JavaScriptCore (JSC, WebKit’s JavaScript engine) needs to be able to parse/unwind the call stack for exception handling or determining stack traces. This is not so simple since JS and native/C++ can be arbitrarily intertwined and JS call frames do not match the system’s calling convention. On the other hand while JSC controls JS stack frames and knows how to unwind them, it can’t do the same thing for native C++ stack frames since this is defined by the compiler (its flags) and/or system.
