https://github.com/mongodb/mongo/blob/master/src/mongo/db/sorter/sorter.cpp#L768

In C++03, the initialization of function statics is not thread-safe (it is in C++11). While some compilers, notably GCC, actually implement thread-safe statics in C++03 mode by default unless asked not to, other compilers do not.

On such platforms, it could be possible for the atomic to be initialized twice, probably leading to inconsistent results.