You iterate over the program (every individual function, really) backwards. A use of a variable means that it is "live" before that point; a definition (i.e., a

You iterate over the program (every individual function, really) backwards. A use of a variable means that it is "live" before that point; a definition (i.e., a write into) a variable means that it is "dead" before that point. That is, at any point, a variable being "live" means that its value at that point may be used in the future. Liveness is especially important for register allocation: If two variables are both live at some program point (and cannot be proved to have the same value), the compiler must place them in different registers or stack slots. As an aside, liveness is also useful for some other things. For example, a variable that is live at the start of a function is one that may be used without being initialized, and the compiler can emit a warning for it. https://en.wikipedia.org/wiki/Live_variable_analysis https://en.wikipedia.org/wiki/Live_variable_analysis Edit: BTW, it really is "liveness", not "liveliness".