SSA works great for keeping track of the old/newness of local variables inside a single function, but it doesn't really help with tracking old/newness of state

SSA works great for keeping track of the old/newness of local variables inside a single function, but it doesn't really help with tracking old/newness of state variables (globals, heap values, stuff on the stack that persists through multiple calls to F) across function calls. You could in theory literally represent your program as a single giant function F that stores the entire program state as local variables and then apply SSA to that, but it would probably be a mess.