LOAD_FAST accounts for
of all bytecodes executed. Including superinstructions brings this up to 14.6+4.7+4.6+2.4+0.9 = 27.1%
TARGET(LOAD_FAST) { PyObject*value=GETLOCAL(oparg); if (value==NULL) { goto unbound_local_error; } Py_INCREF(value); PUSH(value); DISPATCH(); } We can turn this NULL-check into an assertion in many cases, where we can determine at compile time that the local variable is already initialized. Preliminary tests show that almost all LOAD_FAST instructions can be statically analyzed to be loading already-initialized variables.
The one twist is handling del frame.f_locals["x"] or frame.f_lineno = 17, where previously-safe loads could become unsafe. For now, we can just replace all the LOAD_FAST (no null check) with LOAD_FAST_CHECK in that particular code object.
See also
PR: