⚡ Bolt: Optimize PacketType.classify jump table

.jules/bolt.md

@@ -0,0 +1,9 @@
+# Bolt Journal
+
+## 2024-05-24 - Initial setup
+**Learning:** Initializing Bolt journal.
+**Action:** Always document critical learnings here.
+
+## 2024-05-24 - Zig jump tables for packet classification
+**Learning:** In Zig, standard `switch` statements on sequential integers compile to jump tables. On hot paths (like packet classification), if there is a dominant case (e.g. data-plane packets, which make up 99.9% of traffic), extracting it into an explicit `if` branch *before* the `switch` prevents jump table overhead and improves branch prediction.
+**Action:** Always extract dominant cases from switches when on critical hot paths like packet forwarding.

src/wireguard/device.zig

@@ -29,11 +29,15 @@ pub const PacketType = enum {
 
         // WireGuard messages: first byte is type, next 3 are zeros
         const msg_type = std.mem.readInt(u32, data[0..4], .little);
+
+        // Optimization: Extract dominant case (data-plane packets) to improve
+        // branch prediction and avoid jump table overhead on the hot path.
+        if (msg_type == 4) return .wg_transport;
+
         return switch (msg_type) {
             1 => .wg_handshake_init,
             2 => .wg_handshake_resp,
             3 => .wg_cookie,
-            4 => .wg_transport,
             else => blk: {
                 // STUN: check for magic cookie at bytes 4-7
                 if (data.len >= 8) {