LLVM 20 miscompile on num-rational crate

[durin42]

Code

In rust-num/num-rational@4d55ad2 (which looks like it matches the current release version to me):

RUSTFLAGS='-Coverflow-checks=yes'  cargo +trashpanda test --release

where my trashpanda toolchain is built from LLVM HEAD (soon to be LLVM 20) and Rust HEAD.
I expected to see this happen: tests pass

Instead, this happened:

---- test::test_ratio_to_f64 stdout ----

thread 'test::test_ratio_to_f64' panicked at src/lib.rs:1607:34:
called `Option::unwrap()` on a `None` value


failures:
    test::test_ratio_to_f64

test result: FAILED. 51 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s

Note that just setting -Copt-level=3 isn't enough to tickle the bug, so there's something else happening in --release that contributes to the miscompile.

Version it worked on

All

Version with regression

rustc --version --verbose:

a7a6c64a657f68113301c2ffe0745b49a16442d1, built against 6fe0fc60341b05bf30ccc16012dab9eeb55a338d

I bisected over LLVM revisions, and this was broken by llvm/llvm-project@66badf2 but that's as much as I know at this point. That revision isn't trivial to revert anymore (at least with patch -R) so I haven't verified that just backing out that patch at HEAD resolves things.

[lqd]

We haven’t merged the switch to llvm 20 yet, right? i.e. this is not a regression?

[durin42]

That's correct, my company ships LLVM and Rust from HEAD on a rolling basis, so we noticed this early.

[TimNN]

I've reduced this to a single file that only depends on std: https://gist.github.com/TimNN/c14b4d79cc12941e2f4363f57f5d100a

It requires -C opt-level=2 -C debuginfo=1 -C debug-assertions and is very sensitive to even small changes (including changes to dead code, to static assertions, and to whether parameters are passed by value or reference)

[TimNN]

I've reduced this further by applying a bit of std::hint::black_box (it also doesn't require debug info anymore) (this was tested on aarch64-apple-darwin):

// Reduced from https://github.com/rust-num/num-rational/blob/master/src/lib.rs
// with some inlined code from other num libraries.

// Compile with:
// rustc --edition=2021 -C opt-level=2 -C debug-assertions main.rs

fn main() {
    let numer = std::hint::black_box(144115188075855873i128);
    let denom = std::hint::black_box(18014398509481984i128);

    let numer_bits = u64::from(128 - numer.leading_zeros());
    let denom_bits = u64::from(128 - denom.leading_zeros());

    let (is_diff_positive, absolute_diff) = match numer_bits.checked_sub(denom_bits) {
        Some(diff) => (true, diff),
        None => (
            false,
            denom_bits - /* numer_bits */ (u64::from(128 - numer.leading_zeros())),
        ),
    };

    if std::hint::black_box(is_diff_positive) {
        let i = &absolute_diff;

        if *i <= (isize::MAX as u64) {
        } else {
            panic!("REDUCE-FAILURE-MARKER")
        }
    };
}

[dianqk]

Upstream issue: llvm/llvm-project#124267