possible ASAN miscompile

[krasimirgg]

I tried this code with ASAN nightly rust (https://godbolt.org/z/KE6P9xYsY):

// Doesn't fail without -Copt-level=0

#[repr(C)] // doesn't fail without this
#[derive(Clone, Debug)]
pub enum A {
    A(i8),
}

#[repr(C)]  // doesn't fail without this
pub enum B {
    B(A),
}

// doesn't fail if extern "C" is dropped
pub extern "C" fn f(b: B) -> A {
    let a = match b {
        B::B(a) => Some(a),
    };
    a.unwrap()
}

fn main() {
    let b = B::B(A::A(3));
    // doesn't fail if the body of f is inlined.
    let a = f(b);
    println!("a: {:?}", &a);
}

I expected to see this happen: program executes successfully.

Instead, this happened: program failed with ASAN warning.

Meta

Requires nightly with -Zsanitizer=address --target x86_64-unknown-linux-gnu -Copt-level=0: https://godbolt.org/z/KE6P9xYsY.

It looks this is not a recent regression, it's present in rust from 2021.

% export dt=2021-06-10 ; rustup install nightly-$dt &&  rustc +nightly-$dt  -Zsanitizer=address --target x86_64-unknown-linux-gnu -Copt-level=0 -g main.rs && ./main

[krasimirgg]

@cuviper could this be a dup of #75839?

[cuviper]

I don't know -- the symptoms look different, but UB could have the same root cause.

It looks this is not a recent regression, it's present in rust from 2021.

% export dt=2021-06-10

Is that a specific bisected date, or just one that you happened to try?

[krasimirgg]

I don't know -- the symptoms look different, but UB could have the same root cause.

It looks this is not a recent regression, it's present in rust from 2021.

% export dt=2021-06-10

Is that a specific bisected date, or just one that you happened to try?
Just one I happened to try.

[nikic]

This does look the same as #75839 to me. We really shouldn't be generating aggregate loads like that.

[nikic]

I think the load is generated here:

rust/compiler/rustc_codegen_ssa/src/mir/block.rs

Lines 436 to 437 in bb89df6

	let ty = bx.cast_backend_type(cast_ty);
	bx.load(ty, llslot, self.fn_abi.ret.layout.align.abi)

[nikic]

Eh, I meant this one:

rust/compiler/rustc_codegen_ssa/src/mir/block.rs

Lines 1402 to 1403 in bb89df6

	let llty = bx.cast_backend_type(ty);
	llval = bx.load(llty, llval, align.min(arg.layout.align.abi));

The above is for returns rather than argument passing -- though it probably has the same/similar issue.

[jieyouxu]

@rustbot label +requires-nightly +A-sanitizers +T-compiler +C-optimization -needs-triage