> gcc doesn't know anything special about rdtsc. We would have to see the > code to understand the difference. My bad here, sorry. My experiment was incorrect. I rechecked it and rdtsc is indeed treated the same way as rdrand. > For your example, the second rdrand instruction is removed by the > common-subexpression-elimination pass. By default gcc assumes that an > asm instruction reads nothing other than its inputs and changes only its > outputs. By that reasoning, your second rdrand instruction will produce > exactly the same result as your first one, so there is no need for it, > so gcc eliminates it. The volatile qualifier tells gcc that the asm > instruction does something which is not explicitly expressed in the > inputs and outputs. Thank you Ian, Georg and Segher for the helpful explanations - its clear now.
