A.2 Tips for efficient lisp code

Here are ways of improving the execution speed of Lisp programs.

• Use byte compilation whenever you can.

  Beside the static byte compilation yielding .elc files, you can also use the byte-compile function to create and use compiled objects inline.

            ;; We create a byte compiled predicate and bind it to the variable
            ;; is-minus-one-p.
            ;; Note that we do _NOT_ create a function is-minus-one-p.
            (setq is-minus-one-p
                  (byte-compile
                    #'(lambda (a) (zerop (1+ a)))))
              ⇒ #<compiled-function (a) "...(5)" [a zerop] 2>
            
            ;; We use the compiled function object of is-minus-one-p.
            (require 'cl)
            (some is-minus-one-p '(1 2 3 4 -1))
              ⇒ t
  

• Certain built-in functions are handled specially in byte-compiled code, avoiding the need for an ordinary function call. It is a good idea to use these functions rather than alternatives. To see whether a function is handled specially by the compiler, examine its byte-compile property. If the property is non-nil, then the function is handled specially.

  For example, the following input will show you that aref is compiled specially (see Array Functions) while assoc is not (see Sequence Functions):

            (get 'aref 'byte-compile)
                 ⇒ byte-compile-two-args
            
            (get 'assoc 'byte-compile)
                 ⇒ nil
  

• Teach yourself to tag functions with the special quote function (short form is #') instead of using an ordinary quote (short '). In byte compilation, the byte-compiler will look for function quotes and compile them. Ordinarily quoted function names will not be investigated nor byte compiled.

  The same applies to anonymous function objects (lambda lists) in non-obvious forms like backquoted macro bodies, or fset constructions in macros of the form (list 'lambda argname ...).

  On the other hand, if cheating the byte compiler is explicitly what you want emacs lisp will also accept the normal quote to refer to the function cell in most cases.

• If calling a small function accounts for a substantial part of your program's running time, make the function inline (use defsubst. This eliminates the function call overhead. Since making a function inline reduces the flexibility of changing the program, do not do it unless it gives a noticeable speedup in something slow enough that users care about the speed. See Inline Functions.
• Use iteration rather than recursion whenever possible. Function calls are slow in SXEmacs Lisp even when a compiled function is calling another compiled function.
• Using the primitive list-searching functions memq, member, assq, or assoc is even faster than explicit iteration. It may be worth rearranging a data structure so that one of these primitive search functions can be used.
• Use the profile library to profile your program. See the file profile.el for instructions.
• If absolute performance timings are of interest do not use profile.el since it redefines the investigated functions and hence slows down their execution. With a minimal overhead you can use the current-btime function to obtain an absolute time stamp with a microsecond resolution.

  For example we measure the execution time of nreverse for a list of 1000 random numbers.

            ;; initialise the list
            (setq lrn nil)
            (dotimes (i 1000)
              (setq lrn (cons (random) lrn)))
              ⇒ omitted
            
            ;; We use two time variables start and end.
            (progn
              (setq start (current-btime))
              (setq lrn (nreverse lrn))
              (setq end (current-btime)))
            
            ;; We investigate the time by simply subtracting start from end.
            (- end start)
              ⇒ 190
            
            ;; This means nreverse took about 190 microseconds.
            
            ;; The real value is highly likely a little less than this.
            ;; Let's measure the overhead.
            (progn
              (setq start (current-btime))
              (setq end (current-btime))
              (- end start))
              ⇒ 17
  

• Deliberate about your data representation.

  SXEmacs provides a multiplicity of types and containers to store your data. Which of them suits your purpose best depends on many things

  • Do you want to be compatible to other Emacs flavours?

    Well, in this case you should definitely have a glance at the flavour you want to be compatible with. Also see Compatibility Tips.

    If compatibility is not an issue, you can benefit in many cases from the large variety of dedicated data types in SXEmacs. See next item.

  • Do you want speed or comfort or flexibility or memory frugality?

    (Not yet prepared)

• Consider external tools. But think first!

  In some cases it is simply appropriate to use external tools. For example it does not make sense to re-implement the block zipping algorithm of bzip2 in elisp, instead create a process and call the bzip2 binary.

  However, if external tools provide a shared object library the process approach will highly likely be a snail compared to an FFI implementation. The advantage is simply that FFI calls will not create that much overhead because the available data is simply reused and does not need further steps, e.g. preparing the input data for the external tool, or parsing the results and converting them to lisp data, again.

  Again, think twice! An FFI implementation can waste a large amount of development time. Moreover, it requires a detailed knowledge of the internal concept of the external resource. It is very easy to crash your SXEmacs with FFI!

  A safer way of using external libraries in the way FFI does is to create an emodule. In very time critical scenarios, emodules may even outperform FFI because large parts of a procedure can run at C level without interacting with the lisp engine at all. You can even use all the usual advantages of C, such as parallelism, multi-threading and similar. Moreover, if desired you can wrap intermediate results to special containers (using dynacats), and pass these to the lisp level. Indeed, dynacats which escaped to the lisp level are safe since there is no way to access or modify them. Of course, you can envisage dedicated interface functions within your emodule but even in that case the actual processing will always take place in your emodule at C level.

  Anyway, emodules suffer from a serious drawback: they have to be built first in order to use them. In contrast, both the process approach and the FFI implementation just work out of the box.