I genuinely forgot to specify the type of the function argument, but that's not considered an error in C which will default to int when the type is omitted. If the compiler you're using objects to that it's probably for the better, but it's still legal C.
There is no undefined behavior in the example. It's meant to further illustrate my point that C is not a light wrapper around memory and registers. In the example, it's just a deceptively imperative looking way of describing the return value to the optimizer.
The compiler in this case was gcc with -O3.
For an example of behavior that is straight forward in the machine but is undefined in C, try signed integer overflow. Intuitively on x64 it should behave just like ADD, possibly adjust the carry and overflow flags and wrap around. In C, the behavior is entirely undefined. Some operation that results in UB is low hanging fruit for the optimizer. If it can deduce that the operation will result in UB, in this case signed integer overflow, it might omit the operation altogether.
