Following latest fixes in std::inplace_merge and std::stable_sort you 
propose Jonathan to enhance branching in the first.

Here is a proposal based on yours to do so in both algos.

     libstdc++: Enhance branching in std::inplace_merge and std::stable_sort

     libstdc++-v3/ChangeLog:

             * include/bits/stl_algo.h
             (__merge_adaptive): Adapt to merge only when buffer is 
large enough..
             (__merge_adaptive_resize): New, adapt merge when buffer is 
too small.
             (__inplace_merge): Adapt, use latter.
             (__stable_sort_adaptive): Adapt to sort only when buffer is 
large enough.
             (__stable_sort_adaptive_resize): New, adapt sort when 
buffer is too small.
             (__stable_sort): Adapt, use latter.

Tested under Linux x64.

Ok to commit ?

François