[neon] reciprocal square-root estimate (#121)

Author: gnzlbg

ci/run.sh

@@ -9,5 +9,16 @@ export RUSTFLAGS="$RUSTFLAGS -C codegen-units=1"
 # having only one thread increases debuggability to be worth it.
 export RUST_TEST_THREADS=1
 
+# FIXME(rust-lang-nursery/stdsimd#120) run-time feature detection for ARM Neon
+case ${TARGET} in
+    aarch*)
+        export RUSTFLAGS="${RUSTFLAGS} -C target-feature=+neon"
+        ;;
+    *)
+        ;;
+esac
+
+echo "RUSTFLAGS=${RUSTFLAGS}"
+
 cargo test --target $TARGET
 cargo test --release --target $TARGET

src/arm/mod.rs

@@ -3,8 +3,13 @@ pub use self::v6::*;
 pub use self::v7::*;
 #[cfg(target_arch = "aarch64")]
 pub use self::v8::*;
+#[cfg(target_feature = "neon")]
+pub use self::neon::*;
 
 mod v6;
 mod v7;
 #[cfg(target_arch = "aarch64")]
 mod v8;
+
+#[cfg(target_feature = "neon")]
+mod neon;

src/arm/neon.rs

@@ -0,0 +1,38 @@
+//! ARM NEON intrinsics
+//!
+//! The references is [ARM's NEON Intrinsics Reference](http://infocenter.arm.com/help/topic/com.arm.doc.ihi0073a/IHI0073A_arm_neon_intrinsics_ref.pdf). [ARM's NEON Intrinsics Online Database](https://developer.arm.com/technologies/neon/intrinsics) is also useful.
+
+#[cfg(test)]
+use stdsimd_test::assert_instr;
+
+use v64::{f32x2};
+
+#[allow(improper_ctypes)]
+extern "C" {
+    #[link_name = "llvm.aarch64.neon.frsqrte.v2f32"]
+    fn frsqrte_v2f32(a: f32x2) -> f32x2;
+}
+
+/// Reciprocal square-root estimate.
+#[inline(always)]
+#[target_feature = "+neon"]
+#[cfg_attr(test, assert_instr(frsqrte))]
+pub unsafe fn vrsqrte_f32(a: f32x2) -> f32x2 {
+    frsqrte_v2f32(a)
+}
+
+#[cfg(test)]
+mod tests {
+    use stdsimd_test::simd_test;
+
+    use v64::{f32x2};
+    use arm::neon;
+
+    #[test]
+    fn vrsqrt_f32() {
+        let a = f32x2::new(1.0, 2.0);
+        let e = f32x2::new(0.9980469, 0.7050781);
+        let r = unsafe { neon::vrsqrte_f32(a) };
+        assert_eq!(r, e);
+    }
+}