Date: Thu, 24 Sep 2009 13:48:34 -0700
From: Joe Marshall <jmarsh...@alum.mit.edu>
Has anyone measured the performance difference between smashing
the string head and just taking a substring? I'd bet it's a pretty minimal
improvement against the background of the other things that happen during
I/O.
I conducted a thoroughly unscientific comparison whose source code is
attached: making a string of length 2^(i + 1), taking a prefix of
length 2^i, and then discarding it, repeating the process ten thousand
times. Using SUBSTRING becomes slower than SET-STRING-MAXIMUM-LENGTH!
when i is about 5. This is in line with SUBSTRING-MOVE-{LEFT,RIGHT}!,
actually, which defer to the primitive as soon as the string's length
exceeds 2^5. Winning on just GC time starts to happen when i is about
10. (Your mileage may vary, of course. I ran this code compiled, by
the way.)
So how about the following definition?
(define (string-head! string end)
(guarantee-string string 'STRING-HEAD!)
(guarantee-substring-end-index end (string-length string) 'STRING-HEAD!)
(if (and (fix:< end (string-length string))
(or (fix:< end 32) ;32 computed by science (ha!).
(begin
;; Some implementations of the microcode don't
;; support SET-STRING-MAXIMUM-LENGTH!. In this case,
;; defer to %STRING-HEAD.
((ucode-primitive set-string-maximum-length! 2) string end)
(not (fix:= end (string-maximum-length string))))))
(%string-head string end)
string))
(declare (usual-integrations))
(define (test-substring-string-head prefix-length total-length)
(let loop ((i 0))
(if (fix:< i 10000)
(begin
(substring (make-string total-length) 0 prefix-length)
(loop (fix:+ i 1))))))
(define-syntax ucode-primitive
(sc-macro-transformer
(lambda (form environment)
environment ;environment
(apply make-primitive-procedure (cdr form)))))
(define (test-maxlength-string-head prefix-length total-length)
(let loop ((i 0))
(if (fix:< i 10000)
(begin
((ucode-primitive set-string-maximum-length! 2)
(make-string total-length)
prefix-length)
(loop (fix:+ i 1))))))
(define (compare-em)
(define (run-test name procedure)
(newline)
(write-string ";** ")
(pp name)
(do ((i 0 (fix:+ i 1)))
((fix:>= i 20))
;; Sorry for the ugly output. SHOW-TIME doesn't like having a
;; prefix on each line.
(fresh-line)
(pp i)
(gc-flip)
(show-time
(lambda ()
(procedure (fix:lsh 1 i) (fix:lsh 1 (fix:+ i 1))))))
(print-gc-statistics))
(run-test 'SUBSTRING test-substring-string-head)
(run-test 'SET-STRING-MAXIMUM-LENGTH! test-maxlength-string-head))
_______________________________________________
MIT-Scheme-devel mailing list
MIT-Scheme-devel@gnu.org
http://lists.gnu.org/mailman/listinfo/mit-scheme-devel
