Skip to content

set subsections_via_symbols for ld64 helper sections #139752

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
merged 1 commit into from
Apr 25, 2025
Merged

set subsections_via_symbols for ld64 helper sections #139752

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
6 changes: 6 additions & 0 deletions compiler/rustc_codegen_ssa/src/back/link.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -2015,6 +2015,12 @@ fn add_linked_symbol_object(
file.set_mangling(object::write::Mangling::None);
}

if file.format() == object::BinaryFormat::MachO {
// Divide up the sections into sub-sections via symbols for dead code stripping.
// Without this flag, unused `#[no_mangle]` or `#[used]` cannot be discard on MachO targets.
file.set_subsections_via_symbols();
}

// ld64 requires a relocation to load undefined symbols, see below.
// Not strictly needed if linking with lld, but might as well do it there too.
let ld64_section_helper = if file.format() == object::BinaryFormat::MachO {
Expand Down
20 changes: 20 additions & 0 deletions tests/ui/linking/cdylib-no-mangle.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,20 @@
//@ only-apple
//@ build-fail
//@ dont-check-compiler-stderr
//@ dont-check-compiler-stdout

// Regression test for <https://github.com/rust-lang/rust/issues/139744>.
// Functions in the dynamic library marked with no_mangle should not be GC-ed.

#![crate_type = "cdylib"]

unsafe extern "C" {
unsafe static THIS_SYMBOL_SHOULD_BE_UNDEFINED: usize;
}

#[unsafe(no_mangle)]
pub unsafe fn function_marked_with_no_mangle() {
println!("FUNCTION_MARKED_WITH_NO_MANGLE = {}", unsafe { THIS_SYMBOL_SHOULD_BE_UNDEFINED });
}

//~? ERROR linking
27 changes: 27 additions & 0 deletions tests/ui/linking/executable-no-mangle-strip.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,27 @@
//@ run-pass
//@ ignore-windows-gnu: only statics marked with used can be GC-ed on windows-gnu

// Regression test for <https://github.com/rust-lang/rust/issues/139744>.
// Functions in the binary marked with no_mangle should be GC-ed if they
// are not indirectly referenced by main.

#![feature(used_with_arg)]

#[cfg_attr(windows, link(name = "this_lib_does_not_exist", kind = "raw-dylib"))]
unsafe extern "C" {
unsafe static THIS_SYMBOL_SHOULD_BE_UNDEFINED: usize;
}

#[unsafe(no_mangle)]
pub unsafe fn function_marked_with_no_mangle() {
println!("FUNCTION_MARKED_WITH_NO_MANGLE = {}", unsafe { THIS_SYMBOL_SHOULD_BE_UNDEFINED });
}
Copy link
Contributor

Choose a reason for hiding this comment

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

Nit: I know that #[no_mangle] implies it, but could we also add an explicit test for #[used]?

Copy link
Contributor Author

@usamoi usamoi Apr 14, 2025 •
edited
Loading

Choose a reason for hiding this comment

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

#[used] emits errors, while #[no_mangle] does not, on MacOS.

Don't know why.

Edit: using pr + lld, nightly + lld, stable + lld, stable + ld64, #[used] emits errors, too.

Copy link
Contributor Author

@usamoi usamoi Apr 14, 2025 •
edited
Loading

Choose a reason for hiding this comment

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

I know why now.

On MacOS, #[used] emits llvm.used. So #[used(compiler)] is needed here. Is it expected behavior?

Copy link
Member

@bjorn3 bjorn3 Apr 14, 2025 •
edited
Loading

Choose a reason for hiding this comment

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

TIL that #[used] is an alias for #[used(linker)] on some platforms:

rust/compiler/rustc_codegen_ssa/src/codegen_attrs.rs

Lines 195 to 223 in f836ae4

// Unfortunately, unconditionally using `llvm.used` causes
// issues in handling `.init_array` with the gold linker,
// but using `llvm.compiler.used` caused a nontrivial amount
// of unintentional ecosystem breakage -- particularly on
// Mach-O targets.
//
// As a result, we emit `llvm.compiler.used` only on ELF
// targets. This is somewhat ad-hoc, but actually follows
// our pre-LLVM 13 behavior (prior to the ecosystem
// breakage), and seems to match `clang`'s behavior as well
// (both before and after LLVM 13), possibly because they
// have similar compatibility concerns to us. See
// https://github.com/rust-lang/rust/issues/47384#issuecomment-1019080146
// and following comments for some discussion of this, as
// well as the comments in `rustc_codegen_llvm` where these
// flags are handled.
//
// Anyway, to be clear: this is still up in the air
// somewhat, and is subject to change in the future (which
// is a good thing, because this would ideally be a bit
// more firmed up).
let is_like_elf = !(tcx.sess.target.is_like_darwin
|| tcx.sess.target.is_like_windows
|| tcx.sess.target.is_like_wasm);
codegen_fn_attrs.flags |= if is_like_elf {
CodegenFnAttrFlags::USED
} else {
CodegenFnAttrFlags::USED_LINKER
};
Eventually #[used] should be changed to #[used(linker)] unconditionally anyway. Gold is deprecated upstream and broken with current rustc versions anyway: #139425


#[used(compiler)]
pub static FUNCTION_MARKED_WITH_USED: unsafe fn() = || {
println!("FUNCTION_MARKED_WITH_USED = {}", unsafe { THIS_SYMBOL_SHOULD_BE_UNDEFINED });
};

fn main() {
println!("MAIN");
}
Loading