Infinite recursion can trigger undefined behavior (I think)

[davidlattimore]

pub fn bar(i: i32) -> i32 {
    if i < 1000 {
        bar(i % 2)
    } else {
        1234567
    }
}

fn main() {
    println!("RESULT: {}", bar(10));
}

If I cargo run this, I get a stack overflow as expected. If I cargo run --release however, I get:
RESULT: 1234567

The else should be unreachable (with input 10). I suspect LLVM is correctly identifying that it will recurse indefinitely, but that it perhaps considers that undefined behavior, so decides that it's free to remove the if statement and just unconditionally return the constant value. Indeed, if I look at the assembly produced for bar, it's just:

	movl	$1234567, %eax
	retq

rustc version 1.28.0

[nagisa]

A duplicate of #18785. I’m surprised we ended up closing #18785 without really checking whether the underlying reason is the same. I’ll reopen that issue and close this in favour of that.

Thanks for the report!