upgrade to aho-corasick 0.7 #566
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No need to build the benchmark suite 4 times.
This is a "dumb" update in that we retain exactly the same functionality as before.
This adds a couple new methods on HIR expressions for determining whether they are literals or not. This is useful for determining whether to apply optimizations such as Aho-Corasick without re-analyzing the syntax.
Finally, if a regex is just `foo|bar|baz|...|quux`, we will now use plain old Aho-Corasick. The reason why we weren't doing this before is because Aho-Corasick didn't support proper leftmost-first match semantics. But since aho-corasick 0.7, it does, so we can now use it as a drop-in replacement. This basically fixes a pretty bad performance bug in a really common case, but it is otherwise really hacked. First of all, this only happens when a regex is literally `foo|bar|...|baz`. Even something like `foo|b(a)r|...|baz` will prevent this optimization from happening, which is a little silly. Second of all, this optimization only kicks in after we've compiled the full pattern, which adds quite a bit of overhead. Fixing this isn't trivial, since we may need the compiled program to resolve capturing groups. The way to do this is probably to specialize compilation for certain types of expressions. Maybe. Anyway, we hack this in for now, and punt on further improvements until we can really re-think how this should all work.
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We also take this opportunity to add an optimization I've long wanted to add: cause regexes like
foo|bar|baz|...|quuxto just dispatch to Aho-Corasick instead of using the lazy DFA, which can be quite a bit slower for a large dictionary (such as/usr/share/dict/words). The optimization is fairly brittle, but it's a somewhat common case. We couldn't do this before because Aho-Corasick didn't support leftmost-first match semantics, so it couldn't serve as a drop-in replacement for a regex alternation.