hashtable optimization

[François Dumont]

Hello

     I am still studying hashtable performances and especially how to 
reduce overhead compared to tr1 implementation. Most of the overhead is 
coming from the additional modulo operations required with the new data 
model. Having a closer look at PR 57885 bench I realized that we can 
quite easily avoid an important modulo operation in 
_M_insert_bucket_begin thanks to an additional std::size_t in the container.

     The patch is quite straightforward, it optimizes insertion of a 
node in an empty bucket which is the most important kind of insertion as 
long as hash functor is doing a good job as per default we should have 
at most 1 element per buckets:

Without patch:
Container:std::unordered_map<int,int>  Key:int
Insertion: 1106 671 634 634 635  min=634 max=1106

Container:std::tr1::unordered_map<int,int>  Key:int
Insertion: 885 491 487 490 511  min=487 max=885

With patch:
Container:std::unordered_map<int,int>  Key:int
Insertion: 1099 581 583 584 592  min=581 max=1099

Container:std::tr1::unordered_map<int,int>  Key:int
Insertion: 889 491 519 492 493  min=491 max=889

     I prefer to propose it now because it impacts ABI.

2015-02-19  François Dumont  <fdumont@gcc.gnu.org>

     * include/bits/hashtable.h (_Hashtable<>::_M_bbegin_bkt): New, bucket
     pointing to _M_before_begin.
     (_Hashtable<>): Maintain and use later.

Tested under Linux x86_64.

François