remove redundant backchain attribute in codegen

[fneddy]

llvm will look at both

1. the values of "target-features" and
2. the function string attributes.

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

rust/compiler/rustc_codegen_llvm/src/attributes.rs

Line 590 in d34f1f9

to_add.extend(target_features_attr(cx, tcx, function_features));

rust/compiler/rustc_codegen_llvm/src/attributes.rs

Lines 326 to 337 in d34f1f9

	pub(crate) fn target_features_attr<'ll, 'tcx>(
	cx: &SimpleCx<'ll>,
	tcx: TyCtxt<'tcx>,
	function_features: Vec<String>,
	) -> Option<&'ll Attribute> {
	let global_features = tcx.global_backend_features(()).iter().map(String::as_str);
	let function_features = function_features.iter().map(String::as_str);
	let target_features =
	global_features.chain(function_features).intersperse(",").collect::<String>();
	(!target_features.is_empty())
	.then(|| llvm::CreateAttrStringValue(cx.llcx, "target-features", &target_features))
	}

[rustbot]

r? @dingxiangfei2009

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[dingxiangfei2009]

Thank you so much for your PR! 🙇

I don't see anything underhanded here.

@fneddy are you planning to add s390x codegen tests to demonstrate that all six target features are usable and propagated to the LLVM IR? It doesn't need to happen within this PR.

[fneddy]

Hi,

backchain is already tested here. Is that sufficient or is there need for more? IMHO the test tests everything there is to backchain.

I am working also on packedstack attribute(#152432). There I will create excessive tests for all the possible combinations of softfloat+packedstack+backchain.

[fneddy]

Ahh I see the test is for assembly. I could certainly create a test for the llvmir if wanted.