LibJS: Skip iterator result object allocation in for..of and for..in

Introduce special instruction for `for..of` and `for..in` loop that
skips `{ value, done }` result object allocation if iterator is builtin
(array, map, set, string). This reduces GC pressure significantly and
avoids extracting the `value` and `done` properties.

This change makes this micro benchmark 48% faster on my computer:
```js
const arr = new Array(10_000_000);
let counter = 0;
for (let _ of arr) {
    counter++;
}
```

Libraries/LibJS/Bytecode/ASTCodegen.cpp

@@ -3183,11 +3183,23 @@ static Bytecode::CodeGenerationErrorOr<Optional<ScopedOperand>> for_in_of_body_e
     generator.begin_continuable_scope(Bytecode::Label { loop_update }, label_set);
 
     // a. Let nextResult be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]]).
-    auto next_result = generator.allocate_register();
+    auto next_value = generator.allocate_register();
-    generator.emit<Bytecode::Op::IteratorNext>(next_result, *head_result.iterator);
+    auto done = generator.allocate_register();
 
-    // b. If iteratorKind is async, set nextResult to ? Await(nextResult).
+    if (iterator_kind == IteratorHint::Sync) {
-    if (iterator_kind == IteratorHint::Async) {
+        generator.emit<Bytecode::Op::ForOfNext>(next_value, done, *head_result.iterator);
+
+        auto& loop_continue = generator.make_block();
+        generator.emit_jump_if(
+            done,
+            Bytecode::Label { loop_end },
+            Bytecode::Label { loop_continue });
+        generator.switch_to_basic_block(loop_continue);
+    } else {
+        auto next_result = generator.allocate_register();
+        generator.emit<Bytecode::Op::IteratorNext>(next_result, *head_result.iterator);
+
+        // b. If iteratorKind is async, set nextResult to ? Await(nextResult).
         auto received_completion = generator.allocate_register();
         auto received_completion_type = generator.allocate_register();
         auto received_completion_value = generator.allocate_register();
@@ -3195,27 +3207,25 @@ static Bytecode::CodeGenerationErrorOr<Optional<ScopedOperand>> for_in_of_body_e
         generator.emit_mov(received_completion, generator.accumulator());
         auto new_result = generate_await(generator, next_result, received_completion, received_completion_type, received_completion_value);
         generator.emit_mov(next_result, new_result);
+
+        // c. If Type(nextResult) is not Object, throw a TypeError exception.
+        generator.emit<Bytecode::Op::ThrowIfNotObject>(next_result);
+
+        // d. Let done be ? IteratorComplete(nextResult).
+        generator.emit_iterator_complete(done, next_result);
+
+        // e. If done is true, return V.
+        auto& loop_continue = generator.make_block();
+        generator.emit_jump_if(
+            done,
+            Bytecode::Label { loop_end },
+            Bytecode::Label { loop_continue });
+        generator.switch_to_basic_block(loop_continue);
+
+        // f. Let nextValue be ? IteratorValue(nextResult).
+        generator.emit_iterator_value(next_value, next_result);
     }
 
-    // c. If Type(nextResult) is not Object, throw a TypeError exception.
-    generator.emit<Bytecode::Op::ThrowIfNotObject>(next_result);
-
-    // d. Let done be ? IteratorComplete(nextResult).
-    auto done = generator.allocate_register();
-    generator.emit_iterator_complete(done, next_result);
-
-    // e. If done is true, return V.
-    auto& loop_continue = generator.make_block();
-    generator.emit_jump_if(
-        done,
-        Bytecode::Label { loop_end },
-        Bytecode::Label { loop_continue });
-    generator.switch_to_basic_block(loop_continue);
-
-    // f. Let nextValue be ? IteratorValue(nextResult).
-    auto next_value = generator.allocate_register();
-    generator.emit_iterator_value(next_value, next_result);
-
     // g. If lhsKind is either assignment or varBinding, then
     if (head_result.lhs_kind != LHSKind::LexicalBinding) {
         // i. If destructuring is false, then

Libraries/LibJS/Bytecode/Instruction.h

@@ -51,6 +51,7 @@
     O(EnterObjectEnvironment)          \
     O(EnterUnwindContext)              \
     O(Exp)                             \
+    O(ForOfNext)                       \
     O(GetById)                         \
     O(GetByIdWithThis)                 \
     O(GetByValue)                      \

Libraries/LibJS/Bytecode/Interpreter.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2021-2025, Andreas Kling <andreas@ladybird.org>
+ * Copyright (c) 2025, Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -611,6 +612,7 @@ FLATTEN_ON_CLANG void Interpreter::run_bytecode(size_t entry_point)
             HANDLE_INSTRUCTION_WITHOUT_EXCEPTION_CHECK(Dump);
             HANDLE_INSTRUCTION(EnterObjectEnvironment);
             HANDLE_INSTRUCTION(Exp);
+            HANDLE_INSTRUCTION(ForOfNext);
             HANDLE_INSTRUCTION(GetById);
             HANDLE_INSTRUCTION(GetByIdWithThis);
             HANDLE_INSTRUCTION(GetByValue);
@@ -2980,6 +2982,27 @@ ThrowCompletionOr<void> IteratorNext::execute_impl(Bytecode::Interpreter& interp
     return {};
 }
 
+ThrowCompletionOr<void> ForOfNext::execute_impl(Bytecode::Interpreter& interpreter) const
+{
+    auto& vm = interpreter.vm();
+    auto& iterator_record = static_cast<IteratorRecord&>(interpreter.get(m_iterator_record).as_cell());
+
+    Value value;
+    bool done = false;
+    if (auto* builtin_iterator = iterator_record.iterator->as_builtin_iterator()) {
+        TRY(builtin_iterator->next(vm, done, value));
+    } else {
+        auto result = TRY(iterator_next(vm, iterator_record));
+        value = TRY(result->internal_get(vm.names.value, {}));
+        done = TRY(result->internal_get(vm.names.done, {})).to_boolean();
+    }
+
+    interpreter.set(dst_done(), Value(done));
+    interpreter.set(dst_value(), value);
+
+    return {};
+}
+
 ThrowCompletionOr<void> NewClass::execute_impl(Bytecode::Interpreter& interpreter) const
 {
     Value super_class;
@@ -3754,6 +3777,14 @@ ByteString IteratorNext::to_byte_string_impl(Executable const& executable) const
         format_operand("iterator_record"sv, m_iterator_record, executable));
 }
 
+ByteString ForOfNext::to_byte_string_impl(Executable const& executable) const
+{
+    return ByteString::formatted("ForOfNext {}, {}, {}",
+        format_operand("dst_value"sv, m_dst_value, executable),
+        format_operand("dst_done"sv, m_dst_done, executable),
+        format_operand("iterator_record"sv, m_iterator_record, executable));
+}
+
 ByteString ResolveThisBinding::to_byte_string_impl(Bytecode::Executable const&) const
 {
     return "ResolveThisBinding"sv;

Libraries/LibJS/Bytecode/Op.h

@@ -2721,6 +2721,35 @@ private:
     Operand m_iterator_record;
 };
 
+class ForOfNext final : public Instruction {
+public:
+    ForOfNext(Operand dst_value, Operand dst_done, Operand iterator_record)
+        : Instruction(Type::ForOfNext)
+        , m_dst_value(dst_value)
+        , m_dst_done(dst_done)
+        , m_iterator_record(iterator_record)
+    {
+    }
+
+    ThrowCompletionOr<void> execute_impl(Bytecode::Interpreter&) const;
+    ByteString to_byte_string_impl(Bytecode::Executable const&) const;
+    void visit_operands_impl(Function<void(Operand&)> visitor)
+    {
+        visitor(m_dst_value);
+        visitor(m_dst_done);
+        visitor(m_iterator_record);
+    }
+
+    Operand dst_value() const { return m_dst_value; }
+    Operand dst_done() const { return m_dst_done; }
+    Operand iterator_record() const { return m_iterator_record; }
+
+private:
+    Operand m_dst_value;
+    Operand m_dst_done;
+    Operand m_iterator_record;
+};
+
 class ResolveThisBinding final : public Instruction {
 public:
     ResolveThisBinding()