naked functions: emit `.private_extern` on macos by folkertdev · Pull Request #148339 · rust-lang/rust

folkertdev · 2025-10-31T18:11:09Z

Emit .private_extern on macos when the naked function uses Linkage::Internal. Failing to do so can cause issues with LTO.

The documentation on this directive is kind of sparse, but I believe this is at least not incorrect, and does fix the issue.

r? @Amanieu
cc @bjorn3

rustbot · 2025-10-31T18:11:12Z

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

bjorn3 · 2025-11-02T12:38:46Z

Is this really only an issue on macOS? I would have assumes this is an issue on all platforms.

folkertdev · 2025-11-02T14:31:48Z

The reproduction given in the issue builds successfully for me on linux.

I did just notice that we do already have

rust/compiler/rustc_codegen_ssa/src/mir/naked_asm.rs

Lines 245 to 248 in 73e6c9e

match item_data.visibility{
Visibility::Default | Visibility::Protected => {}
Visibility::Hidden => writeln!(begin,".private_extern {asm_name}").unwrap(),
}

That clearly did not actually trigger in this case. Is that right?

bjorn3 · 2025-11-02T18:15:34Z

Yeah, the issue here I believe is that rustc correctly computes the symbol visibility for the naked function as private, but then when ThinLTO decides to inline the caller of the naked function, it is unable to change the naked function to be public (+ add a .llvm.<somehash> suffix to the symbol name to prevent conflicts) like it would do for regular functions. So we have to mark all naked functions as visible within the same DSO rather than just within the same object file, aka use external linkage + hidden visibility on ELF and equivalent with other object file formats.

folkertdev · 2025-11-02T19:50:11Z

aka use external linkage + hidden visibility on ELF and equivalent with other object file formats.

Hmm, we do emit the following:

rust/compiler/rustc_middle/src/mir/mono.rs

Lines 151 to 159 in 6a884ad

// If the function is #[naked] or contains any other attribute that requires exactly-once
// instantiation:
// We emit an unused_attributes lint for this case, which should be kept in sync if possible.
let codegen_fn_attrs = tcx.codegen_instance_attrs(instance.def);
if codegen_fn_attrs.contains_extern_indicator()
 || codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED)
{
returnInstantiationMode::GloballyShared{may_conflict:false};
}

but apparently that is not enough.

Is compiler/rustc_monomorphize/src/partitioning.rs the right place to fix this?

bjorn3 · 2025-11-02T20:09:18Z

That gets overridden at

rust/compiler/rustc_monomorphize/src/partitioning.rs

Lines 583 to 586 in 6a884ad

// If we got here, we did not find any uses from other CGUs, so
// it's fine to make this monomorphization internal.
 data.linkage = Linkage::Internal;
 data.visibility = Visibility::Default;

folkertdev · 2025-11-02T21:00:31Z

- // write nothing
+ // LTO can fail when internal linkage is used.
+ emit_fatal("naked functions may not have internal linkage")


is this too strict? We could allow it, but I think this is only reachable now with an explicit linkage attribute (which is unstable).

I think this is fine for now. We can always revise it later if it becomes a problem.

Amanieu · 2025-11-04T01:52:19Z

I think the issue with inlining is bigger than that and we effectively have to always use global visibility for naked functions. Consider dylib crates where the naked function is private but is called by an inlinable function. If that function gets inlined into another crate then the naked function needs to have global visibility so that it can be referenced from another dylib.

bjorn3 · 2025-11-04T08:43:32Z

Consider dylib crates where the naked function is private but is called by an inlinable function. If that function gets inlined into another crate then the naked function needs to have global visibility so that it can be referenced from another dylib.

If rustc considers the caller to be cross-crate codegenable, it would already mark the callee as public be it a regular function be it a naked function. The problematic case is where the codegen backend decides to inline a function into another codegen unit when rustc didn't consider it cross-crate codegenable. This can only happen within a single dylib through ThinLTO, not between dylibs. And it is only an issue for naked functions as ThinLTO can't make the function public itself as it would do for regular functions.

folkertdev · 2026-01-06T09:11:36Z

@Amanieu will you be able to review this? it is starting to block some things apparently.

Amanieu · 2026-01-06T13:29:00Z

@bors r+

bors · 2026-01-06T13:29:03Z

📌 Commit d96ae8f has been approved by Amanieu

It is now in the queue for this repository.

…ern, r=Amanieu naked functions: emit `.private_extern` on macos fixesrust-lang#148307 Emit `.private_extern` on macos when the naked function uses `Linkage::Internal`. Failing to do so can cause issues with LTO. The documentation on this directive is kind of sparse, but I believe this is at least not incorrect, and does fix the issue. r? `@Amanieu` cc `@bjorn3`

…uwer Rollup of 11 pull requests Successful merges: - #144113 (Impls and impl items inherit `dead_code` lint level of the corresponding traits and trait items) - #148339 (naked functions: emit `.private_extern` on macos) - #149880 (rustc_codegen_llvm: update alignment for double on AIX) - #150122 (Refactor function names of `rustc_ast_lowering`) - #150412 (use PIDFD_GET_INFO ioctl when available) - #150670 (THIR pattern building: Move all `thir::Pat` creation into `rustc_mir_build::thir::pattern`) - #150695 (MGCA: pretty printing for struct expressions and tuple calls ) - #150698 (Improve comment clarity in candidate_may_shadow) - #150706 (Update wasm-component-ld) - #150707 (Fix ICE when transmute Assume field is invalid) - #150708 (Enable merge queue in new bors) r? `@ghost` `@rustbot` modify labels: rollup

JonathanBrouwer · 2026-01-06T15:19:21Z

@bors r-
#150724 (comment)

Rollup of 11 pull requests Successful merges: - #144113 (Impls and impl items inherit `dead_code` lint level of the corresponding traits and trait items) - #148339 (naked functions: emit `.private_extern` on macos) - #149880 (rustc_codegen_llvm: update alignment for double on AIX) - #150122 (Refactor function names of `rustc_ast_lowering`) - #150412 (use PIDFD_GET_INFO ioctl when available) - #150670 (THIR pattern building: Move all `thir::Pat` creation into `rustc_mir_build::thir::pattern`) - #150695 (MGCA: pretty printing for struct expressions and tuple calls ) - #150698 (Improve comment clarity in candidate_may_shadow) - #150706 (Update wasm-component-ld) - #150707 (Fix ICE when transmute Assume field is invalid) - #150708 (Enable merge queue in new bors) r? `@ghost` `@rustbot` modify labels: rollup

Amanieu · 2026-01-06T15:34:28Z

ERROR: unknown compiletest directive add-core-stubs at /checkout/tests/assembly-llvm/naked-functions/hidden.rs:2

naked functions: emit `.private_extern` on macos fixes#148307 Emit `.private_extern` on macos when the naked function uses `Linkage::Internal`. Failing to do so can cause issues with LTO. The documentation on this directive is kind of sparse, but I believe this is at least not incorrect, and does fix the issue. r? `@Amanieu` cc `@bjorn3`

bors · 2026-01-06T18:53:05Z

⌛ Testing commit 76d0843 with merge 23720f5...

rust-log-analyzer · 2026-01-06T19:39:31Z

The job dist-powerpc64-linux failed! Check out the build log: (web)(plain enhanced)(plain)

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

[RUSTC-TIMING] rustc_driver_impl test:false 41.837
[RUSTC-TIMING] rustc_query_impl test:false 129.366
error: linking with `powerpc64-unknown-linux-gnu-gcc` failed: exit status: 1
|
= note: "powerpc64-unknown-linux-gnu-gcc" "-Wl,--version-script=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-rustc/powerpc64-unknown-linux-gnu/release/deps/rustcTNzRaV/list" "-Wl,--no-undefined-version" "-m64" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-rustc/powerpc64-unknown-linux-gnu/release/deps/rustcTNzRaV/symbols.o" "<1 object files omitted>" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-rustc/powerpc64-unknown-linux-gnu/release/deps/rustcTNzRaV/rmeta.o" "<1 object files omitted>" "-Wl,--as-needed" "-Wl,-Bstatic" 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"<sysroot>/lib/rustlib/powerpc64-unknown-linux-gnu/lib/{libstd-*,libpanic_unwind-*,libobject-*,libmemchr-*,libaddr2line-*,libgimli-*,libcfg_if-*,librustc_demangle-*,libstd_detect-*,libhashbrown-*,librustc_std_workspace_alloc-*,libminiz_oxide-*,libadler2-*,libunwind-*,liblibc-*,librustc_std_workspace_core-*,liballoc-*,libcore-*,libcompiler_builtins-*}.rlib" "-Wl,-Bdynamic" "-ldl" "-ldl" "-lgcc_s" "-lutil" "-lrt" "-lpthread" "-lm" "-ldl" "-lc" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-rustc/powerpc64-unknown-linux-gnu/release/deps/rustcTNzRaV/raw-dylibs" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-rustc/powerpc64-unknown-linux-gnu/release/build/psm-018b79a1943a9aa9/out" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-rustc/powerpc64-unknown-linux-gnu/release/build/rustc_llvm-c8668ae673789711/out" "-L" "/checkout/obj/build/powerpc64-unknown-linux-gnu/llvm/lib" "-L" "/x-tools/powerpc64-unknown-linux-gnu/lib/gcc/powerpc64-unknown-linux-gnu/8.5.0/../../../../powerpc64-unknown-linux-gnu/lib/../lib64" "-L" "<sysroot>/lib/rustlib/powerpc64-unknown-linux-gnu/lib" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-rustc/powerpc64-unknown-linux-gnu/release/deps/librustc_driver-8987a4d9fd0f20fe.so" "-shared" "-Wl,-soname=librustc_driver-8987a4d9fd0f20fe.so" "-Wl,-z,relro,-z,now" "-Wl,-O1" "-nodefaultlibs" "-Wl,-z,origin" "-Wl,-rpath,$ORIGIN/../lib"
= note: some arguments are omitted. use `--verbose` to show all linker arguments
= note: /x-tools/powerpc64-unknown-linux-gnu/lib/gcc/powerpc64-unknown-linux-gnu/8.5.0/../../../../powerpc64-unknown-linux-gnu/bin/ld: BFD (crosstool-NG UNKNOWN) 2.32 assertion fail /tmp/build/.build/powerpc64-unknown-linux-gnu/src/binutils/bfd/elf64-ppc.c:10251
/x-tools/powerpc64-unknown-linux-gnu/lib/gcc/powerpc64-unknown-linux-gnu/8.5.0/../../../../powerpc64-unknown-linux-gnu/bin/ld: BFD (crosstool-NG UNKNOWN) 2.32 assertion fail /tmp/build/.build/powerpc64-unknown-linux-gnu/src/binutils/bfd/elf64-ppc.c:10251
collect2: error: ld returned 1 exit status
[RUSTC-TIMING] rustc_driver test:false 20.623
error: could not compile `rustc_driver` (lib) due to 1 previous error

bors · 2026-01-06T19:40:22Z

💔 Test failed - checks-actions

folkertdev · 2026-01-06T19:43:21Z

Hmm, this PR makes changes for macos only, so I'd guess this is a spurious failure?

folkertdev · 2026-01-06T21:25:00Z

@bors2 try job=dist-powerpc64-linux

rust-bors · 2026-01-06T21:25:02Z

Unknown command "2". Run @bors help to see available commands.

naked functions: emit `.private_extern` on macos try-job: dist-powerpc64-linux

rust-bors · 2026-01-06T23:05:59Z

☀️ Try build successful (CI)
Build commit: fb973ae (fb973ae9e7b77ae1dcb9c74b5a434f44a8a4c2c9, parent: 4d73a008eaf6ba3ca067a3221e5c00602e33a5f2)

folkertdev · 2026-01-06T23:07:24Z

@bors r=Amanieu

bors · 2026-01-06T23:07:27Z

💡 This pull request was already approved, no need to approve it again.

This pull request previously failed. You should add more commits to fix the bug, or use retry to trigger a build again.
There's another pull request that is currently being tested, blocking this pull request: Port #[cfg] and #[cfg_attr] trace attributes to the new attribute parsers #150310

bors · 2026-01-06T23:07:27Z

📌 Commit 76d0843 has been approved by Amanieu

It is now in the queue for this repository.

rust-bors · 2026-01-07T23:58:42Z

☀️ Test successful - CI
Approved by: Amanieu
Pushing fecb335 to main...

rust-log-analyzer · 2026-01-08T00:10:10Z

A job failed! Check out the build log: (web)(plain enhanced)(plain)

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

 Running `target/release/citool post-merge-report fecb335cbad3d84ef3da39191ba094e6c726a5b4`
error: the following required arguments were not provided:
<CURRENT>
Usage: citool post-merge-report <PARENT> <CURRENT>
For more information, try '--help'.
##[error]Process completed with exit code 2.
Post job cleanup.

rust-timer · 2026-01-08T02:01:44Z

Finished benchmarking commit (fecb335): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 1.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.5%	[1.5%, 1.5%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.5%	[1.5%, 1.5%]	1

Cycles

Results (secondary 3.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.0%	[3.0%, 3.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 487.524s -> 473.64s (-2.85%)
Artifact size: 390.86 MiB -> 390.86 MiB (0.00%)

folkertdev · 2026-02-01T14:33:18Z

+ // When LTO inlines the caller of a naked function, it will attempt but fail to make the
+ // naked function symbol visible. To ensure that LTO works correctly, do not default
+ // naked functions to internal linkage and default visibility.
+ if let MonoItem::Fn(instance) = item {
+ let flags = cx.tcx.codegen_instance_attrs(instance.def).flags;
+ if flags.contains(CodegenFnAttrFlags::NAKED) {
+ continue;
+ }
+ }


@bjorn3 why would we handle naked functions differently from some other external symbol defined in global assembly?
extern"C"{fnfoo();}global_asm!("foo:","ret");
is it just that nobody ran into this issue with global assembly so far? In any case this fix is causing issues elsewhere, see #151946

global_asm!("foo:","ret");
this defines a local symbol, which means that only if foo gets called within the exact same object file will it work. There is never a guarantee that the global_asm!() gets added to the same codegen unit, so without .globl foo or (if you don't use dylibs) .hidden foo your example is effectively invalid.

folkertdev added the A-naked Area: `#[naked]`, prologue and epilogue-free, functions, https://git.io/vAzzS label Oct 31, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 31, 2025

rustbot assigned Amanieu Oct 31, 2025

folkertdev force-pushed the naked-macos-private-extern branch from 502098d to d96ae8fCompare November 2, 2025 20:59

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JonathanBrouwer mentioned this pull request Jan 6, 2026
Rollup of 11 pull requests #150724
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bors added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jan 6, 2026

naked functions: emit .private_extern on macos
76d0843

JonathanBrouwer mentioned this pull request Jan 6, 2026
Rollup of 4 pull requests #150729
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rust-borsBot added a commit that referenced this pull request Jan 6, 2026
Auto merge of#148339- folkertdev:naked-macos-private-extern, r=<try>
fb973ae
naked functions: emit `.private_extern` on macos try-job: dist-powerpc64-linux

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 6, 2026

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rust-borsBot added the merged-by-bors This PR was explicitly merged by bors. label Jan 7, 2026

rust-borsBot merged commit fecb335 into rust-lang:mainJan 7, 2026
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rustbot added this to the 1.94.0 milestone Jan 7, 2026

github-actionsBot mentioned this pull request Jan 27, 2026
Toolchain upgrade to nightly-2026-02-09 flux-rs/flux#1345
Closed

folkertdev mentioned this pull request Feb 1, 2026
unsafe(naked) functions might not be called via a thunk #151946
Closed

folkertdev commented Feb 1, 2026
View reviewed changes

folkertdev mentioned this pull request Feb 2, 2026
Set hidden visibility on naked functions in compiler-builtins #151998
Merged

folkertdev added the F-naked_functions `#![feature(naked_functions)]` label May 7, 2026

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Overall result: ✅ improvements - no action needed

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Cycles

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