From 7fc596fb67d31a9d5266302771022ec14e22e1e8 Mon Sep 17 00:00:00 2001
From: Folkert <folkert@folkertdev.nl>
Date: Wed, 31 Jul 2024 20:05:29 +0200
Subject: [PATCH] use `LinkageKind` to our advantage for naked functions

---
 compiler/rustc_codegen_llvm/src/mono_item.rs | 10 +-------
 compiler/rustc_codegen_ssa/src/mono_item.rs  | 24 ++++++++++++++++++--
 2 files changed, 23 insertions(+), 11 deletions(-)

diff --git a/compiler/rustc_codegen_llvm/src/mono_item.rs b/compiler/rustc_codegen_llvm/src/mono_item.rs
index fda3176ece82f..8e8558d6a6672 100644
--- a/compiler/rustc_codegen_llvm/src/mono_item.rs
+++ b/compiler/rustc_codegen_llvm/src/mono_item.rs
@@ -2,7 +2,6 @@ use rustc_codegen_ssa::traits::*;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
 use rustc_middle::bug;
-use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::mir::mono::{Linkage, Visibility};
 use rustc_middle::ty::layout::{FnAbiOf, LayoutOf};
 use rustc_middle::ty::{self, Instance, TypeVisitableExt};
@@ -63,14 +62,7 @@ impl<'tcx> PreDefineMethods<'tcx> for CodegenCx<'_, 'tcx> {
         let fn_abi = self.fn_abi_of_instance(instance, ty::List::empty());
         let lldecl = self.declare_fn(symbol_name, fn_abi, Some(instance));
         let attrs = self.tcx.codegen_fn_attrs(instance.def_id());
-        let llvm_linkage =
-            if attrs.flags.contains(CodegenFnAttrFlags::NAKED) && linkage == Linkage::Internal {
-                // this is effectively an extern fn, and must have external linkage
-                llvm::Linkage::ExternalLinkage
-            } else {
-                base::linkage_to_llvm(linkage)
-            };
-        unsafe { llvm::LLVMRustSetLinkage(lldecl, llvm_linkage) };
+        unsafe { llvm::LLVMRustSetLinkage(lldecl, base::linkage_to_llvm(linkage)) };
         base::set_link_section(lldecl, attrs);
         if linkage == Linkage::LinkOnceODR || linkage == Linkage::WeakODR {
             llvm::SetUniqueComdat(self.llmod, lldecl);
diff --git a/compiler/rustc_codegen_ssa/src/mono_item.rs b/compiler/rustc_codegen_ssa/src/mono_item.rs
index 5a0d46ea999c9..889ff9865f53b 100644
--- a/compiler/rustc_codegen_ssa/src/mono_item.rs
+++ b/compiler/rustc_codegen_ssa/src/mono_item.rs
@@ -1,6 +1,7 @@
 use rustc_hir as hir;
+use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::mir::interpret::ErrorHandled;
-use rustc_middle::mir::mono::{Linkage, MonoItem, Visibility};
+use rustc_middle::mir::mono::{Linkage, LinkageKind, MonoItem, Visibility};
 use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf};
 use rustc_middle::ty::Instance;
 use rustc_middle::{span_bug, ty};
@@ -135,7 +136,26 @@ impl<'a, 'tcx: 'a> MonoItemExt<'a, 'tcx> for MonoItem<'tcx> {
                 cx.predefine_static(def_id, linkage_kind.into_linkage(), visibility, symbol_name);
             }
             MonoItem::Fn(instance) => {
-                cx.predefine_fn(instance, linkage_kind.into_linkage(), visibility, symbol_name);
+                let linkage = match linkage_kind {
+                    LinkageKind::Explicit(linkage) => linkage,
+                    LinkageKind::ImplicitExternal => Linkage::External,
+                    LinkageKind::ImplicitInternal => {
+                        let attrs = cx.tcx().codegen_fn_attrs(instance.def_id());
+                        if attrs.flags.contains(CodegenFnAttrFlags::NAKED) {
+                            // to connect the extern fn declaration and the global asm definition of
+                            // a naked function, the linkage must be some flavor of external.
+                            // `LinkageKind::ImplicitInternal` indicates that rust does not really care about
+                            // the linkage, and did not find any references outside of the current CGU. That
+                            // means we're free to pick a different linkage here that is just for the backend,
+                            // and should not influence the symbol visibility in a meaningful way.
+                            Linkage::External
+                        } else {
+                            Linkage::Internal
+                        }
+                    }
+                };
+
+                cx.predefine_fn(instance, linkage, visibility, symbol_name);
             }
             MonoItem::GlobalAsm(..) => {}
         }