Why does the compiler reserve a little stack space but not the whole array size?

Question

The following code

int main() {
  int arr[120];
  return arr[0];
}

Compiles into this:

  sub     rsp, 360
  mov     eax, D         


        

Answer 1

Below the stack area used by a function, there is a 128-byte red zone that is reserved for program use. Since main calls no other function, it has no need to move the stack pointer by more than it needs, though it doesn't matter in this case. It only subtracts enough from rsp to ensure that the array is protected by the red zone.

You can see the difference by adding a function call to main

int test() {
  int arr[120];
  return arr[0]+arr[119];
}

int main() {
  int arr[120];
  test();
  return arr[0]+arr[119];
}

This gives:

test:
  push rbp
  mov rbp, rsp
  sub rsp, 360
  mov edx, DWORD PTR [rbp-480]
  mov eax, DWORD PTR [rbp-4]
  add eax, edx
  leave
  ret
main:
  push rbp
  mov rbp, rsp
  sub rsp, 480
  mov eax, 0
  call test
  mov edx, DWORD PTR [rbp-480]
  mov eax, DWORD PTR [rbp-4]
  add eax, edx
  leave
  ret

You can see that the main function subtracts by 480 because it needs the array to be in its stack space, but test doesn't need to because it doesn't call any functions.

The additional usage of array elements does not significantly change the output, but it was added to make it clear that it's not pretending that those elements don't exist.