examples/fastwc/README - master - niceSOFT/flex

examples/fastwc/README
56 строк2 KB
Will Estes
re-add these files
25 июл 2001, 00:23
25 июл 2001, 00:235fd717c
This directory contains some examples illustrating techniques for extracting
high-performance from flex scanners.  Each program implements a simplified
version of the Unix "wc" tool: read text from stdin and print the number of
characters, words, and lines present in the text.  All programs were compiled
using gcc (version unavailable, sorry) with the -O flag, and run on a
SPARCstation 1+.  The input used was a PostScript file, mainly containing
figures, with the following "wc" counts:

	lines  words  characters
	214217 635954 2592172


The basic principles illustrated by these programs are:

	- match as much text with each rule as possible
	- adding rules does not slow you down!
	- avoid backing up

and the big caveat that comes with them is:

	- you buy performance with decreased maintainability; make
	  sure you really need it before applying the above techniques.

See the "Performance Considerations" section of flexdoc for more
details regarding these principles.


The different versions of "wc":

	mywc.c
		a simple but fairly efficient C version

	wc1.l	a naive flex "wc" implementation

	wc2.l	somewhat faster; adds rules to match multiple tokens at once

	wc3.l	faster still; adds more rules to match longer runs of tokens

	wc4.l	fastest; still more rules added; hard to do much better
		using flex (or, I suspect, hand-coding)

	wc5.l	identical to wc3.l except one rule has been slightly
		shortened, introducing backing-up

Timing results (all times in user CPU seconds):

	program	  time 	 notes
	-------   ----   -----
	wc1       16.4   default flex table compression (= -Cem)
	wc1        6.7   -Cf compression option
	/bin/wc	   5.8	 Sun's standard "wc" tool
	mywc	   4.6   simple but better C implementation!
	wc2	   4.6   as good as C implementation; built using -Cf
	wc3	   3.8   -Cf
	wc4	   3.3   -Cf
	wc5	   5.7   -Cf; ouch, backing up is expensive