c - GCC optimization of iterative functions -

i have following code fibonacci both recursive , iterative versions:

#include <stdio.h>  typedef long long int;  long long recursive (long long i) {     if (i == 0) return 0;     if (i == 1) return 1;     return recursive (i-1) + recursive (i-2); }   long long iterative (long long i) {     int counter = i-1;     int fib1 = 0;     int fib2 = 0;      // first iteration     fib1 = 0;     fib2 = 1;      while (counter > 0) {         int temp1 = fib1;         int temp2 = fib2;         fib1 = fib2;         fib2 = temp1 + temp2;          counter--;     }  }  int main (int argc, char **argv) {      printf("result: %lli\n", iterative(10));      return 0; } 

i tried compiling gcc -o2 optimization see if recursion perform better iteration, noticed interesting occurrence: when compiled -o2, iterative function outputs 0 while if it's compiled without flag, outputs proper number.

gcc -o2 fibonacci.c -o fib && ./fib: result: 0

gcc fibonacci.c -o fib && ./fib: result: 55

recursion slower iterative or tail recursion version (which gets optimized iterative version). examples of both in thread:

fibonacci computation time