> NaN is unordered: it is not equal to, greater than, or less than anything, including itself. x == x is false if the value of x is NaN [0]

My read of this is that comparisons involving NaN on either side always evaluate to false.

In the first one if X or Y is NaN then you'll get do_something_else, and in the second one you'll get do_something.

As far as why one order would be more optimal than the other, I'm not sure. Maybe something to do with branch prediction?

[0] https://www.gnu.org/software/libc/manual/html_node/Infinity-...