Setting a slice of memory without having to enumerate

[dsyer]

It's nice that the WASM memory can be inspected with a slice operator:

from wasmtime import *
engine = Engine()
store = Store(engine)
module = Module.from_file(engine, "./hello.wasm")
instance = Instance(store, module, [])
exports = instance.exports(store)
memory = exports["memory"]

Then

>>> memory.data_ptr(store)[1024:1029]
[114, 97, 116, 115, 0]

but it's not possible to set it that way which seems like a bug, or at least a missing feature:

>>> data = memory.data_ptr(store)
>>> data[1024:1028] = [114, 97, 116, 116]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: sequence index must be integer, not 'slice'

You can work around it by iterating:

for i,v in enumerate("fats".encode('utf-8')): data[i] = v

[alexcrichton]

Ah sorry I don't know much about python bulk array assignment, but if a different type should be returned by the methods here that's more idiomatic that seems like a reasonable thing to do!

[dsyer]

Agree, but I have no idea how to achieve it. All I know is bulk assignment works with an actual list of bytes, and wasmtime._memory.Memory .data_ptr() is masquerading as a list of bytes. So it should be possible.

[dsyer]

This seems to work:

>>> import ctypes
>>> data = (ctypes.c_ubyte*memory.data_len(store)).from_address(ctypes.addressof(memory.data_ptr(store).contents))
>>> data[0:10] = "helloworld".encode('utf-8')
>>> data[0:10]
[104, 101, 108, 108, 111, 119, 111, 114, 108, 100]
>>> memory.data_ptr(store)[0:10]
[104, 101, 108, 108, 111, 119, 111, 114, 108, 100]

[Tinche]

I've also been playing around with wasmtime, got a toy program running but I was reading and writing memory one byte at a time.

Glad to see there's a bulk read method available (it didn't occur to me to use a slice), a bulk write would also be very welcome!

[Tinche]

Playing around with this a little more, memory.data_ptr(store)[:res_length] returns an actual list of integer objects. You can call bytes on it, but it's very inefficient. Realistically no one's going to be treating this as a list of integers, so I'd suggest changing the API to just return a bytes or bytearray instance directly.

[muayyad-alsadi]

this is very important as it's the most obvious bottleneck
python have many ways of doing it, for example
bytearray(b'helloworld') or array.array('B', b'helloworld')

I believe if set_item is passed a slice instead of index, it should iterate instead of raising exception

    ptr = memory.data_len(store)
    ptr[start:end]=bytearray(b'helloworld')
TypeError: sequence index must be integer, not 'slice'

>>> buff=bytearray(b'hello world')
>>> buff[0:5]=b'HELLO'
>>> for i in buff: print(i)
72
69
76
76
79
32
119
111
114
108
100

[muayyad-alsadi]

for comparison, on my example of slice setting of 15MB from images

• wasm3 slice set took ~1ms
• wasmtime it took ~900ms

this is 900x slower!!

[muayyad-alsadi]

here they suggest to cast the C pointer to memoryview

memory_ptr = memory.data_ptr(store)
buff = memoryview(ctypes.cast(memory_ptr, ctypes.POINTER(ctypes.c_ubyte*size)))
buff_bytes = buff.bytes()

https://stackoverflow.com/questions/43810423/what-is-the-most-efficient-way-to-copy-an-externally-provided-buffer-to-bytes

but I got endianess problems

https://bugs.python.org/issue39610

NotImplementedError: memoryview: unsupported format &<B

any clue how to set a slice to bytearray or bytes or similar list of bytes?

[muayyad-alsadi]

@dsyer the following line was too slow, it was even slower than setting the slice

>>> data = (ctypes.c_ubyte*memory.data_len(store)).from_address(ctypes.addressof(memory.data_ptr(store).contents))

how can we make it into memoryview without creating endianess problems?

[muayyad-alsadi]

here is my implementation which is 300x times faster than @dsyer

def set_slice(val, start=0, end=None):
    size = memory.data_len(store)
    if end is None: end=start+len(val)
    val_size = len(val)
    if end-start>val_size or end>size:
        raise IndexError("out of memory size")
    src_ptr = (ctypes.c_ubyte * val_size).from_buffer(val)
    dst_ptr = ctypes.addressof((ctypes.c_ubyte*val_size).from_address(ctypes.addressof(memory_ptr.contents)+start))
    ctypes.memmove(dst_ptr, src_ptr, val_size)
    return

it seems that there was endianess difference thats why @dsyer was slower
and that's why memoryview was failing

[thewtex]

Hi folks, a related note here: it would be very nice if wasmtime-py could expose the wasm module memory with Python's Buffer Protocol. This would enable efficient binary array access, slicing and more, via the standard numpy interface. wasmer-python has this implemented.

[j-blue-arz]

Hi folks, a related note here: it would be very nice if wasmtime-py could expose the wasm module memory with Python's Buffer Protocol. This would enable efficient binary array access, slicing and more, via the standard numpy interface. wasmer-python has this implemented.

While I agree with the above comment, I think it should not be in the scope of this issue here and the already in-progress PR. @thewtex Maybe you can create a new issue?

[muayyad-alsadi]

wasmer-python has this implemented.

@thewtex no, it was worse in terms of performance, please check my article here

[thewtex]

While I agree with the above comment, I think it should not be in the scope of this issue here and the already in-progress #134. @thewtex Maybe you can create a new issue?

@j-blue-arz yes, that's right. This would be in addition to @muayyad-alsadi nice setitem / slicing improvements. Issue created in #135 .

please check my article here

@muayyad-alsadi awesome article!!

Sorry if I was not clear, I meant the use of the buffer protocol with numpy instead of setitem / slicing. An example is here:

muayyad-alsadi/wasm-demos#1

The results:

Slicing
_______
img2bytearray took 9 ms
grow:  238
slice set took 352 ms
wasm took 10 ms
slice to array 78 ms
array to image 7 ms
all took 458 ms

img2bytearray took 7 ms
slice set took 355 ms
wasm took 9 ms
slice to array 77 ms
array to image 7 ms
all took 460 ms

Buffer Protocol
_______
img2bytearray took 9 ms
slice set took 0 ms
wasm took 9 ms
slice to array 0 ms
array to image 8 ms
all took 28 ms

img2bytearray took 5 ms
slice set took 0 ms
wasm took 9 ms
slice to array 0 ms
array to image 8 ms
all took 24 ms

[muayyad-alsadi]

@thewtex thank you that was very useful

here is the wasmtime code for

memory = instance.exports(store)['memory']
ptr_type = ctypes.c_ubyte * (memory.data_len(store))
memory_ptr = (ptr_type).from_address(ctypes.addressof(memory.data_ptr(store).contents))
np_mem = np.frombuffer(memory_ptr, dtype=np.uint8, offset=hb)