rustc_codegen_llvm: properly passing backchain attribute to LLVM ...

... this is a special attribute that was made to be a target-feature in
LLVM 18+, but in all previous versions, this "feature" is a naked
attribute. We will have to handle this situation differently than all
other target-features.

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -271,6 +271,17 @@ fn stackprotector_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> Option<&'ll Attribute> {
     Some(sspattr.create_attr(cx.llcx))
 }
 
+fn backchain_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> Option<&'ll Attribute> {
+    if cx.sess().target.arch != "s390x" {
+        return None;
+    }
+
+    let requested_features = cx.sess().opts.cg.target_feature.split(',');
+    let found_positive = requested_features.clone().any(|r| r == "+backchain");
+
+    if found_positive { Some(llvm::CreateAttrString(cx.llcx, "backchain")) } else { None }
+}
+
 pub fn target_cpu_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> &'ll Attribute {
     let target_cpu = llvm_util::target_cpu(cx.tcx.sess);
     llvm::CreateAttrStringValue(cx.llcx, "target-cpu", target_cpu)
@@ -447,6 +458,9 @@ pub fn from_fn_attrs<'ll, 'tcx>(
     if let Some(align) = codegen_fn_attrs.alignment {
         llvm::set_alignment(llfn, align);
     }
+    if let Some(backchain) = backchain_attr(cx) {
+        to_add.push(backchain);
+    }
     to_add.extend(sanitize_attrs(cx, codegen_fn_attrs.no_sanitize));
     to_add.extend(patchable_function_entry_attrs(cx, codegen_fn_attrs.patchable_function_entry));
 

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -14,7 +14,7 @@ use rustc_session::config::{PrintKind, PrintRequest};
 use rustc_session::Session;
 use rustc_span::symbol::Symbol;
 use rustc_target::spec::{MergeFunctions, PanicStrategy};
-use rustc_target::target_features::RUSTC_SPECIFIC_FEATURES;
+use rustc_target::target_features::{RUSTC_SPECIAL_FEATURES, RUSTC_SPECIFIC_FEATURES};
 
 use std::ffi::{c_char, c_void, CStr, CString};
 use std::fmt::Write;
@@ -321,6 +321,10 @@ pub fn target_features(sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
             }
         })
         .filter(|feature| {
+            // skip checking special features, as LLVM may not understands them
+            if RUSTC_SPECIAL_FEATURES.contains(feature) {
+                return true;
+            }
             // check that all features in a given smallvec are enabled
             for llvm_feature in to_llvm_features(sess, feature) {
                 let cstr = SmallCStr::new(llvm_feature);
@@ -546,6 +550,7 @@ pub(crate) fn global_llvm_features(sess: &Session, diagnostics: bool) -> Vec<Str
 
     // -Ctarget-features
     let supported_features = sess.target.supported_target_features();
+    let (llvm_major, _, _) = get_version();
     let mut featsmap = FxHashMap::default();
     let feats = sess
         .opts
@@ -604,6 +609,13 @@ pub(crate) fn global_llvm_features(sess: &Session, diagnostics: bool) -> Vec<Str
             if RUSTC_SPECIFIC_FEATURES.contains(&feature) {
                 return None;
             }
+
+            // if the target-feature is "backchain" and LLVM version is greater than 18
+            // then we also need to add "+backchain" to the target-features attribute.
+            // otherwise, we will only add the naked `backchain` attribute to the attribute-group.
+            if feature == "backchain" && llvm_major < 18 {
+                return None;
+            }
             // ... otherwise though we run through `to_llvm_features` when
             // passing requests down to LLVM. This means that all in-language
             // features also work on the command line instead of having two

compiler/rustc_target/src/target_features.rs

@@ -4,6 +4,9 @@ use rustc_span::symbol::Symbol;
 /// Features that control behaviour of rustc, rather than the codegen.
 pub const RUSTC_SPECIFIC_FEATURES: &[&str] = &["crt-static"];
 
+/// Features that require special handling when passing to LLVM.
+pub const RUSTC_SPECIAL_FEATURES: &[&str] = &["backchain"];
+
 /// Stability information for target features.
 #[derive(Debug, Clone, Copy)]
 pub enum Stability {