PR 54075 Restore 4.6 growth factor

[François Dumont]

Hi

     Here is the patch to restore the 4.6 growth factor of 2. I prefer 
to validate the restored behavior by adding a performance test. Without 
the patch the result was:

unordered_set.cc             unordered_set 10000000 insertions  403r  
329u   73s 402825280mem    0pf

after the patch:

unordered_set.cc             unordered_set 10000000 insertions  112r   
86u   25s 402825104mem    0pf

It validates the 3x times performance hint.

Tested under Linux x86_64.

2012-07-28  François Dumont  <fdumont@gcc.gnu.org>

     PR libstdc++/54075
     * include/bits/hashtable_policy.h
     (_Prime_rehash_policy::_M_next_bkt): Add a growth factor set to 2
     to boost growth in the number of buckets.
     * testsuite/performance/23_containers/insert/unordered_set.cc: New.

Even if it is not a Standard conformity issue I think we can apply it to 
the 4.7 branch too.

François

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