Calculating CPU frequency in C with RDTSC always returns 0

Question

The following piece of code was given to us from our instructor so we could measure some algorithms performance:

#include 
#include 

        

Answer 1

I can't say for certain what exactly is wrong with your code, but you're doing quite a bit of unnecessary work for such a simple instruction. I recommend you simplify your rdtsc code substantially. You don't need to do 64-bit math carries your self, and you don't need to store the result of that operation as a double. You don't need to use separate outputs in your inline asm, you can tell GCC to use eax and edx.

Here is a greatly simplified version of this code:

#include 

uint64_t rdtsc() {
    uint64_t ret;

# if __WORDSIZE == 64
    asm ("rdtsc; shl $32, %%rdx; or %%rdx, %%rax;"
        : "=A"(ret)
        : /* no input */
        : "%edx"
    );
#else
    asm ("rdtsc" 
        : "=A"(ret)
    );
#endif
    return ret;
}

Also you should consider printing out the values you're getting out of this so you can see if you're getting out 0s, or something else.