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A. Efficiency Concerns


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A.1 Macros

Many of the advanced features of this package, such as defun*, loop, and setf, are implemented as Lisp macros. In byte-compiled code, these complex notations will be expanded into equivalent Lisp code which is simple and efficient. For example, the forms

 
(incf i n)
(push x (car p))

are expanded at compile-time to the Lisp forms

 
(setq i (+ i n))
(setcar p (cons x (car p)))

which are the most efficient ways of doing these respective operations in Lisp. Thus, there is no performance penalty for using the more readable incf and push forms in your compiled code.

Interpreted code, on the other hand, must expand these macros every time they are executed. For this reason it is strongly recommended that code making heavy use of macros be compiled. (The features labelled "Special Form" instead of "Function" in this manual are macros.) A loop using incf a hundred times will execute considerably faster if compiled, and will also garbage-collect less because the macro expansion will not have to be generated, used, and thrown away a hundred times.

You can find out how a macro expands by using the cl-prettyexpand function.

Function: cl-prettyexpand form &optional full
This function takes a single Lisp form as an argument and inserts a nicely formatted copy of it in the current buffer (which must be in Lisp mode so that indentation works properly). It also expands all Lisp macros which appear in the form. The easiest way to use this function is to go to the *scratch* buffer and type, say,

 
(cl-prettyexpand '(loop for x below 10 collect x))

and type C-x C-e immediately after the closing parenthesis; the expansion

 
(block nil
  (let* ((x 0)
         (G1004 nil))
    (while (< x 10)
      (setq G1004 (cons x G1004))
      (setq x (+ x 1)))
    (nreverse G1004)))

will be inserted into the buffer. (The block macro is expanded differently in the interpreter and compiler, so cl-prettyexpand just leaves it alone. The temporary variable G1004 was created by gensym.)

If the optional argument full is true, then all macros are expanded, including block, eval-when, and compiler macros. Expansion is done as if form were a top-level form in a file being compiled. For example,

 
(cl-prettyexpand '(pushnew 'x list))
     -| (setq list (adjoin 'x list))
(cl-prettyexpand '(pushnew 'x list) t)
     -| (setq list (if (memq 'x list) list (cons 'x list)))
(cl-prettyexpand '(caddr (member* 'a list)) t)
     -| (car (cdr (cdr (memq 'a list))))

Note that adjoin, caddr, and member* all have built-in compiler macros to optimize them in common cases.

 

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A.2 Error Checking

Common Lisp compliance has in general not been sacrificed for the sake of efficiency. A few exceptions have been made for cases where substantial gains were possible at the expense of marginal incompatibility. One example is the use of memq (which is treated very efficiently by the byte-compiler) to scan for keyword arguments; this can become confused in rare cases when keyword symbols are used as both keywords and data values at once. This is extremely unlikely to occur in practical code, and the use of memq allows functions with keyword arguments to be nearly as fast as functions that use &optional arguments.

The Common Lisp standard (as embodied in Steele's book) uses the phrase "it is an error if" to indicate a situation which is not supposed to arise in complying programs; implementations are strongly encouraged but not required to signal an error in these situations. This package sometimes omits such error checking in the interest of compactness and efficiency. For example, do variable specifiers are supposed to be lists of one, two, or three forms; extra forms are ignored by this package rather than signalling a syntax error. The endp function is simply a synonym for null in this package. Functions taking keyword arguments will accept an odd number of arguments, treating the trailing keyword as if it were followed by the value nil.

Argument lists (as processed by defun* and friends) are checked rigorously except for the minor point just mentioned; in particular, keyword arguments are checked for validity, and &allow-other-keys and :allow-other-keys are fully implemented. Keyword validity checking is slightly time consuming (though not too bad in byte-compiled code); you can use &allow-other-keys to omit this check. Functions defined in this package such as find and member* do check their keyword arguments for validity.

 

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A.3 Optimizing Compiler

The byte-compiler that comes with Emacs 18 normally fails to expand macros that appear in top-level positions in the file (i.e., outside of defuns or other enclosing forms). This would have disastrous consequences to programs that used such top-level macros as defun*, eval-when, and defstruct. To work around this problem, the CL package patches the Emacs 18 compiler to expand top-level macros. This patch will apply to your own macros, too, if they are used in a top-level context. The patch will not harm versions of the Emacs 18 compiler which have already had a similar patch applied, nor will it affect the optimizing Emacs 19 byte-compiler written by Jamie Zawinski and Hallvard Furuseth. The patch is applied to the byte compiler's code in Emacs' memory, not to the `bytecomp.elc' file stored on disk.

The Emacs 19 compiler (for Emacs 18) is available from various Emacs Lisp archive sites such as archive.cis.ohio-state.edu. Its use is highly recommended; many of the Common Lisp macros emit code which can be improved by optimization. In particular, blocks (whether explicit or implicit in constructs like defun* and loop) carry a fair run-time penalty; the optimizing compiler removes blocks which are not actually referenced by return or return-from inside the block.


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