libstdc++: Optimize std::regex_traits lookup functions

Optimize regex_traits::lookup_collatename and
regex_traits::lookup_classname.

For lookup_collatename we can hoist the static array into a
non-dependent function, then call that for the regex_traits<char>
specialization without performing any narrowing operations via the ctype
facet. For the regex_traits<wchar_t> specialization we populate a
std::string for the narrowed result, and call the non-dependent function.

For lookup_classname we can avoid the static array entirely, replacing
the iteration over that array with a nested switch that implements a
kind of manually-unrolled trie, only match one char at a time until we
either match a full string or get a mismatch. This avoids narrowing the
entire input and storing it in a temporary string. This improves
performance by 2-3x for -O2 and below (the benefit is much smaller for
-O3).

We can also check the input length for random access iterators, and
reject any strings longer than "xdigit" without doing any work at all.
For silly cases like [:xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx:]
this gives a 100x improvement.

libstdc++-v3/ChangeLog:

	* include/bits/regex.tcc (__detail::__lookup_collatename): New
	function.
	(regex_traits::lookup_collatename): Use new function. Elide
	redundant narrowing via ctype facet for regex_traits<char>.
	(regex_traits::lookup_classname): Replace lookup table with
	handwritten prefix match.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

0 files changed, 0 insertions, 0 deletions