fib-optimized.c

#include <stdio.h>
#include <inttypes.h>

uint32_t fib(int n)
{
  register uint32_t r = 0;

  __asm__(
    "mov $1, %%ebx\n\t"
    ".lp%=:\n\t"
    "  xaddl  %%ebx, %0\n\t"
    "  sub    $1, %1\n\t"
    "  testl  %1, %1\n\t"
    "  jnz    .lp%="

    : "+r" (r),
      "+r" (n)
  );

  return r;
}

int main(void)
{
  int x;

  scanf("%d", &x);
  printf("fib(%d) = %" PRIu32 "\n", x, fib(x));

  return 0;
}