[Python-ideas] Performance improvements via static typing

[Stefan Behnel]

Michael Hall schrieb am 19.07.2018 um 15:51:
> While I am aware of projects like Cython and mypy, it seems to make sense
> for CPython to allow optional enforcement of type hints, with compiler
> optimizations related to it to be used. While this would not receive the
> same level of performance benefits as using ctypes directly, there do
> appear to be various gains available here.

Well, first of all, a C level type check at runtime is quite fast compared
to a byte code dispatch with stack argument handling, and can be done
regardless of any type hints. There are various code patterns that would
suggest a certain type ("x.append()" probably appends to a list, "x.get()"
will likely be a dict lookup, etc.), and that can be optimised for without
static type declarations and even without any type hints.

Then, the mere fact that user code says "a: List" does not help in any way.
Even "a: list" would not help, because any behaviour of that "list" might
have been overridden by the list subtype that the function eventually receives.

The same applies to "x: float". Here, in order to gain speed, the compiler
would have to generate two separate code paths through the entire function,
one for C double computations, and one for Python float subtypes. And then
exponentiate that by the number of other typed arguments that may or may
not contain subtypes. Quite some overhead.

It's unclear if the gain would have any reasonable relation to the effort
in the end. Sure, type hints could be used as a bare trigger for certain
optimistic optimisations, but then, it's cheap enough to always apply these
optimisations regardless, via a C type check.

Stefan