[cmucl] #81: Reversing a string is slow

[cmucl]

#81: Reversing a string is slow
--------------------+-------------------------------------------------------
 Reporter:  rtoy    |       Owner:  somebody
     Type:  defect  |      Status:  new     
 Priority:  major   |   Milestone:          
Component:  Core    |     Version:  2013-05 
 Keywords:          |  
--------------------+-------------------------------------------------------
 Consider this:
 {{{
 (defparameter *s* (make-string 1000000))
 (defun time-rev (s)
   (dotimes (k 100 nil)
     (declare (fixnum k))
     (setf s (reverse s)))
   s)
 (compile 'time-rev)
 (time (prog1 t (time-rev *s*)))
 ; Evaluation took:
 ;   10.23 seconds of real time
 ;   10.151641 seconds of user run time
 ;   0.079628 seconds of system run time
 ;   31,310,669,055 CPU cycles
 ;   [Run times include 0.08 seconds GC run time]
 ;   0 page faults and
 ;   200,114,224 bytes consed.
 }}}

 Since strings are basically arrays of unsigned 16-bit integers, compare
 the time when reversing an array:
 {{{
 (defparameter *v* (make-array 1000000 :element-type '(unsigned-byte 16)))
 (time (prog1 t (time-rev *v*)))
 ; Evaluation took:
 ;   3.62 seconds of real time
 ;   3.581727 seconds of user run time
 ;   0.039302 seconds of system run time
 ;   11,089,005,122 CPU cycles
 ;   [Run times include 0.05 seconds GC run time]
 ;   0 page faults and
 ;   200,116,120 bytes consed.
 }}}

 Reversing a string is 3 times slower.  There is some cost since strings
 are utf-16 encoded, but we shouldn't be 3 times slower.

-- 
Ticket URL: <http://trac.common-lisp.net/cmucl/ticket/81>
cmucl <http://common-lisp.net/project/cmucl>
Cmucl is a high-performance, free Common Lisp implementation.