hashtable optimization

[Jonathan Wakely]

On 18/02/15 10:35 +0100, François Dumont wrote:
>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.

I'm nervous about this kind of change at this stage. The patch looks
right, but are you sure you've covered *every* case where the cached
value needs to be updated?

Since this only affects performance, not correctness, it might be hard
to convince the release managers it is necessary during stage 4, and
I'm not entirely convinced myself either.