With higher optimization levels (-O2 or higher), code involving both protected calls and loops can result in an internal error indicating that the program "ran out of registers".

Code that will trigger this error is structured similarly to:

void prot_function() __attribute__((c29_protected_call));
void loop_function(int n) {
  prot_function();
  for (int i = 0; i < n; i++) {
    ...
  }
}

Where the 'for' loop can be any loop concept, but importantly will always use 'n'.