Compiling to Apple's `arm64e` got linking error!

[Wyvern]

When compiling to Apple's arm64e targets arm64e-apple-ios arm64e-apple-tvos arm64e-apple-darwin using -Zbuild-stdgot linking error:

linking with `cc` failed: exit status: 1
          rust-lld: error: INVALID relocation has width 8 bytes, but must be 0 bytes at offset 128 of __DATA,__data in /var/folders/r4/ph658m2d3vs0vj00g75q0zyr0000gn/T/rustcj3lmdR/symbols.o
          rust-lld: error: INVALID relocation has width 8 bytes, but must be 0 bytes at offset 0 of __DATA,__data in /var/folders/r4/ph658m2d3vs0vj00g75q0zyr0000gn/T/rustcj3lmdR/symbols.o
          clang: error: linker command failed with exit code 1 (use -v to see invocation)

Rust: 1.87.0-nightly (cbfdf0b 2025-03-13)

[bjorn3]

Probably something wrong in

rust/compiler/rustc_codegen_ssa/src/back/apple.rs

Lines 30 to 111 in f7b4354

	/// Add relocation and section data needed for a symbol to be considered
	/// undefined by ld64.
	///
	/// The relocation must be valid, and hence must point to a valid piece of
	/// machine code, and hence this is unfortunately very architecture-specific.
	///
	///
	/// # New architectures
	///
	/// The values here are basically the same as emitted by the following program:
	///
	/// ```c
	/// // clang -c foo.c -target $CLANG_TARGET
	/// void foo(void);
	///
	/// extern int bar;
	///
	/// void* foobar[2] = {
	/// (void*)foo,
	/// (void*)&bar,
	/// // ...
	/// };
	/// ```
	///
	/// Can be inspected with:
	/// ```console
	/// objdump --macho --reloc foo.o
	/// objdump --macho --full-contents foo.o
	/// ```
	pub(super) fn add_data_and_relocation(
	file: &mut object::write::Object<'_>,
	section: object::write::SectionId,
	symbol: object::write::SymbolId,
	target: &Target,
	kind: SymbolExportKind,
	) -> object::write::Result<()> {
	let authenticated_pointer =
	kind == SymbolExportKind::Text && target.llvm_target.starts_with("arm64e");
	let data: &[u8] = match target.pointer_width {
	_ if authenticated_pointer => &[0, 0, 0, 0, 0, 0, 0, 0x80],
	32 => &[0; 4],
	64 => &[0; 8],
	pointer_width => unimplemented!("unsupported Apple pointer width {pointer_width:?}"),
	};
	if target.arch == "x86_64" {
	// Force alignment for the entire section to be 16 on x86_64.
	file.section_mut(section).append_data(&[], 16);
	} else {
	// Elsewhere, the section alignment is the same as the pointer width.
	file.section_mut(section).append_data(&[], target.pointer_width as u64);
	}
	let offset = file.section_mut(section).append_data(data, data.len() as u64);
	let flags = if authenticated_pointer {
	object::write::RelocationFlags::MachO {
	r_type: object::macho::ARM64_RELOC_AUTHENTICATED_POINTER,
	r_pcrel: false,
	r_length: 3,
	}
	} else if target.arch == "arm" {
	// FIXME(madsmtm): Remove once `object` supports 32-bit ARM relocations:
	// https://github.com/gimli-rs/object/pull/757
	object::write::RelocationFlags::MachO {
	r_type: object::macho::ARM_RELOC_VANILLA,
	r_pcrel: false,
	r_length: 2,
	}
	} else {
	object::write::RelocationFlags::Generic {
	kind: object::RelocationKind::Absolute,
	encoding: object::RelocationEncoding::Generic,
	size: target.pointer_width as u8,
	}
	};
	file.add_relocation(section, object::write::Relocation { offset, addend: 0, symbol, flags })?;
	Ok(())
	}

[madsmtm]

It seems to be a bug in lld. E.g. the following:

void foo(void);

extern int bar;

void* foobar[2] = {
    (void*)foo,
    (void*)&bar,
};

int main() {
    return 0;
}

Fails when linking with rust-lld too:

$ clang -target arm64e-apple-macosx -fuse-ld=$HOME/.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/bin/gcc-ld/ld64.lld foo.c
rust-lld: warning: directory not found for option -L/usr/local/lib
rust-lld: error: INVALID relocation has width 8 bytes, but must be 0 bytes at offset 0 of __DATA,__data in /var/folders/0j/tk3sfgz540712zgqd1hrry0m0000gn/T/foo-e172f0.o
rust-lld: error: undefined symbol: bar
>>> referenced by /var/folders/0j/tk3sfgz540712zgqd1hrry0m0000gn/T/foo-e172f0.o:(symbol foobar+0x8)
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[madsmtm]

Fundamentally because lld doesn't support arm64e at all: llvm/llvm-project#79543.

We could "fix" this in rustc by not doing the relocation stuff on arm64e, but I'm inclined to not do so, linking with lld is horribly broken here and doesn't actually get you an arm64e binary. The upstream issue(s) should be resolved instead.

@rustbot label +A-llvm +A-linkage -S-needs-repro -needs-triage