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D.4 Tips for Making Compiled Code Fast

Here are ways of improving the execution speed of byte-compiled Lisp programs.

  • Profile your program, to find out where the time is being spent. See Profiling.

  • Use iteration rather than recursion whenever possible. Function calls are slow in Emacs 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 can be worth rearranging a data structure so that one of these primitive search functions can be used.

  • 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 Functions that Operate on Arrays):

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

    Note that in this case (and many others), you must first load the bytecomp library, which defines the byte-compile property.

  • If calling a small function accounts for a substantial part of your program's running time, make the function inline. This eliminates the function call overhead. Since making a function inline reduces the flexibility of changing the program, don't do it unless it gives a noticeable speedup in something slow enough that users care about the speed. See Inline Functions.