| 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 |
