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Auto merge of #140872 - bjorn3:elf_use_used_linker, r=nikic

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Make #[used(linker)] the default on ELF too

`#[used]` currently is an alias for `#[used(linker)]` on all platforms except ELF based ones where it is an alias for `#[used(compiler)]`. The latter has surprising behavior and the LLVM LangRef explicitly states that it "should only be used in rare circumstances, and should not be exposed to source languages." [^2]

The reason `#[used]` still was an alias to `#[used(compiler)]` on ELF is because the gold linker has issues with it. Luckily gold has been deprecated with GCC 15 [^1] and seems to be unable to bootstrap rustc anyway [^3]. As such we shouldn't really care about supporting gold.

This would also allow re-enabling start-stop-gc with lld.

cc https://github.com/rust-lang/rust/issues/93798
Likely fixes https://github.com/rust-lang/rust/issues/85045

[^1]: https://lists.gnu.org/archive/html/info-gnu/2025-02/msg00001.html
[^2]: https://llvm.org/docs/LangRef.html#the-llvm-compiler-used-global-variable
[^3]: https://github.com/rust-lang/rust/issues/139425
This commit is contained in:
bors
2025-06-05 22:52:17 +00:00
parent ccf3198de3 f8e9778eb1
commit cf423712b9
3 changed files with 13 additions and 37 deletions
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5
compiler/rustc_codegen_ssa/src/back/link.rs
View File

@@ -1961,7 +1961,7 @@ fn add_post_link_args(cmd: &mut dyn Linker, sess: &Session, flavor: LinkerFlavor
/// This method creates a synthetic object file, which contains undefined references to all symbols
/// that are necessary for the linking. They are only present in symbol table but not actually
/// used in any sections, so the linker will therefore pick relevant rlibs for linking, but
/// unused `#[no_mangle]` or `#[used]` can still be discard by GC sections.
/// unused `#[no_mangle]` or `#[used(compiler)]` can still be discard by GC sections.
///
/// There's a few internal crates in the standard library (aka libcore and
/// libstd) which actually have a circular dependence upon one another. This
@@ -1995,7 +1995,8 @@ fn add_linked_symbol_object(
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.
// Without this flag, unused `#[no_mangle]` or `#[used(compiler)]` cannot be
// discard on MachO targets.
file.set_subsections_via_symbols();
}

33
compiler/rustc_codegen_ssa/src/codegen_attrs.rs
View File

@@ -195,35 +195,10 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
tcx.dcx().emit_err(errors::ExpectedUsedSymbol { span: attr.span() });
}
None => {
// 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_COMPILER
} else {
CodegenFnAttrFlags::USED_LINKER
};
// Unconditionally using `llvm.used` causes issues in handling
// `.init_array` with the gold linker. Luckily gold has been
// deprecated with GCC 15 and rustc now warns about using gold.
codegen_fn_attrs.flags |= CodegenFnAttrFlags::USED_LINKER
}
}
}