nightly aarch64: broken in debug mode

[SWW13]

The code below does not run correctly under aarch64 on the nightly compiler in debug mode.
Every other combination works (only tried nightly and stable releases).

I minimized the sample as far as I could and verified it on real hardware.
Sorry for the title, I have no clue how to name this issue.

I tried this code:

#[derive(Debug, PartialEq)]
pub struct B {
    pub b1: u16,
    pub b2: u8,
}

impl B {
    fn new(b1: u16, b2: u8) -> B {
        B { b1, b2 }
    }
}

#[test]
fn welp() {
    assert_eq!(B::new(0xDEAD, 0xFE), B { b1: 0xDEAD, b2: 0xFE });
}

I expected to see this happen:

$ cargo +nightly test
[...]
test welp ... ok

Instead, this happened:

(qemu-aarch64)

$ cargo +nightly test
[...]
test welp ... thread 'welp' panicked at 'assertion failed: `(left == right)`
  left: `B { b1: 0, b2: 0 }`,
 right: `B { b1: 57005, b2: 254 }`', src/lib.rs:15:5

(aarch64 cpu)

  left: `B { b1: 127, b2: 0 }`,
 right: `B { b1: 57005, b2: 254 }`', src/lib.rs:15:5

Meta

You can reproduce this with qemu-user (aarch64) with the following config:

$ cat .cargo/config
[target.aarch64-unknown-linux-gnu]
linker = "aarch64-linux-gnu-gcc"
runner = "qemu-aarch64"
$ LD_LIBRARY_PATH="/usr/aarch64-linux-gnu/lib:$LD_LIBRARY_PATH" cargo +nightly test --target aarch64-unknown-linux-gnu

CPU:

$ lscpu
Architecture:          aarch64
Byte Order:            Little Endian
[...]
$ cat /proc/cpuinfo
[...]
Features	: fp asimd evtstrm aes pmull sha1 sha2 crc32
CPU implementer	: 0x41
CPU architecture: 8
CPU variant	: 0x0
CPU part	: 0xd03
CPU revision	: 4
[...]

rustc --version --verbose:

rustc 1.26.0-nightly (3eeb5a665 2018-03-01)
binary: rustc
commit-hash: 3eeb5a665e313c5b281820099e04d4c6c8188b46
commit-date: 2018-03-01
host: x86_64-unknown-linux-gnu
release: 1.26.0-nightly
LLVM version: 6.0

---

Edit: smaller example

[parched]

I narrowed this down a bit to being caused by the LLVM 6 upgrade. I'll try create a minimal LLVM IR repro.

[parched]

Looks like LLVM 6 enables GlobalISel so I tried with RUSTFLAGS="-C llvm-args=-fast-isel" and the problem goes away.

[parched]

There's some changes to the AArch64InstructionSelector in #48782 for inserting/extracting 16bit types so hopefully that will fix it.

[nikomatsakis]

triage: P-high

Check-in from compiler team meeting: thanks for the investigative work! We'll keep an eye on this and see what happens when #48782 lands.

[parched]

I tested this on the latest nightly and it's fixed, so it seems the upgrade to released LLVM 6.0 indeed did the trick.

[nikomatsakis]

OK, then I'm going to close the issue! 🎷