  Since macro calls are expanded when they are compiled,
the macros need to be loaded into Emacs or the byte compiler will not
do the right thing.  The usual way to handle this is with
@code{require} forms which specify the files containing the needed
macro definitions (@pxref{Named Features}).  Normally, the
byte compiler does not evaluate the code that it is compiling, but it
handles @code{require} forms specially, by loading the specified
libraries.  To avoid loading the macro definition files when someone
@emph{runs} the compiled program, write @code{eval-when-compile}
around the @code{require} calls (@pxref{Eval During Compile}).  For
more details, @xref{Compiling Macros}.

  Inline (@code{defsubst}) functions are less troublesome; if you
compile a call to such a function before its definition is known, the
call will still work right, it will just run slower.

@defun byte-compile symbol
This function byte-compiles the function definition of @var{symbol},
replacing the previous definition with the compiled one.  The function
definition of @var{symbol} must be the actual code for the function;
@code{byte-compile} does not handle function indirection.  The return
value is the byte-code function object which is the compiled
definition of @var{symbol} (@pxref{Closure Objects}).

@example
@group
(defun factorial (integer)
  "Compute factorial of INTEGER."
  (if (= 1 integer) 1
    (* integer (factorial (1- integer)))))
@result{} factorial
@end group

@group
(byte-compile 'factorial)
@result{}
#[257
  "\211\300U\203^H^@@\300\207\211\301^BS!_\207"
  [1 factorial] 4
  "Compute factorial of INTEGER.\n\n(fn INTEGER)"]
@end group
@end example

If @var{symbol}'s definition is a byte-code function object,
@code{byte-compile} does nothing and returns @code{nil}.  It does not
compile the symbol's definition again, since the original
(non-compiled) code has already been replaced in the symbol's function
cell by the byte-compiled code.

The argument to @code{byte-compile} can also be a @code{lambda}
expression.  In that case, the function returns the corresponding
compiled code but does not store it anywhere.
@end defun

@deffn 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.

@code{compile-defun} normally displays the result of evaluation in the
echo area, but if @var{arg} is non-@code{nil}, it inserts the result
in the current buffer after the form it has compiled.
@end deffn

@deffn Command byte-compile-file filename
This function compiles a file of Lisp code named @var{filename} into a
file of byte-code.  The output file's name is made by changing the
@samp{.el} suffix into @samp{.elc}; if @var{filename} does not end in
@samp{.el}, it adds @samp{.elc} to the end of @var{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 @code{t} if there were no errors and @code{nil}
otherwise.  When called interactively, it prompts for the file name.

@example
@group
$ ls -l push*
-rw-r--r-- 1 lewis lewis 791 Oct  5 20:31 push.el
@end group

@group
(byte-compile-file "~/emacs/push.el")
     @result{} t
@end group

@group
$ ls -l push*
-rw-r--r-- 1 lewis lewis 791 Oct  5 20:31 push.el
-rw-rw-rw- 1 lewis lewis 638 Oct  8 20:25 push.elc
@end group
@end example
@end deffn

@deffn Command byte-recompile-directory directory &optional flag force follow-symlinks
@cindex library compilation
This command recompiles every @samp{.el} file in @var{directory} (or
its subdirectories) that needs recompilation.  A file needs
recompilation if a @samp{.elc} file exists but is older than the
@samp{.el} file.

When a @samp{.el} file has no corresponding @samp{.elc} file,
@var{flag} says what to do.  If it is @code{nil}, this command ignores
these files.  If @var{flag} is 0, it compiles them.  If it is neither
@code{nil} nor 0, it asks the user whether to compile each such file,
and asks about each subdirectory as well.

Interactively, @code{byte-recompile-directory} prompts for
@var{directory} and @var{flag} is the prefix argument.

If @var{force} is non-@code{nil}, this command recompiles every
@samp{.el} file that has a @samp{.elc} file.

This command will normally not compile @samp{.el} files that are
symlinked.  If the optional @var{follow-symlink} parameter is
non-@code{nil}, symlinked @samp{.el} will also be compiled.

The returned value is unpredictable.
@end deffn

@defun batch-byte-compile &optional noforce
This function runs @code{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.

If @var{noforce} is non-@code{nil}, this function does not recompile
files that have an up-to-date @samp{.elc} file.

@example
$ emacs -batch -f batch-byte-compile *.el
@end example
@end defun

@node Docs and Compilation
@section Documentation Strings and Compilation
@cindex dynamic loading of documentation

  When Emacs loads functions and variables from a byte-compiled file,
it normally does not load their documentation strings into memory.
Each documentation string is dynamically loaded from the
byte-compiled file only when needed.  This saves memory, and speeds up
loading by skipping the processing of the documentation strings.

  This feature has a drawback: if you delete, move, or alter the
compiled file (such as by compiling a new version), Emacs may no
longer be able to access the documentation string of previously-loaded
functions or variables.  Such a problem normally only occurs if you
build Emacs yourself, and happen to edit and/or recompile the Lisp
source files.  To solve it, just reload each file after recompilation.

  Dynamic loading of documentation strings from byte-compiled files is
determined, at compile time, for each byte-compiled file.  It can be
disabled via the option @code{byte-compile-dynamic-docstrings}.

@defopt byte-compile-dynamic-docstrings
If this is non-@code{nil}, the byte compiler generates compiled files
that are set up for dynamic loading of documentation strings.

To disable the dynamic loading feature for a specific file, set this
option to @code{nil} in its header line (@pxref{File Variables, ,
Local Variables in Files, emacs, The GNU Emacs Manual}), like this:

@smallexample
-*-byte-compile-dynamic-docstrings: nil;-*-
@end smallexample

This is useful mainly if you expect to change the file, and you want
Emacs sessions that have already loaded it to keep working when the
file changes.
@end defopt

@cindex @samp{#@@@var{count}}
@cindex @samp{#$}
Internally, the dynamic loading of documentation strings is
accomplished by writing compiled files with a special Lisp reader
construct, @samp{#@@@var{count}}.  This construct skips the next
@var{count} characters.  It also uses the @samp{#$} construct, which
stands for the name of this file, as a string.  Do not use these
constructs in Lisp source files; they are not designed to be clear to
humans reading the file.

@node Eval During Compile
@section Evaluation During Compilation
@cindex eval during compilation

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

@defmac eval-and-compile body@dots{}
This form marks @var{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 @var{body} in a separate file
and referring to that file with @code{require}.  That method is
preferable when @var{body} is large.  Effectively @code{require} is
automatically @code{eval-and-compile}, the package is loaded both when
compiling and executing.

@code{autoload} is also effectively @code{eval-and-compile} too.  It's
recognized when compiling, so uses of such a function don't produce
``not known to be defined'' warnings.

Most uses of @code{eval-and-compile} are fairly sophisticated.

If a macro has a helper function to build its result, and that macro
is used both locally and outside the package, then
@code{eval-and-compile} should be used to get the helper both when
compiling and then later when running.

If functions are defined programmatically (with @code{fset} say), then
@code{eval-and-compile} can be used to have that done at compile-time
as well as run-time, so calls to those functions are checked (and
warnings about ``not known to be defined'' suppressed).
@end defmac

@defmac eval-when-compile body@dots{}
This form marks @var{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, @var{body} is
evaluated normally.

@cindex compile-time constant
If you have a constant that needs some calculation to produce,
@code{eval-when-compile} can do that at compile-time.  For example,

@lisp
(defvar gauss-schoolboy-problem
  (eval-when-compile (apply #'+ (number-sequence 1 100))))
@end lisp

@cindex macros, at compile time
If you're using another package, but only need macros from it (the
byte compiler will expand those), then @code{eval-when-compile} can be
used to load it for compiling, but not executing.  For example,

@lisp
(eval-when-compile
  (require 'my-macro-package))
@end lisp

The same sort of thing goes for macros and @code{defsubst} functions
defined locally and only for use within the file.  They are needed for
compiling the file, but in most cases they are not needed for
execution of the compiled file.  For example,

@lisp
(eval-when-compile
  (unless (fboundp 'some-new-thing)
    (defmacro some-new-thing ()
      (compatibility code))))
@end lisp

@noindent
This is often good for code that's only a fallback for compatibility
with other versions of Emacs.

@strong{Common Lisp Note:} At top level, @code{eval-when-compile} is analogous to the Common
Lisp idiom @code{(eval-when (compile eval) @dots{})}.  Elsewhere, the
Common Lisp @samp{#.} reader macro (but not when interpreting) is closer
to what @code{eval-when-compile} does.
@end defmac

@node Compiler Errors
@section Compiler Errors
@cindex compiler errors
@cindex byte-compiler errors

  Error and warning messages from byte compilation are printed in a
buffer named @file{*Compile-Log*}.  These messages include file names
and line numbers identifying the location of the problem.  The usual
Emacs commands for operating on compiler output can be used on these
messages.

  When an error is due to invalid syntax in the program, the byte
compiler might get confused about the error's exact location.  One way
to investigate is to switch to the buffer @w{@file{ *Compiler
Input*}}.  (This buffer name starts with a space, so it does not show
up in the Buffer Menu.)  This buffer contains the program being
compiled, and point shows how far the byte compiler was able to read;
the cause of the error might be nearby.  @xref{Syntax Errors}, for
some tips for locating syntax errors.

@cindex byte-compiler warnings
@cindex free variable, byte-compiler warning
@cindex reference to free variable, compilation warning
@cindex function not known to be defined, compilation warning
  A common type of warning issued by the byte compiler is for
functions and variables that were used but not defined.  Such warnings
report the line number for the end of the file, not the locations
where the missing functions or variables were used; to find these, you
must search the file manually.

  If you are sure that a warning message about a missing function or
variable is unjustified, there are several ways to suppress it:

@itemize @bullet
@item
You can suppress the warning for a specific call to a function
@var{func} by conditionalizing it on an @code{fboundp} test, like
this:

@example
(if (fboundp '@var{func}) ...(@var{func} ...)...)
@end example

@noindent
The call to @var{func} must be in the @var{then-form} of the
@code{if}, and @var{func} must appear quoted in the call to
@code{fboundp}.  (This feature operates for @code{cond} as well.)

@item
Likewise, you can suppress the warning for a specific use of a
variable @var{variable} by conditionalizing it on a @code{boundp}
test:

@example
(if (boundp '@var{variable}) ...@var{variable}...)
@end example

@noindent
The reference to @var{variable} must be in the @var{then-form} of the
@code{if}, and @var{variable} must appear quoted in the call to
@code{boundp}.

@item
You can tell the compiler that a function is defined using
@code{declare-function}.  @xref{Declaring Functions}.

@item
Likewise, you can tell the compiler that a variable is defined using
@code{defvar} with no initial value.  (Note that this marks the
variable as special, i.e.@: dynamically bound, but only within the
current lexical scope, or file if at top-level.)  @xref{Defining
Variables}.
@end itemize

  You can also suppress compiler warnings within a certain expression
using the @code{with-suppressed-warnings} macro:

@defspec with-suppressed-warnings warnings body@dots{}
In execution, this is equivalent to @code{(progn @var{body}...)}, but
the compiler does not issue warnings for the specified conditions in
@var{body}.  @var{warnings} is an association list of warning symbols
and function/variable symbols they apply to.  For instance, if you
wish to call an obsolete function called @code{foo}, but want to
suppress the compilation warning, say:

@lisp
(with-suppressed-warnings ((obsolete foo))
  (foo ...))
@end lisp
@end defspec

For more coarse-grained suppression of compiler warnings, you can use
the @code{with-no-warnings} construct:

@c This is implemented with a defun, but conceptually it is
@c a special form.

@defspec with-no-warnings body@dots{}
In execution, this is equivalent to @code{(progn @var{body}...)},
but the compiler does not issue warnings for anything that occurs
inside @var{body}.

We recommend that you use @code{with-suppressed-warnings} instead, but
if you do use this construct, that you use it around the smallest
possible piece of code to avoid missing possible warnings other than
one you intend to suppress.
@end defspec

  Byte compiler warnings can be controlled more precisely by setting
the variable @code{byte-compile-warnings}.  See its documentation
string for details.

@vindex byte-compile-error-on-warn
  Sometimes you may wish the byte-compiler warnings to be reported
using @code{error}.  If so, set @code{byte-compile-error-on-warn} to a
non-@code{nil} value.

@node Closure Objects
@section Closure Function Objects
@cindex compiled function
@cindex byte-code function
@cindex byte-code object

  Byte-compiled functions use a special data type: they are closures.
Closures are used both for byte-compiled Lisp functions as well as for
interpreted Lisp functions.  Whenever such an object appears as
a function to be called, Emacs uses the appropriate interpreter to
execute either the byte-code or the non-compiled Lisp code.

  Internally, a closure is much like a vector; its
elements can be accessed using @code{aref}.  Its printed
representation is like that for a vector, with an additional @samp{#}
before the opening @samp{[}.  It must have at least three elements;
there is no maximum number, but only the first six elements have any
normal use.  They are:

@table @var
@item argdesc
The descriptor of the arguments.  This can either be a list of
arguments, as described in @ref{Argument List}, or an integer encoding
the required number of arguments.  In the latter case, the value of
the descriptor specifies the minimum number of arguments in the bits
zero to 6, and the maximum number of arguments in bits 8 to 14.  If
the argument list uses @code{&rest}, then bit 7 is set; otherwise it's
cleared.

When the closure is a byte-code function,
if @var{argdesc} is a list, the arguments will be dynamically bound
before executing the byte code.  If @var{argdesc} is an integer, the
arguments will be instead pushed onto the stack of the byte-code
interpreter, before executing the code.

@item code
For interpreted functions, this element is the (non-empty) list of Lisp
forms that make up the function's body.  For byte-compiled functions, it
is the string containing the byte-code instructions.

@item constants
For byte-compiled functions, this holds the vector of Lisp objects
referenced by the byte code.  These include symbols used as function
names and variable names.
For interpreted functions, this is @code{nil} if the function is using the old
dynamically scoped dialect of Emacs Lisp, and otherwise it holds the
function's lexical environment.

@item stacksize
The maximum stack size this function needs.  This element is left unused
for interpreted functions.

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

@item interactive
The interactive spec (if any).  This can be a string or a Lisp
expression.  It is @code{nil} for a function that isn't interactive.
@end table

Here's an example of a byte-code function object, in printed
representation.  It is the definition of the command
@code{backward-sexp}.

@example
#[256
  "\211\204^G^@@\300\262^A\301^A[!\207"
  [1 forward-sexp]
  3
  1793299
  "^p"]
@end example

  The primitive way to create a byte-code object is with
@code{make-byte-code}:

@defun make-byte-code &rest elements
This function constructs and returns a closure which represents the
byte-code function object with @var{elements} as its elements.
@end defun

  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).

The primitive way to create an interpreted function is with
@code{make-interpreted-closure}:

@defun make-interpreted-closure args body env &optional docstring iform
This function constructs and returns a closure representing the
interpreted function with arguments @var{args} and whose body is made of
@var{body} which must be a non-@code{nil} list of Lisp forms.  @var{env} is the
lexical environment in the same form as used with @code{eval}
(@pxref{Eval}).  The documentation @var{docstring} if non-@code{nil} should be
a string, and the interactive form @var{iform} if non-@code{nil} should be of
the form @w{@code{(interactive @var{arg-descriptor})}} (@pxref{Using
Interactive}).
@end defun

@node Disassembly
@section Disassembled Byte-Code
@cindex 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
human-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.

@deffn Command disassemble object &optional buffer-or-name
This command displays the disassembled code for @var{object}.  In
interactive use, or if @var{buffer-or-name} is @code{nil} or omitted,
the output goes in a buffer named @file{*Disassemble*}.  If
@var{buffer-or-name} is non-@code{nil}, it must be a buffer or the
name of an existing buffer.  Then the output goes there, at point, and
point is left before the output.

The argument @var{object} can be a function name, a lambda expression
(@pxref{Lambda Expressions}), or a byte-code object (@pxref{Closure
Objects}).  If it is a lambda expression, @code{disassemble} compiles
it and disassembles the resulting compiled code.
@end deffn

  Here are two examples of using the @code{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 @code{disassemble}.

@example
@group
(defun factorial (integer)
  "Compute factorial of an integer."
  (if (= 1 integer) 1
    (* integer (factorial (1- integer)))))
     @result{} factorial
@end group

@group
(factorial 4)
     @result{} 24
@end group

@group
(disassemble 'factorial)
     @print{} byte-code for factorial:
 doc: Compute factorial of an integer.
 args: (arg1)
@end group

@group
0   dup                   ; @r{Get the value of @code{integer} and}
                          ;   @r{push it onto the stack.}
1   constant 1            ; @r{Push 1 onto stack.}
@end group
@group
2   eqlsign               ; @r{Pop top two values off stack, compare}
                          ;   @r{them, and push result onto stack.}
@end group
@group
3   goto-if-nil 1         ; @r{Pop and test top of stack;}
                          ;   @r{if @code{nil}, go to 1, else continue.}
6   constant 1            ; @r{Push 1 onto top of stack.}
7   return                ; @r{Return the top element of the stack.}
@end group
@group
8:1 dup                   ; @r{Push value of @code{integer} onto stack.}
9   constant factorial    ; @r{Push @code{factorial} onto stack.}
10  stack-ref 2           ; @r{Push value of @code{integer} onto stack.}
11  sub1                  ; @r{Pop @code{integer}, decrement value,}
                          ;   @r{push new value onto stack.}
12  call     1            ; @r{Call function @code{factorial} using first}
                          ;   @r{(i.e., top) stack element as argument;}
                          ;   @r{push returned value onto stack.}
@end group
@group
13  mult                  ; @r{Pop top two values off stack, multiply}
                          ;   @r{them, and push result onto stack.}
14  return                ; @r{Return the top element of the stack.}
@end group
@end example

The @code{silly-loop} function is somewhat more complex:

@example
@group
(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))))
     @result{} silly-loop
@end group

@group
(disassemble 'silly-loop)
     @print{} byte-code for silly-loop:
 doc: Return time before and after N iterations of a loop.
 args: (arg1)
@end group

@group
0   constant current-time-string  ; @r{Push @code{current-time-string}}
                                  ;   @r{onto top of stack.}
@end group
@group
1   call     0            ; @r{Call @code{current-time-string} with no}
                          ;   @r{argument, push result onto stack as @code{t1}.}
@end group
@group
2:1 stack-ref 1           ; @r{Get value of the argument @code{n}}
                          ;   @r{and push the value on the stack.}
3   sub1                  ; @r{Subtract 1 from top of stack.}
@end group
@group
4   dup                   ; @r{Duplicate top of stack; i.e., copy the top}
                          ;   @r{of the stack and push copy onto stack.}
5   stack-set 3           ; @r{Pop the top of the stack,}
                          ;   @r{and set @code{n} to the value.}

;; @r{(In effect, the sequence @code{dup stack-set} copies the top of}
;; @r{the stack into the value of @code{n} without popping it.)}
@end group

@group
7   constant 0            ; @r{Push 0 onto stack.}
8   gtr                   ; @r{Pop top two values off stack,}
                          ;   @r{test if @var{n} is greater than 0}
                          ;   @r{and push result onto stack.}
@end group
@group
9   goto-if-not-nil 1     ; @r{Goto 1 if @code{n} > 0}
                          ;   @r{(this continues the while loop)}
                          ;   @r{else continue.}
@end group
@group
12  dup                   ; @r{Push value of @code{t1} onto stack.}
13  constant current-time-string  ; @r{Push @code{current-time-string}}
                                  ;   @r{onto the top of the stack.}
14  call     0            ; @r{Call @code{current-time-string} again.}
@end group
@group
15  list2                 ; @r{Pop top two elements off stack, create a}
                          ;   @r{list of them, and push it onto stack.}
16  return                ; @r{Return value of the top of stack.}
@end group
@end example

@node Native Compilation
@chapter Compilation of Lisp to Native Code
@cindex native compilation
@cindex compilation to native code (Emacs Lisp)

@cindex native code
  In addition to the byte-compilation, described in @ref{Byte
Compilation, the previous chapter}, Emacs can also optionally compile
Lisp function definitions into a true compiled code, known as
@dfn{native code}.  This feature uses the @file{libgccjit} library,
which is part of the GCC distribution, and requires that Emacs be
built with support for using that library.  It also requires GCC and
Binutils (the assembler and linker) to be available on your system for
you to be able to native-compile Lisp code.

@vindex native-compile@r{, a Lisp feature}
  To determine whether the current Emacs process can produce and load
natively-compiled Lisp code, call
@code{native-comp-available-p} (@pxref{Native-Compilation Functions}).

  Unlike byte-compiled code, natively-compiled Lisp code is executed
directly by the machine's hardware, and therefore runs at full speed
that the host CPU can provide.  The resulting speedup generally
depends on what the Lisp code does, but is usually 2.5 to 5 times
faster than the corresponding byte-compiled code.

  Since native code is generally incompatible between different
systems, the natively-compiled code is @emph{not} transportable from
one machine to another, it can only be used on the same machine where
it was produced or on very similar ones (having the same CPU and
run-time libraries).  The transportability of natively-compiled code
is the same as that of shared libraries (@file{.so} or @file{.dll}
files).

  Libraries of natively-compiled code include crucial dependencies on
Emacs Lisp primitives (@pxref{What Is a Function}) and their calling
conventions, and thus Emacs usually won't load natively-compiled code
produced by earlier or later Emacs versions; native compilation of the
same Lisp code by a different Emacs version will usually produce a
natively-compiled library under a unique file name that only that
version of Emacs will be able to load.  However, the use of unique
file names enables several versions of the same Lisp library
natively-compiled by several different versions of Emacs to be placed
within the same directory.

@vindex no-native-compile
  A non-@code{nil} file-local variable binding of
@code{no-byte-compile} (@pxref{Byte Compilation}) also disables the
native compilation of that file.  In addition, a similar variable
@code{no-native-compile} disables just the native compilation of the
file.  If both @code{no-byte-compile} and @code{no-native-compile} are
specified, the former takes precedence.

@cindex native compilation, prevent writing @file{*.eln} files
  Sometimes there could be a need to prevent the native compilation
from writing its results, the @file{*.eln} files, into a subdirectory
of @code{user-emacs-directory} (@pxref{Init File}).  You can do that
by either changing the value of @code{native-comp-eln-load-path}
(@pxref{Native-Compilation Variables}) or by temporarily pointing the
@env{HOME} environment variable to a non-existing directory.  Note
that the latter technique might still produce a small number of
@file{*.eln} files if Emacs needs to generate @dfn{trampolines}, which
are used if Lisp primitives are advised or redefined in your Lisp code
that is being natively compiled.  @xref{Native-Compilation Variables,
trampolines}.  Alternatively, you can specify that the @file{*.eln}
files are written to a non-default directory using the
@code{startup-redirect-eln-cache} function; @pxref{Native-Compilation
Functions}.

@menu
* Native-Compilation Functions::  Functions to natively-compile Lisp.
* Native-Compilation Variables::  Variables controlling native compilation.
@end menu

@node Native-Compilation Functions
@section Native-Compilation Functions
@cindex native-compilation functions

   Native-Compilation is implemented as a side effect of
byte-compilation (@pxref{Byte Compilation}).  Thus, compiling Lisp
code natively always produces its byte code as well, and therefore all
the rules and caveats of preparing Lisp code for byte compilation
(@pxref{Compilation Functions}) are valid for native-compilation as
well.

   You can natively-compile either a single function or macro
definition, or a whole file of Lisp code, with the
@code{native-compile} function.  Natively-compiling a file will
produce the @file{.eln} file with native code.

@findex native-comp-limple-mode
@vindex native-comp-verbose
  Native compilation might produce warning or error messages; these
are normally recorded in the buffer called
@file{*Native-compile-Log*}.  In interactive sessions, it uses the
special LIMPLE mode (@code{native-comp-limple-mode}), which sets up
@code{font-lock} as appropriate for this log, and is otherwise the
same as Fundamental mode.  Logging of messages resulting from
native-compilation can be controlled by the @code{native-comp-verbose}
variable (@pxref{Native-Compilation Variables}).

  When Emacs is run non-interactively, messages produced by
native-compilation are reported by calling @code{message}
(@pxref{Displaying Messages}), and are usually displayed on the
standard error stream of the terminal from which Emacs was invoked.

@defun native-compile function-or-file &optional output
This function compiles @var{function-or-file} into native code.  The
argument @var{function-or-file} can be a function symbol, a Lisp form,
or a name (a string) of the file which contains the Emacs Lisp source
code to compile.  If the optional argument @var{output} is provided,
it must be a string specifying the name of the file to write the
compiled code into.  Otherwise, if @var{function-or-file} is a
function or a Lisp form, this function returns the compiled object,
and if @var{function-or-file} is a file name, the function returns the
full absolute name of the file it created for the compiled code.  The
output file is by default given the @file{.eln} extension.

This function runs the final phase of the native compilation, which
invokes GCC via @file{libgccjit}, in a separate subprocess, which
invokes the same Emacs executable as the process that called this
function.
@end defun

@defun batch-native-compile &optional for-tarball
This function runs native-compilation on files specified on the Emacs
command line in batch mode.  It must be used only in a batch execution
of Emacs, as it kills Emacs upon completion of the compilation.  If
one or more of the files fail to compile, the Emacs process will
attempt to compile all the other files, and will terminate with a
non-zero status code.  The optional argument @var{for-tarball}, if
non-@code{nil}, tells the function to place the resulting @file{.eln}
files in the last directory mentioned in
@code{native-comp-eln-load-path} (@pxref{Library Search}); this is
meant to be used as part of building an Emacs source tarball for the
first time, when the natively-compiled files, which are absent from
the source tarball, should be generated in the build tree instead of
the user's cache directory.
@end defun

Native compilation can be run entirely asynchronously, in a subprocess
of the main Emacs process.  This leaves the main Emacs process free to
use while the compilation runs in the background.  This is the method
used by Emacs to natively-compile any Lisp file or byte-compiled Lisp
file that is loaded into Emacs, when no natively-compiled file for it
is available.  Note that because of this use of a subprocess, native
compilation may produce warning and errors which byte-compilation does
not, and Lisp code may thus need to be modified to work correctly.  See
@code{native-comp-async-report-warnings-errors} in @pxref{Native-Compilation
Variables} for more details.

@defun native-compile-async files &optional recursively load selector
This function compiles the named @var{files} asynchronously.  The
argument @var{files} should be a single file name (a string) or a list
of one or more file and/or directory names.  If directories are
present in the list, the optional argument @var{recursively} should be
non-@code{nil} to cause the compilation to recurse into those
directories.  If @var{load} is non-@code{nil}, Emacs will load each
file that it succeeded to compile.  The optional argument
@var{selector} allows control of which of @var{files} will be
compiled; it can have one of the following values:

@table @asis
@item @code{nil} or omitted
Select all the files and directories in @var{files}.
@item a regular expression string
Select the files and directories whose names match the regexp.
@item a function
A predicate function, which will be called with each file and
directory in @var{files}, and should return non-@code{nil} if the file
or the directory should be selected for compilation.
@end table

On systems with multiple CPU execution units, when @var{files} names
more than one file, this function will normally start several
compilation subprocesses in parallel, under the control of
@code{native-comp-async-jobs-number} (@pxref{Native-Compilation
Variables}).
@end defun

@defun native-compile-directory directory
This function compiles into native code all the @file{*.el} files in the
specified @var{directory} and, recursively, in all of its
subdirectories, if a corresponding @file{.eln} file could not be found
in any of the directories mentioned in the
@code{native-comp-eln-load-path} list (@pxref{Library Search}).
@end defun

@deffn Command emacs-lisp-native-compile
This command compiles the file visited by the current buffer into
native code, if the file was changed since the last time it was
natively-compiled.
@end deffn

@deffn Command emacs-lisp-native-compile-and-load
This command compiles the file visited by the current buffer into
native code, like @code{emacs-lisp-native-compile}, but it also loads
the native code when the compilation finishes.
@end deffn

  The following function allows Lisp programs to test whether
native-compilation is available at runtime.

@defun native-comp-available-p
This function returns non-@code{nil} if the running Emacs process has
the native-compilation support compiled into it.  On systems that load
@file{libgccjit} dynamically, it also makes sure that library is
available and can be loaded.  Lisp programs that need to know up front
whether native-compilation is available should use this predicate.
@end defun

  By default, asynchronous native compilation writes the @file{*.eln}
files it produces to a subdirectory of the first writable directory
specified by the @code{native-comp-eln-load-path} variable
(@pxref{Native-Compilation Variables}).  You can change this by using
the following function in your startup files:

@defun startup-redirect-eln-cache cache-directory
This function arranges for the asynchronous native compilation to
write the produced @file{*.eln} files to @var{cache-directory}, which
must be a single directory, a string.  It also destructively modifies
@code{native-comp-eln-load-path} such that its first element is
@var{cache-directory}.  If @var{cache-directory} is not an absolute
file name, it is interpreted relative to @code{user-emacs-directory}
(@pxref{Init File}).
@end defun

@node Native-Compilation Variables
@section Native-Compilation Variables
@cindex native-compilation variables

  This section documents the variables that control
native-compilation.

@defopt native-comp-speed
This variable specifies the optimization level for native compilation.
Its value should be a number between @minus{}1 and 3.  Values between
0 and 3 specify the optimization levels equivalent to the
corresponding compiler @option{-O0}, @option{-O1}, etc.@: command-line
options of the compiler.  The value @minus{}1 means disable
native-compilation: functions and files will be only byte-compiled;
however, the @file{*.eln} files will still be produced, they will just
contain the compiled code in bytecode form.  (This can be achieved at
function granularity by using the @w{@code{(declare (speed -1))}}
form, @pxref{Declare Form}.)
The default value is 2.
@end defopt

@anchor{compilation-safety}
@defopt compilation-safety
This variable specifies the safety level to be used for the emitted
native code.  The value should be a number, either 0 or 1 with the
following meanings:

@table @asis
@item 0
Emitted code can misbehave (up to even crashing Emacs) if a function's
declaration does not describe correctly what the function does or how it
is called, and the function is natively compiled.
@item 1
Emitted code must be generated in a safe manner even if functions are
mis-declared.
@end table

This can also be controlled at a function granularity, by using the
@code{safety} @code{declare} form, @pxref{Declare Form}.
@end defopt

@defopt native-comp-debug
This variable specifies the level of debugging information produced by
native-compilation.  Its value should be a number between zero and 3,
with the following meaning:

@table @asis
@item 0
No debugging output.  This is the default.
@item 1
Emit debugging symbols with the native code.  This allows easier
debugging of the native code with debuggers such as @command{gdb}.
@item 2
Like 1, and in addition dump pseudo-C code.
@item 3
Like 2, and in addition dump the GCC intermediate passes and
@file{libgccjit} log file.
@end table

When generated, the pseudo-C code is deposited in the same directory
as the corresponding @file{.eln} file.
@end defopt

@defopt native-comp-verbose
This variable controls the verbosity of native-compilation by
suppressing some or all of the log messages emitted by it.  If its
value is zero, the default, all of the log messages are suppressed.
Setting it to a value between 1 and 3 will allow logging of the
messages whose level is above the value.  The values have the
following interpretations:

@table @asis
@item 0
No logging.  This is the default.
@item 1
Log the final @acronym{LIMPLE} representation of the code.
@item 2
Log the @acronym{LAP}, the final @acronym{LIMPLE}, and some additional
pass info.
@item 3
Maximum verbosity: log everything.
@end table
@end defopt

@defopt native-comp-async-jobs-number
This variable determines the maximum number of native-compilation
subprocesses that will be started simultaneously.  It should be a
non-negative number.  The default value is zero, which means use half
the number of the CPU execution units, or 1 if the CPU has only one
execution unit.
@end defopt

@defopt native-comp-async-report-warnings-errors
If this variable's value is non-@code{nil}, warnings and errors from
asynchronous native-compilation subprocesses are reported in the main
Emacs session in a buffer named @file{*Warnings*}.  The default value
@code{t} means display the resulting buffer.  To log warnings without
popping up the @file{*Warnings*} buffer, set this variable to
@code{silent}.

  A common cause for asynchronous native-compilation to produce
warnings is compiling a file that is missing some @code{require} of a
necessary feature.  The feature may be loaded into the main emacs, but
because native compilation always starts from a subprocess with a
pristine environment, that may not be true for the subprocess.
@end defopt

@defopt native-comp-async-query-on-exit
If this variable's value is non-@code{nil}, Emacs will query upon exiting
whether to exit and kill any asynchronous native-compilation
subprocesses that are still running, thus preventing the corresponding
@file{.eln} files from being written.  If the value is @code{nil}, the
default, Emacs will kill these subprocesses without querying.
@end defopt

The variable @code{native-comp-eln-load-path} holds the list of
directories where Emacs looks for the @file{*.eln} files
(@pxref{Library Search}); in that role it is the equivalent of
@code{load-path} used to look for @file{*.el} and @file{*.elc} files.
The directories in this list are also used for writing the
@file{*.eln} files produced by asynchronous native-compilation;
specifically, Emacs will write these files into the first writable
directory in the list.  Thus, you can control where native-compilation
stores the results by changing the value of this variable.

@cindex disable asynchronous native compilation
@cindex inhibit asynchronous native compilation
@cindex asynchronous native compilation, disable
@defvar native-comp-jit-compilation
This variable, if non-@code{nil}, enables asynchronous (a.k.a.@:
@dfn{just-in-time}, or @acronym{JIT}) native compilation of the
@file{*.elc} files loaded by Emacs for which the corresponding
@file{*.eln} files do not already exist.  This JIT compilation uses
separate Emacs sub-processes running in batch mode, according to the
value of @code{native-comp-async-jobs-number}.  When the JIT
compilation of a Lisp file finishes successfully, the resulting
@file{.eln} file is loaded and its code replaces the definition of
functions provided by the @file{.elc} file.
@end defvar

@cindex trampolines, in native compilation
  Setting the value of @code{native-comp-jit-compilation} to @code{nil}
disables JIT native compilation.  However, even when JIT native
compilation is disabled, Emacs might still need to start asynchronous
native compilation subprocesses to produce @dfn{trampolines}.  To
control this, use a separate variable, described below.

@defvar native-comp-enable-subr-trampolines
This variable controls generation of trampolines.  A trampoline is a
small piece of native code required to allow calling Lisp primitives,
which were advised or redefined, from Lisp code that was
natively-compiled with @code{native-comp-speed} set to 2 or greater.
Emacs stores the generated trampolines on separate @file{*.eln} files.
By default, this variable's value is @code{t}, which enables the
generation of trampoline files; setting it to @code{nil} disables the
generation of trampolines.  Note that if a trampoline needed for
advising or redefining a primitive is not available and cannot be
generated, calls to that primitive from natively-compiled Lisp will
ignore redefinitions and advices, and will behave as if the primitive
was called directly from C.  Therefore, we don't recommend disabling
the trampoline generation, unless you know that all the trampolines
needed by your Lisp programs are already compiled and accessible to
Emacs.

The value of this variable can also be a string, in which case it
specifies the name of a directory in which to store the generated
trampoline @file{*.eln} files, overriding the directories in
@code{native-comp-eln-load-path}.  This is useful if you want the
trampolines to be generated as needed, but don't want to store them
under the user's @env{HOME} directory or in the other public
directories where @file{*.eln} files are kept.  However, unlike with
directories in @code{native-comp-eln-load-path}, the trampolines will
be stored in the directory given by the value of this variable, not in
its version-specific subdirectory.  If the name of this directory is
not absolute, it is interpreted relative to
@code{invocation-directory} (@pxref{System Environment})

If this variable is non-@code{nil}, and Emacs needs to produce a
trampoline, but it cannot find any writable directory to store the
trampoline, it will store it inside @code{temporary-file-directory}
(@pxref{Unique File Names}).

Trampolines produced when no writable directory is found to store
them, or when this variable is a string, will only be available for
the duration of the current Emacs session, because Emacs doesn't look
for trampolines in either of these places.
@end defvar