[patch 1/N] std::regex refactoring - _BracketMatcher

[Jonathan Wakely]

Hi,

I've been looking through the regex code and have a few ideas for
simplifications or optimisations that I'd like to share.

This first patch is for _BracketMatcher. We only use std::bitset when
is_same<_CharT, char> so 8 * sizeof(_CharT) should be __CHAR_BIT__
instead. We also only user _UnsignedCharT when is_same<_CharT, char>
so that can just be simplified to unsigned char.

The contents of _BracketMatcher::_M_char_set are not sorted and can
contain duplicates in the current code. Making that a sorted, unique
list in _BracketMatcher::_M_ready() allows a binary search instead of
linear search. This improves worst case performance for pathological
regular expressions like std::wregex('['+std::wstring(1000, 'a')+"b]")
but I'm not sure if it helps in the common case.

Finally, in the non-char case the _CacheT member is an unused empty
object, so having that as the first member requires 7 bytes of
padding. Re-ordering the members reduces the size of a non-char
_BracketMatcher by 8 bytes (but it's still a whopping 96 bytes).

(For a char _BracketMatcher the bitset cache makes it 128 bytes,
this patch doesn't change that).

Thoughts?


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