rustc apparently optimizes away the loop in 3.3 #28713
Description
I just implemented the code in section 3.3 to compare Ruby to Rust. It appears that rustc optimizes away the inner loop of the thread, because I kept increasing the limit and the completion time remained constant. I introduced a simple if statement in the loop, and then I got expected timings.
It might be worth modifying the loop so it's not optimized away for a fairer comparison. Here's what I ended up with:
pub extern fn process() {
let handles: Vec<_> = (0..10).map(|_| {
thread::spawn(|| {
let mut x = 0u64;
for i in (0..7_700_000_000u64) {
if i < 1 {
x += 2
} else {
x += 1
}
}
x
})
}).collect();
for h in handles {
println!("Thread finished with count={}",
h.join().map_err(|_| "Could not join a thread!").unwrap());
}
println!("done!");
}