1 files changed, 77 insertions, 0 deletions
diff --git a/library/portable-simd/crates/core_simd/examples/spectral_norm.rs b/library/portable-simd/crates/core_simd/examples/spectral_norm.rs
new file mode 100644
index 000000000..012182e09
--- /dev/null
+++ b/library/portable-simd/crates/core_simd/examples/spectral_norm.rs
@@ -0,0 +1,77 @@
+#![feature(portable_simd)]
+
+use core_simd::simd::*;
+
+fn a(i: usize, j: usize) -> f64 {
+    ((i + j) * (i + j + 1) / 2 + i + 1) as f64
+}
+
+fn mult_av(v: &[f64], out: &mut [f64]) {
+    assert!(v.len() == out.len());
+    assert!(v.len() % 2 == 0);
+
+    for (i, out) in out.iter_mut().enumerate() {
+        let mut sum = f64x2::splat(0.0);
+
+        let mut j = 0;
+        while j < v.len() {
+            let b = f64x2::from_slice(&v[j..]);
+            let a = f64x2::from_array([a(i, j), a(i, j + 1)]);
+            sum += b / a;
+            j += 2
+        }
+        *out = sum.reduce_sum();
+    }
+}
+
+fn mult_atv(v: &[f64], out: &mut [f64]) {
+    assert!(v.len() == out.len());
+    assert!(v.len() % 2 == 0);
+
+    for (i, out) in out.iter_mut().enumerate() {
+        let mut sum = f64x2::splat(0.0);
+
+        let mut j = 0;
+        while j < v.len() {
+            let b = f64x2::from_slice(&v[j..]);
+            let a = f64x2::from_array([a(j, i), a(j + 1, i)]);
+            sum += b / a;
+            j += 2
+        }
+        *out = sum.reduce_sum();
+    }
+}
+
+fn mult_atav(v: &[f64], out: &mut [f64], tmp: &mut [f64]) {
+    mult_av(v, tmp);
+    mult_atv(tmp, out);
+}
+
+pub fn spectral_norm(n: usize) -> f64 {
+    assert!(n % 2 == 0, "only even lengths are accepted");
+
+    let mut u = vec![1.0; n];
+    let mut v = u.clone();
+    let mut tmp = u.clone();
+
+    for _ in 0..10 {
+        mult_atav(&u, &mut v, &mut tmp);
+        mult_atav(&v, &mut u, &mut tmp);
+    }
+    (dot(&u, &v) / dot(&v, &v)).sqrt()
+}
+
+fn dot(x: &[f64], y: &[f64]) -> f64 {
+    // This is auto-vectorized:
+    x.iter().zip(y).map(|(&x, &y)| x * y).sum()
+}
+
+#[cfg(test)]
+#[test]
+fn test() {
+    assert_eq!(&format!("{:.9}", spectral_norm(100)), "1.274219991");
+}
+
+fn main() {
+    // Empty main to make cargo happy
+}