The code runs slower when optimised

[vi]

fn main() {
    let mut va = [0.0, .. 102];
    let mut vb = [0.0, .. 102];
    let mut p=0.0;
    va[51]=1.0;

    do int::range(1,300001) |k| {
        do int::range(2,101) |i| {
            vb[i] = (1.0/6.0)*va[i-1] + (4.0/6.0)*va[i] + (1.0/6.0)*va[i+1];
            true
        }
        let n = (1.0/6.0)*va[2] + (1.0/6.0)*va[100];
        p += n * (k as float);
        vb[0] += n;
        va=vb;
        if k % 5000 == 0 {
            io::println(fmt!("%.10f %.10f %d", va[0], p, k));
        }
        true
    }
    io::println(fmt!("%.10f %.10f", va[0], p));
}

Built with rustc --opt-level 0, it runs fast.
With any other --opt-level, it runs slow.

Checked on c2f5a87 and 5f13e9c.

[luqmana]

It seems to be because of stack switching.

With -O:

-> % env RUST_MIN_STACK=8000000 time ./foo >/dev/null
        0.12 real         0.12 user         0.00 sys

-> % time ./foo >/dev/null
./foo > /dev/null  0.80s user 0.00s system 99% cpu 0.801 total

[vi]

Maybe it can print some warning that suggest me to set RUST_MIN_STACK?

[luqmana]

Now that pcwalton's improvements have landed, building with optimizations doesn't run slower than without optimizations.