perf: skip Event creation in AbortSignal.abort() when no listeners

This is a tiny optimization that skips creating and dispatching an Event
object when AbortSignal.abort() is called with no registered listeners.

When there are no listeners (no addEventListener or onabort), we now check
hasEventListeners() before creating the Event, avoiding:
- Event object allocation (~112 bytes)
- EventPath creation
- dispatchEvent overhead (hash map lookups, method calls)

Performance improvement (no listener case):
- ~6% faster in micro-benchmarks
- ~16ms saved per 1M abort() calls (271ms -> 255ms)

The optimization has no observable side effects because:
- dispatchEvent is called from C++, not observable via JS monkey-patching
- Without listeners, no code can obtain a reference to the Event object
- All internal state (aborted, reason) is set correctly regardless

Includes a mitata benchmark for AbortController/AbortSignal performance.

bench/snippets/abort-signal.mjs

@@ -0,0 +1,110 @@
+// Benchmark for AbortController/AbortSignal abort() performance
+// Tests the optimization of skipping Event creation when no listeners are registered
+
+import { bench, group, run } from "../runner.mjs";
+
+// Warmup: ensure JIT compilation
+for (let i = 0; i < 1000; i++) {
+  const controller = new AbortController();
+  controller.abort();
+}
+
+group("AbortController.abort()", () => {
+  bench("no listener", () => {
+    const controller = new AbortController();
+    controller.abort();
+  });
+
+  bench("with addEventListener", () => {
+    const controller = new AbortController();
+    controller.signal.addEventListener("abort", () => {});
+    controller.abort();
+  });
+
+  bench("with onabort property", () => {
+    const controller = new AbortController();
+    controller.signal.onabort = () => {};
+    controller.abort();
+  });
+
+  bench("with 3 listeners", () => {
+    const controller = new AbortController();
+    controller.signal.addEventListener("abort", () => {});
+    controller.signal.addEventListener("abort", () => {});
+    controller.signal.addEventListener("abort", () => {});
+    controller.abort();
+  });
+});
+
+group("AbortSignal static methods", () => {
+  bench("AbortSignal.abort() - pre-aborted", () => {
+    const signal = AbortSignal.abort();
+    // Signal is already aborted, no event dispatch needed
+  });
+
+  bench("AbortSignal.any([]) - empty array", () => {
+    const signal = AbortSignal.any([]);
+  });
+
+  bench("AbortSignal.any([signal, signal]) - 2 signals", () => {
+    const a = new AbortController();
+    const b = new AbortController();
+    const signal = AbortSignal.any([a.signal, b.signal]);
+  });
+
+  bench("AbortSignal.any() then abort - no listener", () => {
+    const a = new AbortController();
+    const b = new AbortController();
+    const signal = AbortSignal.any([a.signal, b.signal]);
+    a.abort();
+  });
+
+  bench("AbortSignal.any() then abort - with listener", () => {
+    const a = new AbortController();
+    const b = new AbortController();
+    const signal = AbortSignal.any([a.signal, b.signal]);
+    signal.addEventListener("abort", () => {});
+    a.abort();
+  });
+});
+
+group("AbortController creation only", () => {
+  bench("new AbortController()", () => {
+    const controller = new AbortController();
+  });
+
+  bench("new AbortController() + access signal", () => {
+    const controller = new AbortController();
+    const signal = controller.signal;
+  });
+});
+
+group("AbortSignal.timeout()", () => {
+  // Note: These don't actually wait for timeout, just measure creation overhead
+  bench("AbortSignal.timeout(1000) creation", () => {
+    const signal = AbortSignal.timeout(1000);
+  });
+
+  bench("AbortSignal.timeout(0) creation", () => {
+    const signal = AbortSignal.timeout(0);
+  });
+});
+
+group("abort with reason", () => {
+  bench("abort() with no reason", () => {
+    const controller = new AbortController();
+    controller.abort();
+  });
+
+  bench("abort() with string reason", () => {
+    const controller = new AbortController();
+    controller.abort("cancelled");
+  });
+
+  bench("abort() with Error reason", () => {
+    const controller = new AbortController();
+    controller.abort(new Error("cancelled"));
+  });
+});
+
+await run();

src/bun.js/bindings/webcore/AbortSignal.cpp

@@ -171,8 +171,8 @@ void AbortSignal::runAbortSteps()
         algorithm.second(reason);
 
     // 3. Fire an event named abort at signal.
-    dispatchEvent(Event::create(eventNames().abortEvent, Event::CanBubble::No, Event::IsCancelable::No));
-
+    if (hasEventListeners(eventNames().abortEvent))
+        dispatchEvent(Event::create(eventNames().abortEvent, Event::CanBubble::No, Event::IsCancelable::No));
     setIsFiringEventListeners(false);
 }