Several platforms still have incorrectly aligned u128/i128

[beetrees]

I tried this code:

fn main() {
    dbg!(std::mem::align_of::<u128>());
}

I expected to see this happen: The printed alignment to match the alignment used for __int128_t by GCC and Clang.

Instead, this happened: On 64-bit PowerPC, 64-bit SPARC and 64-bit MIPS, Rust thinks the alignment is 8 whereas GCC and Clang think the alignment is 16. The PowerPC 64-bit ABI specifications (both ELFv1 and ELFv2) agree with GCC and Clang (I'm not aware of any specification for 128-bit integers on SPARC64 or MIPS64, but GCC/Clang's behaviour seems to be the de-facto standard). This is because the LLVM data layout for the affected platforms doesn't correctly specify the alignment. This is the same as #54341 but on different architectures (cc rust-lang/lang-team#255). I initially discovered this when running abi-cafe on PowerPC64 to test #128643. I've filed an LLVM bug at llvm/llvm-project#102783.

Meta

rustc --version --verbose:

rustc 1.82.0-nightly (ca5d25e2c 2024-08-09)
binary: rustc
commit-hash: ca5d25e2c41f5a6b4ce65c681bf2f94c7ead1f14
commit-date: 2024-08-09
host: x86_64-unknown-linux-gnu
release: 1.82.0-nightly
LLVM version: 19.1.0

[beetrees]

@rustbot label +A-abi +A-ffi +A-llvm +T-compiler

[tgross35]

For reference: https://godbolt.org/z/jvrxv7jP7

[beetrees]

Fixed in LLVM 20.

@rustbot label +llvm-fixed-upstream

[bjorn3]

We still need to change the data-layout on the rustc side.

[beetrees]

The data layouts have now all been updated in rustc (SPARC, MIPS, PowerPC).

[tgross35]

What else is left to do for this issue?

[tgross35]

Windows has an issue with its i128 ABI, I don't think it's tracked anywhere (outside of an issue in the lang team repo) so maybe this issue could be repurposed.

[beetrees]

This issue is just waiting on LLVM 20 now. Regarding the Windows issue: I'm guessing your referring to rust-lang/lang-team#255 (comment)? It's not really the same issue as this (alignment mismatch vs return ABI mismatch), so it seems better to open a separate rust-lang/rust issue about it so it can be tracked separately. Both x86_64-pc-windows-gnu and x86_64-pc-windows-msvc targets on Clang and MinGW GCC seem to agree that i128 is returned in xmm0, and compiler-builtins even has a special hack to use make sure u128/i128 are returned in xmm0 so that the builtins ABI is what LLVM expects.

[tgross35]

Right, I forgot we are waiting on a LLVM upgrade. Regarding Windows, Wesley thinks that Clang is in the wrong here on the MSVC targets https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/MSVC.20.60f16.60.20and.20.60f128.60.20ABI/near/480542167.

[tgross35]

Opened #134288 for the Windows issue.