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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
177c0ea7 | 3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. |
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4 | @c See the file elisp.texi for copying conditions. |
5 | @setfilename ../info/compile | |
a9f0a989 | 6 | @node Byte Compilation, Advising Functions, Loading, Top |
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7 | @chapter Byte Compilation |
8 | @cindex byte-code | |
9 | @cindex compilation | |
10 | ||
969fe9b5 | 11 | Emacs Lisp has a @dfn{compiler} that translates functions written |
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12 | in Lisp into a special representation called @dfn{byte-code} that can be |
13 | executed more efficiently. The compiler replaces Lisp function | |
14 | definitions with byte-code. When a byte-code function is called, its | |
15 | definition is evaluated by the @dfn{byte-code interpreter}. | |
16 | ||
17 | Because the byte-compiled code is evaluated by the byte-code | |
18 | interpreter, instead of being executed directly by the machine's | |
19 | hardware (as true compiled code is), byte-code is completely | |
20 | transportable from machine to machine without recompilation. It is not, | |
21 | however, as fast as true compiled code. | |
22 | ||
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23 | Compiling a Lisp file with the Emacs byte compiler always reads the |
24 | file as multibyte text, even if Emacs was started with @samp{--unibyte}, | |
25 | unless the file specifies otherwise. This is so that compilation gives | |
26 | results compatible with running the same file without compilation. | |
27 | @xref{Loading Non-ASCII}. | |
28 | ||
53f60086 | 29 | In general, any version of Emacs can run byte-compiled code produced |
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30 | by recent earlier versions of Emacs, but the reverse is not true. A |
31 | major incompatible change was introduced in Emacs version 19.29, and | |
32 | files compiled with versions since that one will definitely not run | |
33 | in earlier versions unless you specify a special option. | |
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34 | @iftex |
35 | @xref{Docs and Compilation}. | |
36 | @end iftex | |
969fe9b5 RS |
37 | In addition, the modifier bits in keyboard characters were renumbered in |
38 | Emacs 19.29; as a result, files compiled in versions before 19.29 will | |
39 | not work in subsequent versions if they contain character constants with | |
40 | modifier bits. | |
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41 | |
42 | @xref{Compilation Errors}, for how to investigate errors occurring in | |
43 | byte compilation. | |
44 | ||
45 | @menu | |
a0acfc98 | 46 | * Speed of Byte-Code:: An example of speedup from byte compilation. |
53f60086 | 47 | * Compilation Functions:: Byte compilation functions. |
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48 | * Docs and Compilation:: Dynamic loading of documentation strings. |
49 | * Dynamic Loading:: Dynamic loading of individual functions. | |
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50 | * Eval During Compile:: Code to be evaluated when you compile. |
51 | * Byte-Code Objects:: The data type used for byte-compiled functions. | |
52 | * Disassembly:: Disassembling byte-code; how to read byte-code. | |
53 | @end menu | |
54 | ||
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55 | @node Speed of Byte-Code |
56 | @section Performance of Byte-Compiled Code | |
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57 | |
58 | A byte-compiled function is not as efficient as a primitive function | |
59 | written in C, but runs much faster than the version written in Lisp. | |
a0acfc98 | 60 | Here is an example: |
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61 | |
62 | @example | |
63 | @group | |
64 | (defun silly-loop (n) | |
65 | "Return time before and after N iterations of a loop." | |
66 | (let ((t1 (current-time-string))) | |
177c0ea7 | 67 | (while (> (setq n (1- n)) |
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68 | 0)) |
69 | (list t1 (current-time-string)))) | |
70 | @result{} silly-loop | |
71 | @end group | |
72 | ||
73 | @group | |
74 | (silly-loop 100000) | |
a0acfc98 RS |
75 | @result{} ("Fri Mar 18 17:25:57 1994" |
76 | "Fri Mar 18 17:26:28 1994") ; @r{31 seconds} | |
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77 | @end group |
78 | ||
79 | @group | |
80 | (byte-compile 'silly-loop) | |
81 | @result{} @r{[Compiled code not shown]} | |
82 | @end group | |
83 | ||
84 | @group | |
85 | (silly-loop 100000) | |
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86 | @result{} ("Fri Mar 18 17:26:52 1994" |
87 | "Fri Mar 18 17:26:58 1994") ; @r{6 seconds} | |
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88 | @end group |
89 | @end example | |
90 | ||
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91 | In this example, the interpreted code required 31 seconds to run, |
92 | whereas the byte-compiled code required 6 seconds. These results are | |
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93 | representative, but actual results will vary greatly. |
94 | ||
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95 | @node Compilation Functions |
96 | @comment node-name, next, previous, up | |
97 | @section The Compilation Functions | |
98 | @cindex compilation functions | |
99 | ||
100 | You can byte-compile an individual function or macro definition with | |
101 | the @code{byte-compile} function. You can compile a whole file with | |
102 | @code{byte-compile-file}, or several files with | |
103 | @code{byte-recompile-directory} or @code{batch-byte-compile}. | |
104 | ||
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105 | The byte compiler produces error messages and warnings about each file |
106 | in a buffer called @samp{*Compile-Log*}. These report things in your | |
107 | program that suggest a problem but are not necessarily erroneous. | |
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108 | |
109 | @cindex macro compilation | |
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110 | Be careful when writing macro calls in files that you may someday |
111 | byte-compile. Macro calls are expanded when they are compiled, so the | |
112 | macros must already be defined for proper compilation. For more | |
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113 | details, see @ref{Compiling Macros}. If a program does not work the |
114 | same way when compiled as it does when interpreted, erroneous macro | |
115 | definitions are one likely cause (@pxref{Problems with Macros}). | |
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116 | |
117 | Normally, compiling a file does not evaluate the file's contents or | |
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118 | load the file. But it does execute any @code{require} calls at top |
119 | level in the file. One way to ensure that necessary macro definitions | |
120 | are available during compilation is to require the file that defines | |
121 | them (@pxref{Named Features}). To avoid loading the macro definition files | |
122 | when someone @emph{runs} the compiled program, write | |
123 | @code{eval-when-compile} around the @code{require} calls (@pxref{Eval | |
124 | During Compile}). | |
a0acfc98 | 125 | |
53f60086 | 126 | @defun byte-compile symbol |
a0acfc98 | 127 | This function byte-compiles the function definition of @var{symbol}, |
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128 | replacing the previous definition with the compiled one. The function |
129 | definition of @var{symbol} must be the actual code for the function; | |
130 | i.e., the compiler does not follow indirection to another symbol. | |
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131 | @code{byte-compile} returns the new, compiled definition of |
132 | @var{symbol}. | |
133 | ||
22697dac | 134 | If @var{symbol}'s definition is a byte-code function object, |
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135 | @code{byte-compile} does nothing and returns @code{nil}. Lisp records |
136 | only one function definition for any symbol, and if that is already | |
137 | compiled, non-compiled code is not available anywhere. So there is no | |
138 | way to ``compile the same definition again.'' | |
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139 | |
140 | @example | |
141 | @group | |
142 | (defun factorial (integer) | |
143 | "Compute factorial of INTEGER." | |
144 | (if (= 1 integer) 1 | |
145 | (* integer (factorial (1- integer))))) | |
a0acfc98 | 146 | @result{} factorial |
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147 | @end group |
148 | ||
149 | @group | |
150 | (byte-compile 'factorial) | |
a0acfc98 | 151 | @result{} |
53f60086 RS |
152 | #[(integer) |
153 | "^H\301U\203^H^@@\301\207\302^H\303^HS!\"\207" | |
154 | [integer 1 * factorial] | |
155 | 4 "Compute factorial of INTEGER."] | |
156 | @end group | |
157 | @end example | |
158 | ||
159 | @noindent | |
a0acfc98 RS |
160 | The result is a byte-code function object. The string it contains is |
161 | the actual byte-code; each character in it is an instruction or an | |
162 | operand of an instruction. The vector contains all the constants, | |
163 | variable names and function names used by the function, except for | |
164 | certain primitives that are coded as special instructions. | |
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165 | @end defun |
166 | ||
167 | @deffn Command compile-defun | |
168 | This command reads the defun containing point, compiles it, and | |
169 | evaluates the result. If you use this on a defun that is actually a | |
170 | function definition, the effect is to install a compiled version of that | |
171 | function. | |
172 | @end deffn | |
173 | ||
174 | @deffn Command byte-compile-file filename | |
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175 | This function compiles a file of Lisp code named @var{filename} into a |
176 | file of byte-code. The output file's name is made by changing the | |
177 | @samp{.el} suffix into @samp{.elc}; if @var{filename} does not end in | |
178 | @samp{.el}, it adds @samp{.elc} to the end of @var{filename}. | |
53f60086 | 179 | |
a0acfc98 | 180 | Compilation works by reading the input file one form at a time. If it |
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181 | is a definition of a function or macro, the compiled function or macro |
182 | definition is written out. Other forms are batched together, then each | |
183 | batch is compiled, and written so that its compiled code will be | |
184 | executed when the file is read. All comments are discarded when the | |
185 | input file is read. | |
186 | ||
a0acfc98 | 187 | This command returns @code{t}. When called interactively, it prompts |
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188 | for the file name. |
189 | ||
190 | @example | |
191 | @group | |
192 | % ls -l push* | |
193 | -rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el | |
194 | @end group | |
195 | ||
196 | @group | |
197 | (byte-compile-file "~/emacs/push.el") | |
198 | @result{} t | |
199 | @end group | |
200 | ||
201 | @group | |
202 | % ls -l push* | |
203 | -rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el | |
204 | -rw-rw-rw- 1 lewis 638 Oct 8 20:25 push.elc | |
205 | @end group | |
206 | @end example | |
207 | @end deffn | |
208 | ||
209 | @deffn Command byte-recompile-directory directory flag | |
210 | @cindex library compilation | |
a0acfc98 | 211 | This function recompiles every @samp{.el} file in @var{directory} that |
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212 | needs recompilation. A file needs recompilation if a @samp{.elc} file |
213 | exists but is older than the @samp{.el} file. | |
214 | ||
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215 | When a @samp{.el} file has no corresponding @samp{.elc} file, @var{flag} |
216 | says what to do. If it is @code{nil}, these files are ignored. If it | |
217 | is non-@code{nil}, the user is asked whether to compile each such file. | |
53f60086 | 218 | |
a0acfc98 | 219 | The returned value of this command is unpredictable. |
53f60086 RS |
220 | @end deffn |
221 | ||
222 | @defun batch-byte-compile | |
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223 | This function runs @code{byte-compile-file} on files specified on the |
224 | command line. This function must be used only in a batch execution of | |
225 | Emacs, as it kills Emacs on completion. An error in one file does not | |
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226 | prevent processing of subsequent files, but no output file will be |
227 | generated for it, and the Emacs process will terminate with a nonzero | |
228 | status code. | |
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229 | |
230 | @example | |
231 | % emacs -batch -f batch-byte-compile *.el | |
232 | @end example | |
233 | @end defun | |
234 | ||
235 | @defun byte-code code-string data-vector max-stack | |
236 | @cindex byte-code interpreter | |
a0acfc98 | 237 | This function actually interprets byte-code. A byte-compiled function |
53f60086 | 238 | is actually defined with a body that calls @code{byte-code}. Don't call |
969fe9b5 | 239 | this function yourself---only the byte compiler knows how to generate |
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240 | valid calls to this function. |
241 | ||
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242 | In Emacs version 18, byte-code was always executed by way of a call to |
243 | the function @code{byte-code}. Nowadays, byte-code is usually executed | |
244 | as part of a byte-code function object, and only rarely through an | |
245 | explicit call to @code{byte-code}. | |
53f60086 RS |
246 | @end defun |
247 | ||
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248 | @node Docs and Compilation |
249 | @section Documentation Strings and Compilation | |
250 | @cindex dynamic loading of documentation | |
251 | ||
252 | Functions and variables loaded from a byte-compiled file access their | |
253 | documentation strings dynamically from the file whenever needed. This | |
cc8c51f1 | 254 | saves space within Emacs, and makes loading faster because the |
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255 | documentation strings themselves need not be processed while loading the |
256 | file. Actual access to the documentation strings becomes slower as a | |
257 | result, but this normally is not enough to bother users. | |
258 | ||
259 | Dynamic access to documentation strings does have drawbacks: | |
260 | ||
261 | @itemize @bullet | |
262 | @item | |
263 | If you delete or move the compiled file after loading it, Emacs can no | |
264 | longer access the documentation strings for the functions and variables | |
265 | in the file. | |
266 | ||
267 | @item | |
268 | If you alter the compiled file (such as by compiling a new version), | |
269 | then further access to documentation strings in this file will give | |
270 | nonsense results. | |
271 | @end itemize | |
272 | ||
273 | If your site installs Emacs following the usual procedures, these | |
274 | problems will never normally occur. Installing a new version uses a new | |
275 | directory with a different name; as long as the old version remains | |
276 | installed, its files will remain unmodified in the places where they are | |
277 | expected to be. | |
278 | ||
89dbfd18 | 279 | However, if you have built Emacs yourself and use it from the |
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280 | directory where you built it, you will experience this problem |
281 | occasionally if you edit and recompile Lisp files. When it happens, you | |
282 | can cure the problem by reloading the file after recompiling it. | |
283 | ||
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284 | Byte-compiled files made with recent versions of Emacs (since 19.29) |
285 | will not load into older versions because the older versions don't | |
286 | support this feature. You can turn off this feature at compile time by | |
287 | setting @code{byte-compile-dynamic-docstrings} to @code{nil}; then you | |
288 | can compile files that will load into older Emacs versions. You can do | |
289 | this globally, or for one source file by specifying a file-local binding | |
290 | for the variable. One way to do that is by adding this string to the | |
291 | file's first line: | |
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292 | |
293 | @example | |
294 | -*-byte-compile-dynamic-docstrings: nil;-*- | |
295 | @end example | |
296 | ||
297 | @defvar byte-compile-dynamic-docstrings | |
298 | If this is non-@code{nil}, the byte compiler generates compiled files | |
299 | that are set up for dynamic loading of documentation strings. | |
300 | @end defvar | |
301 | ||
302 | @cindex @samp{#@@@var{count}} | |
303 | @cindex @samp{#$} | |
304 | The dynamic documentation string feature writes compiled files that | |
305 | use a special Lisp reader construct, @samp{#@@@var{count}}. This | |
306 | construct skips the next @var{count} characters. It also uses the | |
307 | @samp{#$} construct, which stands for ``the name of this file, as a | |
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308 | string.'' It is usually best not to use these constructs in Lisp source |
309 | files, since they are not designed to be clear to humans reading the | |
310 | file. | |
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311 | |
312 | @node Dynamic Loading | |
313 | @section Dynamic Loading of Individual Functions | |
314 | ||
315 | @cindex dynamic loading of functions | |
316 | @cindex lazy loading | |
317 | When you compile a file, you can optionally enable the @dfn{dynamic | |
318 | function loading} feature (also known as @dfn{lazy loading}). With | |
319 | dynamic function loading, loading the file doesn't fully read the | |
320 | function definitions in the file. Instead, each function definition | |
321 | contains a place-holder which refers to the file. The first time each | |
322 | function is called, it reads the full definition from the file, to | |
323 | replace the place-holder. | |
324 | ||
325 | The advantage of dynamic function loading is that loading the file | |
326 | becomes much faster. This is a good thing for a file which contains | |
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327 | many separate user-callable functions, if using one of them does not |
328 | imply you will probably also use the rest. A specialized mode which | |
329 | provides many keyboard commands often has that usage pattern: a user may | |
330 | invoke the mode, but use only a few of the commands it provides. | |
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331 | |
332 | The dynamic loading feature has certain disadvantages: | |
333 | ||
334 | @itemize @bullet | |
335 | @item | |
336 | If you delete or move the compiled file after loading it, Emacs can no | |
337 | longer load the remaining function definitions not already loaded. | |
338 | ||
339 | @item | |
340 | If you alter the compiled file (such as by compiling a new version), | |
969fe9b5 | 341 | then trying to load any function not already loaded will yield nonsense |
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342 | results. |
343 | @end itemize | |
344 | ||
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345 | These problems will never happen in normal circumstances with |
346 | installed Emacs files. But they are quite likely to happen with Lisp | |
347 | files that you are changing. The easiest way to prevent these problems | |
348 | is to reload the new compiled file immediately after each recompilation. | |
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349 | |
350 | The byte compiler uses the dynamic function loading feature if the | |
351 | variable @code{byte-compile-dynamic} is non-@code{nil} at compilation | |
352 | time. Do not set this variable globally, since dynamic loading is | |
353 | desirable only for certain files. Instead, enable the feature for | |
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354 | specific source files with file-local variable bindings. For example, |
355 | you could do it by writing this text in the source file's first line: | |
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356 | |
357 | @example | |
358 | -*-byte-compile-dynamic: t;-*- | |
359 | @end example | |
360 | ||
361 | @defvar byte-compile-dynamic | |
362 | If this is non-@code{nil}, the byte compiler generates compiled files | |
363 | that are set up for dynamic function loading. | |
364 | @end defvar | |
365 | ||
366 | @defun fetch-bytecode function | |
367 | This immediately finishes loading the definition of @var{function} from | |
368 | its byte-compiled file, if it is not fully loaded already. The argument | |
369 | @var{function} may be a byte-code function object or a function name. | |
370 | @end defun | |
371 | ||
53f60086 RS |
372 | @node Eval During Compile |
373 | @section Evaluation During Compilation | |
374 | ||
22697dac | 375 | These features permit you to write code to be evaluated during |
53f60086 RS |
376 | compilation of a program. |
377 | ||
378 | @defspec eval-and-compile body | |
379 | This form marks @var{body} to be evaluated both when you compile the | |
380 | containing code and when you run it (whether compiled or not). | |
381 | ||
382 | You can get a similar result by putting @var{body} in a separate file | |
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383 | and referring to that file with @code{require}. That method is |
384 | preferable when @var{body} is large. | |
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385 | @end defspec |
386 | ||
387 | @defspec eval-when-compile body | |
f9f59935 | 388 | This form marks @var{body} to be evaluated at compile time but not when |
78c71a98 | 389 | the compiled program is loaded. The result of evaluation by the |
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390 | compiler becomes a constant which appears in the compiled program. If |
391 | you load the source file, rather than compiling it, @var{body} is | |
392 | evaluated normally. | |
53f60086 | 393 | |
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394 | @strong{Common Lisp Note:} At top level, this is analogous to the Common |
395 | Lisp idiom @code{(eval-when (compile eval) @dots{})}. Elsewhere, the | |
396 | Common Lisp @samp{#.} reader macro (but not when interpreting) is closer | |
397 | to what @code{eval-when-compile} does. | |
53f60086 RS |
398 | @end defspec |
399 | ||
400 | @node Byte-Code Objects | |
bfe721d1 | 401 | @section Byte-Code Function Objects |
53f60086 RS |
402 | @cindex compiled function |
403 | @cindex byte-code function | |
404 | ||
405 | Byte-compiled functions have a special data type: they are | |
406 | @dfn{byte-code function objects}. | |
407 | ||
408 | Internally, a byte-code function object is much like a vector; | |
409 | however, the evaluator handles this data type specially when it appears | |
410 | as a function to be called. The printed representation for a byte-code | |
411 | function object is like that for a vector, with an additional @samp{#} | |
412 | before the opening @samp{[}. | |
413 | ||
53f60086 | 414 | A byte-code function object must have at least four elements; there is |
969fe9b5 | 415 | no maximum number, but only the first six elements have any normal use. |
53f60086 RS |
416 | They are: |
417 | ||
418 | @table @var | |
419 | @item arglist | |
420 | The list of argument symbols. | |
421 | ||
422 | @item byte-code | |
423 | The string containing the byte-code instructions. | |
424 | ||
425 | @item constants | |
78c71a98 RS |
426 | The vector of Lisp objects referenced by the byte code. These include |
427 | symbols used as function names and variable names. | |
53f60086 RS |
428 | |
429 | @item stacksize | |
430 | The maximum stack size this function needs. | |
431 | ||
432 | @item docstring | |
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433 | The documentation string (if any); otherwise, @code{nil}. The value may |
434 | be a number or a list, in case the documentation string is stored in a | |
435 | file. Use the function @code{documentation} to get the real | |
436 | documentation string (@pxref{Accessing Documentation}). | |
53f60086 RS |
437 | |
438 | @item interactive | |
439 | The interactive spec (if any). This can be a string or a Lisp | |
440 | expression. It is @code{nil} for a function that isn't interactive. | |
441 | @end table | |
442 | ||
443 | Here's an example of a byte-code function object, in printed | |
444 | representation. It is the definition of the command | |
445 | @code{backward-sexp}. | |
446 | ||
447 | @example | |
448 | #[(&optional arg) | |
449 | "^H\204^F^@@\301^P\302^H[!\207" | |
450 | [arg 1 forward-sexp] | |
451 | 2 | |
452 | 254435 | |
453 | "p"] | |
454 | @end example | |
455 | ||
456 | The primitive way to create a byte-code object is with | |
457 | @code{make-byte-code}: | |
458 | ||
459 | @defun make-byte-code &rest elements | |
460 | This function constructs and returns a byte-code function object | |
461 | with @var{elements} as its elements. | |
462 | @end defun | |
463 | ||
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464 | You should not try to come up with the elements for a byte-code |
465 | function yourself, because if they are inconsistent, Emacs may crash | |
78c71a98 | 466 | when you call the function. Always leave it to the byte compiler to |
a0acfc98 | 467 | create these objects; it makes the elements consistent (we hope). |
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468 | |
469 | You can access the elements of a byte-code object using @code{aref}; | |
470 | you can also use @code{vconcat} to create a vector with the same | |
471 | elements. | |
472 | ||
473 | @node Disassembly | |
474 | @section Disassembled Byte-Code | |
475 | @cindex disassembled byte-code | |
476 | ||
477 | People do not write byte-code; that job is left to the byte compiler. | |
478 | But we provide a disassembler to satisfy a cat-like curiosity. The | |
479 | disassembler converts the byte-compiled code into humanly readable | |
480 | form. | |
481 | ||
482 | The byte-code interpreter is implemented as a simple stack machine. | |
a0acfc98 | 483 | It pushes values onto a stack of its own, then pops them off to use them |
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484 | in calculations whose results are themselves pushed back on the stack. |
485 | When a byte-code function returns, it pops a value off the stack and | |
486 | returns it as the value of the function. | |
53f60086 | 487 | |
78c71a98 | 488 | In addition to the stack, byte-code functions can use, bind, and set |
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489 | ordinary Lisp variables, by transferring values between variables and |
490 | the stack. | |
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491 | |
492 | @deffn Command disassemble object &optional stream | |
493 | This function prints the disassembled code for @var{object}. If | |
494 | @var{stream} is supplied, then output goes there. Otherwise, the | |
495 | disassembled code is printed to the stream @code{standard-output}. The | |
496 | argument @var{object} can be a function name or a lambda expression. | |
497 | ||
498 | As a special exception, if this function is used interactively, | |
499 | it outputs to a buffer named @samp{*Disassemble*}. | |
500 | @end deffn | |
501 | ||
502 | Here are two examples of using the @code{disassemble} function. We | |
503 | have added explanatory comments to help you relate the byte-code to the | |
504 | Lisp source; these do not appear in the output of @code{disassemble}. | |
505 | These examples show unoptimized byte-code. Nowadays byte-code is | |
506 | usually optimized, but we did not want to rewrite these examples, since | |
507 | they still serve their purpose. | |
508 | ||
509 | @example | |
510 | @group | |
511 | (defun factorial (integer) | |
512 | "Compute factorial of an integer." | |
513 | (if (= 1 integer) 1 | |
514 | (* integer (factorial (1- integer))))) | |
515 | @result{} factorial | |
516 | @end group | |
517 | ||
518 | @group | |
519 | (factorial 4) | |
520 | @result{} 24 | |
521 | @end group | |
522 | ||
523 | @group | |
524 | (disassemble 'factorial) | |
525 | @print{} byte-code for factorial: | |
526 | doc: Compute factorial of an integer. | |
527 | args: (integer) | |
528 | @end group | |
529 | ||
530 | @group | |
531 | 0 constant 1 ; @r{Push 1 onto stack.} | |
532 | ||
177c0ea7 | 533 | 1 varref integer ; @r{Get value of @code{integer}} |
53f60086 RS |
534 | ; @r{from the environment} |
535 | ; @r{and push the value} | |
536 | ; @r{onto the stack.} | |
537 | @end group | |
538 | ||
539 | @group | |
540 | 2 eqlsign ; @r{Pop top two values off stack,} | |
541 | ; @r{compare them,} | |
542 | ; @r{and push result onto stack.} | |
543 | @end group | |
544 | ||
545 | @group | |
546 | 3 goto-if-nil 10 ; @r{Pop and test top of stack;} | |
547 | ; @r{if @code{nil}, go to 10,} | |
548 | ; @r{else continue.} | |
549 | @end group | |
550 | ||
551 | @group | |
552 | 6 constant 1 ; @r{Push 1 onto top of stack.} | |
553 | ||
554 | 7 goto 17 ; @r{Go to 17 (in this case, 1 will be} | |
555 | ; @r{returned by the function).} | |
556 | @end group | |
557 | ||
558 | @group | |
559 | 10 constant * ; @r{Push symbol @code{*} onto stack.} | |
560 | ||
561 | 11 varref integer ; @r{Push value of @code{integer} onto stack.} | |
562 | @end group | |
563 | ||
564 | @group | |
565 | 12 constant factorial ; @r{Push @code{factorial} onto stack.} | |
566 | ||
567 | 13 varref integer ; @r{Push value of @code{integer} onto stack.} | |
568 | ||
569 | 14 sub1 ; @r{Pop @code{integer}, decrement value,} | |
570 | ; @r{push new value onto stack.} | |
571 | @end group | |
572 | ||
573 | @group | |
574 | ; @r{Stack now contains:} | |
575 | ; @minus{} @r{decremented value of @code{integer}} | |
177c0ea7 | 576 | ; @minus{} @r{@code{factorial}} |
53f60086 RS |
577 | ; @minus{} @r{value of @code{integer}} |
578 | ; @minus{} @r{@code{*}} | |
579 | @end group | |
580 | ||
581 | @group | |
582 | 15 call 1 ; @r{Call function @code{factorial} using} | |
583 | ; @r{the first (i.e., the top) element} | |
584 | ; @r{of the stack as the argument;} | |
585 | ; @r{push returned value onto stack.} | |
586 | @end group | |
587 | ||
588 | @group | |
589 | ; @r{Stack now contains:} | |
78c71a98 | 590 | ; @minus{} @r{result of recursive} |
53f60086 RS |
591 | ; @r{call to @code{factorial}} |
592 | ; @minus{} @r{value of @code{integer}} | |
593 | ; @minus{} @r{@code{*}} | |
594 | @end group | |
595 | ||
596 | @group | |
597 | 16 call 2 ; @r{Using the first two} | |
598 | ; @r{(i.e., the top two)} | |
599 | ; @r{elements of the stack} | |
600 | ; @r{as arguments,} | |
601 | ; @r{call the function @code{*},} | |
602 | ; @r{pushing the result onto the stack.} | |
603 | @end group | |
604 | ||
605 | @group | |
606 | 17 return ; @r{Return the top element} | |
607 | ; @r{of the stack.} | |
608 | @result{} nil | |
609 | @end group | |
610 | @end example | |
611 | ||
612 | The @code{silly-loop} function is somewhat more complex: | |
613 | ||
614 | @example | |
615 | @group | |
616 | (defun silly-loop (n) | |
617 | "Return time before and after N iterations of a loop." | |
618 | (let ((t1 (current-time-string))) | |
177c0ea7 | 619 | (while (> (setq n (1- n)) |
53f60086 RS |
620 | 0)) |
621 | (list t1 (current-time-string)))) | |
622 | @result{} silly-loop | |
623 | @end group | |
624 | ||
625 | @group | |
626 | (disassemble 'silly-loop) | |
627 | @print{} byte-code for silly-loop: | |
628 | doc: Return time before and after N iterations of a loop. | |
629 | args: (n) | |
630 | ||
631 | 0 constant current-time-string ; @r{Push} | |
632 | ; @r{@code{current-time-string}} | |
633 | ; @r{onto top of stack.} | |
634 | @end group | |
635 | ||
636 | @group | |
637 | 1 call 0 ; @r{Call @code{current-time-string}} | |
638 | ; @r{ with no argument,} | |
639 | ; @r{ pushing result onto stack.} | |
640 | @end group | |
641 | ||
642 | @group | |
643 | 2 varbind t1 ; @r{Pop stack and bind @code{t1}} | |
644 | ; @r{to popped value.} | |
645 | @end group | |
646 | ||
647 | @group | |
648 | 3 varref n ; @r{Get value of @code{n} from} | |
649 | ; @r{the environment and push} | |
650 | ; @r{the value onto the stack.} | |
651 | @end group | |
652 | ||
653 | @group | |
654 | 4 sub1 ; @r{Subtract 1 from top of stack.} | |
655 | @end group | |
656 | ||
657 | @group | |
658 | 5 dup ; @r{Duplicate the top of the stack;} | |
a0acfc98 | 659 | ; @r{i.e., copy the top of} |
53f60086 RS |
660 | ; @r{the stack and push the} |
661 | ; @r{copy onto the stack.} | |
662 | @end group | |
663 | ||
664 | @group | |
665 | 6 varset n ; @r{Pop the top of the stack,} | |
666 | ; @r{and bind @code{n} to the value.} | |
667 | ||
668 | ; @r{In effect, the sequence @code{dup varset}} | |
669 | ; @r{copies the top of the stack} | |
670 | ; @r{into the value of @code{n}} | |
671 | ; @r{without popping it.} | |
672 | @end group | |
673 | ||
674 | @group | |
675 | 7 constant 0 ; @r{Push 0 onto stack.} | |
676 | @end group | |
677 | ||
678 | @group | |
679 | 8 gtr ; @r{Pop top two values off stack,} | |
680 | ; @r{test if @var{n} is greater than 0} | |
681 | ; @r{and push result onto stack.} | |
682 | @end group | |
683 | ||
684 | @group | |
78c71a98 RS |
685 | 9 goto-if-nil-else-pop 17 ; @r{Goto 17 if @code{n} <= 0} |
686 | ; @r{(this exits the while loop).} | |
53f60086 RS |
687 | ; @r{else pop top of stack} |
688 | ; @r{and continue} | |
53f60086 RS |
689 | @end group |
690 | ||
691 | @group | |
692 | 12 constant nil ; @r{Push @code{nil} onto stack} | |
693 | ; @r{(this is the body of the loop).} | |
694 | @end group | |
695 | ||
696 | @group | |
697 | 13 discard ; @r{Discard result of the body} | |
698 | ; @r{of the loop (a while loop} | |
699 | ; @r{is always evaluated for} | |
700 | ; @r{its side effects).} | |
701 | @end group | |
702 | ||
703 | @group | |
704 | 14 goto 3 ; @r{Jump back to beginning} | |
705 | ; @r{of while loop.} | |
706 | @end group | |
707 | ||
708 | @group | |
709 | 17 discard ; @r{Discard result of while loop} | |
710 | ; @r{by popping top of stack.} | |
78c71a98 RS |
711 | ; @r{This result is the value @code{nil} that} |
712 | ; @r{was not popped by the goto at 9.} | |
53f60086 RS |
713 | @end group |
714 | ||
715 | @group | |
716 | 18 varref t1 ; @r{Push value of @code{t1} onto stack.} | |
717 | @end group | |
718 | ||
719 | @group | |
177c0ea7 | 720 | 19 constant current-time-string ; @r{Push} |
53f60086 RS |
721 | ; @r{@code{current-time-string}} |
722 | ; @r{onto top of stack.} | |
723 | @end group | |
724 | ||
725 | @group | |
726 | 20 call 0 ; @r{Call @code{current-time-string} again.} | |
727 | @end group | |
728 | ||
729 | @group | |
730 | 21 list2 ; @r{Pop top two elements off stack,} | |
731 | ; @r{create a list of them,} | |
732 | ; @r{and push list onto stack.} | |
733 | @end group | |
734 | ||
735 | @group | |
736 | 22 unbind 1 ; @r{Unbind @code{t1} in local environment.} | |
737 | ||
738 | 23 return ; @r{Return value of the top of stack.} | |
739 | ||
740 | @result{} nil | |
741 | @end group | |
742 | @end example | |
743 | ||
744 |