Incorrect code generated from inline assembly with inout register on riscv32im-unknown-none-elf

I tried compiling this code with a target of riscv32im-unknown-none-elf:

#![no_std]

static mut MY_BUFFER: [u32; 4] = [0u32; 4];

#[no_mangle]
unsafe fn using_inout() {
    let mut start = MY_BUFFER.as_mut_ptr();
    ::core::arch::asm!(
            "ecall",
            inout("a0") start);
    _ = start;
}

The assembly generated is incorrect, since it doesn't add in the lower 16 bits of the address of MY_BUFFER before calling the ecall:

using_inout:
        lui     a0, %hi(example::MY_BUFFER::he36bd30b506a9b69)
        ecall
        ret

As a workaround, changing the inout to inlateout seems to fix the problem:

using_inlateout:
        lui     a0, %hi(example::MY_BUFFER::he36bd30b506a9b69)
        addi    a0, a0, %lo(example::MY_BUFFER::he36bd30b506a9b69)
        ecall
        ret

Here is a compiler explorer link for rust nightly that demonstrates the problem:

https://godbolt.org/z/xMjoTo5oY

(For reference in case it gets fixed in nightly, it's also present in rust 1.80.0)

I haven't looked into this too much yet, but do you know if it reproduces using C's inline assembly fed through clang? This seems like it could be an LLVM problem.

It appears that this bug starts to appear in 1.77.

Using riscv32i-unknown-none-elf target:

1.76: https://godbolt.org/z/bKosKqvfW
1.77: https://godbolt.org/z/MYacc9na7

Could it be that this is the same underlying issue?
#126221

Did you mean between 1.77 and 1.78? The two links look the same to me.

It looks like this has to be a regression in LLVM, which was bumped to version 18 in 1.78, because we seem to generate the exact same IR. Could you file an issue with LLVM?

https://godbolt.org/z/Te9efcnMz

Could it be that this is the same underlying issue? #126221

It looks like it, I will close that one because this one has more detail.

@rustbot

WG-prioritization assigning priority (Zulip discussion).

Being an unsound issue, maybe let's see what we can do. Compile target is Tier 2.

@rustbot label -I-prioritize +P-high

Workaround for LLVM compiler bug for rv32im

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Workaround for LLVM compiler bug for rv32im

Workaround for LLVM compiler bug for rv32im (#2130)

Fix applied to LLVM 19.x llvm/llvm-project#100843

Auto merge of rust-lang#127513 - nikic:llvm-19, r=cuviper

Auto merge of #127513 - nikic:llvm-19, r=cuviper

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Incorrect code generated from inline assembly with `inout` register on riscv32im-unknown-none-elf #128212

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Incorrect code generated from inline assembly with inout register on riscv32im-unknown-none-elf #128212

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tgross35 commented on Jul 25, 2024

flaub commented on Jul 26, 2024

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Incorrect code generated from inline assembly with `inout` register on riscv32im-unknown-none-elf #128212