c-variadic: use `emit_ptr_va_arg` for `va_arg` on `sparc` by folkertdev · Pull Request #160660 · rust-lang/rust

folkertdev · 2026-08-06T23:29:31Z

I did not use an LLM to create a change in this PR.
I used an LLM to create a change in this PR, and I have explained below how it was used.

I've built GCC for the target and validated pretty extensively that this works. Using our helper does generate worse code for i64 unfortunately, I've reported that as llvm/llvm-project#214594. But, given that this is a tier-3 target etc. I don't think it makes sense to go out of our way to do better, we can just wait for LLVM to resolve that issue.

folkertdev · 2026-08-06T23:43:20Z

+        Arch::Sparc => {
+            std::assert_matches!(stability, CVariadicStatus::Unstable { .. });
+            emit_ptr_va_arg(
+                bx,
+                addr,
+                target_ty,
+                PassMode::Direct,
+                SlotSize::Bytes4,
+                AllowHigherAlign::No,
+                ForceRightAdjust::No,
+            )
+        }


Sparc is still gated by the c_variadic_experimental_arch feature.

The arguments to emit_ptr_va_arg here are based on the LLVM implementation. Many targets have an implementation of va_arg in Clang, but for sparc it is really LLVM that expands va_arg.

https://github.com/llvm/llvm-project/blob/5194e33faad9d84fd8d21c0305621a17e0d8a49f/llvm/lib/Target/Sparc/SparcISelLowering.cpp#L2768-L2790

it just loads from a pointer regardless of the size of the argument, hence unconditional PassMode::Direct

The pointer is increased by just the VT size in bytes. Due to argument promotion, the smallest type that can actually be read using va_arg is 4 bytes, and no attempt is made to align to something higher than that. So SlotSize::Bytes4.

AllowHigherAlign::Yes would align e.g. an i128 to a 16-byte boundary. This function does not take the alignment into account at all. So, AllowHigherAlign::No

This setting is true on BE targets with a slot size of 8, where a 4-byte value could be either in the low or high bytes. Despite being a BE target, the setting is not relevant for sparc because all values that could be passed divide cleanly into 4-byte slots.

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rustbot · 2026-08-07T12:46:59Z

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folkertdev · 2026-08-07T12:48:15Z

cc target maintainer @jonathanpallant

folkertdev · 2026-08-07T12:48:51Z

As context, I'd like to just stabilize c-variadic functions for this target, and for that we don't want to rely on LLVM's va_arg.

khyperia · 2026-08-10T04:31:56Z

looks good in principle! but sorry, I have absolutely no clue on sparc/etc. and don't feel confident on this myself:

@rustbot reroll

adwinwhite · 2026-08-10T10:07:06Z

r? codegen

rustbot · 2026-09-03T17:14:09Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

folkertdev · 2026-09-03T17:15:32Z

The LLVM PR has now merged, and the implementation here now matches it

https://github.com/llvm/llvm-project/blob/d6b2176126d716110eea8bf90c611eb6833f10ba/clang/lib/CodeGen/Targets/Sparc.cpp#L99-L116

folkertdev · 2026-09-03T17:16:56Z

+            // f128 is passed indirectly.
+            let pass_mode = match layout.layout.backend_repr() {
+                BackendRepr::Scalar(scalar) => match scalar.primitive() {
+                    Primitive::Float(Float::F128) => PassMode::Indirect,
+                    _ => PassMode::Direct,
+                },
+                _ => PassMode::Direct,
+            };


f128 does not yet implement VaArgSafe, so this is unreachable in practice, but:

that's in the works implement VaArgSafe for f128 #161424

this way we properly match the LLVM implementation

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saethlin · 2026-09-04T02:07:29Z

@bors r+

(I didn't realize how small this was at first)

rust-bors · 2026-09-04T02:07:32Z

📌 Commit fda15d9 has been approved by saethlin

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 10. This pull request will be tested once the tree is reopened.

Reason for tree closure: broken external dependency

c-variadic: use `emit_ptr_va_arg` for `va_arg` on `sparc` I've built GCC for the target and validated pretty extensively that this works. Using our helper does generate worse code for `i64` unfortunately, I've reported that as llvm/llvm-project#214594. But, given that this is a tier-3 target etc. I don't think it makes sense to go out of our way to do better, we can just wait for LLVM to resolve that issue.

…uwer Rollup of 6 pull requests Successful merges: - #157808 (sanitizers: Implement support for the sanitize ignorelist) - #160660 (c-variadic: use `emit_ptr_va_arg` for `va_arg` on `sparc`) - #162237 (make -Ctarget-feature warnings more explicitly FCWs) - #160111 (Generalize Decodable impl for arrays to all types) - #162174 (Update internal docs & tests to use `!` rather than `Infallible` in relation to `Try`) - #162226 (Clean up the AST visitor)

…uwer Rollup of 7 pull requests Successful merges: - #157808 (sanitizers: Implement support for the sanitize ignorelist) - #160660 (c-variadic: use `emit_ptr_va_arg` for `va_arg` on `sparc`) - #162237 (make -Ctarget-feature warnings more explicitly FCWs) - #160111 (Generalize Decodable impl for arrays to all types) - #162174 (Update internal docs & tests to use `!` rather than `Infallible` in relation to `Try`) - #162226 (Clean up the AST visitor) - #162281 (rustc-dev-guide subtree update)

Rollup merge of #160660 - folkertdev:sparc-va-arg, r=saethlin c-variadic: use `emit_ptr_va_arg` for `va_arg` on `sparc` I've built GCC for the target and validated pretty extensively that this works. Using our helper does generate worse code for `i64` unfortunately, I've reported that as llvm/llvm-project#214594. But, given that this is a tier-3 target etc. I don't think it makes sense to go out of our way to do better, we can just wait for LLVM to resolve that issue.

folkertdev added the F-c_variadic `#![feature(c_variadic)]` label Aug 6, 2026

folkertdev force-pushed the sparc-va-arg branch from 3ec8bc2 to dc6688c Compare August 6, 2026 23:33

rustbot added the A-run-make Area: port run-make Makefiles to rmake.rs label Aug 6, 2026

folkertdev commented Aug 6, 2026

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folkertdev marked this pull request as ready for review August 7, 2026 12:46

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 7, 2026

rustbot assigned khyperia Aug 7, 2026

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Aug 7, 2026

rustbot assigned adwinwhite and unassigned khyperia Aug 10, 2026

rustbot assigned saethlin and unassigned adwinwhite Aug 10, 2026

c-variadic: use emit_ptr_va_arg for va_arg on sparc

fda15d9

folkertdev force-pushed the sparc-va-arg branch from dc6688c to fda15d9 Compare September 3, 2026 17:14

folkertdev commented Sep 3, 2026

View reviewed changes

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 4, 2026

JonathanBrouwer mentioned this pull request Sep 4, 2026

Rollup of 5 pull requests #162278

Closed

This was referenced Sep 4, 2026

Rollup of 6 pull requests #162280

Closed

Rollup of 7 pull requests #162284

Merged

rust-bors Bot merged commit a63c406 into rust-lang:main Sep 4, 2026
13 checks passed

rustbot added this to the 1.100.0 milestone Sep 4, 2026

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c-variadic: use `emit_ptr_va_arg` for `va_arg` on `sparc` - #160660

c-variadic: use `emit_ptr_va_arg` for `va_arg` on `sparc`#160660
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