add rustc option -Zpacked-stack by fneddy · Pull Request #152432 · rust-lang/rust

fneddy · 2026-02-10T10:19:08Z

this enables -Zpacked-stack just as -mpacked-stack in clang and gcc. packed-stack is needed on s390x for kernel development.

For reference: #151154 and #150766

look at @uweigand s post for full explanation of what this does. Here a wrap-up:

#150766 (comment)

[...]
packed-stack [...] modifies how the compiler-generated function prolog/epilog code makes use of the 160 byte register save area provided by a caller to the callee [...] this variant is not actually incompatible with the ABI - packed-stack and regular functions can freely call each other without ABI issues.
[...]
combination of -mpacked-stack and -mbackchain [...] the location in the stack frame where the backchain link ought to be stored is not available. [...] is not supported at all with the default ABI
[...]
However, in the special case of also using soft-float, our (implied) soft-float ABI provides a different location for the backchain that is compatible with -mpacked-stack, so that combination should be supported
[...]

rustbot · 2026-02-10T10:19:14Z

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fneddy · 2026-02-10T10:21:56Z

CC @cuviper @RalfJung @folkertdev @workingjubilee

fneddy · 2026-02-10T10:25:16Z

I'll add tests for assembly output and for target incompatibility.

compiler/rustc_codegen_llvm/src/attributes.rs

compiler/rustc_target/src/spec/mod.rs

compiler/rustc_session/src/session.rs

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

rustbot · 2026-02-16T09:35:32Z

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.

fneddy · 2026-02-16T09:38:59Z

ok, i think most should be ok now.

compiler/rustc_interface/src/util.rs

RalfJung · 2026-02-16T09:50:38Z

compiler/rustc_interface/src/util.rs

+    if sess.target.arch == Arch::S390x
+        && sess.opts.unstable_opts.packed_stack
+        && sess.unstable_target_features.contains(&Symbol::intern(&"backchain"))
+        && sess.target.abi != Abi::SoftFloat


target.abi is just in charge of cfg(target_abi), it doesn't control the actual ABI. Please check rustc_abi which controls the actual ABI.

changed it to check on the soft-float unstable_target_feature which llvm will evaluate to switch abi for s390x.

I think checking rustc_abi makes most sense.

tests/ui/target-feature/packedstack-combinations.rs

compiler/rustc_interface/src/errors.rs

RalfJung · 2026-02-16T09:54:36Z

compiler/rustc_interface/src/util.rs

+    // this is the first place in the `run_compiler` flow where all this is available
+    if sess.target.arch == Arch::S390x
+        && sess.opts.unstable_opts.packed_stack
+        && sess.unstable_target_features.contains(&Symbol::intern(&"backchain"))


What happens if someone tries to enable backchain just for a particular function via #[target_feature] instead of -Ctarget-feature?

ufff yes we have to check this for every call to llfn_attrs_from_instance at compiler runtime. is it ok to do ....contains(&Symbol::intern(... every time?

from_target_feature_attr is probably a better, higher-level place to check this?

You can add backchain as a pre-interned symbol in compiler/rustc_span/src/symbol.rs so you don't have to re-intern it every time.

I am back on working on this. yes in from_target_feature_attr we can check for the #[target_feature(...)] attributes. BUT unfortunately not for the -Ctarget-feature... attributes ..

Yeah the handling for -Ctarget-feature and #[target_feature] is entirely separate. Maybe it's possible to introduce a new helper that gets called form those 2 places and that performs these kinds of consistency checks?

Ah I see you're now doing the check in packed_stack_attr. That also seems reasonable.

chenyukang · 2026-02-24T04:40:54Z

looks good for me, but it's better to be reviewed by someone with related domain knowledge.
@rustbot reroll

fneddy · 2026-02-24T15:11:35Z

ok now this is something. i think i added a good check for all the different possibilities of the attribute with a check here and test here.

fneddy · 2026-02-24T15:31:26Z

hold on I messed up that test with the last commit. I'll fix that tomorrow morning.

this enables packed-stack just as -mpacked-stack in clang and gcc. packed-stack is needed on s390x for kernel development. Co-authored-by: Ralf Jung <post@ralfj.de>

fneddy · 2026-02-25T10:38:38Z

ok I think its ready now for next review round.

@rustbot ready

RalfJung · 2026-02-27T14:00:57Z

compiler/rustc_codegen_llvm/src/attributes.rs

+        return None;
+    }
+
+    // The backchain and softfloat flags can be set via -Ctarget-features=...


I'd suggest an early return on !sess.opts.unstable_opts.packed_stack to avoid doing any of this work if we don't have to.

removed comment that was not needed in the end

rustbot assigned chenyukang Feb 10, 2026

fneddy marked this pull request as draft February 10, 2026 10:24

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 Feb 10, 2026

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fneddy mentioned this pull request Feb 11, 2026

remove redundant backchain attribute in codegen #152486

Merged

fneddy marked this pull request as ready for review February 13, 2026 11:19

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fneddy force-pushed the s390x_packed_stack_attribute branch from 0d6d167 to 0c182af Compare February 13, 2026 12:58

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fneddy force-pushed the s390x_packed_stack_attribute branch from 0c182af to 7fef6ea Compare February 16, 2026 09:35

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fneddy force-pushed the s390x_packed_stack_attribute branch from d886fbf to f39c52b Compare February 16, 2026 14:18

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add rustc option -Zpacked-stack

6753aec

this enables packed-stack just as -mpacked-stack in clang and gcc. packed-stack is needed on s390x for kernel development. Co-authored-by: Ralf Jung <post@ralfj.de>

fneddy force-pushed the s390x_packed_stack_attribute branch from ebb7f42 to 6753aec Compare February 25, 2026 08:22

RalfJung reviewed Feb 27, 2026

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added early return for packed stack

462efd0

removed comment that was not needed in the end

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fneddy commented Feb 10, 2026 •

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