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.

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

RalfJung · 2026-02-10T17:18:52Z

compiler/rustc_session/src/session.rs

+    if sess.opts.unstable_opts.packed_stack {
+        if sess.target.arch != Arch::S390x {
+            sess.dcx().emit_err(errors::UnsupportedPackedStack);
+        }


This seems like the most reasonable place to check the backchain condition?

yes i will move the other check here

~~unfortunately feature flags are only parsed during codegen_backend.init(sess) which is one step after rustc_session::build_session(...). So no feature flags available to check at this point.~~

~~-> i need to check them in codegen_llvm or codegen_ssa.~~

Please avoid duplicating the check across the codegen backends.

The check seems to be somewhat similar in nature to

rust/compiler/rustc_interface/src/interface.rs

Line 500 in 9a114c6

util::check_abi_required_features(&sess);

. So maybe rename check_abi_required_features to something slightly more general and do the check there?

yes this seems, also to me, the only option left.

there is another way.

I could try to talk to the llvm guys if its possible to make packed-stack a proper target-feature so we dont have to go over a cli argument.

Not sure if that would help. Target feature conflicts currently also need ad-hoc checks, we don't have a framework for dealing with them at the moment.

RalfJung · 2026-02-10T17:19:37Z

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

 If not specified, overflow checks are enabled if
 [debug-assertions](#debug-assertions) are enabled, disabled otherwise.

+## packed-stack


Since the option isn't stable yet, this is the wrong place to document it I think?

If i dont have packed-stack here x test tidy will complain. I try to find another location to do it.

Something must be going wrong there. The page is explicitly about -C flags, not -Z flags.

you where right. The error was not about my changes but about some uncommitted local changes. :( sorry

everything works as expected.

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

rustbot · 2026-02-13T12:58:48Z

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.

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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bjorn3 reviewed Feb 10, 2026

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fneddy force-pushed the s390x_packed_stack_attribute branch from 5ddcabb to 4ebec9d Compare February 10, 2026 11:29

RalfJung reviewed 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

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 Feb 13, 2026

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fneddy added 3 commits February 13, 2026 13:55

add rustc option -Zpacked-stack

64e69b8

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

Added tests

21aea26

removed unstable description

0c182af

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

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