Optimization example

main.rs

#![feature(asm)]

use std::env::args;

#[cfg(target_arch = "x86_64")]
fn main() {
    match args().nth(1).unwrap().as_ref() {
        "a" => fa(),
        "b" => fb(),
        x => panic!("unknown arg: {}", x),
    }
}

fn fa() {
    println!("Starting!");
    let mut x = 0u32;
    for i in 0..1_000_000_000u64 {
        if i % 3 == 0 {
            x += 1;
        }
    }
    println!("{}", x);
    println!("Done!");
}

fn fb() {
    println!("Starting 2!");
    let mut x = 0u32;
    for i in 0..1_000_000_000u64 {
        unsafe {
            // ASM Equivelent naively
            asm!("mov rdx, 0
                  mov rcx, 3
                  div rcx
                  cmp rdx, 0
                  jne .out
                  add rbx, 1
            .out:
                  nop" : "={rbx}"(x) : "{rax}"(i), "{rbx}"(x) : "rcx", "rax", "rbx", "rdx", "cc" : "intel");
        }

    }
    println!("{}", x);
    println!("Done 2!");
}

#[cfg(not(target_arch = "x86_64"))]
fn main() {
    println!("Unsupported target");
}