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LibJS: Add an optimization to avoid needless arguments object creation

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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 :^)
This commit is contained in:
Linus Groh 2021-10-05 08:44:58 +01:00
parent fcb355f193
commit 4fa5748093
9 changed files with 47 additions and 26 deletions
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8
Userland/Libraries/LibJS/AST.cpp
View file

@ -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());