This safe program reports UB when run with Miri (note that you need to use RUSTFLAGS=-Zmir-enable-passes=+GVN cargo +nightly miri run because Miri explicitly turns off GVN for a different reason)
#[repr(C, packed)]
#[derive(Clone, Copy)]
pub struct Packed {
pub a: u8,
pub b: (u32, u32),
}
#[derive(Clone, Copy)]
pub struct Outer {
pub x: u64,
pub p: Packed,
}
const C: Outer = Outer {
x: 7,
p: Packed { a: 1, b: (2, 3) },
};
#[inline(never)]
fn get() -> (u32, u32) {
C.p.b
}
fn main() {
println!("{:?}", get());
}error: Undefined Behavior: accessing memory based on pointer with alignment 1, but alignment 4 is required
--> src/main.rs:18:5
|
18 | C.p.b
| ^^^^^ Undefined Behavior occurred here
|
= help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
= help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
= note: stack backtrace:
0: get
at src/main.rs:18:5: 18:10
1: main
at src/main.rs:22:22: 22:27
Whatever Miri is seeing here, I can't see it in our MIR dumps. I wonder if our pretty-printing is obscuring the problem?
fn get() -> (u32, u32) {
let mut _0: (u32, u32);
let mut _1: Outer;
bb0: {
StorageLive(_1);
_1 = const C;
_0 = const (2_u32, 3_u32);
StorageDead(_1);
return;
}
}
alloc1 (size: 24, align: 8) {
0x00 │ 07 00 00 00 00 00 00 00 01 02 00 00 00 03 00 00 │ ................
0x10 │ 00 __ __ __ __ __ __ __ │ .░░░░░░░
}
This was found by an LLM
This safe program reports UB when run with Miri (note that you need to use
RUSTFLAGS=-Zmir-enable-passes=+GVN cargo +nightly miri runbecause Miri explicitly turns off GVN for a different reason)Whatever Miri is seeing here, I can't see it in our MIR dumps. I wonder if our pretty-printing is obscuring the problem?
This was found by an LLM