From 2d476222e8458aa873e6760aac189b5e9d0a9930 Mon Sep 17 00:00:00 2001
From: Miguel Ojeda <ojeda@kernel.org>
Date: Wed, 18 Oct 2023 16:58:17 +0200
Subject: 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>
---
 compiler/rustc_codegen_llvm/src/attributes.rs | 12 +++++++++++-
 compiler/rustc_codegen_llvm/src/llvm/ffi.rs   |  1 +
 2 files changed, 12 insertions(+), 1 deletion(-)

(limited to 'compiler/rustc_codegen_llvm/src')

diff --git a/compiler/rustc_codegen_llvm/src/attributes.rs b/compiler/rustc_codegen_llvm/src/attributes.rs
index 3e8884e02e0..481741bb127 100644
--- a/compiler/rustc_codegen_llvm/src/attributes.rs
+++ b/compiler/rustc_codegen_llvm/src/attributes.rs
@@ -4,7 +4,7 @@ use rustc_codegen_ssa::traits::*;
 use rustc_hir::def_id::DefId;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::ty::{self, TyCtxt};
-use rustc_session::config::OptLevel;
+use rustc_session::config::{FunctionReturn, OptLevel};
 use rustc_span::symbol::sym;
 use rustc_target::spec::abi::Abi;
 use rustc_target::spec::{FramePointer, SanitizerSet, StackProbeType, StackProtector};
@@ -118,6 +118,15 @@ pub fn frame_pointer_type_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> Option<&'ll Attr
     Some(llvm::CreateAttrStringValue(cx.llcx, "frame-pointer", attr_value))
 }
 
+fn function_return_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> Option<&'ll Attribute> {
+    let function_return_attr = match cx.sess().opts.unstable_opts.function_return {
+        FunctionReturn::Keep => return None,
+        FunctionReturn::ThunkExtern => AttributeKind::FnRetThunkExtern,
+    };
+
+    Some(function_return_attr.create_attr(cx.llcx))
+}
+
 /// Tell LLVM what instrument function to insert.
 #[inline]
 fn instrument_function_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> SmallVec<[&'ll Attribute; 4]> {
@@ -333,6 +342,7 @@ pub fn from_fn_attrs<'ll, 'tcx>(
 
     // FIXME: none of these functions interact with source level attributes.
     to_add.extend(frame_pointer_type_attr(cx));
+    to_add.extend(function_return_attr(cx));
     to_add.extend(instrument_function_attr(cx));
     to_add.extend(nojumptables_attr(cx));
     to_add.extend(probestack_attr(cx));
diff --git a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
index df4cd8ea0bd..d701b20e953 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
@@ -200,6 +200,7 @@ pub enum AttributeKind {
     AllocatedPointer = 38,
     AllocAlign = 39,
     SanitizeSafeStack = 40,
+    FnRetThunkExtern = 41,
 }
 
 /// LLVMIntPredicate
-- 
cgit 1.4.1-3-g733a5