When is stack space allocated for local variables?

Question

I have a question about the following C code:

void my_function()
{
    int i1;
    int j1;

    // Do something...

    if (check_something())
    {
        int i2;
        int j2;

        // Do something else...
    }

    // Do some more stuff...
}

Are there any guarantees about when stack space is allocated/deallocated for i2 and j2 or does it depend on the compiler? I would expect the stack pointer to be adjusted down when i2 and j2 come into scope and adjusted back up when they go out of scope, but then thought some compilers may just "optimize" the whole thing and account for variables in a nested scope when the function is first entered.

I know I can look at the assembly code generated by my compiler, but was wondering if the implementation can be left up to the compiler.

Thanks!

Answer 1

There are no guarantees.

Different optimization flags will likely result in different methods of saving variables.

The compiler can even make 1 or more variables not use the stack at all and keep them in registers for the whole duration of the function execution.

Answer 2

The compiler is free to do whatever it wants, as long as the semantics of the language are reserved. In other words, i2 and j2 can be bound to memory places before the execution reaches the entry point of their block, and can be unbounded any time as long as that doesn't affect the semantics of your code.