From f8e9778eb12573eabac183f5d617e1a7404505c7 Mon Sep 17 00:00:00 2001 From: bjorn3 <17426603+bjorn3@users.noreply.github.com> Date: Fri, 9 May 2025 20:37:06 +0000 Subject: [PATCH] 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." 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 and seems to be unable to bootstrap rustc anyway. As such we shouldn't really care about supporting gold. --- compiler/rustc_codegen_llvm/src/consts.rs | 12 +++---- compiler/rustc_codegen_ssa/src/back/link.rs | 5 +-- .../rustc_codegen_ssa/src/codegen_attrs.rs | 33 +++---------------- 3 files changed, 13 insertions(+), 37 deletions(-) diff --git a/compiler/rustc_codegen_llvm/src/consts.rs b/compiler/rustc_codegen_llvm/src/consts.rs index fe2f20273276a..866696276e172 100644 --- a/compiler/rustc_codegen_llvm/src/consts.rs +++ b/compiler/rustc_codegen_llvm/src/consts.rs @@ -541,12 +541,12 @@ impl<'ll> CodegenCx<'ll, '_> { // in the handling of `.init_array` (the static constructor list) in versions of // the gold linker (prior to the one released with binutils 2.36). // - // That said, we only ever emit these when compiling for ELF targets, unless - // `#[used(compiler)]` is explicitly requested. This is to avoid similar breakage - // on other targets, in particular MachO targets have *their* static constructor - // lists broken if `llvm.compiler.used` is emitted rather than `llvm.used`. However, - // that check happens when assigning the `CodegenFnAttrFlags` in - // `rustc_hir_analysis`, so we don't need to take care of it here. + // That said, we only ever emit these when `#[used(compiler)]` is explicitly + // requested. This is to avoid similar breakage on other targets, in particular + // MachO targets have *their* static constructor lists broken if `llvm.compiler.used` + // is emitted rather than `llvm.used`. However, that check happens when assigning + // the `CodegenFnAttrFlags` in the `codegen_fn_attrs` query, so we don't need to + // take care of it here. self.add_compiler_used_global(g); } if attrs.flags.contains(CodegenFnAttrFlags::USED_LINKER) { diff --git a/compiler/rustc_codegen_ssa/src/back/link.rs b/compiler/rustc_codegen_ssa/src/back/link.rs index c5792da267819..30128c4bea2fa 100644 --- a/compiler/rustc_codegen_ssa/src/back/link.rs +++ b/compiler/rustc_codegen_ssa/src/back/link.rs @@ -1986,7 +1986,7 @@ 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 @@ -2020,7 +2020,8 @@ 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(); } diff --git a/compiler/rustc_codegen_ssa/src/codegen_attrs.rs b/compiler/rustc_codegen_ssa/src/codegen_attrs.rs index 4bcafa3be3662..0b31fa8fa886b 100644 --- a/compiler/rustc_codegen_ssa/src/codegen_attrs.rs +++ b/compiler/rustc_codegen_ssa/src/codegen_attrs.rs @@ -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 } } }