bitwise operation takes wrong value

[ya-mouse]

I'm on caf316a

Found a strange value calculation when bitwising with wrong casting:

https://gist.github.com/ya-mouse/8444726

[jfager]

This doesn't appear to be a problem anymore, can it be closed?

[alexcrichton]

This still reproduces for me when compiling with optimizations.

[alexcrichton]

fn main() {
    let m : [u8, ..1] = [ 0u8 ];
    println!("{}", ((m[0] >> 8) | 0) as u16);
    println!("{}", ((m[0] >> 8) | m[0]) as u16);
    assert!((((m[0] >> 8) | 0) as u16) == 0);
    assert!((((m[0] >> 8) | m[0]) as u16) == 255);
}

$ rustc -O foo.rs
$ ./foo
0
255

[thestinger]

Bit shifts with a RHS equal to or larger than the width of the type in bytes have undefined behaviour in C and LLVM. The code here is shifting an 8-bit integer by 8 bits. This issue is reported as #10183 already.