make naked functions always have external linkage *in LLVM*. If we do…

… it earlier, then some other logic causes invalid visibility for the item (exporting when it shouldn't).

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -402,13 +402,9 @@ pub fn from_fn_attrs<'ll, 'tcx>(
         to_add.push(MemoryEffects::None.create_attr(cx.llcx));
     }
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED) {
-        to_add.push(AttributeKind::Naked.create_attr(cx.llcx));
-        // HACK(jubilee): "indirect branch tracking" works by attaching prologues to functions.
-        // And it is a module-level attribute, so the alternative is pulling naked functions into new LLVM modules.
-        // Otherwise LLVM's "naked" functions come with endbr prefixes per https://github.com/rust-lang/rust/issues/98768
-        to_add.push(AttributeKind::NoCfCheck.create_attr(cx.llcx));
-        // Need this for AArch64.
-        to_add.push(llvm::CreateAttrStringValue(cx.llcx, "branch-target-enforcement", "false"));
+        // do nothing; a naked function is converted into an extern function
+        // and a global assembly block. LLVM's support for naked functions is
+        // not used.
     }
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::ALLOCATOR)
         || codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::ALLOCATOR_ZEROED)

compiler/rustc_codegen_llvm/src/mono_item.rs

@@ -8,6 +8,7 @@ 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};
@@ -62,8 +63,15 @@ 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));
-        unsafe { llvm::LLVMRustSetLinkage(lldecl, base::linkage_to_llvm(linkage)) };
         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) };
         base::set_link_section(lldecl, attrs);
         if linkage == Linkage::LinkOnceODR || linkage == Linkage::WeakODR {
             llvm::SetUniqueComdat(self.llmod, lldecl);

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -109,14 +109,7 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
             sym::rustc_allocator_zeroed => {
                 codegen_fn_attrs.flags |= CodegenFnAttrFlags::ALLOCATOR_ZEROED
             }
-            sym::naked => {
-                codegen_fn_attrs.flags |= CodegenFnAttrFlags::NAKED;
-
-                // naked functions are generated as an extern function, and a global asm block that
-                // contains the naked function's body. In order to link these together, the linkage
-                // of the extern function must be external.
-                codegen_fn_attrs.linkage = Some(Linkage::External);
-            }
+            sym::naked => codegen_fn_attrs.flags |= CodegenFnAttrFlags::NAKED,
             sym::no_mangle => {
                 if tcx.opt_item_name(did.to_def_id()).is_some() {
                     codegen_fn_attrs.flags |= CodegenFnAttrFlags::NO_MANGLE
@@ -563,7 +556,9 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
         }
     });
 
-    // naked function MUST NOT be inlined!
+    // naked function MUST NOT be inlined! This attribute is required for the rust compiler itself,
+    // but not for the code generation backend because at that point the naked function will just be
+    // a declaration, with a definition provided in global assembly.
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED) {
         codegen_fn_attrs.inline = InlineAttr::Never;
     }