From a64b2a95ff3155fd6dc85312cb4ceac232d28d02 Mon Sep 17 00:00:00 2001
From: Thom Chiovoloni <chiovolonit@gmail.com>
Date: Wed, 11 May 2022 01:55:00 -0700
Subject: [PATCH] Move `#[used]` check for Mach-O to `rustc_typeck` from
`rustc_codegen_llvm`
---
compiler/rustc_codegen_llvm/src/consts.rs | 34 +++++------------------
compiler/rustc_typeck/src/collect.rs | 32 ++++++++++++++++++++-
2 files changed, 38 insertions(+), 28 deletions(-)
diff --git a/compiler/rustc_codegen_llvm/src/consts.rs b/compiler/rustc_codegen_llvm/src/consts.rs
index a1ab590335a..ea5df912438 100644
--- a/compiler/rustc_codegen_llvm/src/consts.rs
+++ b/compiler/rustc_codegen_llvm/src/consts.rs
@@ -543,33 +543,13 @@ impl<'ll> StaticMethods for 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).
//
- // However, unconditional use of `llvm.compiler.used` caused a nontrivial amount of
- // ecosystem breakage, especially on Mach-O targets. To resolve this, we compile it
- // as llvm.compiler.used on ELF targets and llvm.used elsewhere, which should be
- // equivalent to how we compiled `#[used]` before LLVM 13, as `llvm.used` and
- // `llvm.compiler.used` were treated the same on ELF targets prior in earlier LLVM
- // versions (additionally, it seems to be how Clang handles `__attribute__((used))`,
- // perhaps for similar compatibility-motivated reasons).
- //
- // See https://github.com/rust-lang/rust/issues/47384#issuecomment-1019080146 and
- // following comments for some discussion of this.
- //
- // The final wrinkle is it's not really clear how to tell if we're going to output
- // ELF, so it's been approximated as "not like wasm, osx, or windows", which is
- // not exactly correct, but is pretty close and hopefully handles all the platforms
- // platforms where old versions of `ld.gold` are likely to show up.
- //
- // All this is subject to change in the future. Which is a good thing, because this
- // probably should be firmed up somehow!
- let seems_like_elf = !(self.tcx.sess.target.is_like_osx
- || self.tcx.sess.target.is_like_windows
- || self.tcx.sess.target.is_like_wasm);
-
- if seems_like_elf {
- self.add_compiler_used_global(g);
- } else {
- self.add_used_global(g);
- }
+ // 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_typeck`,
+ // so we don't need to take care of it here.
+ self.add_compiler_used_global(g);
}
if attrs.flags.contains(CodegenFnAttrFlags::USED_LINKER) {
// `USED` and `USED_LINKER` can't be used together.
diff --git a/compiler/rustc_typeck/src/collect.rs b/compiler/rustc_typeck/src/collect.rs
index 2e0e026631b..e13c8322e33 100644
--- a/compiler/rustc_typeck/src/collect.rs
+++ b/compiler/rustc_typeck/src/collect.rs
@@ -2850,7 +2850,37 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: DefId) -> CodegenFnAttrs {
)
.emit();
}
- None => codegen_fn_attrs.flags |= CodegenFnAttrFlags::USED,
+ None => {
+ // Unfortunately, unconditionally using `llvm.used` causes
+ // issues in handling `.init_array` with the gold linker,
+ // but using `llvm.compiler.used` caused a nontrival 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_osx
+ || 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
+ };
+ }
}
} else if attr.has_name(sym::cmse_nonsecure_entry) {
if !matches!(tcx.fn_sig(did).abi(), abi::Abi::C { .. }) {