diff options
| -rw-r--r-- | TODO | 2 | ||||
| -rw-r--r-- | flex.texi | 26 |
2 files changed, 23 insertions, 5 deletions
@@ -4,8 +4,6 @@ * the manual: -** revisit discussion of yylineno performance %% - ** clean up the faqs section. The information is good; the texinfo could use some touching up. @@ -2240,7 +2240,7 @@ cause a serious loss of performance in the resulting scanner. If you give the flag twice, you will also get comments regarding features that lead to minor performance losses. -Note that the use of @code{REJECT}, @code{%option yylineno}, and +Note that the use of @code{REJECT}, and variable trailing context (@pxref{Limitations}) entails a substantial performance penalty; use of @code{yymore()}, the @samp{^} operator, and the @samp{--interactive} flag entail minor performance penalties. @@ -2767,11 +2767,12 @@ which degrade performance. These are, from most expensive to least: @example @verbatim REJECT - %option yylineno arbitrary trailing context pattern sets that require backing up + %option yylineno %array + %option interactive %option always-interactive @@ -2780,7 +2781,7 @@ which degrade performance. These are, from most expensive to least: @end verbatim @end example -with the first three all being quite expensive and the last two being +with the first two all being quite expensive and the last two being quite cheap. Note also that @code{unput()} is implemented as a routine call that potentially does quite a bit of work, while @code{yyless()} is a quite-cheap macro. So if you are just putting back some excess text @@ -2789,6 +2790,25 @@ you scanned, use @code{ss()}. @code{REJECT} should be avoided at all costs when performance is important. It is a particularly expensive option. +There is one case when @code{%option yylineno} can be expensive. That is when +your patterns match long tokens that could @emph{possibly} contain a newline +character. There is no performance penalty for rules that can not possibly +match newlines, since flex does not need to check them for newlines. In +general, you should avoid rules such as @code{[^f]+}, which match very long +tokens, including newlines, and may possibly match your entire file! A better +approach is to separate @code{[^f]+} into two rules: + +@example +@verbatim +%option yylineno +%% + [^f\n]+ + \n+ +@end verbatim +@end example + +The above scanner does not incur a performance penalty. + @cindex patterns, tuning for performance @cindex performance, backing up @cindex backing up, example of eliminating |