Avoid closure allocation in RCTDeviceEventEmitter.emit when tracing is disabled

[secitr]

Summary:

RCTDeviceEventEmitter.emit is one of the hottest paths in React Native:
NativeEventEmitter.emit delegates to it, so every event delivered from
native (scroll, touch, keyboard, app state, ...) goes through this method.
The current implementation wraps every call in Systrace.trace, which
always allocates two closures (a lazy event-name thunk and a callback)
on every emit — even when tracing is disabled, which is the common case.

This changes emit to guard on Systrace.isEnabled() and use
beginEvent/endEvent directly:

• tracing disabled (common case): one branch check + a plain call —
  zero allocations
• tracing enabled: same trace section name (RCTDeviceEventEmitter.emit#)
  and the same begin/end semantics, including endEvent() in a finally
  block when a listener throws

Behavior is unchanged; only the fast-path allocations are removed.

Changelog:

[General] [Changed] - RCTDeviceEventEmitter.emit no longer allocates closures on every emit when tracing is disabled, reducing per-event allocation on the native-to-JS event path

Test Plan:

• New Fantom integration tests in packages/react-native/Libraries/EventEmitter/tests/RCTDeviceEventEmitter-itest.js (public API, runs against Hermes):
  • event + args are forwarded to listeners
  • no trace section calls when tracing is disabled
  • trace section is begun/ended with the correct name when tracing is enabled
  • end section is still emitted when a listener throws
  • __RCTProfileIsProfiling fallback still enables tracing
• yarn test packages/react-native/Libraries — 433 tests pass (30 suites)
• yarn flow-check — 0 errors
• yarn lint — 0 warnings
• Micro-benchmark (Node v24, tracing disabled, 5M emits with a scroll-like payload):
  • before: 158ms total (~31.6 ns/emit)
  • after: 117ms total (~23.4 ns/emit)
  • ~26% faster per emit; on Hermes/mobile CPUs the win comes from avoiding
    two heap-allocated closures per event

[cortinico]

let's have this method in a else branch and avoid the early return please

[secitr]

Done — moved the fast path into the else branch and removed the early return. Pushed in 4359d54.

[secitr]

The iOS RNTester e2e (Debug) failed on retry_2 while Release passed and the same suite passed on the two previous runs — looks like a flaky simulator. A re-run might clear it. Let me know if anything on the code side needs attention.

[rubennorte]

This change looks fine but please don't add new Jest tests as we're moving to Fantom to make sure the environment that runtime code runs in tests is as close as the real environment as possible. In this case, the test should be renamed to have the -itest suffix and use public APIs instead.

[secitr]

This change looks fine but please don't add new Jest tests as we're moving to Fantom to make sure the environment that runtime code runs in tests is as close as the real environment as possible. In this case, the test should be renamed to have the -itest suffix and use public APIs instead.

Done — converted the added Jest test to a Fantom itest (RCTDeviceEventEmitter-itest.js), renamed with the -itest suffix, and it now uses the public API (DeviceEventEmitter from react-native). All 5 tests pass locally against the Hermes runtime; pushed in 844dd96.

[meta-codesync]

@cortinico has imported this pull request. If you are a Meta employee, you can view this in D121799063.

[meta-codesync]

@cortinico merged this pull request in 7f6043b.