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Re: skip-chars-forward v. re-search-forward
From: |
Kenichi Handa |
Subject: |
Re: skip-chars-forward v. re-search-forward |
Date: |
Thu, 24 Apr 2003 10:41:33 +0900 (JST) |
User-agent: |
SEMI/1.14.3 (Ushinoya) FLIM/1.14.2 (Yagi-Nishiguchi) APEL/10.2 Emacs/21.2.92 (sparc-sun-solaris2.6) MULE/5.0 (SAKAKI) |
In article <E19126X-0005aH-00@fencepost.gnu.org>, Richard Stallman
<rms@gnu.org> writes:
> I've tended to use skip-chars-forward instead of re-search-forward
> since it's byte-coded and I assumed it's more efficient. However, I
> made a measurement and found that skip-chars-forward was actually
> slower than re-search-forward.
> That is really surprising. Can you figure out what makes
> skip-chars-forward so slow? We ought to be able to make it faster
> than re-search-forward without much effort, so we may as well try.
I've just installed a fix for skip-chars-forward. I think
the reason of the slowness was that it uses FETCH_CHAR
naively. I changed the code to use the common technique of
*p, *stop, *endp.
The attached benchmark code (using a 25M-byte file that
doesn't contain "%" nor syntax `"') shows that now
skip-chars-forward/backward is slightly faster than
re-search-forward/backward.
(search-forward "%") 0.561530
(skip-chars-forward "^%") 1.174494
(re-search-forward "[%]") 1.239955
(skip-syntax-forward "^\"") 2.712070
(re-search-forward "\\s\"") 26.080168
(search-backward "%") 0.992879
(skip-chars-backward "^%") 3.079179
(re-search-backward "[%]") 10.005354
It's still surprising that search-forward/backward is more
than twice faster. I have no idea what kind of magic the
boyer-moore search is using.
I tried it on GNU/Linux (devian). Could someone please try
it on the different system?
---
Ken'ichi HANDA
handa@m17n.org
(defun benchtest ()
(require 'benchmark)
(save-excursion
(let ((coding-system-for-read 'utf-8))
(set-buffer
(find-file-noselect "/project/mule/UNIDATA/Unihan-3.2.0.txt")))
;; It is sure that the buffer doesn't contain "%" nor syntax `"'.
(concat
(format "%-30S %f\n" '(search-forward "%")
(car (benchmark-run
10 (progn (goto-char 1)
(search-forward "%" nil 'move)))))
(format "%-30S %f\n" '(skip-chars-forward "^%")
(car (benchmark-run
10 (progn (goto-char 1)
(skip-chars-forward "^%")))))
(format "%-30S %f\n" '(re-search-forward "[%]")
(car (benchmark-run
10 (progn (goto-char 1)
(re-search-forward "[%]" nil 'move)))))
(format "%-30S %f\n" '(skip-syntax-forward "^\"")
(car (benchmark-run
10 (progn (goto-char 1)
(skip-syntax-forward "^\"")))))
(format "%-30S %f\n" '(re-search-forward "\\s\"")
(car (benchmark-run
10 (progn (goto-char 1)
(re-search-forward "\\s\"" nil 'move)))))
(format "%-30S %f\n" '(search-backward "%")
(car (benchmark-run
10 (progn (goto-char (point-max))
(search-backward "%" nil 'move)))))
(format "%-30S %f\n" '(skip-chars-backward "^%")
(car (benchmark-run
10 (progn (goto-char (point-max))
(skip-chars-backward "^%")))))
(format "%-30S %f\n" '(re-search-backward "[%]")
(car (benchmark-run
10 (progn (goto-char (point-max))
(re-search-backward "[%]" nil 'move))))))))