Directly calling extern "riscv-interrupt-{m,s}" fn... compiles?

[workingjubilee]

I tried this code with rustc --target riscv64gc-unknown-linux-gnu rust_code.rs:

#![feature(abi_riscv_interrupt)]
pub extern "riscv-interrupt-m" fn interrupt_machine() {}

pub extern "riscv-interrupt-s" fn interrupt_supervisor() {}

pub fn main() {
    interrupt_machine();
    interrupt_supervisor();
}

I expected to see this happen: rustc or LLVM reject the direct call.

Instead, this happened:

...wait, why does it compile? This doesn't seem like it should compile...

Meta

rustc --version --verbose:

rustc 1.84.0-nightly (a0d98ff0e 2024-10-31)
binary: rustc
commit-hash: a0d98ff0e5b6e1f2c63fd26f68484792621b235c
commit-date: 2024-10-31
host: x86_64-unknown-linux-gnu
release: 1.84.0-nightly
LLVM version: 19.1.

@rustbot label: +A-hardware-interrupts +A-LLVM +O-riscv +A-ABI +T-compiler

Related Issues

• Tracking Issue for riscv-interrupt-{m,s} ABIs #111889
• Directly calling "x86-interrupt" fn should be invalid #132834

[workingjubilee]

Filed upstream as llvm/llvm-project#115640

[workingjubilee]

#![feature(no_core, lang_items)]
#![feature(abi_avr_interrupt)]
#![no_core]
#[lang = "sized"]
trait Sized {}

pub extern "avr-interrupt" fn avr_interrupt() {}

pub fn call() {
    avr_interrupt();
}

Also compiles...? cc @Patryk27 is this sensical?

@rustbot label: +O-AVR

[workingjubilee]

AVR variant filed upstream as llvm/llvm-project#115641

[Patryk27]

Hmm, I would've thought that interrupt handlers must be unsafe (in which case it's your fault you're calling an unsafe function like that) 🤔

But yeah, extra validation on rustc's and LLVM's side won't hurt.

[workingjubilee]

It seems they need not be unsafe either!

I don't think it makes much sense for them to be unsafe, as there's nothing intrinsically unsafe about an interrupt handler, just... everything it then does probably requires unsafe. But you kind of have to be callable at any time, you know? By nature of being an interrupt handler.

[workingjubilee]

That said, I have no intrinsic objection to adopting such a limitation, just... not enforced today.