16.1 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 50000000)
     ⇒ ("Wed Mar 11 21:10:19 2009"
         "Wed Mar 11 21:10:41 2009")  ; 22 seconds
     
     (byte-compile 'silly-loop)
     ⇒ [Compiled code not shown]
     
     (silly-loop 50000000)
     ⇒ ("Wed Mar 11 21:12:26 2009"
         "Wed Mar 11 21:12:32 2009")  ; 6 seconds

In this example, the interpreted code required 22 seconds to run, whereas the byte-compiled code required 6 seconds. These results are representative, but actual results will vary greatly.