[mypyc]Implement CallC IR

[TH3CHARLie]

relates mypyc/mypyc#709

This PR adds a new IR op CallC to replace some PrimitiveOp that simply calls a C function. To demonstrate this prototype, str.join primitive is now switched from PrimitiveOp to CallC, with identical generated C code:

Test driver code

from typing import List

def test_str_join_helper(helper_arg: List[str]) -> str:
    helper_base = "#"
    return helper_base.join(helper_arg)

def main():
    l = ["a", "bb", "cc"]
    print(test_str_join_helper(l))

main()

generated C code(for the helper function, identical before/after):

PyObject *CPyDef_test_str_join_helper(PyObject *cpy_r_helper_arg) {
    PyObject *cpy_r_r0;
    PyObject *cpy_r_helper_base;
    PyObject *cpy_r_r1;
    PyObject *cpy_r_r2;
CPyL0: ;
    cpy_r_r0 = CPyStatic_unicode_3; /* '#' */
    CPy_INCREF(cpy_r_r0);
    cpy_r_helper_base = cpy_r_r0;
    cpy_r_r1 = PyUnicode_Join(cpy_r_helper_base, cpy_r_helper_arg);
    CPy_DecRef(cpy_r_helper_base);
    if (unlikely(cpy_r_r1 == NULL)) {
        CPy_AddTraceback("foo.py", "test_str_join_helper", 5, CPyStatic_globals);
        goto CPyL2;
    } else
        goto CPyL1;
CPyL1: ;
    return cpy_r_r1;
CPyL2: ;
    cpy_r_r2 = NULL;
    return cpy_r_r2;
}

Some objectives need to be completed after some discussion.

• figure out the differences betweenOpDescrption and new LLOpDescrption, do we need fields including is_var_arg, steals, is_borrowed?(Not supported in this PR yet, but we will gradually update the design when we encounter ops that require such changes)
• support void functions(through a flag?)
• textual IR testcase

[JukkaL]

Overall looks good! Left some comments, mostly about naming. Naming is one of the most important factors that affects the readability of code, so it makes sense to think carefully about it. (It's also okay to later rename things if we come up with better names.)

[JukkaL]

This name doesn't fit the current implementation. Can you propose a new name that communicates that this specifically describes how to call a C function?

[TH3CHARLie]

I am using CFunctionDescription in the updated commits.

[JukkaL]

Thanks for updates! Just a few more naming ideas and comment about a comment, otherwise looks great.

[JukkaL]

Thanks, looks great now!

[TH3CHARLie]

Thanks for the review!