LibJS: Skip declarative env in block statement without lexical decls

The idea here is simple: If the block statement doesn't contain any
lexical declarations, we don't need to allocate, initialize and
eventually garbage collect a new declarative environment.
This even makes lookups across nested blocks slightly faster as we don't
have to traverse a chain of empty environments anymore - instead, the
execution context just stores the outermost non-empty one.

This doesn't speed up test-js considerably, but has a noticeable effect
on test262 and real-world web content :^)

2 files changed, 16 insertions, 5 deletions

diff --git a/Userland/Libraries/LibJS/AST.cpp b/Userland/Libraries/LibJS/AST.cpp
index 5cd3a2d7c9..8b2bdfe7cb 100644
--- a/Userland/Libraries/LibJS/AST.cpp
+++ b/Userland/Libraries/LibJS/AST.cpp
@@ -122,13 +122,21 @@ Value BlockStatement::execute(Interpreter& interpreter, GlobalObject& global_obj
     InterpreterNodeScope node_scope { interpreter, *this };
 
     auto& vm = interpreter.vm();
-    Environment* old_environment = vm.running_execution_context().lexical_environment;
-    ScopeGuard restore_environment = [&] {
+
+    Environment* old_environment { nullptr };
+    ArmedScopeGuard restore_environment = [&] {
         vm.running_execution_context().lexical_environment = old_environment;
     };
-    auto* block_environment = new_declarative_environment(*old_environment);
-    block_declaration_instantiation(global_object, block_environment);
-    vm.running_execution_context().lexical_environment = block_environment;
+
+    // Optimization: We only need a new lexical environment if there are any lexical declarations. :^)
+    if (has_lexical_declarations()) {
+        old_environment = vm.running_execution_context().lexical_environment;
+        auto* block_environment = new_declarative_environment(*old_environment);
+        block_declaration_instantiation(global_object, block_environment);
+        vm.running_execution_context().lexical_environment = block_environment;
+    } else {
+        restore_environment.disarm();
+    }
 
     auto block_value = evaluate_statements(interpreter, global_object);
     if (!labels().is_empty() && vm.should_unwind_until(ScopeType::Breakable, labels()))
diff --git a/Userland/Libraries/LibJS/AST.h b/Userland/Libraries/LibJS/AST.h
index f44c44a991..2baab6fda3 100644
--- a/Userland/Libraries/LibJS/AST.h
+++ b/Userland/Libraries/LibJS/AST.h
@@ -183,6 +183,9 @@ public:
     void add_lexical_declaration(NonnullRefPtr<Declaration> variables);
     void add_hoisted_function(NonnullRefPtr<FunctionDeclaration> declaration);
 
+    [[nodiscard]] bool has_lexical_declarations() const { return !m_lexical_declarations.is_empty(); }
+    [[nodiscard]] bool has_var_declarations() const { return !m_var_declarations.is_empty(); }
+
     void for_each_lexically_scoped_declaration(IteratorOrVoidFunction<Declaration const&>&& callback) const;
     void for_each_lexically_declared_name(IteratorOrVoidFunction<FlyString const&>&& callback) const;