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