NVPTX: Avoid PassMode::Direct for args in C abi

[kjetilkjeka]

Fixes #117480

I must admit that I'm confused about PassMode altogether, is there a good sum-up threads for this anywhere? I'm especially confused about how "indirect" and "byval" goes together. To me it seems like "indirect" basically means "use a indirection through a pointer", while "byval" basically means "do not use indirection through a pointer".

The return used to keep PassMode::Direct for small aggregates. It turns out that make_indirect messes up the tests and one way to fix it is to keep PassMode::Direct for all aggregates. I have mostly seen this PassMode mentioned for args. Is it also a problem for returns? When experimenting with byval as an alternative i ran into this assert

I have added tests for the same kind of types that is already tested for the "ptx-kernel" abi. The tests cannot be enabled until something like #117458 is completed and merged.

CC: @RalfJung since you seem to be the expert on this and have already helped me out tremendously

CC: @RDambrosio016 in case this influence your work on rustc_codegen_nvvm

@rustbot label +O-NVPTX

[rustbot]

r? @compiler-errors

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[kjetilkjeka]

@rustbot label +O-NVPTX

[RalfJung]

I must admit that I'm confused about PassMode altogether, is there a good sum-up threads for this anywhere?

Sadly, no. The best we have are the API docs. I improved them a bit recently, but we are still lacking some proper "big-picture" docs for this. largely this is because while the system was designed with some big-picture ideas ~5 years ago, those were not written down at the time and since then it's been hotfixed and patched without regard for a coherent story. The entire system needs an overhaul. See rust-lang/compiler-team#672: the compiler team accepted the proposal to overhaul this, but we're still looking for a volunteer to do the work.

CC: @RalfJung since you seem to be the expert on this and have already helped me out tremendously

😂 I knew nothing about this until around 3 months ago, and spent way too much time on it since.^^ I still don't feel like an expert, since all I know is reverse engineered from looking at parts of rustc. There are many things about ABI I do not know. But at this point I think I know at least how rustc deals with ABI, and I know some necessary (but not sufficient) conditions for ABI adjustments to make sense.

[RalfJung]

Sadly, for this PR, I can't really tell if it makes any sense. But if it helps fix the assertions and also still generates working code for your tests, then it's at least an improvement...

[bjorn3]

I think you will want to use PassMode::Cast for aggregate types.

[kjetilkjeka]

I did experiment with casting to Uniform with Reg size equal to alignment and size equal to size. This caused miscompilation when passing types like

#[repr(C)]
pub struct SingleU8 {
    f: u8,
}

The problem was that since it was passed as a single register of size 8bit it passed it like a primitive type u8 instead of an aggregate. In ptx these primitives are padded while aggregates are not.

[bjorn3]

In ptx these primitives are padded while aggregates are not.

😱

[compiler-errors]

Sorry, too busy to review this 😰

r? compiler

[cjgillot]

r? compiler

[bors]

☔ The latest upstream changes (presumably #117500) made this pull request unmergeable. Please resolve the merge conflicts.

[davidtwco]

Apologies for the delay in getting to this - we don't have an expert in NVPTX on the team, so I'll take a stab at reviewing this so it isn't stuck forever. It seems reasonable to me.

Does this mean that NVPTX can be re-enabled in tests/ui/abi/compatibility.rs now? If so, could you do that in this PR? Otherwise let me know and I'll approve this.

[kjetilkjeka]

When rebasing there are unfortunately regressions on some of the tests related to returning structs. I will look into re-enabling the compatibility test when I have investigated these regressions.

If there's any silver lining I guess it is that it highlights the importance of #117458 and getting the nvptx tests up and running again in CI.

[RalfJung]

Does this mean that NVPTX can be re-enabled in tests/ui/abi/compatibility.rs now? If so, could you do that in this PR? Otherwise let me know and I'll approve this.

Yeah it should be re-enabled, otherwise the claim that it fixes the issue is not accurate.

[apiraino]

Checking status of this PR. @kjetilkjeka is this PR waiting on #117458? Switching the review flag for a rebase, thanks.

@rustbot author

[kjetilkjeka]

Yes, this is waiting for #117458

It turned out to be more difficult than anticipated to get correct. I will of course spend the time to figure it out, but when I manage to get it right I would hate for it to break again. Therefore I would love to wait until the nvptx tests are enabled in CI so I can write tests and be confident that things will keep working.

[OwenTrokeBillard]

Is this still blocked now that #117458 is merged?

[apiraino]

@kjetilkjeka when you have a sec. can you look at this patch again? Thanks

@rustbot author