[Bug c++/53958] set_slot_part and canon_value_cmp using 90% of compile time

[aoliva at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53958

--- Comment #8 from Alexandre Oliva <aoliva at gcc dot gnu.org> ---
Jakub is right WRT onepart vs non-onepart vars.  Now, I can't think of any why
the union/intersection couldn't be done incrementally, and only for changed
incoming sets (but how would you tell an incoming set changed?).

IIRC, onepart and non-onepart sets are effective disjoint, even if they share
the same data structures, so when we perform union on one and intersection on
another we could avoid doing that repeatedly.