Add support to creating MtlArray using a memory allocated by Array

[ronisbr]

Hi!

The package has already the support to create an Array given the memory allocated by a MtlArray, but not the other way around. It is possible, but required that the allocated memory is page-aligned. The following code is a MWE of how this can be achieved (without worrying about object destruction):

using Metal

# Obtain the page size.
pagesize = ccall(:getpagesize, Cint, ())

# Dimensions and type of the desired array.
dims = (1000, 1000)
T = Float32

# Compute how many pages we need to store the array.
npages = ceil(Int, prod(dims) * sizeof(T) / pagesize)

# Compute the total number of bytes.
nbytes = npages * pagesize

# Allocate memory aligned with the page.
addr = Ref(C_NULL)
ccall(
    :posix_memalign,
    Cint,
    (Ptr{Ptr{Cvoid}}, Csize_t, Csize_t),
    addr,
    pagesize,
    prod(dims) * sizeof(T)
)

# Wrap the allocated memory to a Julia array.
array = unsafe_wrap(Array{T, length(dims)}, reinterpret(Ptr{T}, addr[]), dims, own = false)

# Create the MtlArray.
pbuf = Metal.MTL.mtDeviceNewBufferWithBytesNoCopy(
    current_device(),
    addr[],
    nbytes,
    Metal.Shared | Metal.MTL.DefaultTracking | Metal.MTL.DefaultCPUCache
)
buf = MtlBuffer(pbuf)
marray = MtlArray{T, length(dims)}(buf, dims)

julia> array[1, 1] = 100.0
100.0

julia> marray[1, 1]
100.0

julia> array[1000, 1000] = 1986
1968

julia> marray[1000, 1000]
1986.0

julia> marray[1000, 1000] = 1987
1987

julia> array[1000, 1000]
1987.0

[maleadt]

@jpsamaroo also mentioned that a better way to allocate page-aligned memory is probably https://github.com/mkitti/ArrayAllocators.jl

[dlfivefifty]

Thanks for creating this issue. Not being an expert I was confused why this package wasn't taking advantage of the shared memory of M1/2 chips and this answered that question.

Perhaps a comment in the README would be worthwhile for others interested in this?