I recently played around with what I call "manual tail-call optimization": transform a tail call to a goto to the beginning of the function. Check it out: https://godbolt.org/z/3fY1v1oeW

  int factorial_loop_iterative(int n, int a){
    while(n > 0){
      a = a * n;
      n = n - 1;
    }
    return a;
  }
  
  int factorial_loop_recursive(int n, int a){
    if(n > 0){
      return factorial_loop_recursive(n - 1, a * n);
    }else{
      return a;
    }
  }
  
  int factorial_loop_manual(int n, int a){
  tailcall:
    if(n > 0){
      a = a * n;
      n = n - 1;
      goto tailcall;
    }else{
      return a;
    }
  }
  
  int (*factorial_loop)(int n, int a) = factorial_loop_manual;
  
  int factorial(int n){
    return factorial_loop(n, 0);
  }

I recommend against, of course! Incorrectly sequencing the manual version results in bugs (swap the assignment for n and a), which the recursive version doesn't need to care about.