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LibJS: Elide empty declarative environments inside switch statements

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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.
This commit is contained in:
Andreas Kling 2021-10-09 00:42:10 +02:00
parent 9d06448bc7
commit eafbf372d0
1 changed files with 8 additions and 7 deletions
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15
Userland/Libraries/LibJS/AST.cpp
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@ -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];