rayon_but_why.rs

#[macro_use]
extern crate bencher;
extern crate rayon;
use rayon::prelude::*;
use bencher::Bencher;

fn sieve(bench: &mut Bencher) {
    bench.iter(|| {
        let max = 10_000 as usize;
        // This mut is more of an annotation than anything
        // It's not required, but we're going to mutate it...
        let mut is_prime = vec![true; (max + 1) / 2 as usize];
        let mut j_vec = Vec::new();

        let mut i = 3;
        while i < ((max + 1) as f64).sqrt() as usize {
            if is_prime[i / 2] {
                let mut j = i * i;
                // First we need some data for rayon to iterate over
                j_vec.clear();
                while j <= max {
                    j_vec.push(j);
                    j = j + 2 * i;
                }
                j_vec.par_iter().for_each(|j| {
                    // The writes to is_prime probably qualify as a data race, but we don't care!
                    // No seriously, we don't.
                    unsafe {
                        let is_prime_ptr = is_prime.as_ptr() as *mut bool;
                        *is_prime_ptr.offset((j / 2) as isize) = false;
                    }
                });
            }
            i += 2
        }
    })
}

benchmark_group!(benches, sieve);
benchmark_main!(benches);