added support for s390x target features `soft-float` and `packed-stack` #150766

fneddy · 2026-01-07T14:02:52Z

as we want to enable rust in kernel on s390x we need to do some things:

allow the use of soft-float feature only if also soft-float ABI is enabled
add a -packed-stack rustc option. this adds the packed-stack function-attribute to every function call. I implemented it the same way as it is implemented in clang¹². In clang this is a cli argument. It is not a part of the target feature. Therefore we cannot pass it via the features field in the target.json.
added a check for packed-stack && backchain** && !soft-float as this combination should never occur. However I don't know if I put it in the right spot. ³

rustbot · 2026-01-07T14:02:55Z

These commits modify compiler targets.
(See the Target Tier Policy.)

rustbot · 2026-01-07T14:02:57Z

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compiler/rustc_target/src/spec/mod.rs

folkertdev · 2026-01-07T21:18:52Z

src/doc/rustc/src/codegen-options/index.md

+## packed-stack
+
+This flag enables packed StackFrames on s390x.
+If backchain is also enabled, switch to the soft-float ABI.


I think this should say something about allowed values and what the default is. (if only for consistency).

Also i like your phrasing from above better: enabling both packed-stack and backchain is incompatible with hard-float, but is compatible with soft-float. The current text could be read to mean that soft-float would be switched to automatically.

folkertdev · 2026-01-07T21:24:48Z

compiler/rustc_target/src/target_features.rs

+                // On s390x only the hard-float ABI is valid. However for kernel compilation we need to
+                // allow soft-float if and only if the ABI is explicitly set to soft-float.


Does this refer to the linux kernel? Also I feel like this mixes up the SoftFloat ABI and the soft-float feature, maybe you can clarify which way the implication goes?

tgross35

Having flags that change ABI is a problem because everything quietly breaks if you don't rebuild the world (among other problems); I don't think we want to add this to any more targets, we're trying to rip it out. See context at #116344, cc @RalfJung.

The thing to do is to submit an MCP to create a new separate -softfloat target, like we have with aarch64-unknown-none-softfloat, loongarch64-unknown-none-softfloat, x86_64-unknown-none (softfloat is implied there), and others.

Also cc @uweigand @cuviper target maintainers

View changes since this review

rustbot · 2026-01-08T05:56:07Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

RalfJung · 2026-01-08T07:55:03Z

compiler/rustc_target/src/target_features.rs

-                FeatureConstraints { required: &[], incompatible: &["soft-float"] }
+                // On s390x only the hard-float ABI is valid. However for kernel compilation we need to
+                // allow soft-float if and only if the ABI is explicitly set to soft-float.
+                if matches!(self.abi, Abi::SoftFloat) {


self.abi is just used for cfg(target_abi), it doesn't actually control the ABI.

It looks like s390x is just as bad as x86 when it comes to ABI handling so the logic should probably be similar?

there is only one valid abi on s390x. This ABI uses hard-float.

The idea is that soft-floatremains incompatible unless the user explicitly requests to use the soft-float abi(e.g.¹). This will not change the abi, as there is no other ABI specified. And to my knowledge there wont be a soft-float abi specified that really could be used by llvm.

It is used as a gate, so if you want to use soft-float you also have to explicitly change the ABI. And make it clear that this code is incompatible with the normal code.

Footnotes

https://github.com/torvalds/linux/compare/master...fneddy:linux:add_rustc_s390x#diff-8b2b79dc9fe8130845984440e8df685c928623dfe1ebd12fe937efe4ab352aa9R267 ↩

This will not change the abi, as there is no other ABI specified.

What exactly do you mean by this - floats still get passed in vregs even with soft-float? What does the flag do then?

There might only be one official ABI, but the soft-float ABI is real and we're not going to pretend that it isn't. The soft-float ABI is whatever LLVM does to pass float args across function barriers when the soft-float target feature is on. So there might be no specification for such an ABI, but that doesn't stop LLVM from using it.

That's what happened on x86, anyway. I suspect it is similar on s390x.

so if you want to use soft-float you also have to explicitly change the ABI.

Exactly. But self.abi does not change the ABI, it just tells cfg what the ABI is. llvm_abiname, llvm_floatabi, and rustc_abi change the ABI.

(Well, that's not entirely true. self.abi actually is used in some places in the ABI adjustment logic. It's all a bit messy unfortunately...)

RalfJung · 2026-01-08T07:55:49Z

compiler/rustc_target/src/target_features.rs

Note that further up this file it still says

("soft-float", Forbidden { reason: "currently unsupported ABI-configuration feature" }, &[]),

So using the soft-float target feature still doesn't work even with this PR.

On that note, it would be great to have codegen tests for these kind of flags.

compiler/rustc_session/src/options.rs

RalfJung · 2026-01-08T07:59:09Z

compiler/rustc_codegen_llvm/src/attributes.rs

+    }
+
+    if sess.opts.cg.packed_stack {
+        Some(llvm::CreateAttrString(cx.llcx, "packed-stack"))


What are the ABI considerations for this flag? Is it possible to freely mix code built with and without this flag? If not, it needs to be made a "target modifier" to avoid unsoundness.

yes packed-stack and non packed stack code can be mixed. It just affects the calling code and not the called code.

RalfJung · 2026-01-08T08:00:03Z

compiler/rustc_session/src/options.rs

    overflow_checks: Option<bool> = (None, parse_opt_bool, [TRACKED],
        "use overflow checks for integer arithmetic"),
+    packed_stack: bool = (false, parse_bool, [TRACKED],
+        "use packed stack frames (s390x only) (default: no)"),


It's bad UI to just silently ignore the flag for targets where it does not make sense. Please ensure it raises an error instead.

rust-log-analyzer · 2026-01-08T10:32:19Z

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

    |
319 |     if sess.opts.cg.packed_stack {
    |                     ^^^^^^^^^^^^ unknown field
    |
    = note: available fields are: `ar`, `code_model`, `codegen_units`, `collapse_macro_debuginfo`, `control_flow_guard` ... and 48 others

For more information about this error, try `rustc --explain E0609`.
[RUSTC-TIMING] rustc_codegen_llvm test:false 3.395
error: could not compile `rustc_codegen_llvm` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...

nnethercote · 2026-01-08T11:20:56Z

I don't know anything about this stuff, so I will hand this over to a target maintainer:

r? @cuviper

cuviper · 2026-01-08T19:48:40Z

cc @uweigand, in case you're available to look at this, otherwise I'll try to review it soon.

rustbot assigned nnethercote Jan 7, 2026

fneddy marked this pull request as draft January 7, 2026 14:04

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 7, 2026

added support for s390x target features soft-float and packed-stack

30628ca

fneddy force-pushed the new_target_s390x_softfloat branch from 7fdc148 to 30628ca Compare January 7, 2026 14:09

fneddy marked this pull request as ready for review January 7, 2026 14:10

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jan 7, 2026

folkertdev reviewed Jan 7, 2026

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tgross35 requested changes Jan 8, 2026

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 8, 2026

RalfJung reviewed Jan 8, 2026

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compiler/rustc_session/src/options.rs Outdated Show resolved Hide resolved

RalfJung reviewed Jan 8, 2026

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fixed wording and moved arg to unstable options

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rustbot assigned cuviper and unassigned nnethercote Jan 8, 2026

		// On s390x only the hard-float ABI is valid. However for kernel compilation we need to
		// allow soft-float if and only if the ABI is explicitly set to soft-float.

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added support for s390x target features soft-float and packed-stack #150766

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added support for s390x target features soft-float and packed-stack #150766

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fneddy commented Jan 7, 2026

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added support for s390x target features `soft-float` and `packed-stack` #150766

added support for s390x target features `soft-float` and `packed-stack` #150766

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