Bug 179478 – [JSC] Optimize default constructor implementation for derived class with array iterator protocol check

• Status: NEW
• Component: JavaScriptCore

[Yusuke Suzuki]

Current default constructor implementation for derived class is,

constructor(...args)
{
    super(...args);
}

But the above code allocates arrays until FTL while that is apparently unnecessary for many cases.
It makes super() call construct_varargs with opaque array... Which is not good since DFGByteCodeParser do not handle it well if the constructor is non JS.
So, extends Array case is not optimized well in this case.

We would like to explore the performance improvement by changing this to

constructor(...args)
{
    if (@isArrayIteratorProtocolFastAndNonObservable()) {
        @superForwardArguments();
        return;
    }
    super(@spread());
}