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21. Byte Compilation

XEmacs Lisp has a compiler that translates functions written in Lisp into a special representation called byte-code that can be executed more efficiently. The compiler replaces Lisp function definitions with byte-code. When a byte-coded function is called, its definition is evaluated by the byte-code interpreter.

Because the byte-compiled code is evaluated by the byte-code interpreter, instead of being executed directly by the machine's hardware (as true compiled code is), byte-code is completely transportable from machine to machine without recompilation. It is not, however, as fast as true compiled code.

In general, any version of Emacs can run byte-compiled code produced by recent earlier versions of Emacs, but the reverse is not true. In particular, if you compile a program with XEmacs 20, the compiled code may not run in earlier versions.

The first time a compiled-function object is executed, the byte-code instructions are validated and the byte-code is further optimized. An invalid-byte-code error is signaled if the byte-code is invalid, for example if it contains invalid opcodes. This usually means a bug in the byte compiler.

See section 22.3 Debugging Problems in Compilation, for how to investigate errors occurring in byte compilation.

21.1 Performance of Byte-Compiled Code  An example of speedup from byte compilation.
21.2 The Compilation Functions  Byte compilation functions.
21.3 Options for the Byte Compiler  Controlling the byte compiler's behavior.
21.4 Documentation Strings and Compilation  Dynamic loading of documentation strings.
21.5 Dynamic Loading of Individual Functions  Dynamic loading of individual functions.
21.6 Evaluation During Compilation  Code to be evaluated when you compile.
21.7 Compiled-Function Objects  The data type used for byte-compiled functions.
21.8 Disassembled Byte-Code  Disassembling byte-code; how to read byte-code.
21.9 Different Behavior  When compiled code gives different results.


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21.1 Performance of Byte-Compiled Code

A byte-compiled function is not as efficient as a primitive function written in C, but runs much faster than the version written in Lisp. Here is an example:

 
(defun silly-loop (n)
  "Return time before and after N iterations of a loop."
  (let ((t1 (current-time-string)))
    (while (> (setq n (1- n))
              0))
    (list t1 (current-time-string))))
=> silly-loop

(silly-loop 5000000)
=> ("Mon Sep 14 15:51:49 1998"
    "Mon Sep 14 15:52:07 1998")  ; 18 seconds

(byte-compile 'silly-loop)
=> #<compiled-function
(n)
"...(23)"
[current-time-string t1 n 0]
2
"Return time before and after N iterations of a loop.">

(silly-loop 5000000)
=> ("Mon Sep 14 15:53:43 1998"
    "Mon Sep 14 15:53:49 1998")  ; 6 seconds

In this example, the interpreted code required 18 seconds to run, whereas the byte-compiled code required 6 seconds. These results are representative, but actual results will vary greatly.


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21.2 The Compilation Functions

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

When you run the byte compiler, you may get warnings in a buffer called `*Compile-Log*'. These report things in your program that suggest a problem but are not necessarily erroneous.

Be careful when byte-compiling code that uses macros. Macro calls are expanded when they are compiled, so the macros must already be defined for proper compilation. For more details, see 18.3 Macros and Byte Compilation.

Normally, compiling a file does not evaluate the file's contents or load the file. But it does execute any require calls at top level in the file. One way to ensure that necessary macro definitions are available during compilation is to require the file that defines them (see section 20.4 Features). To avoid loading the macro definition files when someone runs the compiled program, write eval-when-compile around the require calls (see section 21.6 Evaluation During Compilation).

Function: byte-compile symbol
This function byte-compiles the function definition of symbol, replacing the previous definition with the compiled one. The function definition of symbol must be the actual code for the function; i.e., the compiler does not follow indirection to another symbol. byte-compile returns the new, compiled definition of symbol.

If symbol's definition is a compiled-function object, byte-compile does nothing and returns nil. Lisp records only one function definition for any symbol, and if that is already compiled, non-compiled code is not available anywhere. So there is no way to "compile the same definition again."

 
(defun factorial (integer)
  "Compute factorial of INTEGER."
  (if (= 1 integer) 1
    (* integer (factorial (1- integer)))))
=> factorial

(byte-compile 'factorial)
=> #<compiled-function
(integer)
"...(21)"
[integer 1 factorial]
3
"Compute factorial of INTEGER.">

The result is a compiled-function object. The string it contains is the actual byte-code; each character in it is an instruction or an operand of an instruction. The vector contains all the constants, variable names and function names used by the function, except for certain primitives that are coded as special instructions.

Command: compile-defun &optional arg
This command reads the defun containing point, compiles it, and evaluates the result. If you use this on a defun that is actually a function definition, the effect is to install a compiled version of that function.

If arg is non-nil, the result is inserted in the current buffer after the form; otherwise, it is printed in the minibuffer.

Command: byte-compile-file filename &optional load
This function compiles a file of Lisp code named filename into a file of byte-code. The output file's name is made by appending `c' to the end of filename.

If load is non-nil, the file is loaded after having been compiled.

Compilation works by reading the input file one form at a time. If it is a definition of a function or macro, the compiled function or macro definition is written out. Other forms are batched together, then each batch is compiled, and written so that its compiled code will be executed when the file is read. All comments are discarded when the input file is read.

This command returns t. When called interactively, it prompts for the file name.

 
% ls -l push*
-rw-r--r--  1 lewis     791 Oct  5 20:31 push.el

(byte-compile-file "~/emacs/push.el")
     => t

% ls -l push*
-rw-r--r--  1 lewis     791 Oct  5 20:31 push.el
-rw-r--r--  1 lewis     638 Oct  8 20:25 push.elc

Command: byte-recompile-directory directory &optional flag norecursion force
This function recompiles every `.el' file in directory that needs recompilation. A file needs recompilation if a `.elc' file exists but is older than the `.el' file.

Files in subdirectories of directory are also processed unless optional argument norecursion is non-nil.

When a `.el' file has no corresponding `.elc' file, then flag says what to do. If it is nil, these files are ignored. If it is non-nil, the user is asked whether to compile each such file.

If the fourth optional argument force is non-nil, recompile every `.el' file that already has a `.elc' file.

The return value of this command is unpredictable.

Function: batch-byte-compile
This function runs byte-compile-file on files specified on the command line. This function must be used only in a batch execution of Emacs, as it kills Emacs on completion. An error in one file does not prevent processing of subsequent files. (The file that gets the error will not, of course, produce any compiled code.)

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

Function: batch-byte-recompile-directory
This function is similar to batch-byte-compile but runs the command byte-recompile-directory on the files remaining on the command line.

Variable: byte-recompile-directory-ignore-errors-p
When non-nil, byte-recompile-directory will continue compiling even when an error occurs in a file. Default: nil, but bound to t by batch-byte-recompile-directory.

Variable: byte-recompile-directory-recursively
When non-nil, byte-recompile-directory will recurse on subdirectories. Default: t.

Function: byte-code instructions constants stack-depth
This function actually interprets byte-code. Don't call this function yourself. Only the byte compiler knows how to generate valid calls to this function.

In newer Emacs versions (19 and up), byte code is usually executed as part of a compiled-function object, and only rarely due to an explicit call to byte-code. A byte-compiled function was once actually defined with a body that calls byte-code, but in recent versions of Emacs byte-code is only used to run isolated fragments of lisp code without an associated argument list.


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21.3 Options for the Byte Compiler

Warning: this node is a quick draft based on docstrings. There may be inaccuracies, as the docstrings occasionally disagree with each other. This has not been checked yet.

The byte compiler and optimizer are controlled by the following variables. The byte-compiler-options macro described below provides a convenient way to set most of them on a file-by-file basis.

Variable: emacs-lisp-file-regexp
Regexp which matches Emacs Lisp source files. You may want to redefine byte-compile-dest-file if you change this. Default: "\\.el$".

Function: byte-compile-dest-file filename
Convert an Emacs Lisp source file name to a compiled file name. This function may be redefined by the user, if necessary, for compatibility with emacs-lisp-file-regexp.

Variable: byte-compile-verbose
When non-nil, print messages describing progress of byte-compiler. Default: t if interactive on a not-too-slow terminal (see search-slow-speed), otherwise nil.

Variable: byte-optimize
Level of optimization in the byte compiler.

nil
Do no optimization.

t
Do all optimizations.

source
Do optimizations manipulating the source code only.

byte
Do optimizations manipulating the byte code (actually, LAP code) only.
Default: t.

Variable: byte-compile-delete-errors
When non-nil, the optimizer may delete forms that may signal an error if that is the only change in the function's behavior. This includes variable references and calls to functions such as car. Default: t.

Variable: byte-optimize-log nil
When non-nil, the byte-compiler logs optimizations into `*Compile-Log*'.

nil
Log no optimization.

t
Log all optimizations.

source
Log optimizations manipulating the source code only.

byte
Log optimizations manipulating the byte code (actually, LAP code) only.
Default: nil.

Variable: byte-compile-error-on-warn
When non-nil, the byte-compiler reports warnings with error. Default: nil.

Variable: byte-compile-default-warnings
The warnings used when byte-compile-warnings is t. Called byte-compile-warning-types in GNU Emacs. Default: (redefine callargs subr-callargs free-vars unresolved unused-vars obsolete).

Variable: byte-compile-warnings

List of warnings that the compiler should issue (t for the default set). Elements of the list may be:

free-vars
References to variables not in the current lexical scope.

unused-vars
References to non-global variables bound but not referenced.

unresolved
Calls to unknown functions.

callargs
Lambda calls with args that don't match the definition.

subr-callargs
Calls to subrs with args that don't match the definition.

redefine
Function cell redefined from a macro to a lambda or vice versa, or redefined to take a different number of arguments.

obsolete
Use of an obsolete function or variable.

pedantic
Warn of use of compatible symbols.

The default set is specified by byte-compile-default-warnings and normally encompasses all possible warnings.

See also the macro byte-compiler-options. Default: t.

The compiler can generate a call graph, which gives information about which functions call which functions.

Variable: byte-compile-generate-call-tree
When non-nil, the compiler generates a call graph. This records functions that were called and from where. If the value is t, compilation displays the call graph when it finishes. If the value is neither t nor nil, compilation asks you whether to display the graph.

The call tree only lists functions called, not macros used. Those functions which the byte-code interpreter knows about directly (eq, cons, etc.) are not reported.

The call tree also lists those functions which are not known to be called (that is, to which no calls have been compiled). Functions which can be invoked interactively are excluded from this list. Default: nil.

Variable: byte-compile-call-tree nil

Alist of functions and their call tree, used internally. Each element takes the form

(function callers calls)

where callers is a list of functions that call function, and calls is a list of functions for which calls were generated while compiling function.

Variable: byte-compile-call-tree-sort
When non-nil, sort the call tree. The values name, callers, calls, and calls+callers specify different fields to sort on.") Default: name.

byte-compile-overwrite-file controls treatment of existing compiled files.

Variable: byte-compile-overwrite-file
When non-nil, do not preserve backups of `.elc's. Precisely, if nil, old `.elc' files are deleted before the new one is saved, and `.elc' files will have the same modes as the corresponding `.el' file. Otherwise, existing `.elc' files will simply be overwritten, and the existing modes will not be changed. If this variable is nil, then an `.elc' file which is a symbolic link will be turned into a normal file, instead of the file which the link points to being overwritten. Default: t.

Variables controlling recompiling directories are described elsewhere See section 21.2 The Compilation Functions. They are byte-recompile-directory-ignore-errors-p and byte-recompile-directory-recursively.

The dynamic loading features are described elsewhere. These are controlled by the variables byte-compile-dynamic (see section 21.5 Dynamic Loading of Individual Functions) and byte-compile-dynamic-docstrings (see section 21.4 Documentation Strings and Compilation).

The byte compiler is a relatively recent development, and has evolved significantly over the period covering Emacs versions 19 and 20. The following variables control use of newer functionality by the byte compiler. These are rarely needed since the release of XEmacs 21.

Another set of compatibility issues arises between Mule and non-Mule XEmacsen; there are no known compatibility issues specific to the byte compiler. There are also compatibility issues between XEmacs and GNU Emacs's versions of the byte compiler. While almost all of the byte codes are the same, and code compiled by one version often runs perfectly well on the other, this is very dangerous, and can result in crashes or data loss. Always recompile your Lisp when moving between XEmacs and GNU Emacs.

Variable: byte-compile-single-version nil
When non-nil, the choice of emacs version (v19 or v20) byte-codes will be hard-coded into bytecomp when it compiles itself. If the compiler itself is compiled with optimization, this causes a speedup. Default: nil.

Variable: byte-compile-emacs19-compatibility
When non-nil generate output that can run in Emacs 19. Default: nil when Emacs version is 20 or above, otherwise t.

Variable: byte-compile-print-gensym
When non-nil, the compiler may generate code that creates unique symbols at run-time. This is achieved by printing uninterned symbols using the #:symbol notation, so that they will be read uninterned when run.

With this feature, code that uses uninterned symbols in macros will not be runnable under pre-21.0 XEmacsen.

Default: When byte-compile-emacs19-compatibility is non-nil, this variable is ignored and considered to be nil. Otherwise t.

Variable: byte-compile-new-bytecodes
This is completely ignored. For backwards compatibility.

Function: byte-compiler-options &rest args
Set some compilation-parameters for this file. This will affect only the file in which it appears; this does nothing when evaluated, or when loaded from a `.el' file.

Each argument to this macro must be a list of a key and a value. (#### Need to check whether the newer variables are settable here.)

 
  Keys:		  Values:		Corresponding variable:

  verbose	  t, nil		byte-compile-verbose
  optimize	  t, nil, source, byte	byte-optimize
  warnings	  list of warnings	byte-compile-warnings
  file-format	  emacs19, emacs20	byte-compile-emacs19-compatibility

The value specified with the warningsoption must be a list, containing some subset of the following flags:

 
  free-vars	references to variables not in the current lexical scope.
  unused-vars	references to non-global variables bound but not referenced.
  unresolved	calls to unknown functions.
  callargs	lambda calls with args that don't match the definition.
  redefine	function cell redefined from a macro to a lambda or vice
		versa, or redefined to take a different number of arguments.

If the first element if the list is + or ` then the specified elements are added to or removed from the current set of warnings, instead of the entire set of warnings being overwritten. (#### Need to check whether the newer warnings are settable here.)

For example, something like this might appear at the top of a source file:

 
    (byte-compiler-options
      (optimize t)
      (warnings (- callargs))		; Don't warn about arglist mismatch
      (warnings (+ unused-vars))	; Do warn about unused bindings
      (file-format emacs19))


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21.4 Documentation Strings and Compilation

Functions and variables loaded from a byte-compiled file access their documentation strings dynamically from the file whenever needed. This saves space within Emacs, and makes loading faster because the documentation strings themselves need not be processed while loading the file. Actual access to the documentation strings becomes slower as a result, but normally not enough to bother users.

Dynamic access to documentation strings does have drawbacks:

If your site installs Emacs following the usual procedures, these problems will never normally occur. Installing a new version uses a new directory with a different name; as long as the old version remains installed, its files will remain unmodified in the places where they are expected to be.

However, if you have built Emacs yourself and use it from the directory where you built it, you will experience this problem occasionally if you edit and recompile Lisp files. When it happens, you can cure the problem by reloading the file after recompiling it.

Versions of Emacs up to and including XEmacs 19.14 and FSF Emacs 19.28 do not support the dynamic docstrings feature, and so will not be able to load bytecode created by more recent Emacs versions. You can turn off the dynamic docstring feature by setting byte-compile-dynamic-docstrings to nil. Once this is done, you can compile files that will load into older Emacs versions. You can do this globally, or for one source file by specifying a file-local binding for the variable. Here's one way to do that:

 
-*-byte-compile-dynamic-docstrings: nil;-*-

Variable: byte-compile-dynamic-docstrings
If this is non-nil, the byte compiler generates compiled files that are set up for dynamic loading of documentation strings. Default: t.

The dynamic documentation string feature writes compiled files that use a special Lisp reader construct, `#@count'. This construct skips the next count characters. It also uses the `#$' construct, which stands for "the name of this file, as a string." It is best not to use these constructs in Lisp source files.


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21.5 Dynamic Loading of Individual Functions

When you compile a file, you can optionally enable the dynamic function loading feature (also known as lazy loading). With dynamic function loading, loading the file doesn't fully read the function definitions in the file. Instead, each function definition contains a place-holder which refers to the file. The first time each function is called, it reads the full definition from the file, to replace the place-holder.

The advantage of dynamic function loading is that loading the file becomes much faster. This is a good thing for a file which contains many separate commands, provided that using one of them does not imply you will soon (or ever) use the rest. A specialized mode which provides many keyboard commands often has that usage pattern: a user may invoke the mode, but use only a few of the commands it provides.

The dynamic loading feature has certain disadvantages:

If you compile a new version of the file, the best thing to do is immediately load the new compiled file. That will prevent any future problems.

The byte compiler uses the dynamic function loading feature if the variable byte-compile-dynamic is non-nil at compilation time. Do not set this variable globally, since dynamic loading is desirable only for certain files. Instead, enable the feature for specific source files with file-local variable bindings, like this:

 
-*-byte-compile-dynamic: t;-*-

Variable: byte-compile-dynamic
If this is non-nil, the byte compiler generates compiled files that are set up for dynamic function loading. Default: nil.

Function: fetch-bytecode function
This immediately finishes loading the definition of function from its byte-compiled file, if it is not fully loaded already. The argument function may be a compiled-function object or a function name.


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21.6 Evaluation During Compilation

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

Macro: eval-and-compile body
This form marks body to be evaluated both when you compile the containing code and when you run it (whether compiled or not).

You can get a similar result by putting body in a separate file and referring to that file with require. Using require is preferable if there is a substantial amount of code to be executed in this way.

Macro: eval-when-compile body
This form marks body to be evaluated at compile time and not when the compiled program is loaded. The result of evaluation by the compiler becomes a constant which appears in the compiled program. When the program is interpreted, not compiled at all, body is evaluated normally.

At top level, this is analogous to the Common Lisp idiom (eval-when (compile eval) ...). Elsewhere, the Common Lisp `#.' reader macro (but not when interpreting) is closer to what eval-when-compile does.


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21.7 Compiled-Function Objects

Byte-compiled functions have a special data type: they are compiled-function objects. The evaluator handles this data type specially when it appears as a function to be called.

The printed representation for a compiled-function object normally begins with `#<compiled-function' and ends with `>'. However, if the variable print-readably is non-nil, the object is printed beginning with `#[' and ending with `]'. This representation can be read directly by the Lisp reader, and is used in byte-compiled files (those ending in `.elc').

In Emacs version 18, there was no compiled-function object data type; compiled functions used the function byte-code to run the byte code.

A compiled-function object has a number of different attributes. They are:

arglist
The list of argument symbols.

instructions
The string containing the byte-code instructions.

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

stack-depth
The maximum stack size this function needs.

doc-string
The documentation string (if any); otherwise, nil. The value may be a number or a list, in case the documentation string is stored in a file. Use the function documentation to get the real documentation string (see section 34.2 Access to Documentation Strings).

interactive
The interactive spec (if any). This can be a string or a Lisp expression. It is nil for a function that isn't interactive.

domain
The domain (if any). This is only meaningful if I18N3 (message-translation) support was compiled into XEmacs. This is a string defining which domain to find the translation for the documentation string and interactive prompt. See section 62.2.4 Domain Specification.

Here's an example of a compiled-function object, in printed representation. It is the definition of the command backward-sexp.

 
(symbol-function 'backward-sexp)
=> #<compiled-function
(&optional arg)
"...(15)" [arg 1 forward-sexp] 2 854740 "_p">

The primitive way to create a compiled-function object is with make-byte-code:

Function: make-byte-code arglist instructions constants stack-depth &optional doc-string interactive
This function constructs and returns a compiled-function object with the specified attributes.

Please note: Unlike all other Emacs-lisp functions, calling this with five arguments is not the same as calling it with six arguments, the last of which is nil. If the interactive arg is specified as nil, then that means that this function was defined with (interactive). If the arg is not specified, then that means the function is not interactive. This is terrible behavior which is retained for compatibility with old `.elc' files which expected these semantics.

You should not try to come up with the elements for a compiled-function object yourself, because if they are inconsistent, XEmacs may crash when you call the function. Always leave it to the byte compiler to create these objects; it makes the elements consistent (we hope).

The following primitives are provided for accessing the elements of a compiled-function object.

Function: compiled-function-arglist function
This function returns the argument list of compiled-function object function.

Function: compiled-function-instructions function
This function returns a string describing the byte-code instructions of compiled-function object function.

Function: compiled-function-constants function
This function returns the vector of Lisp objects referenced by compiled-function object function.

Function: compiled-function-stack-depth function
This function returns the maximum stack size needed by compiled-function object function.

Function: compiled-function-doc-string function
This function returns the doc string of compiled-function object function, if available.

Function: compiled-function-interactive function
This function returns the interactive spec of compiled-function object function, if any. The return value is nil or a two-element list, the first element of which is the symbol interactive and the second element is the interactive spec (a string or Lisp form).

Function: compiled-function-domain function
This function returns the domain of compiled-function object function, if any. The result will be a string or nil. See section 62.2.4 Domain Specification.


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21.8 Disassembled Byte-Code

People do not write byte-code; that job is left to the byte compiler. But we provide a disassembler to satisfy a cat-like curiosity. The disassembler converts the byte-compiled code into humanly readable form.

The byte-code interpreter is implemented as a simple stack machine. It pushes values onto a stack of its own, then pops them off to use them in calculations whose results are themselves pushed back on the stack. When a byte-code function returns, it pops a value off the stack and returns it as the value of the function.

In addition to the stack, byte-code functions can use, bind, and set ordinary Lisp variables, by transferring values between variables and the stack.

Command: disassemble object &optional stream
This function prints the disassembled code for object. If stream is supplied, then output goes there. Otherwise, the disassembled code is printed to the stream standard-output. The argument object can be a function name or a lambda expression.

As a special exception, if this function is used interactively, it outputs to a buffer named `*Disassemble*'.

Here are two examples of using the disassemble function. We have added explanatory comments to help you relate the byte-code to the Lisp source; these do not appear in the output of disassemble.

 
(defun factorial (integer)
  "Compute factorial of an integer."
  (if (= 1 integer) 1
    (* integer (factorial (1- integer)))))
     => factorial

(factorial 4)
     => 24

(disassemble 'factorial)
     -| byte-code for factorial:
 doc: Compute factorial of an integer.
 args: (integer)

0   varref   integer        ; Get value of integer
                            ;   from the environment
                            ;   and push the value
                            ;   onto the stack.

1   constant 1              ; Push 1 onto stack.

2   eqlsign                 ; Pop top two values off stack,
                            ;   compare them,
                            ;   and push result onto stack.

3   goto-if-nil 1           ; Pop and test top of stack;
                            ;   if nil,
                            ;   go to label 1 (which is also byte 7),
                            ;   else continue.

5   constant 1              ; Push 1 onto top of stack.

6   return                  ; Return the top element
                            ;   of the stack.

7:1 varref   integer        ; Push value of integer onto stack.

8   constant factorial      ; Push factorial onto stack.

9   varref   integer        ; Push value of integer onto stack.

10  sub1                    ; Pop integer, decrement value,
                            ;   push new value onto stack.

                            ; Stack now contains:
                            ;   - decremented value of integer
                            ;   - factorial
                            ;   - value of integer

15  call     1              ; Call function factorial using
                            ;   the first (i.e., the top) element
                            ;   of the stack as the argument;
                            ;   push returned value onto stack.

                            ; Stack now contains:
                            ;   - result of recursive
                            ;        call to factorial
                            ;   - value of integer

12  mult                    ; Pop top two values off the stack,
                            ;   multiply them,
                            ;   pushing the result onto the stack.

13  return                  ; Return the top element
                            ;   of the stack.
     => nil

The silly-loop function is somewhat more complex:

 
(defun silly-loop (n)
  "Return time before and after N iterations of a loop."
  (let ((t1 (current-time-string)))
    (while (> (setq n (1- n))
              0))
    (list t1 (current-time-string))))
     => silly-loop

(disassemble 'silly-loop)
     -| byte-code for silly-loop:
 doc: Return time before and after N iterations of a loop.
 args: (n)

0   constant current-time-string  ; Push
                                  ;   current-time-string
                                  ;   onto top of stack.

1   call     0              ; Call current-time-string
                            ;    with no argument,
                            ;    pushing result onto stack.

2   varbind  t1             ; Pop stack and bind t1
                            ;   to popped value.

3:1 varref   n              ; Get value of n from
                            ;   the environment and push
                            ;   the value onto the stack.

4   sub1                    ; Subtract 1 from top of stack.

5   dup                     ; Duplicate the top of the stack;
                            ;   i.e., copy the top of
                            ;   the stack and push the
                            ;   copy onto the stack.

6   varset   n              ; Pop the top of the stack,
                            ;   and set n to the value.

                            ; In effect, the sequence dup varset
                            ;   copies the top of the stack
                            ;   into the value of n
                            ;   without popping it.

7   constant 0              ; Push 0 onto stack.

8   gtr                     ; Pop top two values off stack,
                            ;   test if n is greater than 0
                            ;   and push result onto stack.

9   goto-if-not-nil 1       ; Goto label 1 (byte 3) if n <= 0
                            ;   (this exits the while loop).
                            ;   else pop top of stack
                            ;   and continue

11  varref   t1             ; Push value of t1 onto stack.

12  constant current-time-string  ; Push
                                  ;   current-time-string
                                  ;   onto top of stack.

13  call     0              ; Call current-time-string again.

14  unbind   1              ; Unbind t1 in local environment.

15  list2                   ; Pop top two elements off stack,
                            ;   create a list of them,
                            ;   and push list onto stack.

16  return                  ; Return the top element of the stack.

     => nil


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21.9 Different Behavior

The intent is that compiled byte-code and the corresponding code executed by the Lisp interpreter produce identical results. However, there are some circumstances where the results will differ.

For maximal equivalence between interpreted and compiled code, the variables byte-compile-delete-errors and byte-compile-optimize can be set to nil, but this is not recommended.


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