179478 2017-11-09 07:44:48 -0800 [JSC] Optimize default constructor implementation for derived class with array iterator protocol check 2017-11-09 08:28:35 -0800 1 1 1 Unclassified WebKit JavaScriptCore WebKit Nightly Build Unspecified Unspecified NEW P2 Normal --- 178064 1 ysuzuki ysuzuki oldest_to_newest 1369867 0 ysuzuki 2017-11-09 07:44:48 -0800 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()); }