Skip to content

naked functions: emit .private_extern on macos #148339

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
rust-bors merged 1 commit into from
Jan 7, 2026
Merged

naked functions: emit .private_extern on macos #148339

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
3 changes: 2 additions & 1 deletion compiler/rustc_codegen_ssa/src/mir/naked_asm.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -181,7 +181,8 @@ fn prefix_and_suffix<'tcx>(
}
}
Linkage::Internal => {
// write nothing
// LTO can fail when internal linkage is used.
emit_fatal("naked functions may not have internal linkage")
Comment on lines -184 to +185
Copy link
Contributor Author

@folkertdev folkertdev Nov 2, 2025 •
edited
Loading

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

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

Copy link
Member

@Amanieu Amanieu Jan 6, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

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

}
Linkage::Common => emit_fatal("Functions may not have common linkage"),
Linkage::AvailableExternally => {
Expand Down
10 changes: 10 additions & 0 deletions compiler/rustc_monomorphize/src/partitioning.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -580,6 +580,16 @@ fn internalize_symbols<'tcx>(
}
}

// 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;
}
}
Comment on lines +583 to +591
Copy link
Contributor Author

@folkertdev folkertdev Feb 1, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

@bjorn3 why would we handle naked functions differently from some other external symbol defined in global assembly?

extern "C" {
    fn foo();
}

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

Copy link
Member

@bjorn3 bjorn3 Feb 2, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more. 

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.


// 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;
Expand Down
2 changes: 1 addition & 1 deletion tests/assembly-llvm/naked-functions/aix.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -9,7 +9,7 @@
//@[aix] needs-llvm-components: powerpc

#![crate_type = "lib"]
#![feature(no_core, asm_experimental_arch, f128, linkage, fn_align)]
#![feature(no_core, asm_experimental_arch)]
#![no_core]

// tests that naked functions work for the `powerpc64-ibm-aix` target.
Expand Down
41 changes: 41 additions & 0 deletions tests/assembly-llvm/naked-functions/hidden.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,41 @@
//@ revisions: macos-x86 macos-aarch64 linux-x86
//@ add-minicore
//@ assembly-output: emit-asm
//
//@[macos-aarch64] compile-flags: --target aarch64-apple-darwin
//@[macos-aarch64] needs-llvm-components: aarch64
//
//@[macos-x86] compile-flags: --target x86_64-apple-darwin
//@[macos-x86] needs-llvm-components: x86
//
//@[linux-x86] compile-flags: --target x86_64-unknown-linux-gnu
//@[linux-x86] needs-llvm-components: x86

#![crate_type = "lib"]
#![feature(no_core, asm_experimental_arch)]
#![no_core]

// Tests that naked functions that are not externally linked (e.g. via `no_mangle`)
// are marked as `Visibility::Hidden` and emit `.private_extern` or `.hidden`.
//
// Without this directive, LTO may fail because the symbol is not visible.
// See also https://github.com/rust-lang/rust/issues/148307.

extern crate minicore;
use minicore::*;

// CHECK: .p2align 2
// macos-x86,macos-aarch64: .private_extern
// linux-x86: .globl
// linux-x86: .hidden
// CHECK: ret
#[unsafe(naked)]
extern "C" fn ret() {
naked_asm!("ret")
}

// CHECK-LABEL: entry
#[no_mangle]
pub fn entry() {
ret()
}
Loading