> So each time an uninitialized variable gets used, we can just use any machine register—and for different uses, those can be different registers! So, one time we “look” at x it can be at least 150, and then when we look at it again it is less than 120, even though x did not change. x was just uninitialized all the time.

Is this actually true? I thought these would just be poisoned and then the optimizer would just do whatever it liked in the presence of undefined behavior (like optimize the function to return true).