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diff --git a/examples/fastwc/README b/examples/fastwc/README new file mode 100644 index 0000000..0dd3afe --- /dev/null +++ b/examples/fastwc/README @@ -0,0 +1,56 @@ +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 |