Simple recursion but get weird result when optimization is enabled

[vita-dounai]

I tried this code:

fn main() {
    let ret = fn1(42);
    println!("it returns! {}", ret);
}

fn fn1(i: u32) -> u32 {
    fn2(i)
}

fn fn2(i: u32) -> u32 {
    fn1(i)
}

I expected to see this happen: in the code above, fn1 and fn2 just simply call each other, and there is indeed a infinitely recursion. I expect that when the main is executed, the call to f1 inside main should never return, and the stack will be overflowed at some point.

Instead, this happened: when I enable lto or set opt-level to 1 or higher, the call to f1 suprisingly returns, and the return value is always zero.

Meta

rustc --version --verbose:

rustc 1.48.0 (7eac88abb 2020-11-16)
binary: rustc
commit-hash: 7eac88abb2e57e752f3302f02be5f3ce3d7adfb4
commit-date: 2020-11-16
host: x86_64-pc-windows-msvc
release: 1.48.0
LLVM version: 11.0

Backtrace

<backtrace>

[ehuss]

I believe that infinite loops are undefined behavior in LLVM, which can result in anything happening. See issue #28728.

[jonas-schievink]

Yes, closing as a duplicate of that