> At least in Linux, C++11 atomics use pthreads (not the other way around).

I have no idea what you can possibly mean here.

Edit: Oh, you must have meant the stupid default for large atomic objects that just hashes them to an opaque mutex somewhere. An invisible performance cliff like this is not a useful feature, it's a useless footgun. I can't imagine anyone serious about performance using this thing (that's why I always static_assert() on is_always_lock_free() for my atomic types).