[MIPS] `AtomicI8` and `AtomicI16` doctests for `fn fetch_min()` and `fn fetch_max()` began failing in `nightly-2024-04-08`

[martn3]

Hello,

nightly-2024-04-08 regressed on the following tests on MIPS:

failures:
    library/core/src/sync/atomic.rs - sync::atomic::AtomicI16::fetch_max (line 3072)
    library/core/src/sync/atomic.rs - sync::atomic::AtomicI16::fetch_min (line 3072)
    library/core/src/sync/atomic.rs - sync::atomic::AtomicI16::fetch_min (line 3084)
    library/core/src/sync/atomic.rs - sync::atomic::AtomicI8::fetch_max (line 3036)
    library/core/src/sync/atomic.rs - sync::atomic::AtomicI8::fetch_min (line 3036)
    library/core/src/sync/atomic.rs - sync::atomic::AtomicI8::fetch_min (line 3048)

test result: FAILED. 4537 passed; 6 failed; 37 ignored; 0 measured; 6 filtered out; finished in 315.73s

I have bisected this to #123555 in the Rust repo and to llvm/llvm-project@fbb27d1 in the LLVM repo.

How to reproduce

1. (On Debian 12) install a MIPS toolchain:

sudo apt install \
    gcc-mipsel-linux-gnu \
    libc6-mipsel-cross \
    qemu-user

2. Create a file main.rs with the following contents (which is a simplified snippet of the failing doctests mentioned above):

fn main() {
    let x = std::sync::atomic::AtomicI8::new(23);
    assert_eq!(x.fetch_max(42, std::sync::atomic::Ordering::SeqCst), 23);
}

3. Build it like this:

./x build --target mipsel-unknown-linux-gnu
build/x86_64-unknown-linux-gnu/stage1/bin/rustc \
    --target mipsel-unknown-linux-gnu \
    -Clinker=mipsel-linux-gnu-gcc \
    main.rs

4. Run like this:

qemu-mipsel -L /usr/mipsel-linux-gnu main

Expected

The program completes successfully.

Actual

The program fails like this:

thread 'main' panicked at src/main.rs:3:5:
assertion `left == right` failed
  left: 0
 right: 23
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Bisection

With src/llvm-project pointing to llvm/llvm-project@fbb27d1 the test also fails, but when pointing src/llvm-project to the parent commit (llvm/llvm-project@e74c167) the test passes.

So we can conclude that this regression was introduced by llvm/llvm-project@fbb27d1, which originates from llvm/llvm-project#77072.

Edit: See llvm/llvm-project#77072 for more discussion around this.

@rustbot label +O-MIPS +A-atomic +T-compiler +C-bug

[dianqk]

@rustbot label +A-LLVM