Greg Parker: The compiler knows which selectors are optimized by the runtime. It compiles the call site differently, calling objc_msgSend_fixup via a function pointer. At runtime, objc_msgSend_fixup replaces the function pointer with one of the objc_msgSend_vtable functions, if the called selector is one of the optimized selectors. Update (2014-06-25): objc-runtime-new.m (via Colin Wheeler): /*********************************************************************** * … Continue reading objc_msgSend_vtable