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

compiler/rustc_session/src/session.rs

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.

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.

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

RalfJung · 2026-02-16T09:48:51Z

compiler/rustc_interface/src/util.rs

+    // we need to have access to the complete constructed `target` and the session options.
+    // this is the first place in the `run_compiler` flow where all this is available


This comment is very confusing when read outside the context of this PR.

Suggested change

// we need to have access to the complete constructed `target` and the session options.

// this is the first place in the `run_compiler` flow where all this is available

// s390x: packed_stack + backchain only works with "softlofat" ABI.

RalfJung · 2026-02-16T09:49:50Z

compiler/rustc_interface/src/util.rs

@@ -72,7 +72,7 @@ pub(crate) fn add_configuration(

 /// Ensures that all target features required by the ABI are present.
 /// Must be called after `unstable_target_features` has been populated!


Suggested change

/// Must be called after `unstable_target_features` has been populated!

/// Must be called after `unstable_target_features` has been populated!

/// Also enforces other, target-specific constraints on how target features can be combined.

/// (Those checks can only be done once the `Session` is mostly constructed, so there's

/// no earlier place we can put them.)

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.

RalfJung · 2026-02-16T09:52:18Z

tests/ui/packedstack/combination.rs

Is this a new top-level test folder for such a small feature, with just a single test in it? That doesn't seem justified. Please move it to tests/ui/target-feature/, with a file name that indicates what it is about.

RalfJung · 2026-02-16T09:53:16Z

compiler/rustc_interface/src/errors.rs

+#[derive(Diagnostic)]
+#[diag("packedstack with backchain needs softfloat")]
+#[note(
+    "enabling both `packedstack` and `backchain` attributes is incompatible with the default s390x target. Switch to s390x-unknown-none-softfloat if you need both attributes!"


Suggested change

"enabling both `packedstack` and `backchain` attributes is incompatible with the default s390x target. Switch to s390x-unknown-none-softfloat if you need both attributes!"

"enabling both `-Zpacked-stack` and the `backchain` target feature is incompatible with the default s390x ABI. Switch to s390x-unknown-none-softfloat if you need both attributes."

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?

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

Merged

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 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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add rustc option -Zpacked-stack

31a40dc

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

fneddy force-pushed the s390x_packed_stack_attribute branch from 7fef6ea to 31a40dc Compare February 16, 2026 09:50

RalfJung reviewed Feb 16, 2026

View reviewed changes

		// we need to have access to the complete constructed `target` and the session options.
		// this is the first place in the `run_compiler` flow where all this is available

	// we need to have access to the complete constructed `target` and the session options.
	// this is the first place in the `run_compiler` flow where all this is available
	// s390x: packed_stack + backchain only works with "softlofat" ABI.

		@@ -72,7 +72,7 @@ pub(crate) fn add_configuration(

		/// Ensures that all target features required by the ABI are present.
		/// Must be called after `unstable_target_features` has been populated!

-/// Must be called after `unstable_target_features` has been populated!
+/// Must be called after `unstable_target_features` has been populated!
+/// Also enforces other, target-specific constraints on how target features can be combined.
+/// (Those checks can only be done once the `Session` is mostly constructed, so there's
+/// no earlier place we can put them.)

	"enabling both `packedstack` and `backchain` attributes is incompatible with the default s390x target. Switch to s390x-unknown-none-softfloat if you need both attributes!"
	"enabling both `-Zpacked-stack` and the `backchain` target feature is incompatible with the default s390x ABI. Switch to s390x-unknown-none-softfloat if you need both attributes."

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fneddy commented Feb 10, 2026 •

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