perf(packet): wire SIMD Internet checksum for long payloads

common/arcbox-packet/examples/checksum_bench.rs

@@ -0,0 +1,55 @@
+//! Microbench: dispatched, scalar, and SIMD Internet checksum throughput.
+//!
+//! ```text
+//! cargo run -p arcbox-packet --example checksum_bench --release
+//! ```
+
+use std::time::Instant;
+
+use arcbox_packet::checksum::{checksum, checksum_add_scalar, checksum_fold, checksum_simd};
+
+fn bench(label: &str, len: usize, iters: usize, f: impl Fn(&[u8]) -> u16) {
+    let data: Vec<u8> = (0..len).map(|i| (i % 251) as u8).collect();
+    for _ in 0..100 {
+        std::hint::black_box(f(&data));
+    }
+    let t0 = Instant::now();
+    let mut sink = 0u16;
+    for _ in 0..iters {
+        sink ^= f(&data);
+    }
+    let elapsed = t0.elapsed();
+    std::hint::black_box(sink);
+    let bytes = (len as u64).saturating_mul(iters as u64);
+    let gbps = (bytes as f64 * 8.0) / elapsed.as_secs_f64() / 1e9;
+    let ns_per_call = elapsed.as_nanos() as f64 / iters as f64;
+    println!(
+        "{label:<18} len={len:>5}  {iters:>8} iters  {gbps:>8.2} Gbit/s  {ns_per_call:>8.1} ns/call"
+    );
+}
+
+fn checksum_scalar(data: &[u8]) -> u16 {
+    checksum_fold(checksum_add_scalar(data))
+}
+
+fn main() {
+    println!(
+        "# Internet checksum microbench (host = {})",
+        std::env::consts::ARCH
+    );
+    println!();
+
+    for &len in &[63usize, 64, 65, 1500, 9000, 16384, 65536] {
+        let iters = (50_000_000 / len).max(2_000);
+        bench("checksum", len, iters, checksum);
+        bench("scalar", len, iters, checksum_scalar);
+        bench("simd", len, iters, checksum_simd);
+        let data: Vec<u8> = (0..len).map(|i| (i % 251) as u8).collect();
+        assert_eq!(
+            checksum_fold(checksum_add_scalar(&data)),
+            checksum_simd(&data),
+            "scalar/simd mismatch at len={len}"
+        );
+        println!();
+    }
+}