Andreas Kling 3a2f2f3926 LibJS: Add fast path in async function await for non-thenable values ...

Per spec, every `await` goes through PromiseResolve (which wraps the
value in a new Promise via NewPromiseCapability) and then
PerformPromiseThen (which creates PromiseReaction and JobCallback
objects). This results in 13-16 GC cell allocations per await.

Add a fast path that detects two common cases:

1. Primitive values: These can never have a "then" property, so we
   can skip all promise wrapping and directly schedule the async
   function's continuation as a microtask.

2. Already-settled native Promises: If the promise has no own
   properties and its prototype is the intrinsic %Promise.prototype%,
   we can extract the result directly and schedule continuation.

For these cases, we bypass promise_resolve(), new_promise_capability(),
create_resolving_functions(), perform_then(), PromiseReaction creation,
and JobCallback creation -- replacing ~13 GC allocations with 1
(the GC::Function for the microtask job).

2026-03-16 12:01:49 -05:00

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