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732 lines
25 KiB
Plaintext
@c -*-texinfo-*-
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@c This is part of the GNU Emacs Lisp Reference Manual.
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@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
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@c See the file elisp.texi for copying conditions.
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@setfilename ../info/compile
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@node Byte Compilation, Debugging, Loading, Top
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@chapter Byte Compilation
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@cindex byte-code
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@cindex compilation
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GNU Emacs Lisp has a @dfn{compiler} that translates functions written
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in Lisp into a special representation called @dfn{byte-code} that can be
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executed more efficiently. The compiler replaces Lisp function
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definitions with byte-code. When a byte-code function is called, its
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definition is evaluated by the @dfn{byte-code interpreter}.
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Because the byte-compiled code is evaluated by the byte-code
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interpreter, instead of being executed directly by the machine's
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hardware (as true compiled code is), byte-code is completely
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transportable from machine to machine without recompilation. It is not,
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however, as fast as true compiled code.
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In general, any version of Emacs can run byte-compiled code produced
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by recent earlier versions of Emacs, but the reverse is not true. In
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particular, if you compile a program with Emacs 19.29, the compiled
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code does not run in earlier versions.
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@iftex
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@xref{Docs and Compilation}.
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@end iftex
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Files compiled in versions before 19.29 may not work in 19.29 if they
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contain character constants with modifier bits, because the bits were
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renumbered in Emacs 19.29.
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@xref{Compilation Errors}, for how to investigate errors occurring in
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byte compilation.
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@menu
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* Speed of Byte-Code:: An example of speedup from byte compilation.
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* Compilation Functions:: Byte compilation functions.
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* Docs and Compilation:: Dynamic loading of documentation strings.
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* Dynamic Loading:: Dynamic loading of individual functions.
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* Eval During Compile:: Code to be evaluated when you compile.
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* Byte-Code Objects:: The data type used for byte-compiled functions.
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* Disassembly:: Disassembling byte-code; how to read byte-code.
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@end menu
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@node Speed of Byte-Code
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@section Performance of Byte-Compiled Code
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A byte-compiled function is not as efficient as a primitive function
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written in C, but runs much faster than the version written in Lisp.
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Here is an example:
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@example
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@group
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(defun silly-loop (n)
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"Return time before and after N iterations of a loop."
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(let ((t1 (current-time-string)))
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(while (> (setq n (1- n))
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0))
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(list t1 (current-time-string))))
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@result{} silly-loop
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@end group
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@group
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(silly-loop 100000)
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@result{} ("Fri Mar 18 17:25:57 1994"
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"Fri Mar 18 17:26:28 1994") ; @r{31 seconds}
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@end group
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@group
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(byte-compile 'silly-loop)
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@result{} @r{[Compiled code not shown]}
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@end group
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@group
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(silly-loop 100000)
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@result{} ("Fri Mar 18 17:26:52 1994"
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"Fri Mar 18 17:26:58 1994") ; @r{6 seconds}
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@end group
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@end example
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In this example, the interpreted code required 31 seconds to run,
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whereas the byte-compiled code required 6 seconds. These results are
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representative, but actual results will vary greatly.
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@node Compilation Functions
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@comment node-name, next, previous, up
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@section The Compilation Functions
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@cindex compilation functions
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You can byte-compile an individual function or macro definition with
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the @code{byte-compile} function. You can compile a whole file with
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@code{byte-compile-file}, or several files with
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@code{byte-recompile-directory} or @code{batch-byte-compile}.
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When you run the byte compiler, you may get warnings in a buffer
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called @samp{*Compile-Log*}. These report things in your program that
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suggest a problem but are not necessarily erroneous.
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@cindex macro compilation
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Be careful when byte-compiling code that uses macros. Macro calls are
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expanded when they are compiled, so the macros must already be defined
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for proper compilation. For more details, see @ref{Compiling Macros}.
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Normally, compiling a file does not evaluate the file's contents or
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load the file. But it does execute any @code{require} calls at top
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level in the file. One way to ensure that necessary macro definitions
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are available during compilation is to require the file that defines
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them (@pxref{Named Features}). To avoid loading the macro definition files
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when someone @emph{runs} the compiled program, write
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@code{eval-when-compile} around the @code{require} calls (@pxref{Eval
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During Compile}).
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@defun byte-compile symbol
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This function byte-compiles the function definition of @var{symbol},
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replacing the previous definition with the compiled one. The function
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definition of @var{symbol} must be the actual code for the function;
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i.e., the compiler does not follow indirection to another symbol.
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@code{byte-compile} returns the new, compiled definition of
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@var{symbol}.
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If @var{symbol}'s definition is a byte-code function object,
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@code{byte-compile} does nothing and returns @code{nil}. Lisp records
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only one function definition for any symbol, and if that is already
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compiled, non-compiled code is not available anywhere. So there is no
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way to ``compile the same definition again.''
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@example
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@group
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(defun factorial (integer)
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"Compute factorial of INTEGER."
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(if (= 1 integer) 1
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(* integer (factorial (1- integer)))))
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@result{} factorial
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@end group
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@group
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(byte-compile 'factorial)
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@result{}
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#[(integer)
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"^H\301U\203^H^@@\301\207\302^H\303^HS!\"\207"
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[integer 1 * factorial]
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4 "Compute factorial of INTEGER."]
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@end group
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@end example
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@noindent
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The result is a byte-code function object. The string it contains is
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the actual byte-code; each character in it is an instruction or an
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operand of an instruction. The vector contains all the constants,
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variable names and function names used by the function, except for
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certain primitives that are coded as special instructions.
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@end defun
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@deffn Command compile-defun
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This command reads the defun containing point, compiles it, and
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evaluates the result. If you use this on a defun that is actually a
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function definition, the effect is to install a compiled version of that
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function.
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@end deffn
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@deffn Command byte-compile-file filename
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This function compiles a file of Lisp code named @var{filename} into
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a file of byte-code. The output file's name is made by appending
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@samp{c} to the end of @var{filename}.
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Compilation works by reading the input file one form at a time. If it
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is a definition of a function or macro, the compiled function or macro
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definition is written out. Other forms are batched together, then each
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batch is compiled, and written so that its compiled code will be
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executed when the file is read. All comments are discarded when the
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input file is read.
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This command returns @code{t}. When called interactively, it prompts
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for the file name.
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@example
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@group
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% ls -l push*
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-rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el
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@end group
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@group
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(byte-compile-file "~/emacs/push.el")
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@result{} t
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@end group
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@group
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% ls -l push*
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-rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el
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-rw-rw-rw- 1 lewis 638 Oct 8 20:25 push.elc
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@end group
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@end example
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@end deffn
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@deffn Command byte-recompile-directory directory flag
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@cindex library compilation
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This function recompiles every @samp{.el} file in @var{directory} that
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needs recompilation. A file needs recompilation if a @samp{.elc} file
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exists but is older than the @samp{.el} file.
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When a @samp{.el} file has no corresponding @samp{.elc} file, then
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@var{flag} says what to do. If it is @code{nil}, these files are
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ignored. If it is non-@code{nil}, the user is asked whether to compile
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each such file.
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The returned value of this command is unpredictable.
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@end deffn
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@defun batch-byte-compile
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This function runs @code{byte-compile-file} on files specified on the
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command line. This function must be used only in a batch execution of
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Emacs, as it kills Emacs on completion. An error in one file does not
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prevent processing of subsequent files. (The file that gets the error
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will not, of course, produce any compiled code.)
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@example
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% emacs -batch -f batch-byte-compile *.el
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@end example
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@end defun
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@defun byte-code code-string data-vector max-stack
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@cindex byte-code interpreter
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This function actually interprets byte-code. A byte-compiled function
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is actually defined with a body that calls @code{byte-code}. Don't call
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this function yourself. Only the byte compiler knows how to generate
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valid calls to this function.
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In newer Emacs versions (19 and up), byte-code is usually executed as
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part of a byte-code function object, and only rarely due to an explicit
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call to @code{byte-code}.
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@end defun
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@node Docs and Compilation
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@section Documentation Strings and Compilation
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@cindex dynamic loading of documentation
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Functions and variables loaded from a byte-compiled file access their
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documentation strings dynamically from the file whenever needed. This
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saves space within Emacs, and makes loading faster because the
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documentation strings themselves need not be processed while loading the
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file. Actual access to the documentation strings becomes slower as a
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result, but this normally is not enough to bother users.
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Dynamic access to documentation strings does have drawbacks:
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@itemize @bullet
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@item
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If you delete or move the compiled file after loading it, Emacs can no
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longer access the documentation strings for the functions and variables
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in the file.
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@item
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If you alter the compiled file (such as by compiling a new version),
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then further access to documentation strings in this file will give
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nonsense results.
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@end itemize
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If your site installs Emacs following the usual procedures, these
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problems will never normally occur. Installing a new version uses a new
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directory with a different name; as long as the old version remains
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installed, its files will remain unmodified in the places where they are
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expected to be.
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However, if you have built Emacs yourself and use it from the
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directory where you built it, you will experience this problem
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occasionally if you edit and recompile Lisp files. When it happens, you
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can cure the problem by reloading the file after recompiling it.
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Byte-compiled files made with Emacs 19.29 will not load into older
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versions because the older versions don't support this feature. You can
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turn off this feature by setting @code{byte-compile-dynamic-docstrings}
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to @code{nil}. Once this is done, you can compile files that will load
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into older Emacs versions. You can do this globally, or for one source
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file by specifying a file-local binding for the variable. Here's one
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way to do that:
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@example
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-*-byte-compile-dynamic-docstrings: nil;-*-
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@end example
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@defvar byte-compile-dynamic-docstrings
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If this is non-@code{nil}, the byte compiler generates compiled files
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that are set up for dynamic loading of documentation strings.
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@end defvar
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@cindex @samp{#@@@var{count}}
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@cindex @samp{#$}
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The dynamic documentation string feature writes compiled files that
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use a special Lisp reader construct, @samp{#@@@var{count}}. This
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construct skips the next @var{count} characters. It also uses the
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@samp{#$} construct, which stands for ``the name of this file, as a
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string.'' It is best not to use these constructs in Lisp source files.
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@node Dynamic Loading
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@section Dynamic Loading of Individual Functions
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@cindex dynamic loading of functions
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@cindex lazy loading
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When you compile a file, you can optionally enable the @dfn{dynamic
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function loading} feature (also known as @dfn{lazy loading}). With
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dynamic function loading, loading the file doesn't fully read the
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function definitions in the file. Instead, each function definition
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contains a place-holder which refers to the file. The first time each
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function is called, it reads the full definition from the file, to
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replace the place-holder.
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The advantage of dynamic function loading is that loading the file
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becomes much faster. This is a good thing for a file which contains
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many separate commands, provided that using one of them does not imply
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you will soon (or ever) use the rest. A specialized mode which provides
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many keyboard commands often has that usage pattern: a user may invoke
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the mode, but use only a few of the commands it provides.
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The dynamic loading feature has certain disadvantages:
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@itemize @bullet
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@item
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If you delete or move the compiled file after loading it, Emacs can no
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longer load the remaining function definitions not already loaded.
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@item
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If you alter the compiled file (such as by compiling a new version),
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then trying to load any function not already loaded will get nonsense
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results.
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@end itemize
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If you compile a new version of the file, the best thing to do is
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immediately load the new compiled file. That will prevent any future
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problems.
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The byte compiler uses the dynamic function loading feature if the
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variable @code{byte-compile-dynamic} is non-@code{nil} at compilation
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time. Do not set this variable globally, since dynamic loading is
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desirable only for certain files. Instead, enable the feature for
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specific source files with file-local variable bindings, like this:
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@example
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-*-byte-compile-dynamic: t;-*-
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@end example
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@defvar byte-compile-dynamic
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If this is non-@code{nil}, the byte compiler generates compiled files
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that are set up for dynamic function loading.
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@end defvar
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@defun fetch-bytecode function
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This immediately finishes loading the definition of @var{function} from
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its byte-compiled file, if it is not fully loaded already. The argument
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@var{function} may be a byte-code function object or a function name.
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@end defun
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@node Eval During Compile
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@section Evaluation During Compilation
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These features permit you to write code to be evaluated during
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compilation of a program.
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@defspec eval-and-compile body
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This form marks @var{body} to be evaluated both when you compile the
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containing code and when you run it (whether compiled or not).
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You can get a similar result by putting @var{body} in a separate file
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and referring to that file with @code{require}. Using @code{require} is
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preferable if there is a substantial amount of code to be executed in
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this way.
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@end defspec
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@defspec eval-when-compile body
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This form marks @var{body} to be evaluated at compile time and not when
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the compiled program is loaded. The result of evaluation by the
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compiler becomes a constant which appears in the compiled program. When
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the program is interpreted, not compiled at all, @var{body} is evaluated
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normally.
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At top level, this is analogous to the Common Lisp idiom
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@code{(eval-when (compile eval) @dots{})}. Elsewhere, the Common Lisp
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@samp{#.} reader macro (but not when interpreting) is closer to what
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@code{eval-when-compile} does.
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@end defspec
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@node Byte-Code Objects
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@section Byte-Code Function Objects
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@cindex compiled function
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@cindex byte-code function
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Byte-compiled functions have a special data type: they are
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@dfn{byte-code function objects}.
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Internally, a byte-code function object is much like a vector;
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however, the evaluator handles this data type specially when it appears
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as a function to be called. The printed representation for a byte-code
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function object is like that for a vector, with an additional @samp{#}
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before the opening @samp{[}.
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In Emacs version 18, there was no byte-code function object data type;
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compiled functions used the function @code{byte-code} to run the byte
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code.
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A byte-code function object must have at least four elements; there is
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no maximum number, but only the first six elements are actually used.
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They are:
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@table @var
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@item arglist
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The list of argument symbols.
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@item byte-code
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The string containing the byte-code instructions.
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@item constants
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The vector of Lisp objects referenced by the byte code. These include
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symbols used as function names and variable names.
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@item stacksize
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The maximum stack size this function needs.
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@item docstring
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The documentation string (if any); otherwise, @code{nil}. The value may
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be a number or a list, in case the documentation string is stored in a
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file. Use the function @code{documentation} to get the real
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documentation string (@pxref{Accessing Documentation}).
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@item interactive
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The interactive spec (if any). This can be a string or a Lisp
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expression. It is @code{nil} for a function that isn't interactive.
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@end table
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Here's an example of a byte-code function object, in printed
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representation. It is the definition of the command
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@code{backward-sexp}.
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@example
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#[(&optional arg)
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"^H\204^F^@@\301^P\302^H[!\207"
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[arg 1 forward-sexp]
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2
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254435
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"p"]
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@end example
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The primitive way to create a byte-code object is with
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@code{make-byte-code}:
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@defun make-byte-code &rest elements
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This function constructs and returns a byte-code function object
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with @var{elements} as its elements.
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@end defun
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You should not try to come up with the elements for a byte-code
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function yourself, because if they are inconsistent, Emacs may crash
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when you call the function. Always leave it to the byte compiler to
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create these objects; it makes the elements consistent (we hope).
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You can access the elements of a byte-code object using @code{aref};
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you can also use @code{vconcat} to create a vector with the same
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elements.
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@node Disassembly
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@section Disassembled Byte-Code
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@cindex disassembled byte-code
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People do not write byte-code; that job is left to the byte compiler.
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But we provide a disassembler to satisfy a cat-like curiosity. The
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disassembler converts the byte-compiled code into humanly readable
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form.
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The byte-code interpreter is implemented as a simple stack machine.
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It pushes values onto a stack of its own, then pops them off to use them
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in calculations whose results are themselves pushed back on the stack.
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When a byte-code function returns, it pops a value off the stack and
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returns it as the value of the function.
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In addition to the stack, byte-code functions can use, bind, and set
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ordinary Lisp variables, by transferring values between variables and
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the stack.
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@deffn Command disassemble object &optional stream
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This function prints the disassembled code for @var{object}. If
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@var{stream} is supplied, then output goes there. Otherwise, the
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disassembled code is printed to the stream @code{standard-output}. The
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argument @var{object} can be a function name or a lambda expression.
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As a special exception, if this function is used interactively,
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it outputs to a buffer named @samp{*Disassemble*}.
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@end deffn
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Here are two examples of using the @code{disassemble} function. We
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have added explanatory comments to help you relate the byte-code to the
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Lisp source; these do not appear in the output of @code{disassemble}.
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These examples show unoptimized byte-code. Nowadays byte-code is
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usually optimized, but we did not want to rewrite these examples, since
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they still serve their purpose.
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@example
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@group
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(defun factorial (integer)
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"Compute factorial of an integer."
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(if (= 1 integer) 1
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(* integer (factorial (1- integer)))))
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@result{} factorial
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@end group
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@group
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(factorial 4)
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@result{} 24
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@end group
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@group
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(disassemble 'factorial)
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@print{} byte-code for factorial:
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doc: Compute factorial of an integer.
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args: (integer)
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@end group
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@group
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0 constant 1 ; @r{Push 1 onto stack.}
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1 varref integer ; @r{Get value of @code{integer}}
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; @r{from the environment}
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; @r{and push the value}
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; @r{onto the stack.}
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@end group
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@group
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2 eqlsign ; @r{Pop top two values off stack,}
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; @r{compare them,}
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; @r{and push result onto stack.}
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@end group
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@group
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3 goto-if-nil 10 ; @r{Pop and test top of stack;}
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; @r{if @code{nil}, go to 10,}
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; @r{else continue.}
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@end group
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@group
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6 constant 1 ; @r{Push 1 onto top of stack.}
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7 goto 17 ; @r{Go to 17 (in this case, 1 will be}
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; @r{returned by the function).}
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@end group
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@group
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10 constant * ; @r{Push symbol @code{*} onto stack.}
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11 varref integer ; @r{Push value of @code{integer} onto stack.}
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@end group
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@group
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12 constant factorial ; @r{Push @code{factorial} onto stack.}
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13 varref integer ; @r{Push value of @code{integer} onto stack.}
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14 sub1 ; @r{Pop @code{integer}, decrement value,}
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; @r{push new value onto stack.}
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@end group
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@group
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; @r{Stack now contains:}
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; @minus{} @r{decremented value of @code{integer}}
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; @minus{} @r{@code{factorial}}
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; @minus{} @r{value of @code{integer}}
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; @minus{} @r{@code{*}}
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@end group
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@group
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15 call 1 ; @r{Call function @code{factorial} using}
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; @r{the first (i.e., the top) element}
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; @r{of the stack as the argument;}
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; @r{push returned value onto stack.}
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@end group
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@group
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; @r{Stack now contains:}
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; @minus{} @r{result of recursive}
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; @r{call to @code{factorial}}
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; @minus{} @r{value of @code{integer}}
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; @minus{} @r{@code{*}}
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@end group
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@group
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16 call 2 ; @r{Using the first two}
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; @r{(i.e., the top two)}
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; @r{elements of the stack}
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; @r{as arguments,}
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; @r{call the function @code{*},}
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; @r{pushing the result onto the stack.}
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@end group
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@group
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17 return ; @r{Return the top element}
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; @r{of the stack.}
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@result{} nil
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@end group
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@end example
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The @code{silly-loop} function is somewhat more complex:
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@example
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@group
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(defun silly-loop (n)
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"Return time before and after N iterations of a loop."
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(let ((t1 (current-time-string)))
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(while (> (setq n (1- n))
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0))
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(list t1 (current-time-string))))
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@result{} silly-loop
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@end group
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@group
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(disassemble 'silly-loop)
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@print{} byte-code for silly-loop:
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doc: Return time before and after N iterations of a loop.
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args: (n)
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0 constant current-time-string ; @r{Push}
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; @r{@code{current-time-string}}
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; @r{onto top of stack.}
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@end group
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@group
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1 call 0 ; @r{Call @code{current-time-string}}
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; @r{ with no argument,}
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; @r{ pushing result onto stack.}
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@end group
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@group
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2 varbind t1 ; @r{Pop stack and bind @code{t1}}
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; @r{to popped value.}
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@end group
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@group
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3 varref n ; @r{Get value of @code{n} from}
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; @r{the environment and push}
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; @r{the value onto the stack.}
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@end group
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@group
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4 sub1 ; @r{Subtract 1 from top of stack.}
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@end group
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@group
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5 dup ; @r{Duplicate the top of the stack;}
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; @r{i.e., copy the top of}
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; @r{the stack and push the}
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; @r{copy onto the stack.}
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@end group
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@group
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6 varset n ; @r{Pop the top of the stack,}
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; @r{and bind @code{n} to the value.}
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; @r{In effect, the sequence @code{dup varset}}
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; @r{copies the top of the stack}
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; @r{into the value of @code{n}}
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; @r{without popping it.}
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@end group
|
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@group
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7 constant 0 ; @r{Push 0 onto stack.}
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@end group
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@group
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8 gtr ; @r{Pop top two values off stack,}
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; @r{test if @var{n} is greater than 0}
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; @r{and push result onto stack.}
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@end group
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@group
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9 goto-if-nil-else-pop 17 ; @r{Goto 17 if @code{n} <= 0}
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; @r{(this exits the while loop).}
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; @r{else pop top of stack}
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; @r{and continue}
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@end group
|
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@group
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12 constant nil ; @r{Push @code{nil} onto stack}
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; @r{(this is the body of the loop).}
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@end group
|
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@group
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13 discard ; @r{Discard result of the body}
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; @r{of the loop (a while loop}
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; @r{is always evaluated for}
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; @r{its side effects).}
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@end group
|
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@group
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14 goto 3 ; @r{Jump back to beginning}
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; @r{of while loop.}
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@end group
|
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|
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@group
|
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17 discard ; @r{Discard result of while loop}
|
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; @r{by popping top of stack.}
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; @r{This result is the value @code{nil} that}
|
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; @r{was not popped by the goto at 9.}
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@end group
|
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|
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@group
|
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18 varref t1 ; @r{Push value of @code{t1} onto stack.}
|
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@end group
|
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|
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@group
|
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19 constant current-time-string ; @r{Push}
|
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; @r{@code{current-time-string}}
|
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; @r{onto top of stack.}
|
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@end group
|
|
|
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@group
|
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20 call 0 ; @r{Call @code{current-time-string} again.}
|
|
@end group
|
|
|
|
@group
|
|
21 list2 ; @r{Pop top two elements off stack,}
|
|
; @r{create a list of them,}
|
|
; @r{and push list onto stack.}
|
|
@end group
|
|
|
|
@group
|
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22 unbind 1 ; @r{Unbind @code{t1} in local environment.}
|
|
|
|
23 return ; @r{Return value of the top of stack.}
|
|
|
|
@result{} nil
|
|
@end group
|
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@end example
|
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