Miscompile from LLVM assuming that globals don't get merged #162078

use std::mem::MaybeUninit;

fn get_first_if_half(p: &[u32]) -> u32 {
    let half = const { &[MaybeUninit::uninit(), MaybeUninit::new(0)] };
    
    // Simply observe that the addresses are equal, without making
    // assumptions about what that implies
    assert_eq!(p.as_ptr().addr(), half.as_ptr().addr());

    // LLVM will happily use that to inline the `undef` from `half` here.
    // But uninit globals can get merged into init globals by the linker!
    p[0]
}

pub fn main() {
    // Linker will merge this with `half` from the other function
    let full = std::hint::black_box(const { &[0, 0] });

    // Below is just exploitation to make it miscompile. 
    // LLVM is returning `undef` for the `u32`, which is already UB

    if get_first_if_half(full) != get_first_if_half(full) {
        // SAFETY: We loaded the first item in `full` twice; 
        // 0 != 0 is impossible.
        unsafe { std::hint::unreachable_unchecked() }
    }
}