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Fix miscompilation of inline assembly with outputs in cases where we emit an inv...

 2 years ago
source link: https://github.com/rust-lang/rust/pull/95864
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@luqmana luqmana commented 13 days ago •

edited

We ran into this bug where rustc would segfault while trying to compile certain uses of inline assembly.

Here is a simple repro that demonstrates the issue:

#![feature(asm_unwind)]

fn main() {
    let _x = String::from("string here just cause we need something with a non-trivial drop");
    let foo: u64;
    unsafe {
        std::arch::asm!(
            "mov {}, 1",
            out(reg) foo,
            options(may_unwind)
        );
    }
    println!("{}", foo);
}

(playground link)

But crucially feature(asm_unwind) is not actually needed and this can be triggered on stable as a result of the way async functions/generators are handled in the compiler. e.g.:

extern crate futures; // 0.3.21

async fn bar() {
    let foo: u64;
    unsafe {
        std::arch::asm!(
            "mov {}, 1",
            out(reg) foo,
        );
    }
    println!("{}", foo);
}

fn main() {
    futures::executor::block_on(bar());
}

(playground link)

An example of the incorrect LLVM generated:

bb1:                                              ; preds = %start
  %1 = invoke i64 asm sideeffect alignstack inteldialect unwind "mov ${0:q}, 1", "=&r,~{dirflag},~{fpsr},~{flags},~{memory}"()
          to label %bb2 unwind label %cleanup, !srcloc !9
  store i64 %1, i64* %foo, align 8

bb2:
[...snip...]

The store should not be placed after the asm invoke but rather should be in the normal control flow basic block (bb2 in this case).

Here is a writeup of the investigation that lead to finding this.

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