remove redundant backchain attribute in codegen

llvm will look at both
1. the values of "target-features" and
2. the function string attributes.

this removes the redundant function string attribute because it is not needed at all.
rustc sets the `+backchain` attribute through `target_features_attr(...)`

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -301,17 +301,6 @@ fn stackprotector_attr<'ll>(cx: &SimpleCx<'ll>, sess: &Session) -> Option<&'ll A
     Some(sspattr.create_attr(cx.llcx))
 }
 
-fn backchain_attr<'ll>(cx: &SimpleCx<'ll>, sess: &Session) -> Option<&'ll Attribute> {
-    if sess.target.arch != Arch::S390x {
-        return None;
-    }
-
-    let requested_features = 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(crate) fn target_cpu_attr<'ll>(cx: &SimpleCx<'ll>, sess: &Session) -> &'ll Attribute {
     let target_cpu = llvm_util::target_cpu(sess);
     llvm::CreateAttrStringValue(cx.llcx, "target-cpu", target_cpu)
@@ -530,9 +519,6 @@ pub(crate) fn llfn_attrs_from_instance<'ll, 'tcx>(
     if let Some(align) = codegen_fn_attrs.alignment {
         llvm::set_alignment(llfn, align);
     }
-    if let Some(backchain) = backchain_attr(cx, sess) {
-        to_add.push(backchain);
-    }
     to_add.extend(patchable_function_entry_attrs(
         cx,
         sess,

tests/codegen-llvm/backchain.rs

@@ -0,0 +1,15 @@
+//@ add-minicore
+//@ compile-flags: -Copt-level=3 --crate-type=lib --target=s390x-unknown-linux-gnu -Ctarget-feature=+backchain
+//@ needs-llvm-components: systemz
+#![crate_type = "lib"]
+#![feature(no_core, lang_items)]
+#![no_core]
+
+extern crate minicore;
+use minicore::*;
+
+#[no_mangle]
+pub fn test_backchain() {
+    // CHECK: @test_backchain() unnamed_addr #0
+}
+// CHECK: attributes #0 = { {{.*}}"target-features"="{{[^"]*}}+backchain{{.*}} }