3 ms·
The interesting part of this is that all this dynamism and dependence on runtime actually improves performance in comparison to C++. For example it is perfectly
by dfox 9y ago
The interesting part of this is that all this dynamism and dependence on runtime actually improves performance in comparison to C++. For example it is perfectly possible to implement objc_msgSend in portable C such that it is on modern superscalar and heavily cache dependent CPUs on average faster than C++-style virtual method call.
- mikeash 9y agoReally? That seems highly unlikely, and doesn't match with any of the speed testing I've done. When you get down to it, an ObjC message send performs a superset of the work of a C++ virtual call. A C++ virtual call gets the vtable pointer from the object, indexes into that table by a constant offset, loads the function pointer at that offset, and calls it. An ObjC message send gets the class pointer from the object, indexes into that table by a constant offset, loads the method cache information at that index, uses the selector to look up the entry in the cache's hash table, and then if all goes well, it loads the function pointer from the table and jumps to it.