```__neg__``` failing for empty arrays

[mitkotak]

Here's the MWE

>>> import pycuda.autoinit
>>> import pycuda.gpuarray as gpuarray
>>> import numpy as np
>>> empty_array = np.array([])
>>> empty_array_gpu = gpuarray.to_gpu(empty_array)
>>> neg_empty_array = -empty_array # array([], dtype=float64)
>>> neg_empty_array_gpu = -empty_array_gpu # Fails

Here's the error trace

---------------------------------------------------------------------------
error                                     Traceback (most recent call last)
Input In [17], in <cell line: 1>()
----> 1 -gpuarray.to_gpu(a)

File ~/pycuda/pycuda/gpuarray.py:643, in GPUArray.__neg__(self)
    641 def __neg__(self):
    642     result = self._new_like_me()
--> 643     return self._axpbz(-1, 0, result)

File ~/pycuda/pycuda/gpuarray.py:468, in GPUArray._axpbz(self, selffac, other, out, stream)
    463     raise RuntimeError(
    464         "only contiguous arrays may " "be used as arguments to this operation"
    465     )
    467 func = elementwise.get_axpbz_kernel(self.dtype, out.dtype)
--> 468 func.prepared_async_call(
    469     self._grid,
    470     self._block,
    471     stream,
    472     selffac,
    473     self.gpudata,
    474     other,
    475     out.gpudata,
    476     self.mem_size,
    477 )
    479 return out

File ~/pycuda/pycuda/driver.py:626, in _add_functionality.<locals>.function_prepared_async_call(func, grid, block, stream, *args, **kwargs)
    620     raise TypeError(
    621         "unknown keyword arguments: " + ", ".join(kwargs.keys())
    622     )
    624 from pycuda._pvt_struct import pack
--> 626 arg_buf = pack(func.arg_format, *args)
    628 for texref in func.texrefs:
    629     func.param_set_texref(texref)

error: required argument is not an integer

[kaushikcfd]

I see the following two approaches:

1. Rewrite the kernel launches in pycuda.gpuarray to guard from passing None as an argument, or,
2. Modify Function.prepared_call to accept None as a valid argument.

Taking (1) would lead to an uglier implementation of GPUArray and taking (2) might lead to user mishaps that could be reflected as segfaults.

Maybe we could introduce another kw-only argument to prepared_call called accept_none which is False by default and if True, either doesn't launch the kernel or maps such arguments to 0.

@inducer: Any comments?

[inducer]

If you can do 2 without introducing per-argument processing in Python (such as by modifying the custom struct packing), that could be viable.