env.diff

commit eafbf372d0bb0013f50f0952f4115dd359772095
Author: Andreas Kling <kling@serenityos.org>
Date:   Sat Oct 9 00:42:10 2021 +0200

    LibJS: Elide empty declarative environments inside switch statements
    
    Most switch statements don't have any lexically scoped declarations,
    so let's avoid allocating an environment in the common case where we
    don't have to.

diff --git a/Userland/Libraries/LibJS/AST.cpp b/Userland/Libraries/LibJS/AST.cpp
index 99803c807e..6e82d9bc7f 100644
--- a/Userland/Libraries/LibJS/AST.cpp
+++ b/Userland/Libraries/LibJS/AST.cpp
@@ -2856,14 +2856,15 @@ Value SwitchStatement::execute(Interpreter& interpreter, GlobalObject& global_ob
     if (interpreter.exception())
         return {};
 
-    auto* old_environment = interpreter.lexical_environment();
-    ScopeGuard restore_environment = [&] {
-        interpreter.vm().running_execution_context().lexical_environment = old_environment;
-    };
-    auto* block_environment = new_declarative_environment(*old_environment);
-    block_declaration_instantiation(global_object, block_environment);
+    // Optimization: Avoid creating a lexical environment if there are no lexical declarations.
+    Optional<TemporaryChange<Environment*>> lexical_environment_changer;
+    if (has_lexical_declarations()) {
+        auto* old_environment = interpreter.lexical_environment();
+        auto* block_environment = new_declarative_environment(*old_environment);
+        block_declaration_instantiation(global_object, block_environment);
+        lexical_environment_changer.emplace(interpreter.vm().running_execution_context().lexical_environment, block_environment);
+    }
 
-    interpreter.vm().running_execution_context().lexical_environment = block_environment;
     Optional<size_t> first_passing_case;
     for (size_t i = 0; i < m_cases.size(); ++i) {
         auto& switch_case = m_cases[i];