compiler: fix lazy DFA false quits on ASCII text #768
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One of the things the lazy DFA can't handle is Unicode word boundaries,
since it requires multi-byte look-around. However, it turns out that on
pure ASCII text, Unicode word boundaries are equivalent to ASCII word
boundaries. So the DFA has a heuristic: it treats Unicode word
boundaries as ASCII boundaries until it sees a non-ASCII byte. When it
does, it quits, and some other (slower) regex engine needs to take over.
In a bug report against ripgrep[1], it was discovered that the lazy DFA
was quitting and falling back to a slower engine even though the
haystack was pure ASCII.
It turned out that our equivalence byte class optimization was at fault.
Namely, a '{' (which appears very frequently in the input) was being
grouped in with other non-ASCII bytes. So whenever the DFA saw it, it
treated it as a non-ASCII byte and thus stopped.
The fix for this is simple: when we see a Unicode word boundary in the
compiler, we set a boundary on our byte classes such that ASCII bytes
are guaranteed to be in a different class from non-ASCII bytes. And
indeed, this fixes the performance problem reported in [1].
[1] - BurntSushi/ripgrep#1860
BurntSushi
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May 1, 2021
This brings in a performance bug fix, merged in rust-lang/regex#768. Fixes #1860.
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One of the things the lazy DFA can't handle is Unicode word boundaries,
since it requires multi-byte look-around. However, it turns out that on
pure ASCII text, Unicode word boundaries are equivalent to ASCII word
boundaries. So the DFA has a heuristic: it treats Unicode word
boundaries as ASCII boundaries until it sees a non-ASCII byte. When it
does, it quits, and some other (slower) regex engine needs to take over.
In a bug report against ripgrep[1], it was discovered that the lazy DFA
was quitting and falling back to a slower engine even though the
haystack was pure ASCII.
It turned out that our equivalence byte class optimization was at fault.
Namely, a '{' (which appears very frequently in the input) was being
grouped in with other non-ASCII bytes. So whenever the DFA saw it, it
treated it as a non-ASCII byte and thus stopped.
The fix for this is simple: when we see a Unicode word boundary in the
compiler, we set a boundary on our byte classes such that ASCII bytes
are guaranteed to be in a different class from non-ASCII bytes. And
indeed, this fixes the performance problem reported in [1].
[1] - BurntSushi/ripgrep#1860