Support nullptr testing

[Smit-create]

Fixes #1781

[certik]

The cmpop has various things like >. What does that mean for CPtr? I think we only want == and !=.

[Smit-create]

C has pointer comparisons for different cmpop as seen here.

[certik]

I see. We can allow it in ASR, since eventually we want to have a C frontend too. But in LPython I would mainly support == and != for now for CPtr. The other ones we can, but don't have to right now.

[Smit-create]

@certik The current diff does work fine for C and LLVM but fails in CPython because of two object comparisons.

>>> import ctypes
>>> x = ctypes.c_void_p()
>>> y = ctypes.c_void_p()
>>> x == y
False

[czgdp1807]

Here you go,

(lp) 18:59:02:~/lpython_project/lpython % python       
Python 3.10.4 | packaged by conda-forge | (main, Mar 24 2022, 17:39:37) [Clang 12.0.1 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import ctypes
>>> x = ctypes.c_void_p()
>>> y = ctypes.c_void_p()
>>> x.value == y.value
True

[certik]

I see, we need to figure out a robust solution in CPython that will always work. If we have to do the .value trick, then maybe that's what is needed. Otherwise we can create our own wrapper for CPtr to handle == correctly.

[czgdp1807]

Done in 1b162b9.

[Smit-create]

Thanks @czgdp1807!

[certik]

Thanks!