Unable to link code containing naked functions on ARM64 macOS with ThinLTO enabled

[mkaput]

I have originally reported this issue in wasmtime repository and was redirected here: bytecodealliance/wasmtime#11957. I have tried to minimize the reproduction, but I failed to do so in reasonable time. The complexity of wasmtime crates is critical here. I would appreciate any hints that would help me minimize this.

Compiling Wasmtime 38.0.3 produces a linker error on ARM64 macOS when building with ThinLTO enabled. The error indicates that wasmtime_fiber_switch symbol is undefined during linking.

error: linking with `cc` failed: exit status: 1
  |
  = note:  "cc" "<426 object files omitted>" "/var/folders/8c/r_zrp9mj12xd414t08zrks940000gn/T/rustc2wYYXA/{libwasmtime-e843f7bbc21927d3,libzstd_sys-7eabaf414ee67d02,libwasmtime_internal_jit_debug-9043bb9a4e496da8}.rlib" "<sysroot>/lib/rustlib/aarch64-apple-darwin/lib/libcompiler_builtins-*.rlib" "-liconv" "-lSystem" "-lc" "-lm" "-arch" "arm64" "-mmacosx-version-min=11.0.0" "-L" "/Users/mk/Developer/software-mansion/scarb/linker-repro/wasmtime-only-test/target/release/build/wasmtime-15671f59b24bbe0f/out" "-L" "/Users/mk/Developer/software-mansion/scarb/linker-repro/wasmtime-only-test/target/release/build/zstd-sys-1ea01473940232bc/out" "-L" "/Users/mk/Developer/software-mansion/scarb/linker-repro/wasmtime-only-test/target/release/build/wasmtime-internal-jit-debug-2aaf81cce8508d62/out" "-o" "/Users/mk/Developer/software-mansion/scarb/linker-repro/wasmtime-only-test/target/release/deps/wasmtime_only_test-69cba5150eff8962" "-Wl,-dead_strip" "-nodefaultlibs"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: ld: warning: object file (/private/var/folders/8c/r_zrp9mj12xd414t08zrks940000gn/T/rustc2wYYXA/libwasmtime-e843f7bbc21927d3.rlib[2](242b1992def1ef0b-helpers.o)) was built for newer 'macOS' version (26.0) than being linked (11.0)
          ld: warning: object file (/private/var/folders/8c/r_zrp9mj12xd414t08zrks940000gn/T/rustc2wYYXA/libwasmtime_internal_jit_debug-9043bb9a4e496da8.rlib[2](db3b6bfb95261072-gdbjit.o)) was built for newer 'macOS' version (26.0) than being linked (11.0)
          Undefined symbols for architecture arm64:
            "wasmtime_internal_fiber::stackswitch::aarch64::wasmtime_fiber_switch::h0b241ee887e10750", referenced from:
                wasmtime_internal_fiber::unix::Suspend::switch::hfd4514b4efef3e13 in wasmtime_only_test-69cba5150eff8962.wasmtime-e843f7bbc21927d3.wasmtime.7e3b8021754df0c4-cgu.05.rcgu.o.rcgu.o
          ld: symbol(s) not found for architecture arm64
          clang: error: linker command failed with exit code 1 (use -v to see invocation)
          

What's important is that this issue only pops up with ThinLTO enabled. It didn't fail with either LTO disabled, set to FatLTO, or Wasmtime 37.

Steps to Reproduce

• Architecture: ARM64 (Apple Silicon)
• OS: macOS 26.0
• Rust: 1.90.0
• Wasmtime: 38.0.3
• Build Profile: release with lto = "thin"

Cargo.toml:

[package]
name = "linker-repro"
version = "0.1.0"
edition = "2021"

[dependencies]
wasmtime = "38"

[profile.release]
lto = "thin"

src/main.rs:

fn main() {
    let engine = wasmtime::Engine::default();
    println!("Engine created: {:?}", engine);
}

Build command:

cargo build --release

This will fail with the linker error shown above.

I think the wasmtime commit that triggers this regression is: bytecodealliance/wasmtime@3e9eca8

Meta

I have tested this reproduction on all stable+beta+nightly at the time of submitting this issue:

rustc --version --verbose:

rustc 1.90.0 (1159e78c4 2025-09-14)
binary: rustc
commit-hash: 1159e78c4747b02ef996e55082b704c09b970588
commit-date: 2025-09-14
host: aarch64-apple-darwin
release: 1.90.0
LLVM version: 20.1.8

rustc 1.92.0-beta.1 (3b4dd9bf1 2025-10-28)
binary: rustc
commit-hash: 3b4dd9bf1410f8da6329baa36ce5e37673cbbd1f
commit-date: 2025-10-28
host: aarch64-apple-darwin
release: 1.92.0-beta.1
LLVM version: 21.1.3

rustc 1.93.0-nightly (292be5c7c 2025-10-29)
binary: rustc
commit-hash: 292be5c7c05138d753bbd4b30db7a3f1a5c914f7
commit-date: 2025-10-29
host: aarch64-apple-darwin
release: 1.93.0-nightly
LLVM version: 21.1.3

[folkertdev]

I extracted the before and after version of the offending function here https://godbolt.org/z/5a16njzE8

        .section        __TEXT,__text,regular,pure_instructions

        .p2align        4
        .private_extern _wasmtime_fiber_switch
        .globl  _wasmtime_fiber_switch
_wasmtime_fiber_switch:

becomes

        .section        __TEXT,__text,regular,pure_instructions
        .p2align        2
example::wasmtime_fiber_switch_new::h48b3ba9107b841cf:

The .globl and .private_extern lines have disappeared. As you can see, these were added specifically by the old asm_func! macro. Experimentally, either is sufficient to make the example link correctly.

---

a quick fix is to make this function #[unsafe(no_mangle)]: that will emit the .globl.

---

We do have code to emit .globl on macos, here:

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

Lines 160 to 198 in 23c7bad

	let write_linkage = |w: &mut String| -> std::fmt::Result {
	match item_data.linkage {
	Linkage::External => {
	writeln!(w, ".globl {asm_name}")?;
	}
	Linkage::LinkOnceAny | Linkage::LinkOnceODR | Linkage::WeakAny | Linkage::WeakODR => {
	match asm_binary_format {
	BinaryFormat::Elf | BinaryFormat::Coff | BinaryFormat::Wasm => {
	writeln!(w, ".weak {asm_name}")?;
	}
	BinaryFormat::Xcoff => {
	// FIXME: there is currently no way of defining a weak symbol in inline assembly
	// for AIX. See https://github.com/llvm/llvm-project/issues/130269
	emit_fatal(
	"cannot create weak symbols from inline assembly for this target",
	)
	}
	BinaryFormat::MachO => {
	writeln!(w, ".globl {asm_name}")?;
	writeln!(w, ".weak_definition {asm_name}")?;
	}
	}
	}
	Linkage::Internal => {
	// write nothing
	}
	Linkage::Common => emit_fatal("Functions may not have common linkage"),
	Linkage::AvailableExternally => {
	// this would make the function equal an extern definition
	emit_fatal("Functions may not have available_externally linkage")
	}
	Linkage::ExternalWeak => {
	// FIXME: actually this causes a SIGILL in LLVM
	emit_fatal("Functions may not have external weak linkage")
	}
	}
	Ok(())
	};

So somehow the linkage is such that this does not fire, I think? It seems plausible that it is Linkage::Internal, maybe on macos we should emit a directive even then?

[folkertdev]

I suspect that the fix here is to emit .private_extern {asm_name} when the linkage is Linkage::Internal.

[bjorn3]

I think what happens is that rustc sees that the naked function is only callable from functions within the same codegen unit and thus marks the naked function as private, but then ThinLTO decides to inline the caller into another codegen unit. Normally it would make the callee public, but it can't for symbols defined in assembly.

[folkertdev]

thus marks the naked function as private

Does that mean that the linkage becomes Linkage::Internal?

it can't for symbols defined in assembly

Right. The docs on .private_extern are pretty sparse, but to me it seems at least not incorrect to add it to internal naked functions.

[bjorn3]

Does that mean that the linkage becomes Linkage::Internal?

Yes

Right. The docs on .private_extern are pretty sparse, but to me it seems at least not incorrect to add it to internal naked functions.

I think so

[alexcrichton]

A bit smaller as a reproduction:

#[no_mangle]
pub extern "C" fn foo() {
    bar::bar();
}

mod bar {
    pub fn bar() {
        unsafe {
            baz();
        }
    }

    #[unsafe(naked)]
    pub unsafe extern "C" fn baz() {
        std::arch::naked_asm!("ret")
    }
}

yields:

$ rustc foo.rs --crate-type cdylib -O -Ccodegen-units=2
error: linking with `cc` failed: exit status: 1
  |
  = note:  "cc" "-Wl,-exported_symbols_list" "-Wl,/var/folders/70/cx6_8hz93jz7kj_km0_5c9_40000gn/T/rustcCdhCXE/list" "/var/folders/70/cx6_8hz93jz7kj_km0_5c9_40000gn/T/rustcCdhCXE/symbols.o" "<3 object files omitted>" "<sysroot>/lib/rustlib/aarch64-apple-darwin/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" "-lSystem" "-lc" "-lm" "-arch" "arm64" "-mmacosx-version-min=11.0.0" "-o" "libfoo.dylib" "-Wl,-dead_strip" "-dynamiclib" "-nodefaultlibs"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: Undefined symbols for architecture arm64:
            "foo::bar::baz::hdee9697fdd075596", referenced from:
                _foo in foo.foo.2a68415ea2c694de-cgu.1.rcgu.o
          ld: symbol(s) not found for architecture arm64
          clang: error: linker command failed with exit code 1 (use -v to see invocation)


error: aborting due to 1 previous error