[Bug libstdc++/54075] [4.7.1] unordered_map 3x slower than 4.6.2

["fdumont at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54075

--- Comment #9 from FranÃois Dumont <fdumont at gcc dot gnu.org> 2012-07-24 20:15:10 UTC ---
I confirm that the reserve method is broken. I had correctly handle the size
hint that can be given through the hashtable constructor, I set
_M_rehash_policy._M_prev_resize to 0 just to avoid the container to be shrink
until it reaches the targetted size. The same thing should be done in the
rehash method that is called from reserve one. I will submit a patch soon.

Regarding the shrink on insert calls, well, it can be easily avoided by calling
(normally) reserve so I considered that it is better to offer a container with
a symetric behavior.

I will also I think reconsider the grow factor. During one of my propositions
of hashtable redesign the number of empty buckets was a problem for
performance. With the current implementation it is not a problem anymore so we
could grow a little bit faster.