[Bug libstdc++/65641] unordered_map - __detail::_Mod_range_hashing is slow

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65641

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-04-02
                 CC|                            |fdumont at gcc dot gnu.org
     Ever confirmed|0                           |1
           Severity|normal                      |enhancement

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jens Breitbart from comment #2)
> Another possible solution would be to allow the number of buckets to be a
> power of two, as one can easily compute the mask for such cases. This could
> be triggered by the user explicitly calling rehash() with a power of two as
> the parameter. Increasing the number of buckets would only increase to
> another power of two. _Mod_range_hashing could check if the number of
> buckets is a power of two and use masking in that case. This would not
> require an ABI change.

That sounds promising, and worth pursuing.

> Any chance of getting such a change upstream?

I don't see why not, although unless you have a GCC copyright assignment on
file, or plan to get one (immediately, since it can take a while) it's better
*not* to give us a patch, because we can't use it anyway and there can be no
danger of using your code if we don't see it!

> As far as I can see, there
> seems to be no easy way to have the unorered_map use our folding functor
> instead of _Mod_range_hashing or am I missing something?

I think you would need to use the _Hastable class template directly, rather
than via std::unordered_map. In theory that allows you to re-use the internals
with different policies, but in practice it's not very easy.