Make buffers always be a multiple of 16 bytes to support MPS rand fun…

…ctionality

lib/mtl/buffer.jl

@@ -21,14 +21,16 @@ end
 
 
 ## allocation
+bufferbytesize(bytesize::T) where T<:Integer = ceil(T, bytesize / 16) * T(16)
 
 function MTLBuffer(dev::Union{MTLDevice,MTLHeap}, bytesize::Integer;
                    storage=Private, hazard_tracking=DefaultTracking,
                    cache_mode=DefaultCPUCache)
     opts = convert(MTLResourceOptions, storage) | hazard_tracking | cache_mode
 
-    @assert 0 < bytesize <= dev.maxBufferLength # XXX: not supported by MTLHeap
-    ptr = alloc_buffer(dev, bytesize, opts)
+    realbytesize = bufferbytesize(bytesize)
+    @assert 0 < realbytesize <= dev.maxBufferLength # XXX: not supported by MTLHeap
+    ptr = alloc_buffer(dev, realbytesize, opts)
 
     return MTLBuffer(ptr)
 end
@@ -39,8 +41,9 @@ function MTLBuffer(dev::MTLDevice, bytesize::Integer, ptr::Ptr;
     storage == Private && error("Can't create a Private copy-allocated buffer.")
     opts =  convert(MTLResourceOptions, storage) | hazard_tracking | cache_mode
 
-    @assert 0 < bytesize <= dev.maxBufferLength
-    ptr = alloc_buffer(dev, bytesize, opts, ptr)
+    realbytesize = bufferbytesize(bytesize)
+    @assert 0 < realbytesize <= dev.maxBufferLength
+    ptr = alloc_buffer(dev, realbytesize, opts, ptr)
 
     return MTLBuffer(ptr)
 end