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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 2001, 2002, 2003, 2004, | |
5df4f04c | 4 | @c 2005, 2006, 2007, 2008, 2009, 2010, 2011 Free Software Foundation, Inc. |
b8d4c8d0 | 5 | @c See the file elisp.texi for copying conditions. |
6336d8c3 | 6 | @setfilename ../../info/compile |
b8d4c8d0 GM |
7 | @node Byte Compilation, Advising Functions, Loading, Top |
8 | @chapter Byte Compilation | |
9 | @cindex byte compilation | |
10 | @cindex byte-code | |
11 | @cindex compilation (Emacs Lisp) | |
12 | ||
13 | Emacs Lisp has a @dfn{compiler} that translates functions written | |
14 | in Lisp into a special representation called @dfn{byte-code} that can be | |
15 | executed more efficiently. The compiler replaces Lisp function | |
16 | definitions with byte-code. When a byte-code function is called, its | |
17 | definition is evaluated by the @dfn{byte-code interpreter}. | |
18 | ||
19 | Because the byte-compiled code is evaluated by the byte-code | |
20 | interpreter, instead of being executed directly by the machine's | |
21 | hardware (as true compiled code is), byte-code is completely | |
22 | transportable from machine to machine without recompilation. It is not, | |
23 | however, as fast as true compiled code. | |
24 | ||
b8d4c8d0 GM |
25 | In general, any version of Emacs can run byte-compiled code produced |
26 | by recent earlier versions of Emacs, but the reverse is not true. | |
27 | ||
28 | @vindex no-byte-compile | |
29 | If you do not want a Lisp file to be compiled, ever, put a file-local | |
30 | variable binding for @code{no-byte-compile} into it, like this: | |
31 | ||
32 | @example | |
33 | ;; -*-no-byte-compile: t; -*- | |
34 | @end example | |
35 | ||
36 | @xref{Compilation Errors}, for how to investigate errors occurring in | |
37 | byte compilation. | |
38 | ||
39 | @menu | |
40 | * Speed of Byte-Code:: An example of speedup from byte compilation. | |
41 | * Compilation Functions:: Byte compilation functions. | |
42 | * Docs and Compilation:: Dynamic loading of documentation strings. | |
43 | * Dynamic Loading:: Dynamic loading of individual functions. | |
d24880de | 44 | * Eval During Compile:: Code to be evaluated when you compile. |
b8d4c8d0 | 45 | * Compiler Errors:: Handling compiler error messages. |
d24880de | 46 | * Byte-Code Objects:: The data type used for byte-compiled functions. |
b8d4c8d0 GM |
47 | * Disassembly:: Disassembling byte-code; how to read byte-code. |
48 | @end menu | |
49 | ||
50 | @node Speed of Byte-Code | |
51 | @section Performance of Byte-Compiled Code | |
52 | ||
53 | A byte-compiled function is not as efficient as a primitive function | |
54 | written in C, but runs much faster than the version written in Lisp. | |
55 | Here is an example: | |
56 | ||
57 | @example | |
58 | @group | |
59 | (defun silly-loop (n) | |
60 | "Return time before and after N iterations of a loop." | |
61 | (let ((t1 (current-time-string))) | |
62 | (while (> (setq n (1- n)) | |
63 | 0)) | |
64 | (list t1 (current-time-string)))) | |
65 | @result{} silly-loop | |
66 | @end group | |
67 | ||
68 | @group | |
c36745c6 CY |
69 | (silly-loop 50000000) |
70 | @result{} ("Wed Mar 11 21:10:19 2009" | |
71 | "Wed Mar 11 21:10:41 2009") ; @r{22 seconds} | |
b8d4c8d0 GM |
72 | @end group |
73 | ||
74 | @group | |
75 | (byte-compile 'silly-loop) | |
76 | @result{} @r{[Compiled code not shown]} | |
77 | @end group | |
78 | ||
79 | @group | |
c36745c6 CY |
80 | (silly-loop 50000000) |
81 | @result{} ("Wed Mar 11 21:12:26 2009" | |
82 | "Wed Mar 11 21:12:32 2009") ; @r{6 seconds} | |
b8d4c8d0 GM |
83 | @end group |
84 | @end example | |
85 | ||
c36745c6 | 86 | In this example, the interpreted code required 22 seconds to run, |
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87 | whereas the byte-compiled code required 6 seconds. These results are |
88 | representative, but actual results will vary greatly. | |
89 | ||
90 | @node Compilation Functions | |
91 | @comment node-name, next, previous, up | |
92 | @section The Compilation Functions | |
93 | @cindex compilation functions | |
94 | ||
95 | You can byte-compile an individual function or macro definition with | |
96 | the @code{byte-compile} function. You can compile a whole file with | |
97 | @code{byte-compile-file}, or several files with | |
98 | @code{byte-recompile-directory} or @code{batch-byte-compile}. | |
99 | ||
100 | The byte compiler produces error messages and warnings about each file | |
101 | in a buffer called @samp{*Compile-Log*}. These report things in your | |
102 | program that suggest a problem but are not necessarily erroneous. | |
103 | ||
104 | @cindex macro compilation | |
105 | Be careful when writing macro calls in files that you may someday | |
106 | byte-compile. Macro calls are expanded when they are compiled, so the | |
107 | macros must already be defined for proper compilation. For more | |
108 | details, see @ref{Compiling Macros}. If a program does not work the | |
109 | same way when compiled as it does when interpreted, erroneous macro | |
110 | definitions are one likely cause (@pxref{Problems with Macros}). | |
111 | Inline (@code{defsubst}) functions are less troublesome; if you | |
112 | compile a call to such a function before its definition is known, the | |
113 | call will still work right, it will just run slower. | |
114 | ||
115 | Normally, compiling a file does not evaluate the file's contents or | |
116 | load the file. But it does execute any @code{require} calls at top | |
117 | level in the file. One way to ensure that necessary macro definitions | |
118 | are available during compilation is to require the file that defines | |
119 | them (@pxref{Named Features}). To avoid loading the macro definition files | |
120 | when someone @emph{runs} the compiled program, write | |
121 | @code{eval-when-compile} around the @code{require} calls (@pxref{Eval | |
122 | During Compile}). | |
123 | ||
124 | @defun byte-compile symbol | |
125 | This function byte-compiles the function definition of @var{symbol}, | |
126 | replacing the previous definition with the compiled one. The function | |
127 | definition of @var{symbol} must be the actual code for the function; | |
128 | i.e., the compiler does not follow indirection to another symbol. | |
129 | @code{byte-compile} returns the new, compiled definition of | |
130 | @var{symbol}. | |
131 | ||
132 | If @var{symbol}'s definition is a byte-code function object, | |
133 | @code{byte-compile} does nothing and returns @code{nil}. Lisp records | |
134 | only one function definition for any symbol, and if that is already | |
135 | compiled, non-compiled code is not available anywhere. So there is no | |
136 | way to ``compile the same definition again.'' | |
137 | ||
138 | @example | |
139 | @group | |
140 | (defun factorial (integer) | |
141 | "Compute factorial of INTEGER." | |
142 | (if (= 1 integer) 1 | |
143 | (* integer (factorial (1- integer))))) | |
144 | @result{} factorial | |
145 | @end group | |
146 | ||
147 | @group | |
148 | (byte-compile 'factorial) | |
149 | @result{} | |
150 | #[(integer) | |
151 | "^H\301U\203^H^@@\301\207\302^H\303^HS!\"\207" | |
152 | [integer 1 * factorial] | |
153 | 4 "Compute factorial of INTEGER."] | |
154 | @end group | |
155 | @end example | |
156 | ||
157 | @noindent | |
158 | The result is a byte-code function object. The string it contains is | |
159 | the actual byte-code; each character in it is an instruction or an | |
160 | operand of an instruction. The vector contains all the constants, | |
161 | variable names and function names used by the function, except for | |
162 | certain primitives that are coded as special instructions. | |
163 | ||
164 | If the argument to @code{byte-compile} is a @code{lambda} expression, | |
165 | it returns the corresponding compiled code, but does not store | |
166 | it anywhere. | |
167 | @end defun | |
168 | ||
169 | @deffn Command compile-defun &optional arg | |
170 | This command reads the defun containing point, compiles it, and | |
171 | evaluates the result. If you use this on a defun that is actually a | |
172 | function definition, the effect is to install a compiled version of that | |
173 | function. | |
174 | ||
175 | @code{compile-defun} normally displays the result of evaluation in the | |
176 | echo area, but if @var{arg} is non-@code{nil}, it inserts the result | |
177 | in the current buffer after the form it compiled. | |
178 | @end deffn | |
179 | ||
180 | @deffn Command byte-compile-file filename &optional load | |
181 | This function compiles a file of Lisp code named @var{filename} into a | |
182 | file of byte-code. The output file's name is made by changing the | |
183 | @samp{.el} suffix into @samp{.elc}; if @var{filename} does not end in | |
184 | @samp{.el}, it adds @samp{.elc} to the end of @var{filename}. | |
185 | ||
186 | Compilation works by reading the input file one form at a time. If it | |
187 | is a definition of a function or macro, the compiled function or macro | |
188 | definition is written out. Other forms are batched together, then each | |
189 | batch is compiled, and written so that its compiled code will be | |
190 | executed when the file is read. All comments are discarded when the | |
191 | input file is read. | |
192 | ||
193 | This command returns @code{t} if there were no errors and @code{nil} | |
194 | otherwise. When called interactively, it prompts for the file name. | |
195 | ||
196 | If @var{load} is non-@code{nil}, this command loads the compiled file | |
197 | after compiling it. Interactively, @var{load} is the prefix argument. | |
198 | ||
199 | @example | |
200 | @group | |
201 | % ls -l push* | |
202 | -rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el | |
203 | @end group | |
204 | ||
205 | @group | |
206 | (byte-compile-file "~/emacs/push.el") | |
207 | @result{} t | |
208 | @end group | |
209 | ||
210 | @group | |
211 | % ls -l push* | |
212 | -rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el | |
213 | -rw-rw-rw- 1 lewis 638 Oct 8 20:25 push.elc | |
214 | @end group | |
215 | @end example | |
216 | @end deffn | |
217 | ||
218 | @deffn Command byte-recompile-directory directory &optional flag force | |
219 | @cindex library compilation | |
220 | This command recompiles every @samp{.el} file in @var{directory} (or | |
221 | its subdirectories) that needs recompilation. A file needs | |
222 | recompilation if a @samp{.elc} file exists but is older than the | |
223 | @samp{.el} file. | |
224 | ||
225 | When a @samp{.el} file has no corresponding @samp{.elc} file, | |
226 | @var{flag} says what to do. If it is @code{nil}, this command ignores | |
227 | these files. If @var{flag} is 0, it compiles them. If it is neither | |
228 | @code{nil} nor 0, it asks the user whether to compile each such file, | |
229 | and asks about each subdirectory as well. | |
230 | ||
231 | Interactively, @code{byte-recompile-directory} prompts for | |
232 | @var{directory} and @var{flag} is the prefix argument. | |
233 | ||
234 | If @var{force} is non-@code{nil}, this command recompiles every | |
235 | @samp{.el} file that has a @samp{.elc} file. | |
236 | ||
237 | The returned value is unpredictable. | |
238 | @end deffn | |
239 | ||
240 | @defun batch-byte-compile &optional noforce | |
241 | This function runs @code{byte-compile-file} on files specified on the | |
242 | command line. This function must be used only in a batch execution of | |
243 | Emacs, as it kills Emacs on completion. An error in one file does not | |
244 | prevent processing of subsequent files, but no output file will be | |
245 | generated for it, and the Emacs process will terminate with a nonzero | |
246 | status code. | |
247 | ||
248 | If @var{noforce} is non-@code{nil}, this function does not recompile | |
249 | files that have an up-to-date @samp{.elc} file. | |
250 | ||
251 | @example | |
252 | % emacs -batch -f batch-byte-compile *.el | |
253 | @end example | |
254 | @end defun | |
255 | ||
256 | @defun byte-code code-string data-vector max-stack | |
257 | @cindex byte-code interpreter | |
258 | This function actually interprets byte-code. A byte-compiled function | |
259 | is actually defined with a body that calls @code{byte-code}. Don't call | |
260 | this function yourself---only the byte compiler knows how to generate | |
261 | valid calls to this function. | |
262 | ||
263 | In Emacs version 18, byte-code was always executed by way of a call to | |
264 | the function @code{byte-code}. Nowadays, byte-code is usually executed | |
265 | as part of a byte-code function object, and only rarely through an | |
266 | explicit call to @code{byte-code}. | |
267 | @end defun | |
268 | ||
269 | @node Docs and Compilation | |
270 | @section Documentation Strings and Compilation | |
271 | @cindex dynamic loading of documentation | |
272 | ||
273 | Functions and variables loaded from a byte-compiled file access their | |
274 | documentation strings dynamically from the file whenever needed. This | |
275 | saves space within Emacs, and makes loading faster because the | |
276 | documentation strings themselves need not be processed while loading the | |
277 | file. Actual access to the documentation strings becomes slower as a | |
278 | result, but this normally is not enough to bother users. | |
279 | ||
280 | Dynamic access to documentation strings does have drawbacks: | |
281 | ||
282 | @itemize @bullet | |
283 | @item | |
284 | If you delete or move the compiled file after loading it, Emacs can no | |
285 | longer access the documentation strings for the functions and variables | |
286 | in the file. | |
287 | ||
288 | @item | |
289 | If you alter the compiled file (such as by compiling a new version), | |
290 | then further access to documentation strings in this file will | |
291 | probably give nonsense results. | |
292 | @end itemize | |
293 | ||
294 | If your site installs Emacs following the usual procedures, these | |
295 | problems will never normally occur. Installing a new version uses a new | |
296 | directory with a different name; as long as the old version remains | |
297 | installed, its files will remain unmodified in the places where they are | |
298 | expected to be. | |
299 | ||
300 | However, if you have built Emacs yourself and use it from the | |
301 | directory where you built it, you will experience this problem | |
302 | occasionally if you edit and recompile Lisp files. When it happens, you | |
303 | can cure the problem by reloading the file after recompiling it. | |
304 | ||
305 | You can turn off this feature at compile time by setting | |
306 | @code{byte-compile-dynamic-docstrings} to @code{nil}; this is useful | |
307 | mainly if you expect to change the file, and you want Emacs processes | |
308 | that have already loaded it to keep working when the file changes. | |
309 | You can do this globally, or for one source file by specifying a | |
310 | file-local binding for the variable. One way to do that is by adding | |
311 | this string to the file's first line: | |
312 | ||
313 | @example | |
314 | -*-byte-compile-dynamic-docstrings: nil;-*- | |
315 | @end example | |
316 | ||
317 | @defvar byte-compile-dynamic-docstrings | |
318 | If this is non-@code{nil}, the byte compiler generates compiled files | |
319 | that are set up for dynamic loading of documentation strings. | |
320 | @end defvar | |
321 | ||
322 | @cindex @samp{#@@@var{count}} | |
323 | @cindex @samp{#$} | |
324 | The dynamic documentation string feature writes compiled files that | |
325 | use a special Lisp reader construct, @samp{#@@@var{count}}. This | |
326 | construct skips the next @var{count} characters. It also uses the | |
327 | @samp{#$} construct, which stands for ``the name of this file, as a | |
328 | string.'' It is usually best not to use these constructs in Lisp source | |
329 | files, since they are not designed to be clear to humans reading the | |
330 | file. | |
331 | ||
332 | @node Dynamic Loading | |
333 | @section Dynamic Loading of Individual Functions | |
334 | ||
335 | @cindex dynamic loading of functions | |
336 | @cindex lazy loading | |
337 | When you compile a file, you can optionally enable the @dfn{dynamic | |
338 | function loading} feature (also known as @dfn{lazy loading}). With | |
339 | dynamic function loading, loading the file doesn't fully read the | |
340 | function definitions in the file. Instead, each function definition | |
341 | contains a place-holder which refers to the file. The first time each | |
342 | function is called, it reads the full definition from the file, to | |
343 | replace the place-holder. | |
344 | ||
345 | The advantage of dynamic function loading is that loading the file | |
346 | becomes much faster. This is a good thing for a file which contains | |
347 | many separate user-callable functions, if using one of them does not | |
348 | imply you will probably also use the rest. A specialized mode which | |
349 | provides many keyboard commands often has that usage pattern: a user may | |
350 | invoke the mode, but use only a few of the commands it provides. | |
351 | ||
352 | The dynamic loading feature has certain disadvantages: | |
353 | ||
354 | @itemize @bullet | |
355 | @item | |
356 | If you delete or move the compiled file after loading it, Emacs can no | |
357 | longer load the remaining function definitions not already loaded. | |
358 | ||
359 | @item | |
360 | If you alter the compiled file (such as by compiling a new version), | |
361 | then trying to load any function not already loaded will usually yield | |
362 | nonsense results. | |
363 | @end itemize | |
364 | ||
365 | These problems will never happen in normal circumstances with | |
366 | installed Emacs files. But they are quite likely to happen with Lisp | |
367 | files that you are changing. The easiest way to prevent these problems | |
368 | is to reload the new compiled file immediately after each recompilation. | |
369 | ||
370 | The byte compiler uses the dynamic function loading feature if the | |
371 | variable @code{byte-compile-dynamic} is non-@code{nil} at compilation | |
372 | time. Do not set this variable globally, since dynamic loading is | |
373 | desirable only for certain files. Instead, enable the feature for | |
374 | specific source files with file-local variable bindings. For example, | |
375 | you could do it by writing this text in the source file's first line: | |
376 | ||
377 | @example | |
378 | -*-byte-compile-dynamic: t;-*- | |
379 | @end example | |
380 | ||
381 | @defvar byte-compile-dynamic | |
382 | If this is non-@code{nil}, the byte compiler generates compiled files | |
383 | that are set up for dynamic function loading. | |
384 | @end defvar | |
385 | ||
386 | @defun fetch-bytecode function | |
387 | If @var{function} is a byte-code function object, this immediately | |
388 | finishes loading the byte code of @var{function} from its | |
389 | byte-compiled file, if it is not fully loaded already. Otherwise, | |
390 | it does nothing. It always returns @var{function}. | |
391 | @end defun | |
392 | ||
393 | @node Eval During Compile | |
394 | @section Evaluation During Compilation | |
395 | ||
396 | These features permit you to write code to be evaluated during | |
397 | compilation of a program. | |
398 | ||
399 | @defspec eval-and-compile body@dots{} | |
400 | This form marks @var{body} to be evaluated both when you compile the | |
401 | containing code and when you run it (whether compiled or not). | |
402 | ||
403 | You can get a similar result by putting @var{body} in a separate file | |
404 | and referring to that file with @code{require}. That method is | |
405 | preferable when @var{body} is large. Effectively @code{require} is | |
406 | automatically @code{eval-and-compile}, the package is loaded both when | |
407 | compiling and executing. | |
408 | ||
409 | @code{autoload} is also effectively @code{eval-and-compile} too. It's | |
410 | recognized when compiling, so uses of such a function don't produce | |
411 | ``not known to be defined'' warnings. | |
412 | ||
413 | Most uses of @code{eval-and-compile} are fairly sophisticated. | |
414 | ||
415 | If a macro has a helper function to build its result, and that macro | |
416 | is used both locally and outside the package, then | |
417 | @code{eval-and-compile} should be used to get the helper both when | |
418 | compiling and then later when running. | |
419 | ||
420 | If functions are defined programmatically (with @code{fset} say), then | |
421 | @code{eval-and-compile} can be used to have that done at compile-time | |
422 | as well as run-time, so calls to those functions are checked (and | |
423 | warnings about ``not known to be defined'' suppressed). | |
424 | @end defspec | |
425 | ||
426 | @defspec eval-when-compile body@dots{} | |
427 | This form marks @var{body} to be evaluated at compile time but not when | |
428 | the compiled program is loaded. The result of evaluation by the | |
429 | compiler becomes a constant which appears in the compiled program. If | |
430 | you load the source file, rather than compiling it, @var{body} is | |
431 | evaluated normally. | |
432 | ||
433 | @cindex compile-time constant | |
434 | If you have a constant that needs some calculation to produce, | |
435 | @code{eval-when-compile} can do that at compile-time. For example, | |
436 | ||
437 | @lisp | |
438 | (defvar my-regexp | |
439 | (eval-when-compile (regexp-opt '("aaa" "aba" "abb")))) | |
440 | @end lisp | |
441 | ||
442 | @cindex macros, at compile time | |
443 | If you're using another package, but only need macros from it (the | |
444 | byte compiler will expand those), then @code{eval-when-compile} can be | |
445 | used to load it for compiling, but not executing. For example, | |
446 | ||
447 | @lisp | |
448 | (eval-when-compile | |
449 | (require 'my-macro-package)) ;; only macros needed from this | |
450 | @end lisp | |
451 | ||
452 | The same sort of thing goes for macros and @code{defsubst} functions | |
453 | defined locally and only for use within the file. They are needed for | |
454 | compiling the file, but in most cases they are not needed for | |
455 | execution of the compiled file. For example, | |
456 | ||
457 | @lisp | |
458 | (eval-when-compile | |
459 | (unless (fboundp 'some-new-thing) | |
460 | (defmacro 'some-new-thing () | |
461 | (compatibility code)))) | |
462 | @end lisp | |
463 | ||
464 | @noindent | |
465 | This is often good for code that's only a fallback for compatibility | |
466 | with other versions of Emacs. | |
467 | ||
468 | @strong{Common Lisp Note:} At top level, @code{eval-when-compile} is analogous to the Common | |
469 | Lisp idiom @code{(eval-when (compile eval) @dots{})}. Elsewhere, the | |
470 | Common Lisp @samp{#.} reader macro (but not when interpreting) is closer | |
471 | to what @code{eval-when-compile} does. | |
472 | @end defspec | |
473 | ||
474 | @node Compiler Errors | |
475 | @section Compiler Errors | |
476 | @cindex compiler errors | |
477 | ||
478 | Byte compilation outputs all errors and warnings into the buffer | |
479 | @samp{*Compile-Log*}. The messages include file names and line | |
480 | numbers that identify the location of the problem. The usual Emacs | |
481 | commands for operating on compiler diagnostics work properly on | |
482 | these messages. | |
483 | ||
484 | However, the warnings about functions that were used but not | |
485 | defined are always ``located'' at the end of the file, so these | |
486 | commands won't find the places they are really used. To do that, | |
487 | you must search for the function names. | |
488 | ||
489 | You can suppress the compiler warning for calling an undefined | |
490 | function @var{func} by conditionalizing the function call on an | |
491 | @code{fboundp} test, like this: | |
492 | ||
493 | @example | |
494 | (if (fboundp '@var{func}) ...(@var{func} ...)...) | |
495 | @end example | |
496 | ||
497 | @noindent | |
498 | The call to @var{func} must be in the @var{then-form} of the | |
499 | @code{if}, and @var{func} must appear quoted in the call to | |
500 | @code{fboundp}. (This feature operates for @code{cond} as well.) | |
501 | ||
fc37ae72 RS |
502 | You can tell the compiler that a function is defined using |
503 | @code{declare-function} (@pxref{Declaring Functions}). Likewise, you | |
504 | can tell the compiler that a variable is defined using @code{defvar} | |
505 | with no initial value. | |
5bb0cda3 | 506 | |
fc37ae72 RS |
507 | You can suppress the compiler warning for a specific use of an |
508 | undefined variable @var{variable} by conditionalizing its use on a | |
509 | @code{boundp} test, like this: | |
b8d4c8d0 GM |
510 | |
511 | @example | |
512 | (if (boundp '@var{variable}) ...@var{variable}...) | |
513 | @end example | |
514 | ||
515 | @noindent | |
516 | The reference to @var{variable} must be in the @var{then-form} of the | |
517 | @code{if}, and @var{variable} must appear quoted in the call to | |
518 | @code{boundp}. | |
519 | ||
fc37ae72 RS |
520 | You can suppress any and all compiler warnings within a certain |
521 | expression using the construct @code{with-no-warnings}: | |
b8d4c8d0 GM |
522 | |
523 | @c This is implemented with a defun, but conceptually it is | |
524 | @c a special form. | |
525 | ||
526 | @defspec with-no-warnings body@dots{} | |
527 | In execution, this is equivalent to @code{(progn @var{body}...)}, | |
528 | but the compiler does not issue warnings for anything that occurs | |
529 | inside @var{body}. | |
530 | ||
531 | We recommend that you use this construct around the smallest | |
fc37ae72 RS |
532 | possible piece of code, to avoid missing possible warnings other than one |
533 | one you intend to suppress. | |
b8d4c8d0 GM |
534 | @end defspec |
535 | ||
fc37ae72 | 536 | More precise control of warnings is possible by setting the variable |
5bb0cda3 GM |
537 | @code{byte-compile-warnings}. |
538 | ||
b8d4c8d0 GM |
539 | @node Byte-Code Objects |
540 | @section Byte-Code Function Objects | |
541 | @cindex compiled function | |
542 | @cindex byte-code function | |
543 | ||
544 | Byte-compiled functions have a special data type: they are | |
545 | @dfn{byte-code function objects}. | |
546 | ||
547 | Internally, a byte-code function object is much like a vector; | |
548 | however, the evaluator handles this data type specially when it appears | |
549 | as a function to be called. The printed representation for a byte-code | |
550 | function object is like that for a vector, with an additional @samp{#} | |
551 | before the opening @samp{[}. | |
552 | ||
553 | A byte-code function object must have at least four elements; there is | |
554 | no maximum number, but only the first six elements have any normal use. | |
555 | They are: | |
556 | ||
557 | @table @var | |
558 | @item arglist | |
559 | The list of argument symbols. | |
560 | ||
561 | @item byte-code | |
562 | The string containing the byte-code instructions. | |
563 | ||
564 | @item constants | |
565 | The vector of Lisp objects referenced by the byte code. These include | |
566 | symbols used as function names and variable names. | |
567 | ||
568 | @item stacksize | |
569 | The maximum stack size this function needs. | |
570 | ||
571 | @item docstring | |
572 | The documentation string (if any); otherwise, @code{nil}. The value may | |
573 | be a number or a list, in case the documentation string is stored in a | |
574 | file. Use the function @code{documentation} to get the real | |
575 | documentation string (@pxref{Accessing Documentation}). | |
576 | ||
577 | @item interactive | |
578 | The interactive spec (if any). This can be a string or a Lisp | |
579 | expression. It is @code{nil} for a function that isn't interactive. | |
580 | @end table | |
581 | ||
582 | Here's an example of a byte-code function object, in printed | |
583 | representation. It is the definition of the command | |
584 | @code{backward-sexp}. | |
585 | ||
586 | @example | |
587 | #[(&optional arg) | |
588 | "^H\204^F^@@\301^P\302^H[!\207" | |
589 | [arg 1 forward-sexp] | |
590 | 2 | |
591 | 254435 | |
592 | "p"] | |
593 | @end example | |
594 | ||
595 | The primitive way to create a byte-code object is with | |
596 | @code{make-byte-code}: | |
597 | ||
598 | @defun make-byte-code &rest elements | |
599 | This function constructs and returns a byte-code function object | |
600 | with @var{elements} as its elements. | |
601 | @end defun | |
602 | ||
603 | You should not try to come up with the elements for a byte-code | |
604 | function yourself, because if they are inconsistent, Emacs may crash | |
605 | when you call the function. Always leave it to the byte compiler to | |
606 | create these objects; it makes the elements consistent (we hope). | |
607 | ||
608 | You can access the elements of a byte-code object using @code{aref}; | |
609 | you can also use @code{vconcat} to create a vector with the same | |
610 | elements. | |
611 | ||
612 | @node Disassembly | |
613 | @section Disassembled Byte-Code | |
614 | @cindex disassembled byte-code | |
615 | ||
c36745c6 CY |
616 | People do not write byte-code; that job is left to the byte |
617 | compiler. But we provide a disassembler to satisfy a cat-like | |
618 | curiosity. The disassembler converts the byte-compiled code into | |
619 | human-readable form. | |
b8d4c8d0 GM |
620 | |
621 | The byte-code interpreter is implemented as a simple stack machine. | |
622 | It pushes values onto a stack of its own, then pops them off to use them | |
623 | in calculations whose results are themselves pushed back on the stack. | |
624 | When a byte-code function returns, it pops a value off the stack and | |
625 | returns it as the value of the function. | |
626 | ||
627 | In addition to the stack, byte-code functions can use, bind, and set | |
628 | ordinary Lisp variables, by transferring values between variables and | |
629 | the stack. | |
630 | ||
631 | @deffn Command disassemble object &optional buffer-or-name | |
632 | This command displays the disassembled code for @var{object}. In | |
633 | interactive use, or if @var{buffer-or-name} is @code{nil} or omitted, | |
634 | the output goes in a buffer named @samp{*Disassemble*}. If | |
635 | @var{buffer-or-name} is non-@code{nil}, it must be a buffer or the | |
636 | name of an existing buffer. Then the output goes there, at point, and | |
637 | point is left before the output. | |
638 | ||
639 | The argument @var{object} can be a function name, a lambda expression | |
640 | or a byte-code object. If it is a lambda expression, @code{disassemble} | |
641 | compiles it and disassembles the resulting compiled code. | |
642 | @end deffn | |
643 | ||
644 | Here are two examples of using the @code{disassemble} function. We | |
645 | have added explanatory comments to help you relate the byte-code to the | |
646 | Lisp source; these do not appear in the output of @code{disassemble}. | |
b8d4c8d0 GM |
647 | |
648 | @example | |
649 | @group | |
650 | (defun factorial (integer) | |
651 | "Compute factorial of an integer." | |
652 | (if (= 1 integer) 1 | |
653 | (* integer (factorial (1- integer))))) | |
654 | @result{} factorial | |
655 | @end group | |
656 | ||
657 | @group | |
658 | (factorial 4) | |
659 | @result{} 24 | |
660 | @end group | |
661 | ||
662 | @group | |
663 | (disassemble 'factorial) | |
664 | @print{} byte-code for factorial: | |
665 | doc: Compute factorial of an integer. | |
666 | args: (integer) | |
667 | @end group | |
668 | ||
669 | @group | |
c36745c6 CY |
670 | 0 varref integer ; @r{Get the value of @code{integer}} |
671 | ; @r{and push it onto the stack.} | |
672 | 1 constant 1 ; @r{Push 1 onto stack.} | |
b8d4c8d0 GM |
673 | @end group |
674 | ||
675 | @group | |
c36745c6 CY |
676 | 2 eqlsign ; @r{Pop top two values off stack, compare} |
677 | ; @r{them, and push result onto stack.} | |
b8d4c8d0 GM |
678 | @end group |
679 | ||
680 | @group | |
c36745c6 CY |
681 | 3 goto-if-nil 1 ; @r{Pop and test top of stack;} |
682 | ; @r{if @code{nil}, go to 1,} | |
b8d4c8d0 | 683 | ; @r{else continue.} |
b8d4c8d0 | 684 | 6 constant 1 ; @r{Push 1 onto top of stack.} |
c36745c6 CY |
685 | 7 return ; @r{Return the top element} |
686 | ; @r{of the stack.} | |
b8d4c8d0 GM |
687 | @end group |
688 | ||
689 | @group | |
c36745c6 CY |
690 | 8:1 varref integer ; @r{Push value of @code{integer} onto stack.} |
691 | 9 constant factorial ; @r{Push @code{factorial} onto stack.} | |
692 | 10 varref integer ; @r{Push value of @code{integer} onto stack.} | |
693 | 11 sub1 ; @r{Pop @code{integer}, decrement value,} | |
b8d4c8d0 | 694 | ; @r{push new value onto stack.} |
c36745c6 | 695 | 12 call 1 ; @r{Call function @code{factorial} using} |
b8d4c8d0 GM |
696 | ; @r{the first (i.e., the top) element} |
697 | ; @r{of the stack as the argument;} | |
698 | ; @r{push returned value onto stack.} | |
699 | @end group | |
700 | ||
701 | @group | |
c36745c6 CY |
702 | 13 mult ; @r{Pop top two values off stack, multiply} |
703 | ; @r{them, and push result onto stack.} | |
704 | 14 return ; @r{Return the top element of stack.} | |
b8d4c8d0 GM |
705 | @end group |
706 | @end example | |
707 | ||
708 | The @code{silly-loop} function is somewhat more complex: | |
709 | ||
710 | @example | |
711 | @group | |
712 | (defun silly-loop (n) | |
713 | "Return time before and after N iterations of a loop." | |
714 | (let ((t1 (current-time-string))) | |
715 | (while (> (setq n (1- n)) | |
716 | 0)) | |
717 | (list t1 (current-time-string)))) | |
718 | @result{} silly-loop | |
719 | @end group | |
720 | ||
721 | @group | |
722 | (disassemble 'silly-loop) | |
723 | @print{} byte-code for silly-loop: | |
724 | doc: Return time before and after N iterations of a loop. | |
725 | args: (n) | |
726 | ||
727 | 0 constant current-time-string ; @r{Push} | |
728 | ; @r{@code{current-time-string}} | |
729 | ; @r{onto top of stack.} | |
730 | @end group | |
731 | ||
732 | @group | |
733 | 1 call 0 ; @r{Call @code{current-time-string}} | |
c36745c6 CY |
734 | ; @r{with no argument,} |
735 | ; @r{pushing result onto stack.} | |
b8d4c8d0 GM |
736 | @end group |
737 | ||
738 | @group | |
739 | 2 varbind t1 ; @r{Pop stack and bind @code{t1}} | |
740 | ; @r{to popped value.} | |
741 | @end group | |
742 | ||
743 | @group | |
c36745c6 | 744 | 3:1 varref n ; @r{Get value of @code{n} from} |
b8d4c8d0 GM |
745 | ; @r{the environment and push} |
746 | ; @r{the value onto the stack.} | |
b8d4c8d0 GM |
747 | 4 sub1 ; @r{Subtract 1 from top of stack.} |
748 | @end group | |
749 | ||
750 | @group | |
751 | 5 dup ; @r{Duplicate the top of the stack;} | |
752 | ; @r{i.e., copy the top of} | |
753 | ; @r{the stack and push the} | |
754 | ; @r{copy onto the stack.} | |
b8d4c8d0 GM |
755 | 6 varset n ; @r{Pop the top of the stack,} |
756 | ; @r{and bind @code{n} to the value.} | |
757 | ||
758 | ; @r{In effect, the sequence @code{dup varset}} | |
759 | ; @r{copies the top of the stack} | |
760 | ; @r{into the value of @code{n}} | |
761 | ; @r{without popping it.} | |
762 | @end group | |
763 | ||
764 | @group | |
765 | 7 constant 0 ; @r{Push 0 onto stack.} | |
b8d4c8d0 GM |
766 | 8 gtr ; @r{Pop top two values off stack,} |
767 | ; @r{test if @var{n} is greater than 0} | |
768 | ; @r{and push result onto stack.} | |
769 | @end group | |
770 | ||
771 | @group | |
c36745c6 CY |
772 | 9 goto-if-not-nil 1 ; @r{Goto 1 if @code{n} > 0} |
773 | ; @r{(this continues the while loop)} | |
774 | ; @r{else continue.} | |
b8d4c8d0 GM |
775 | @end group |
776 | ||
777 | @group | |
c36745c6 CY |
778 | 12 varref t1 ; @r{Push value of @code{t1} onto stack.} |
779 | 13 constant current-time-string ; @r{Push @code{current-time-string}} | |
b8d4c8d0 | 780 | ; @r{onto top of stack.} |
c36745c6 | 781 | 14 call 0 ; @r{Call @code{current-time-string} again.} |
b8d4c8d0 GM |
782 | @end group |
783 | ||
784 | @group | |
c36745c6 CY |
785 | 15 unbind 1 ; @r{Unbind @code{t1} in local environment.} |
786 | 16 list2 ; @r{Pop top two elements off stack,} | |
b8d4c8d0 GM |
787 | ; @r{create a list of them,} |
788 | ; @r{and push list onto stack.} | |
c36745c6 | 789 | 17 return ; @r{Return value of the top of stack.} |
b8d4c8d0 GM |
790 | @end group |
791 | @end example | |
792 | ||
793 | ||
794 | @ignore | |
795 | arch-tag: f78e3050-2f0a-4dee-be27-d9979a0a2289 | |
796 | @end ignore |