Question

How does an optimizing c++ compiler determine when a stack slot of a function(part of stack frame of a function) is no longer needed by that function, so it can reuse its memory? .
By stack slot I mean a part of stack frame of a function, not necessarily a whole stack frame of a function and an example to clarify the matter is, suppose we have a function that has six integer variables defined in its scope, when it’s time to use sixth variable in the function, fifth variable’s become useless so compiler can use same memory block for fifth and sixth variables.
any information on this subject is appreciated.

Answer 1

The easy part is: When a function exits, all local variables of that function are released. Thus, function exit indicates that the whole stack frame can be freed. That’s a no-brainer, though, and you wouldn’t have mentioned “optimizing compiler” if that’s what you were after.

In theory, a compiler can do flow analysis on a function, find out which chunks of memory are used at what time, and perhaps even re-order stack allocation based on the order in which variables become available. Then, if new automatic variables are introduced somewhere in the middle of the function or other scope nested within the function (rather than at its beginning), those recently freed slots could be re-used.

In practice, this sounds like a lot of spinning gears, and I suspect that stack is simply allocated whenever variables come in scope and popped off en block by decrementing the stack pointer when the scope finishes. But I admit I’m no expert on this topic. Someone with more authoritative knowledge may come along and correct me.