c - How is stack memory consumption calculated?

Question

I need to calculate the stack memory consumption of my program. gcc's -fstack-usage only calculates the stack usage of function, but does not include an additional function call in that function as far as I understand.

void test1(){
    uint32_t stackmemory[100];
    function1();                    //needs aditional stack, say 200 Bytes
    uint32_t stackmemory2[100];
}

void test2(){
    uint32_t stackmemory[100];
    uint32_t stackmemory2[100];
    function1();                   //needs additional stack, say 200 Bytes
}

Which test() function uses less stack? I would say test1(), as the stack is freed after the function1() call. Or does this depend on the optimization level -Os/-O2...?

Does the compiler allocate memory in test1() for all its static variables, as soon as the function is entered? Or is stackmemory2[100] allocated when the line is reached?

Answer 1

In general you need to combine call-graph information with the .su files generated by -fstack-usage to find the deepest stack usage starting from a specific function. Starting at main() or a thread entry-point will then give you the worst-case usage for that thread.

Helpfully the work to create such a tool has been done for you as discussed here, using a Perl script from here.

ARM's armcc compiler (as used in Keil ARM-MDK) has this functionality built-in and can include detailed stack analysis in the link map, including the worst-case call path and warnings of non-deterministic stack usage (due to recursion for example).

In my experience observing the behaviour of several compilers, the stack-frame is typically defined for the lifetime of the function regardless of the lifetime and scope of the variables declared. So the two versions in that case will not differ. Without declaring them volatile the optimiser will likely remove both arrays in any event. However you should not rely on any observations in this respect being universal - it implementation rather then language defined.