naked functions: emit .private_extern on macos

Author: folkertdev

compiler/rustc_codegen_ssa/src/mir/naked_asm.rs

@@ -180,9 +180,13 @@ fn prefix_and_suffix<'tcx>(
                     }
                 }
             }
-            Linkage::Internal => {
-                // write nothing
-            }
+            Linkage::Internal => match asm_binary_format {
+                BinaryFormat::MachO => {
+                    // See https://github.com/rust-lang/rust/issues/148307.
+                    writeln!(w, ".private_extern {asm_name}")?;
+                }
+                _ => { /* write nothing */ }
+            },
             Linkage::Common => emit_fatal("Functions may not have common linkage"),
             Linkage::AvailableExternally => {
                 // this would make the function equal an extern definition

tests/assembly-llvm/naked-functions/aix.rs

@@ -9,7 +9,7 @@
 //@[aix] needs-llvm-components: powerpc
 
 #![crate_type = "lib"]
-#![feature(no_core, asm_experimental_arch, f128, linkage, fn_align)]
+#![feature(no_core, asm_experimental_arch)]
 #![no_core]
 
 // tests that naked functions work for the `powerpc64-ibm-aix` target.

tests/assembly-llvm/naked-functions/macos.rs

@@ -0,0 +1,34 @@
+//@ revisions: x86 aarch64
+//@ add-core-stubs
+//@ assembly-output: emit-asm
+//
+//@[aarch64] compile-flags: --target aarch64-apple-darwin
+//@[aarch64] needs-llvm-components: aarch64
+//
+//@[x86] compile-flags: --target x86_64-apple-darwin
+//@[x86] needs-llvm-components: x86
+
+#![crate_type = "lib"]
+#![feature(no_core, asm_experimental_arch)]
+#![no_core]
+
+// tests that naked functions on macos emit `.private_extern {asm_name}`.
+//
+// Without this directive, LTO may fail because the symbol is not visible.
+// See also https://github.com/rust-lang/rust/issues/148307.
+
+extern crate minicore;
+use minicore::*;
+
+// CHECK: .p2align 2
+// CHECK: .private_extern
+// CHECK: ret
+#[unsafe(naked)]
+extern "C" fn ret() {
+    naked_asm!("ret")
+}
+
+#[no_mangle]
+pub fn entry() {
+    ret()
+}