i32::pow only handles overflow up to an exponent of 1023

[jonas-schievink]

Playpen: http://is.gd/zhwksu

fn main() {
    let x: i32 = 2;

    println!("{}", x.pow(1024));
    println!("{}", x.pow(1023));
}

Prints:

0
thread '<main>' panicked at 'arithmetic operation overflowed', ../src/libcore/num/mod.rs:550

Interestingly, this only seems to happen with an even base. The overflow is detected correctly when x is set to 3 or other odd numbers.

[bluss]

cc @pnkfelix introducing this behavior in c8db89a, it may be deliberate.

[steveklabnik]

/cc @rust-lang/libs @rust-lang/lang is this intended behavior?

[BurntSushi]

I'm not sure this is intended behavior, and if it was, it does seem a little surprising. What seems to happen is if the exponent is even, and each iteration of the pow loop divides by 2, then the if (exp & 1) == 1 { check is never hit, which I think is supposed to cause pow to panic if overflow is observable. Certainly overflow is observable here.

[alexcrichton]

This looks like a bug, seems fine to fix

[nikomatsakis]

I agree with @alexcrichton