> When variables are allocated on the stack, the space for them with all compilers I have ever used (or written) is allocated on the stack once by adjusting the stack pointer - i.e. for all of the variables of the function.

it's not a given

    int algo(int*, int n);
    int computation(int i, int n) {
        int alloc[n];
        return algo(alloc, n);    
    }

on some compilers (Keil for instance afair), alloc will be heap-allocated (and freed when leaving the function) even if it's "automatic storage" (what most people call the stack).