[mypyc] Dict true test

[97littleleaf11]

Description

Closes mypyc/mypyc#768

• Modify builtin_len(dict).
  • In cpython/dictobject.h

     /* Get the number of items of a dictionary. */
    #define PyDict_GET_SIZE(mp)  (assert(PyDict_Check(mp)),((PyDictObject *)mp)->ma_used)

  • Thus replace call_c with LoadMem and add PyDictObject. Related issue: Add specialization for strings in builtin_len mypyc/mypyc#835

Test Plan

An IR test and a functional test.
Similar to 1e96f1d

[JukkaL]

Hmm the original implementation may actually be buggy. We should allow subclasses to override __len__, for consistency with other dict operations, and this means that we should have a type guard. This probably makes it too complex to be done directly in IR, and instead we could use a C helper that uses PyDict_GET_SIZE behind a type guard. Sorry about that!

This should work similar to CPython and print 'here':

class D(dict):
    def __len__(self) -> int:
        return 1

if D():
    print('here') 

[97littleleaf11]

Got it. I will start a new branch from beginning later.

[97littleleaf11]

I tried adding this:

Py_ssize_t CPyDict_Size(PyObject *dict) {
    if (PyDict_CheckExact(dict)) {
        return PyDict_Size(dict);
    }
    return PyObject_Size(dict);
}

class MyDict(dict):
    def __init__(self, *args, **kwargs):
        self.update(*args, **kwargs)

    def __len__(self):
        return 1


def test_dict_subclass_to_bool() -> None:
    d = MyDict()
    if d:
        print("true")

However PyObject_Size won't call the overrided __len__.
Does the cast operation cause that?

L0:
    r0 = tmp5.MyDict :: type
    r1 = PyObject_CallFunctionObjArgs(r0, 0)
    if is_error(r1) goto L7 (error at test_dict_subclass_to_bool:45) else goto L1
L1:
    r2 = cast(dict, r1)
    if is_error(r2) goto L7 (error at test_dict_subclass_to_bool:45) else goto L2
L2:
    d = r2
    r3 = CPyDict_Size(d)
    dec_ref d
    r4 = r3 >= 0 :: signed
    if not r4 goto L7 (error at test_dict_subclass_to_bool:45) else goto L3 :: bool

[JukkaL]

However PyObject_Size won't call the overrided len.

Hmm I wonder if this is a bug in PyObject_Size. Anyway, it's already broken for len(d), so not supporting it for boolean checks seems okay. So let's drop the __len__ override test case and use the original implementation for now. Sorry about the back and forth -- there are some gray areas with respect to CPython compatibility where it's often unclear what's the best course of action.

But can you add a new issue to track the issue of overriding __len__? To properly fix it, we should fix it for len() as well.

Does the cast operation cause that?

No, the cast does not modify the operand.

[JukkaL]

Thanks for the updates! This looks good, and it made a quick benchmark I wrote about 50% faster, which is a nice speedup.