Aarch64: target_feature="neon" enabled on nightly

kryps · May 10, 2021, 6:19am

It seems that neon is always enabled as a target_feature on nightly-aarch64-unknown-linux-gnu and nightly-aarch64-apple-darwin but not on the respective stable toolchains. Is that on purpose and more importantly will that behavior be implemented on stable as well when the aarch64 SIMD intrinsics are stabilized?

pi@raspberrypi:~ $ rustup default stable
[...]
  stable-aarch64-unknown-linux-gnu unchanged - rustc 1.52.0 (88f19c6da 2021-05-03)

pi@raspberrypi:~ $ rustc --print cfg
debug_assertions
target_arch="aarch64"
target_endian="little"
target_env="gnu"
target_family="unix"
target_os="linux"
target_pointer_width="64"
target_vendor="unknown"
unix

pi@raspberrypi:~ $ rustup default nightly
[...]
  nightly-aarch64-unknown-linux-gnu unchanged - rustc 1.54.0-nightly (881c1ac40 2021-05-08)

pi@raspberrypi:~ $ rustc --print cfg
[...]
target_feature="fp"
target_feature="neon"
[...]

bjorn3 · May 10, 2021, 8:44am

Target features are only shown when they are either stable or the compiler is nightly:

github.com

rust-lang/rust/blob/00f2bf40d6374ec3541a72edb5b481fb1370dbca/compiler/rustc_codegen_llvm/src/llvm_util.rs#L165


        (_, s) => s,
    }
}


pub fn target_features(sess: &Session) -> Vec<Symbol> {
    let target_machine = create_informational_target_machine(sess);
    supported_target_features(sess)
        .iter()
        .filter_map(
            |&(feature, gate)| {
                if sess.is_nightly_build() || gate.is_none() { Some(feature) } else { None }
            },
        )
        .filter(|feature| {
            let llvm_feature = to_llvm_feature(sess, feature);
            let cstr = CString::new(llvm_feature).unwrap();
            unsafe { llvm::LLVMRustHasFeature(target_machine, cstr.as_ptr()) }
        })
        .map(|feature| Symbol::intern(feature))
        .collect()
}

system · August 8, 2021, 8:45am

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Aarch64: target_feature="neon" enabled on nightly

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