Works on rustc 1.27.0-nightly (fb44b4c0e 2018-04-04)

rdtsc.rs

#![feature(asm)]
fn rdtsc() -> u64 {

    let eax: u32;
    let edx: u32;

    unsafe {
        asm!("rdtsc; ": /* Assembly */
             "={eax}"(eax), "={edx}"(edx) : /* Output registers */
             : /* Input registers */
             "eax", "edx" /* Clobbers */ );
    }

    ((edx as u64) << 32) | (eax as u64)
}

fn main() {
    let start = rdtsc();
    println!("Hello, world!");
    let stop = rdtsc();

    let diff = stop - start;

    println!("Took {}", diff);
}