remove redundant calls to mtlconvert

src/compiler/execution.jl

@@ -84,7 +84,7 @@ macro metal(ex...)
                 $kernel_tt = Tuple{map(Core.Typeof, $kernel_args)...}
                 $kernel = $mtlfunction($kernel_f, $kernel_tt; $(compiler_kwargs...))
                 if $launch
-                    $kernel($(var_exprs...); $(call_kwargs...))
+                    $kernel($kernel_args...; $(call_kwargs...))
                 end
                 $kernel
             end
@@ -227,7 +227,7 @@ const _kernel_instances = Dict{UInt, Any}()
         else
             # everything else is passed by reference, in an argument buffer
             append!(ex.args, (quote
-                buf = encode_argument!(kernel, mtlconvert($(argex), cce))
+                buf = encode_argument!(kernel, $argex)
                 set_buffer!(cce, buf, 0, $idx)
                 push!(bufs, buf)
             end).args)
@@ -259,8 +259,8 @@ end
     return argument_buffer
 end
 
-@autoreleasepool function (kernel::HostKernel)(args...; groups=1, threads=1,
-                                               queue=global_queue(device()))
+@autoreleasepool function (kernel::HostKernel{F,TT})(args...; groups=1, threads=1,
+                                               queue=global_queue(device())) where {F,TT}
     groups = MTLSize(groups)
     threads = MTLSize(threads)
     (groups.width>0 && groups.height>0 && groups.depth>0) ||
@@ -274,9 +274,24 @@ end
     cmdbuf = MTLCommandBuffer(queue)
     cmdbuf.label = "MTLCommandBuffer($(nameof(kernel.f)))"
     cce = MTLComputeCommandEncoder(cmdbuf)
+
+    argexprs = [:(kernel.f)]
+     for i in 1:length(args)
+         if args[i] != TT.parameters[i]
+             # if there's a type mismatch between the argument and the compiled kernel,
+             # assume we still have to call `cudaconvert`. this is a bit of a hack, but
+             # it's generally true that `cudaconvert` will change the type of the argument,
+             # and it makes it possible to pass both original and converted arguments
+             # to compiled kernel objects, avoiding the need for multiple conversions.
+             push!(argexprs, :(mtlconvert(args[$i])))
+         else
+             push!(argexprs, :(args[$i]))
+         end
+     end
+
     argument_buffers = try
         MTL.set_function!(cce, kernel.pipeline)
-        bufs = encode_arguments!(cce, kernel, kernel.f, args...)
+        bufs = encode_arguments!(cce, kernel, kernel.f, argexprs...)
         MTL.append_current_function!(cce, groups, threads)
         bufs
     finally