Add `-Zfunction-return={keep,thunk-extern}` option

This is intended to be used for Linux kernel RETHUNK builds.

With this commit (optionally backported to Rust 1.73.0), plus a
patched Linux kernel to pass the flag, I get a RETHUNK build with
Rust enabled that is `objtool`-warning-free and is able to boot in
QEMU and load a sample Rust kernel module.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 28 insertions, 0 deletions

diff --git a/tests/codegen/function-return.rs b/tests/codegen/function-return.rs
new file mode 100644
index 00000000000..d832d19ac39
--- /dev/null
+++ b/tests/codegen/function-return.rs
@@ -0,0 +1,28 @@
+// Test that the `fn_ret_thunk_extern` function attribute is (not) emitted when
+// the `-Zfunction-return={keep,thunk-extern}` flag is (not) set.
+
+// revisions: unset keep thunk-extern keep-thunk-extern thunk-extern-keep
+// needs-llvm-components: x86
+// compile-flags: --target x86_64-unknown-linux-gnu
+// [keep] compile-flags: -Zfunction-return=keep
+// [thunk-extern] compile-flags: -Zfunction-return=thunk-extern
+// [keep-thunk-extern] compile-flags: -Zfunction-return=keep -Zfunction-return=thunk-extern
+// [thunk-extern-keep] compile-flags: -Zfunction-return=thunk-extern -Zfunction-return=keep
+
+#![crate_type = "lib"]
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang = "sized"]
+trait Sized {}
+
+#[no_mangle]
+pub fn foo() {
+    // CHECK: @foo() unnamed_addr #0
+
+    // unset-NOT: fn_ret_thunk_extern
+    // keep-NOT: fn_ret_thunk_extern
+    // thunk-extern: attributes #0 = { {{.*}}fn_ret_thunk_extern{{.*}} }
+    // keep-thunk-extern: attributes #0 = { {{.*}}fn_ret_thunk_extern{{.*}} }
+    // thunk-extern-keep-NOT: fn_ret_thunk_extern
+}