[AVR] Invalid asm code "ldd rn, X+N" compiles and produces corrupted binary

[Slickis]

AVR assembler instruction ldd is defined only for registers Y and Z, but not for X.
asm!() macro accepts it.

I tried this code:

#[no_mangle]
extern "C" fn main() {
    unsafe {
        ::core::arch::asm!("
                ldd   {tmp}, X+0
                ldd   {tmp}, X+1
            ",
            tmp = out(reg) _,
            in("X") &0xAABB_u16,
        );
    }
}

I expected to see this happen: compilation error.

Instead, this happened: rustc produces a binary with a corrupted code.

000000a6 <main>:
  a6:    a0 e0           ldi    r26, 0x00    ; 0
  a8:    b1 e0           ldi    r27, 0x01    ; 1
  aa:    88 81           ld     r24, Y
  ac:    89 81           ldd    r24, Y+1     ; 0x01
  ae:    08 95           ret

Pointer (0x0100) to a value is loaded into register X (r26:r27) following by reads from *Y.

Meta

rustc --version --verbose:

rustc 1.70.0-nightly (22f247c6f 2023-03-13)
binary: rustc
commit-hash: 22f247c6f3ed388cb702d01c2ff27da658a8b353
commit-date: 2023-03-13
host: x86_64-unknown-linux-gnu
release: 1.70.0-nightly
LLVM version: 15.0.7

Steps to reproduce:

repo: https://github.com/Slickis/rustc-ldd-x

git clone https://github.com/Slickis/rustc-ldd-x
cd rustc-ldd-x
make lss

[workingjubilee]

Rust does not reject illegal assembly. It does not necessarily even inspect it. For the string of the asm! macro, Rust performs the formatting instantiation and allows the assembler to do the rest. We only control the operands.

[workingjubilee]

I believe this is more of a documentation issue, not a bug we can solve here. This could be reported to LLVM regarding the AVR assembler implementation they use, as I believe they host the assembler for this. However, ultimately, I expect are a million other followup issues like this, and if they don't care to fix it, then, well, it won't be, and I don't think rustc should carry around an assembler linter inside it that can check every instruction you could possibly give it for every single architecture.

To this end I have opened rust-lang/reference#1344

[workingjubilee]

Reported upstream as llvm/llvm-project#62012

Hope I got the details correct!

[Patryk27]

fwiw, this got fixed with #114048 🙂