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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 :^)
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2 changed files with 16 additions and 5 deletions
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@ -122,13 +122,21 @@ Value BlockStatement::execute(Interpreter& interpreter, GlobalObject& global_obj
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InterpreterNodeScope node_scope { interpreter, *this };
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auto& vm = interpreter.vm();
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Environment* old_environment = vm.running_execution_context().lexical_environment;
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ScopeGuard restore_environment = [&] {
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Environment* old_environment { nullptr };
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ArmedScopeGuard restore_environment = [&] {
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vm.running_execution_context().lexical_environment = old_environment;
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};
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auto* block_environment = new_declarative_environment(*old_environment);
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block_declaration_instantiation(global_object, block_environment);
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vm.running_execution_context().lexical_environment = block_environment;
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// Optimization: We only need a new lexical environment if there are any lexical declarations. :^)
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if (has_lexical_declarations()) {
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old_environment = vm.running_execution_context().lexical_environment;
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auto* block_environment = new_declarative_environment(*old_environment);
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block_declaration_instantiation(global_object, block_environment);
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vm.running_execution_context().lexical_environment = block_environment;
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} else {
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restore_environment.disarm();
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}
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auto block_value = evaluate_statements(interpreter, global_object);
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if (!labels().is_empty() && vm.should_unwind_until(ScopeType::Breakable, labels()))
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