bpo-36616: optimize handling of thread state in function call code by jdemeyer · Pull Request #12839 · python/cpython

Those strange redirections were added because somebody wanted to use

register PyObject **stack_pointer;

and it's illegal to take the address of a register variable. But the register keyword is obsolete, compilers are now much more clever than they were before. Also, call_function() should be inlined and then the pointer isn't even a real pointer.