New submission from Dennis Sweeney <sweeney.dennis...@gmail.com>:
Anecdotally, a few people I've talked to have expected that match-case
statements would improve performance in the same way that switch-cases
sometimes do in C-like languages. While this is impossible in general without
changing semantics (since, for example, __eq__ can have side effects), it would
be nice if a jump table was implemented in certain safe cases.
My idea was to implement this whenever the list of cases begins with 2 or more
of the following in a row:
(1) ints
(2) strings
(3) None
(4) OR (`|`) patterns of any of the above
(5?) potentially later add support for tuples
Example:
match x:
case 10: # safe
print("A")
case 20: # safe
print("B")
case 30 | 31: # safe
print("C")
case 40: # safe
print("D")
case MyClass(10, 20): # unsafe
print("E")
case []: # unsafe
print("F")
case whatever: # unsafe
print("G")
This would compile to something like
LOAD_FAST (x)
LOAD_CONST 16 ({10: 16, 20: 38, 30: 80, 31: 80, 40: 102})
ATTEMPT_SAFE_MATCH 110
... all the rest of the code is the same ...
Where the new opcode would be would be something like
case TARGET(ATTEMPT_SAFE_MATCH): {
PyObject *jump_dict = POP();
PyObject *x = TOP();
if (PyLong_CheckExact(x) ||
PyUnicode_CheckExact(x) ||
Py_Is(x, Py_None))
{
PyObject *boxed = PyDict_GetItemWithError(jump_dict, x);
Py_DECREF(jump_dict);
if (res == NULL) {
if (PyErr_Occurred()) {
goto error;
}
else {
JUMPBY(oparg);
}
}
assert(PyLong_CheckExact(boxed));
int target = (int)PyLong_AsLong(boxed);
JUMPBY(target);
}
else {
// fall back to general matching code
Py_DECREF(jump_dict);
DISPATCH();
}
}
I can work on an implementation in the next couple of weeks.
----------
components: Interpreter Core
messages: 394874
nosy: Dennis Sweeney
priority: normal
severity: normal
status: open
title: Add jump table for certain safe match-case statements
type: performance
versions: Python 3.11
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue44283>
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