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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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9 changed files with 47 additions and 26 deletions
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@ -334,7 +334,7 @@ void Call::execute_impl(Bytecode::Interpreter& interpreter) const
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void NewFunction::execute_impl(Bytecode::Interpreter& interpreter) const
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{
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auto& vm = interpreter.vm();
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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());
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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());
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}
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void Return::execute_impl(Bytecode::Interpreter& interpreter) const
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