MIRI fails to detect overflow in signed division

[celinval]

I tried running MIRI in this code:

#![feature(core_intrinsics)]
fn main() {
    let ub = unsafe { unchecked_div(i32::MAX, -1) };
    // ---> Uncomment this line to see MIRI correctly finding the overflow.
    // let ub = unsafe { std::intrinsics::unchecked_div(i32::MIN, -1) };
    println!("{ub}");
}

unsafe fn unchecked_div(a: i32, b: i32) -> i32 {
    std::intrinsics::unchecked_div(a, b)
}

I expected to see this happen: MIRI should fail with the following error:

error: Undefined Behavior: overflow in signed division (dividing MIN by -1)

Instead, this happened: No UB is detected and the program prints -2147483647.

Meta

I ran this using playground. The version reported is:

0.1.0 (2023-06-18 2d0aa57)

[celinval]

@rustbot label A-miri

[celinval]

I wonder if this is related to #112168

[saethlin]

I'm confused. i32::MAX / -1 doesn't overflow, and Miri correctly does not report UB for the unchecked version. i32::MIN / -1 does overflow, and Miri correctly reports UB for the unchecked version.

#![feature(core_intrinsics)]

fn main() {
    println!("{:?}", i32::MAX.checked_div(-1));
    let not_ub = unsafe { std::intrinsics::unchecked_div(i32::MAX, -1) };
    println!("{not_ub}");

    println!("{:?}", i32::MIN.checked_div(-1));
    let ub = unsafe { std::intrinsics::unchecked_div(i32::MIN, -1) };
    println!("{ub}");
}

Reports UB on the second unchecked_div:

 --> src/main.rs:9:23
  |
9 |     let ub = unsafe { std::intrinsics::unchecked_div(i32::MIN, -1) };
  |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflow in signed division (dividing MIN by -1)
  |

[celinval]

Shoot, my bad. I did have a typo there, didn't I. Sorry