From edb5547e370cfc4283cd5beb4fec694a171d4c70 Mon Sep 17 00:00:00 2001
From: Andreas Kling <andreas@ladybird.org>
Date: Sat, 3 May 2025 21:12:00 +0200
Subject: [PATCH] LibJS: Don't incrementally delete PropertyNameIterator while
 iterating

We were spending a lot of time removing each property name from the
iterator's underlying HashMap while iterating over it. This wasn't
actually necessary, so let's stop doing it and instead just iterate
over the property names with a stored HashTable iterator.

1.10x speedup on MicroBench/for-in-indexed-properties.js
---
 Libraries/LibJS/Bytecode/Interpreter.cpp | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/Libraries/LibJS/Bytecode/Interpreter.cpp b/Libraries/LibJS/Bytecode/Interpreter.cpp
index d224b7979e9..1ae0004e45d 100644
--- a/Libraries/LibJS/Bytecode/Interpreter.cpp
+++ b/Libraries/LibJS/Bytecode/Interpreter.cpp
@@ -1728,20 +1728,20 @@ public:
     ThrowCompletionOr<void> next(VM&, bool& done, Value& value) override
     {
         while (true) {
-            if (m_properties.is_empty()) {
+            if (m_iterator == m_properties.end()) {
                 done = true;
                 return {};
             }
 
-            auto it = *m_properties.begin();
-            ScopeGuard remove_first = [&] { m_properties.take_first(); };
+            auto const& entry = *m_iterator;
+            ScopeGuard remove_first = [&] { ++m_iterator; };
 
             // If the property is deleted, don't include it (invariant no. 2)
-            if (!TRY(m_object->has_property(it.key.key)))
+            if (!TRY(m_object->has_property(entry.key.key)))
                 continue;
 
             done = false;
-            value = it.value;
+            value = entry.value;
             return {};
         }
     }
@@ -1751,6 +1751,7 @@ private:
         : Object(realm, nullptr)
         , m_object(object)
         , m_properties(move(properties))
+        , m_iterator(m_properties.begin())
     {
     }
 
@@ -1763,6 +1764,7 @@ private:
 
     GC::Ref<Object> m_object;
     OrderedHashMap<PropertyKeyAndEnumerableFlag, Value> m_properties;
+    decltype(m_properties.begin()) m_iterator;
 };
 
 GC_DEFINE_ALLOCATOR(PropertyNameIterator);