Lowering `StridedMemoryView` attribues to typed efficient C/C++/Cython accessible values

[jakirkham]

Currently the attributes of StridedMemoryView are as follows:

https://github.com/NVIDIA/cuda-python/blob/8c841cdb24f64e65138cd2658d30fdeabd18769b/cuda_core/cuda/core/experimental/_memoryview.pyx#L24-L32

There is a todo in the code noting that this is worth converting to Cython types. Would also support this recommendation

When accessing things like shape or strides in Cython, one ideally wants a C array type of some form (pointer, typed-memoryview, dynamic array, etc.) that can easily be iterated over in C for-loops. As these attributes are currently accessing them requires calling the CPython API to get the length, each value, coerce them to C friendly types, etc.

This is especially important for things like ptr, which gets accessed regularly. So having a fast access C-type really helps

---

If we look at the Python Buffer Protocol (PEP 3118), they have the following definition for Py_buffer (their equivalent type):

typedef struct {
    void *buf;
    PyObject *obj;        /* owned reference */
    Py_ssize_t len;
    Py_ssize_t itemsize;  /* This is Py_ssize_t so it can be
                             pointed to by strides in simple case.*/
    int readonly;
    int ndim;
    char *format;
    Py_ssize_t *shape;
    Py_ssize_t *strides;
    Py_ssize_t *suboffsets;
    void *internal;
} Py_buffer;

They also require users to call PyObject_GetBuffer to produce a buffer object and PyBuffer_Release to release a buffer object. This handles any memory allocation/deallocation for shape, strides, etc.. It also handles refcounting for obj. This functionality is wonderful to use

The lack of these semantics has made working with DLPack a chore

---

Thinking about how to map the C-like struct above to Cython/Python. A few things stick out

Some of these Cython can translate between Python/Cython/C like Py_ssize_t (effectively ssize_t in C (PEP 353)) to Python int's when needed.

Others can be coerced like char* to bytes (though usually one will want to decode/encode to/from str)

Still others could be translated well by Cython as long as they are typed appropriately. For example void* doesn't translate well to Python. However uintptr_t does behave like a pointer in C (sometimes with a cast) and like a Python int. Cython will handle the translation for us. Similarly bint for readonly works better when capturing Python's bool semantics while still working in C.

With a typed memoryview, it is possible to wrangle Py_ssize_t* into something better behaved like Py_ssize_t[::1], which can then move more easily between Python & Cython.

Also note that format above is a string specifying the format type according to the Python Buffer Protocol. NumPy is also able to consume and produce such format strings

This led RAPIDS to this approach:

cdef class Array:
    cdef readonly uintptr_t ptr
    cdef readonly bint readonly
    cdef readonly object obj

    cdef readonly Py_ssize_t itemsize

    cdef readonly Py_ssize_t ndim
    cdef Py_ssize_t[::1] shape_mv
    cdef Py_ssize_t[::1] strides_mv

    cdef readonly bint cuda

---

If StridedMemoryView is used with C/C++ directly, it may make sense to actually type a public C struct in Cython. Then this could be leveraged in C/C++ code that can hand such objects to or receive them from CUDA-Python. For example

# filename: strided_memory_view.pxd

from libc.stdint cimport uintptr_t

cdef public struct CStridedMemoryView:
    uintptr_t ptr
    ssize_t* shape
    ssize_t* strides
    char* format
    int device_id
    bint device_accessible
    bint readonly
    void* obj

cdef class StridedMemoryView:
    CStridedMemoryView data

// filename: my_program.c

#include <stdbool.h>
#include <stdint.h>

#include <sys/types.h>

#include "strided_memory_view.h"


static const char* s = "Hello World!";
static const ssize_t s_len = 13;


int main() {
    CStridedMemoryView st;
    st.ptr = (uintptr_t)s;
    st.shape = &s_len;
    st.readonly = true;

    // ...

    return 0;
}

Though there can be other valid ways to go

[leofang]

On the Oct 23 meeting we discussed and agreed that this is an important feature to support. The information provided by StridedMemoryView should be host-/device- accessible. Temporarily slating this for the beta 3 release.

[leofang]

Unfortunately due to the code freeze approaching and other commitments I need to push this to the next release.