Incorrect optimization in code fragment

[nstoddard]

I found some code that behaves differently with optimization. With -O0, it prints false; with any higher setting, it prints true.

Sorry about the poor variable names; this is (mostly) a translation from some Javascript code. I've tried to remove as much code as I can, but I'm sure it can still be reduced somewhat and still show the same incorrect behavior.

Edit: the old code sample linked to 'hello world' for some reason. I fixed it.

[sfackler]

Is that code sample the right one?

[thomas-huet]

I have reduced the code to this :

fn main() { 
  for d in range(0u, 1) { 
    let u: uint = (d + 1) % 2;
    let mut x = [0i,..2];
    let mut mask = false;
    while x[u] < 2 { 
      mask = x[1] > 0;
      x[u] += 1;
    } 
    println!("This should be true: {}" , mask);
    if !mask { 
      panic!("Failed!");
    } 
  } 
}

runs fine when compiled with rustc but not when compiled with rustc -O

[nstoddard]

Whoops, I'm not sure why my link didn't work. Obviously I didn't find a bug in Hello World :). I fixed the link.

[mahkoh]

It appears that the code is too long for a get request which is why play doesn't work.

[nstoddard]

Is it worth marking this as a release blocker? Incorrect optimizations seem like a big deal that should be fixed before 1.0.

[steveklabnik]

This code sample now prints 'true' in both cases, with no panic.