LibJS: Add an optimization to avoid needless arguments object creation

This gives FunctionNode a "might need arguments object" boolean flag and
sets it based on the simplest possible heuristic for this: if we
encounter an identifier called "arguments" or "eval" up to the next
(nested) function declaration or expression, we won't need an arguments
object. Otherwise, we *might* need one - the final decision is made in
the FunctionDeclarationInstantiation AO.

Now, this is obviously not perfect. Even if you avoid eval, something
like `foo.arguments` will still trigger a false positive - but it's a
start and already massively cuts down on needlessly allocated objects,
especially in real-world code that is often minified, and so a full
"arguments" identifier will be an actual arguments object more often
than not.

To illustrate the actual impact of this change, here's the number of
allocated arguments objects during a full test-js run:

Before:
- Unmapped arguments objects: 78765
- Mapped arguments objects: 2455

After:
- Unmapped arguments objects: 18
- Mapped arguments objects: 37

This results in a ~5% speedup of test-js on my Linux host machine, and
about 3.5% on i686 Serenity in QEMU (warm runs, average of 5).

The following microbenchmark (calling an empty function 1M times) runs
25% faster on Linux and 45% on Serenity:

    function foo() {}
    for (var i = 0; i < 1_000_000; ++i)
        foo();

test262 reports no changes in either direction, apart from a speedup :^)

Userland/Libraries/LibJS/AST.cpp

@@ -187,7 +187,7 @@ Value FunctionExpression::instantiate_ordinary_function_expression(Interpreter&
         scope->create_immutable_binding(global_object, name(), false);
     }
 
-    auto closure = ECMAScriptFunctionObject::create(global_object, used_name, body(), parameters(), function_length(), scope, kind(), is_strict_mode(), is_arrow_function());
+    auto closure = ECMAScriptFunctionObject::create(global_object, used_name, body(), parameters(), function_length(), scope, kind(), is_strict_mode(), might_need_arguments_object(), is_arrow_function());
 
     // FIXME: 6. Perform SetFunctionName(closure, name).
     // FIXME: 7. Perform MakeConstructor(closure).
@@ -1289,7 +1289,7 @@ ThrowCompletionOr<Value> ClassExpression::class_definition_evaluation(Interprete
             auto copy_initializer = field.initializer();
             auto body = create_ast_node<ExpressionStatement>(field.initializer()->source_range(), copy_initializer.release_nonnull());
             // FIXME: A potential optimization is not creating the functions here since these are never directly accessible.
-            initializer = ECMAScriptFunctionObject::create(interpreter.global_object(), property_key.to_display_string(), *body, {}, 0, interpreter.lexical_environment(), FunctionKind::Regular, false);
+            initializer = ECMAScriptFunctionObject::create(interpreter.global_object(), property_key.to_display_string(), *body, {}, 0, interpreter.lexical_environment(), FunctionKind::Regular, false, false);
             initializer->set_home_object(field.is_static() ? class_constructor : &class_prototype.as_object());
         }
 
@@ -3169,7 +3169,7 @@ void ScopeNode::block_declaration_instantiation(GlobalObject& global_object, Env
 
         if (is<FunctionDeclaration>(declaration)) {
             auto& function_declaration = static_cast<FunctionDeclaration const&>(declaration);
-            auto* function = ECMAScriptFunctionObject::create(global_object, function_declaration.name(), function_declaration.body(), function_declaration.parameters(), function_declaration.function_length(), environment, function_declaration.kind(), function_declaration.is_strict_mode());
+            auto* function = ECMAScriptFunctionObject::create(global_object, function_declaration.name(), function_declaration.body(), function_declaration.parameters(), function_declaration.function_length(), environment, function_declaration.kind(), function_declaration.is_strict_mode(), function_declaration.might_need_arguments_object());
             VERIFY(is<DeclarativeEnvironment>(*environment));
             static_cast<DeclarativeEnvironment&>(*environment).initialize_or_set_mutable_binding({}, global_object, function_declaration.name(), function);
         }
@@ -3313,7 +3313,7 @@ ThrowCompletionOr<void> Program::global_declaration_instantiation(Interpreter& i
     });
 
     for (auto& declaration : functions_to_initialize) {
-        auto* function = ECMAScriptFunctionObject::create(global_object, declaration.name(), declaration.body(), declaration.parameters(), declaration.function_length(), &global_environment, declaration.kind(), declaration.is_strict_mode());
+        auto* function = ECMAScriptFunctionObject::create(global_object, declaration.name(), declaration.body(), declaration.parameters(), declaration.function_length(), &global_environment, declaration.kind(), declaration.is_strict_mode(), declaration.might_need_arguments_object());
         global_environment.create_global_function_binding(declaration.name(), function, false);
         if (auto* exception = interpreter.exception())
             return throw_completion(exception->value());

Userland/Libraries/LibJS/AST.h

@@ -432,19 +432,23 @@ public:
     Vector<Parameter> const& parameters() const { return m_parameters; };
     i32 function_length() const { return m_function_length; }
     bool is_strict_mode() const { return m_is_strict_mode; }
+    bool might_need_arguments_object() const { return m_might_need_arguments_object; }
     bool is_arrow_function() const { return m_is_arrow_function; }
     FunctionKind kind() const { return m_kind; }
 
 protected:
-    FunctionNode(FlyString name, NonnullRefPtr<Statement> body, Vector<Parameter> parameters, i32 function_length, FunctionKind kind, bool is_strict_mode, bool is_arrow_function)
+    FunctionNode(FlyString name, NonnullRefPtr<Statement> body, Vector<Parameter> parameters, i32 function_length, FunctionKind kind, bool is_strict_mode, bool might_need_arguments_object, bool is_arrow_function)
         : m_name(move(name))
         , m_body(move(body))
         , m_parameters(move(parameters))
         , m_function_length(function_length)
         , m_kind(kind)
         , m_is_strict_mode(is_strict_mode)
+        , m_might_need_arguments_object(might_need_arguments_object)
         , m_is_arrow_function(is_arrow_function)
     {
+        if (m_is_arrow_function)
+            VERIFY(!m_might_need_arguments_object);
     }
 
     void dump(int indent, String const& class_name) const;
@@ -462,7 +466,8 @@ private:
     Vector<Parameter> const m_parameters;
     const i32 m_function_length;
     FunctionKind m_kind;
-    bool m_is_strict_mode;
+    bool m_is_strict_mode { false };
+    bool m_might_need_arguments_object { false };
     bool m_is_arrow_function { false };
 };
 
@@ -472,9 +477,9 @@ class FunctionDeclaration final
 public:
     static bool must_have_name() { return true; }
 
-    FunctionDeclaration(SourceRange source_range, FlyString const& name, NonnullRefPtr<Statement> body, Vector<Parameter> parameters, i32 function_length, FunctionKind kind, bool is_strict_mode = false)
+    FunctionDeclaration(SourceRange source_range, FlyString const& name, NonnullRefPtr<Statement> body, Vector<Parameter> parameters, i32 function_length, FunctionKind kind, bool is_strict_mode, bool might_need_arguments_object)
         : Declaration(source_range)
-        , FunctionNode(name, move(body), move(parameters), function_length, kind, is_strict_mode, false)
+        , FunctionNode(name, move(body), move(parameters), function_length, kind, is_strict_mode, might_need_arguments_object, false)
     {
     }
 
@@ -498,9 +503,9 @@ class FunctionExpression final
 public:
     static bool must_have_name() { return false; }
 
-    FunctionExpression(SourceRange source_range, FlyString const& name, NonnullRefPtr<Statement> body, Vector<Parameter> parameters, i32 function_length, FunctionKind kind, bool is_strict_mode, bool is_arrow_function = false)
+    FunctionExpression(SourceRange source_range, FlyString const& name, NonnullRefPtr<Statement> body, Vector<Parameter> parameters, i32 function_length, FunctionKind kind, bool is_strict_mode, bool might_need_arguments_object, bool is_arrow_function = false)
         : Expression(source_range)
-        , FunctionNode(name, move(body), move(parameters), function_length, kind, is_strict_mode, is_arrow_function)
+        , FunctionNode(name, move(body), move(parameters), function_length, kind, is_strict_mode, might_need_arguments_object, is_arrow_function)
     {
     }
 

Userland/Libraries/LibJS/Bytecode/Op.cpp

@@ -334,7 +334,7 @@ void Call::execute_impl(Bytecode::Interpreter& interpreter) const
 void NewFunction::execute_impl(Bytecode::Interpreter& interpreter) const
 {
     auto& vm = interpreter.vm();
-    interpreter.accumulator() = ECMAScriptFunctionObject::create(interpreter.global_object(), m_function_node.name(), m_function_node.body(), m_function_node.parameters(), m_function_node.function_length(), vm.lexical_environment(), m_function_node.kind(), m_function_node.is_strict_mode(), m_function_node.is_arrow_function());
+    interpreter.accumulator() = ECMAScriptFunctionObject::create(interpreter.global_object(), m_function_node.name(), m_function_node.body(), m_function_node.parameters(), m_function_node.function_length(), vm.lexical_environment(), m_function_node.kind(), m_function_node.is_strict_mode(), m_function_node.might_need_arguments_object(), m_function_node.is_arrow_function());
 }
 
 void Return::execute_impl(Bytecode::Interpreter& interpreter) const

Userland/Libraries/LibJS/Parser.cpp

@@ -657,7 +657,8 @@ RefPtr<FunctionExpression> Parser::try_parse_arrow_function_expression(bool expe
 
     return create_ast_node<FunctionExpression>(
         { m_state.current_token.filename(), rule_start.position(), position() }, "", move(body),
-        move(parameters), function_length, FunctionKind::Regular, body->in_strict_mode(), true);
+        move(parameters), function_length, FunctionKind::Regular, body->in_strict_mode(),
+        /* might_need_arguments_object */ false, /* is_arrow_function */ true);
 }
 
 RefPtr<Statement> Parser::try_parse_labelled_statement(AllowLabelledFunction allow_function)
@@ -878,7 +879,7 @@ NonnullRefPtr<ClassExpression> Parser::parse_class_expression(bool expect_class_
                     break;
                 }
 
-                //https://tc39.es/ecma262/#sec-class-definitions-static-semantics-early-errors
+                // https://tc39.es/ecma262/#sec-class-definitions-static-semantics-early-errors
                 // ClassElement : static MethodDefinition
                 //   It is a Syntax Error if PropName of MethodDefinition is "prototype".
                 if (is_static && name == "prototype"sv)
@@ -975,11 +976,13 @@ NonnullRefPtr<ClassExpression> Parser::parse_class_expression(bool expect_class_
 
             constructor = create_ast_node<FunctionExpression>(
                 { m_state.current_token.filename(), rule_start.position(), position() }, class_name, move(constructor_body),
-                Vector { FunctionNode::Parameter { FlyString { "args" }, nullptr, true } }, 0, FunctionKind::Regular, true);
+                Vector { FunctionNode::Parameter { FlyString { "args" }, nullptr, true } }, 0, FunctionKind::Regular,
+                /* is_strict_mode */ true, /* might_need_arguments_object */ false);
         } else {
             constructor = create_ast_node<FunctionExpression>(
                 { m_state.current_token.filename(), rule_start.position(), position() }, class_name, move(constructor_body),
-                Vector<FunctionNode::Parameter> {}, 0, FunctionKind::Regular, true);
+                Vector<FunctionNode::Parameter> {}, 0, FunctionKind::Regular,
+                /* is_strict_mode */ true, /* might_need_arguments_object */ false);
         }
     }
 
@@ -1958,6 +1961,7 @@ NonnullRefPtr<FunctionNodeType> Parser::parse_function_node(u8 parse_options)
     TemporaryChange break_context_rollback(m_state.in_break_context, false);
     TemporaryChange continue_context_rollback(m_state.in_continue_context, false);
     TemporaryChange class_field_initializer_rollback(m_state.in_class_field_initializer, false);
+    TemporaryChange might_need_arguments_object_rollback(m_state.function_might_need_arguments_object, false);
 
     constexpr auto is_function_expression = IsSame<FunctionNodeType, FunctionExpression>;
     auto function_kind = (parse_options & FunctionNodeParseOptions::IsGeneratorFunction) != 0 ? FunctionKind::Generator : FunctionKind::Regular;
@@ -1989,7 +1993,7 @@ NonnullRefPtr<FunctionNodeType> Parser::parse_function_node(u8 parse_options)
     if (function_length == -1)
         function_length = parameters.size();
 
-    TemporaryChange change(m_state.in_function_context, true);
+    TemporaryChange function_context_rollback(m_state.in_function_context, true);
 
     auto old_labels_in_scope = move(m_state.labels_in_scope);
     ScopeGuard guard([&]() {
@@ -2006,7 +2010,7 @@ NonnullRefPtr<FunctionNodeType> Parser::parse_function_node(u8 parse_options)
     return create_ast_node<FunctionNodeType>(
         { m_state.current_token.filename(), rule_start.position(), position() },
         name, move(body), move(parameters), function_length,
-        function_kind, has_strict_directive);
+        function_kind, has_strict_directive, m_state.function_might_need_arguments_object);
 }
 
 Vector<FunctionNode::Parameter> Parser::parse_formal_parameters(int& function_length, u8 parse_options)
@@ -3129,6 +3133,11 @@ Token Parser::consume()
 {
     auto old_token = m_state.current_token;
     m_state.current_token = m_state.lexer.next();
+    // NOTE: This is the bare minimum needed to decide whether we might need an arguments object
+    // in a function expression or declaration. ("might" because the AST implements some further
+    // conditions from the spec that rule out the need for allocating one)
+    if (old_token.type() == TokenType::Identifier && old_token.value().is_one_of("arguments"sv, "eval"sv))
+        m_state.function_might_need_arguments_object = true;
     return old_token;
 }
 

Userland/Libraries/LibJS/Parser.h

@@ -253,6 +253,7 @@ private:
         bool in_continue_context { false };
         bool string_legacy_octal_escape_sequence_in_scope { false };
         bool in_class_field_initializer { false };
+        bool function_might_need_arguments_object { false };
 
         ParserState(Lexer, Program::Type);
     };

Userland/Libraries/LibJS/Runtime/AbstractOperations.cpp

@@ -627,7 +627,7 @@ ThrowCompletionOr<void> eval_declaration_instantiation(VM& vm, GlobalObject& glo
         return throw_completion(exception->value());
 
     for (auto& declaration : functions_to_initialize) {
-        auto* function = ECMAScriptFunctionObject::create(global_object, declaration.name(), declaration.body(), declaration.parameters(), declaration.function_length(), lexical_environment, declaration.kind(), declaration.is_strict_mode());
+        auto* function = ECMAScriptFunctionObject::create(global_object, declaration.name(), declaration.body(), declaration.parameters(), declaration.function_length(), lexical_environment, declaration.kind(), declaration.is_strict_mode(), declaration.might_need_arguments_object());
         if (global_var_environment) {
             global_var_environment->create_global_function_binding(declaration.name(), function, true);
             if (auto* exception = vm.exception())

Userland/Libraries/LibJS/Runtime/ECMAScriptFunctionObject.cpp

@@ -24,7 +24,7 @@
 
 namespace JS {
 
-ECMAScriptFunctionObject* ECMAScriptFunctionObject::create(GlobalObject& global_object, FlyString name, Statement const& ecmascript_code, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, Environment* parent_scope, FunctionKind kind, bool is_strict, bool is_arrow_function)
+ECMAScriptFunctionObject* ECMAScriptFunctionObject::create(GlobalObject& global_object, FlyString name, Statement const& ecmascript_code, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, Environment* parent_scope, FunctionKind kind, bool is_strict, bool might_need_arguments_object, bool is_arrow_function)
 {
     Object* prototype = nullptr;
     switch (kind) {
@@ -35,10 +35,10 @@ ECMAScriptFunctionObject* ECMAScriptFunctionObject::create(GlobalObject& global_
         prototype = global_object.generator_function_prototype();
         break;
     }
-    return global_object.heap().allocate<ECMAScriptFunctionObject>(global_object, move(name), ecmascript_code, move(parameters), m_function_length, parent_scope, *prototype, kind, is_strict, is_arrow_function);
+    return global_object.heap().allocate<ECMAScriptFunctionObject>(global_object, move(name), ecmascript_code, move(parameters), m_function_length, parent_scope, *prototype, kind, is_strict, might_need_arguments_object, is_arrow_function);
 }
 
-ECMAScriptFunctionObject::ECMAScriptFunctionObject(FlyString name, Statement const& ecmascript_code, Vector<FunctionNode::Parameter> formal_parameters, i32 function_length, Environment* parent_scope, Object& prototype, FunctionKind kind, bool strict, bool is_arrow_function)
+ECMAScriptFunctionObject::ECMAScriptFunctionObject(FlyString name, Statement const& ecmascript_code, Vector<FunctionNode::Parameter> formal_parameters, i32 function_length, Environment* parent_scope, Object& prototype, FunctionKind kind, bool strict, bool might_need_arguments_object, bool is_arrow_function)
     : FunctionObject(prototype)
     , m_environment(parent_scope)
     , m_formal_parameters(move(formal_parameters))
@@ -48,6 +48,7 @@ ECMAScriptFunctionObject::ECMAScriptFunctionObject(FlyString name, Statement con
     , m_name(move(name))
     , m_function_length(function_length)
     , m_kind(kind)
+    , m_might_need_arguments_object(might_need_arguments_object)
     , m_is_arrow_function(is_arrow_function)
 {
     // NOTE: This logic is from OrdinaryFunctionCreate, https://tc39.es/ecma262/#sec-ordinaryfunctioncreate
@@ -159,7 +160,11 @@ ThrowCompletionOr<void> ECMAScriptFunctionObject::function_declaration_instantia
             });
     }
 
-    auto arguments_object_needed = this_mode() != ThisMode::Lexical;
+    auto arguments_object_needed = m_might_need_arguments_object;
+
+    if (this_mode() == ThisMode::Lexical)
+        arguments_object_needed = false;
+
     if (parameter_names.contains(vm.names.arguments.as_string()))
         arguments_object_needed = false;
 
@@ -360,7 +365,7 @@ ThrowCompletionOr<void> ECMAScriptFunctionObject::function_declaration_instantia
     VERIFY(!vm.exception());
 
     for (auto& declaration : functions_to_initialize) {
-        auto* function = ECMAScriptFunctionObject::create(global_object(), declaration.name(), declaration.body(), declaration.parameters(), declaration.function_length(), lex_environment, declaration.kind(), declaration.is_strict_mode());
+        auto* function = ECMAScriptFunctionObject::create(global_object(), declaration.name(), declaration.body(), declaration.parameters(), declaration.function_length(), lex_environment, declaration.kind(), declaration.is_strict_mode(), declaration.might_need_arguments_object());
         var_environment->set_mutable_binding(global_object(), declaration.name(), function, false);
     }
 

Userland/Libraries/LibJS/Runtime/ECMAScriptFunctionObject.h

@@ -28,9 +28,9 @@ public:
         Global,
     };
 
-    static ECMAScriptFunctionObject* create(GlobalObject&, FlyString name, Statement const& ecmascript_code, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, Environment* parent_scope, FunctionKind, bool is_strict, bool is_arrow_function = false);
+    static ECMAScriptFunctionObject* create(GlobalObject&, FlyString name, Statement const& ecmascript_code, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, Environment* parent_scope, FunctionKind, bool is_strict, bool might_need_arguments_object = true, bool is_arrow_function = false);
 
-    ECMAScriptFunctionObject(FlyString name, Statement const& ecmascript_code, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, Environment* parent_scope, Object& prototype, FunctionKind, bool is_strict, bool is_arrow_function = false);
+    ECMAScriptFunctionObject(FlyString name, Statement const& ecmascript_code, Vector<FunctionNode::Parameter> parameters, i32 m_function_length, Environment* parent_scope, Object& prototype, FunctionKind, bool is_strict, bool might_need_arguments_object, bool is_arrow_function);
     virtual void initialize(GlobalObject&) override;
     virtual ~ECMAScriptFunctionObject();
 
@@ -100,6 +100,7 @@ private:
     Optional<Bytecode::Executable> m_bytecode_executable;
     i32 m_function_length { 0 };
     FunctionKind m_kind { FunctionKind::Regular };
+    bool m_might_need_arguments_object { true };
     bool m_is_arrow_function { false };
     bool m_has_simple_parameter_list { false };
 };

Userland/Libraries/LibJS/Runtime/GeneratorFunctionConstructor.cpp

@@ -62,7 +62,7 @@ Value GeneratorFunctionConstructor::construct(FunctionObject& new_target)
             block.dump(executable);
     }
 
-    return ECMAScriptFunctionObject::create(global_object(), function->name(), function->body(), function->parameters(), function->function_length(), vm().lexical_environment(), FunctionKind::Generator, function->is_strict_mode(), false);
+    return ECMAScriptFunctionObject::create(global_object(), function->name(), function->body(), function->parameters(), function->function_length(), vm().lexical_environment(), FunctionKind::Generator, function->is_strict_mode(), function->might_need_arguments_object());
 }
 
 }