Tips for Making Compiled Code Fast

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

• Use the `profile' library to profile your program. See the file `profile.el' for instructions.
• 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 may 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 section Functions that Operate on Arrays) while elt is not (see section Sequences):

  (get 'aref 'byte-compile)
       => byte-compile-two-args
  
  (get 'elt 'byte-compile)
       => nil
  

• 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 section Inline Functions.

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