Sven van Caem (Nov 05 2024 at 09:41): Sven van Caem (Nov 05 2024 at 09:41):Rust glue currently uses Rust's u128/i128 types to represent Roc's U128/I128 types. This causes rustc to raise a warning:
warning: `extern` block uses type `u128`, which is not FFI-safe
--> src/x86_64.rs:27:50
|
27 | fn roc__mainForHost_1_exposed_generic(_: *mut u128);
| ^^^^^^^^^ not FFI-safe
|
= note: 128-bit integers don't currently have a known stable ABI
= note: `#[warn(improper_ctypes)]` on by default
I've noticed roc_std contains U128/I128 implementations with explicitly specified alignment. I suspect the proper fix here would be to make Rust glue use these types instead?
Anton (Nov 05 2024 at 09:54):I think so, I've solved a difficult bug in the past where we assumed a Roc type (don't remember which one) had the same representation as a similar Rust type but this assumption was incorrect
Sven van Caem (Nov 05 2024 at 09:55): Sven van Caem (Nov 05 2024 at 09:56):Either way, I'll make the change!
Anton (Nov 05 2024 at 10:19):Sven van Caem said:
Yes, that's the one!
Brendan Hansknecht (Nov 05 2024 at 16:55):We may need to update llvm before you can switch that, but I don't recall fully
Brendan Hansknecht (Nov 05 2024 at 16:55):I know that llvm initially got the alignment of u128 wrong, but a fix eventually landed
Brendan Hansknecht (Nov 05 2024 at 16:56):I'm not sure which roc matches currently
Sven van Caem (Nov 07 2024 at 09:08):I've just tested it with the following function:
Numbers : { u128 : U128, i128 : I128 }
main : Numbers -> Numbers
main = \{ u128, i128 } -> { u128: 2 * u128, i128: 2 * i128 }
it seems roc_std's U128/I128 give incorrect results and Rust's u128 and i128 work just fine. I suppose it wasn't an oversight after all? It does mean we're stuck with the improper_ctypes warning though unless we choose to suppress it.
Sven van Caem (Nov 07 2024 at 09:20):Here's the values it fails with:
---- glue_cli_run::u128_i128 stdout ----
thread 'glue_cli_run::u128_i128' panicked at crates/glue/tests/test_glue_cli.rs:277:13:
`roc` command had unexpected stderr: 🔨 Rebuilding platform...
thread 'main' panicked at src/lib.rs:24:5:
assertion `left == right` failed
left: Numbers { i128: 281446803774592, u128: 922337203685477580798 }
right: Numbers { i128: 170141183460469231731687303715884105726, u128: 340282366920938463463374607431768211454 }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
where left is the value roc returned, and right is the value that was expected
Sven van Caem (Nov 07 2024 at 09:23):i128 seems to be slightly different each time, but u128 seems to be the same garbage each time
Sven van Caem (Nov 07 2024 at 09:25):It's not a thorough test, but I'll change RustGlue back to emitting u128 instead of roc_std::U128 for now
Brendan Hansknecht (Nov 09 2024 at 06:40):I think this can be fixed the next time we update llvm
Brendan Hansknecht (Nov 09 2024 at 06:40):Though we may also need to update our u128 alignment in general in the compiler
Last updated: Jul 26 2025 at 12:14 UTC