From 6530f55f34e0dc6ad362d07cbc7d4cbbe4347929 Mon Sep 17 00:00:00 2001
From: Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
Date: Fri, 9 May 2025 04:57:02 +0300
Subject: [PATCH] LibJS: Cache rejected/fulfilled callbacks in
 AsyncFunctionDriverWrapper

Saves us two NativeFunction allocations per each `await`.

With this change, following function goes 80% faster on my computer:
```js
(async () => {
    const resolved = Promise.resolve();
    for (let i = 0; i < 5_000_000; i++) {
        await resolved;
    }
})();
```
---
 Libraries/LibJS/Runtime/AsyncFunctionDriverWrapper.cpp | 10 +++++++---
 Libraries/LibJS/Runtime/AsyncFunctionDriverWrapper.h   |  3 +++
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/Libraries/LibJS/Runtime/AsyncFunctionDriverWrapper.cpp b/Libraries/LibJS/Runtime/AsyncFunctionDriverWrapper.cpp
index 0007ee97d1b..8f3a67216c1 100644
--- a/Libraries/LibJS/Runtime/AsyncFunctionDriverWrapper.cpp
+++ b/Libraries/LibJS/Runtime/AsyncFunctionDriverWrapper.cpp
@@ -75,7 +75,8 @@ ThrowCompletionOr<void> AsyncFunctionDriverWrapper::await(JS::Value value)
     };
 
     // 4. Let onFulfilled be CreateBuiltinFunction(fulfilledClosure, 1, "", « »).
-    auto on_fulfilled = NativeFunction::create(realm, move(fulfilled_closure), 1);
+    if (!m_on_fulfilled)
+        m_on_fulfilled = NativeFunction::create(realm, move(fulfilled_closure), 1);
 
     // 5. Let rejectedClosure be a new Abstract Closure with parameters (reason) that captures asyncContext and performs the
     //    following steps when called:
@@ -103,11 +104,12 @@ ThrowCompletionOr<void> AsyncFunctionDriverWrapper::await(JS::Value value)
     };
 
     // 6. Let onRejected be CreateBuiltinFunction(rejectedClosure, 1, "", « »).
-    auto on_rejected = NativeFunction::create(realm, move(rejected_closure), 1);
+    if (!m_on_rejected)
+        m_on_rejected = NativeFunction::create(realm, move(rejected_closure), 1);
 
     // 7. Perform PerformPromiseThen(promise, onFulfilled, onRejected).
     m_current_promise = as<Promise>(promise_object);
-    m_current_promise->perform_then(on_fulfilled, on_rejected, {});
+    m_current_promise->perform_then(m_on_fulfilled, m_on_rejected, {});
 
     // NOTE: None of these are necessary. 8-12 are handled by step d of the above lambdas.
     // 8. Remove asyncContext from the execution context stack and restore the execution context that is at the top of the
@@ -176,6 +178,8 @@ void AsyncFunctionDriverWrapper::visit_edges(Cell::Visitor& visitor)
         visitor.visit(m_current_promise);
     if (m_suspended_execution_context)
         m_suspended_execution_context->visit_edges(visitor);
+    visitor.visit(m_on_fulfilled);
+    visitor.visit(m_on_rejected);
 }
 
 }
diff --git a/Libraries/LibJS/Runtime/AsyncFunctionDriverWrapper.h b/Libraries/LibJS/Runtime/AsyncFunctionDriverWrapper.h
index ba93f94f5ee..54b0d67d8cd 100644
--- a/Libraries/LibJS/Runtime/AsyncFunctionDriverWrapper.h
+++ b/Libraries/LibJS/Runtime/AsyncFunctionDriverWrapper.h
@@ -34,6 +34,9 @@ private:
     GC::Ref<Promise> m_top_level_promise;
     GC::Ptr<Promise> m_current_promise { nullptr };
     OwnPtr<ExecutionContext> m_suspended_execution_context;
+
+    GC::Ptr<NativeFunction> m_on_fulfilled;
+    GC::Ptr<NativeFunction> m_on_rejected;
 };
 
 }