add rustc option -Zpacked-stack#152432

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.

bjorn3 · 2026-02-10T11:22:03Z

compiler/rustc_codegen_llvm/src/attributes.rs


+fn packed_stack_attr<'ll>(cx: &SimpleCx<'ll>, sess: &Session) -> Option<&'ll Attribute> {
+    if sess.target.arch != Arch::S390x {
+        return None;


Maybe panic here given that the frontend should already have emitted an error.

hmm packed_stack_attr(...) is called every time llfn_attrs_from_instance is called indifferent of target arch.

And I think it would not be a good thing to add an arch check to llfn_attrs_from_instance as there are no other checks in that function. It blindly calls all possibilities and assumes that checks are already handled elsewhere.

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.

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?

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>

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

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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

add rustc option -Zpacked-stack

dc98326

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 f39c52b to dc98326 Compare February 16, 2026 14:19

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fneddy commented Feb 10, 2026 •

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