[Bug libstdc++/50160] vector<bool> comparison very slow (no specialisation)

["marc.glisse at normalesup dot org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50160

--- Comment #7 from Marc Glisse <marc.glisse at normalesup dot org> 2011-08-23 18:35:57 UTC ---
If I understand correctly, operator< is supposed to give a lexicographic order,
and vector<bool> stores {true,false,false} as 1 and {false,false,true} as 4, so
we can't just make operator< compare chunks. Reversing the order of the bits in
an unsigned long is a bit complicated, although it would still lead to code
faster than the current (easiest would be to compare byte by byte, and mirror
each byte using a table).

But you don't seem particularly interested in using the lexicographic order, is
that true? In that case you could try and specify some other order to std::map,
say comparing hashes ;-) I understand that the lexicographic order of the
chunks would be a fine order, but I don't think it can be exposed. I haven't
thought about the potential drawbacks of implementing vector<bool> backwards.