Cannot load default library in Metal using Swift

Question

For my project (being compiled as a framework) I have a file ops.metal:

kernel void add(device float *lhs [[buffer(0)]],
                device float *rhs [[buffer(1)]],
                device float *result [[buffer(2)]],
                uint id [[ thread_position_in_grid ]])
{
    result[id] = lhs[id] + rhs[id];
}

and the following Swift code:

@available(OSX 10.11, *)
public class MTLContext {
    var device: MTLDevice!
    var commandQueue:MTLCommandQueue!
    var library:MTLLibrary!
    var commandBuffer:MTLCommandBuffer
    var commandEncoder:MTLComputeCommandEncoder

    init() {
        if let defaultDevice = MTLCreateSystemDefaultDevice() {
            device = defaultDevice
            print("device created")
        } else {
            print("Metal is not supported")
        }

        commandQueue = device.makeCommandQueue()

        library = device.newDefaultLibrary()
        if let defaultLibrary = device.newDefaultLibrary() {
            library = defaultLibrary
        } else {
            print("could not load default library")
        }

        commandBuffer = commandQueue.makeCommandBuffer()
        commandEncoder = commandBuffer.makeComputeCommandEncoder()
    }

    deinit {
        commandEncoder.endEncoding()
    }
}

When I try to create an instance of MTLContext in a unit test, the device is created, but the default library cannot be created ("could not load default library"). I've checked that the compiled framework has a default.metallib in Resources (which is the most common reason given for newDefaultLibrary).

Unfortunately I haven't been able to find any working examples that are creating compute kernels in a Metal shader file (there are a few examples using the performance shaders, but they don't need to make kernels in the shader file).

Any suggestions would be greatly appreciated!

Answer 1

newDefaultLibrary() loads from the main bundle of the currently running application. It doesn't search any embedded frameworks or other locations for libraries.

If you want to use a metallib that was compiled into an embedded framework, the easiest thing to do is to get a reference to its containing Bundle and ask for the default library of that bundle instead:

let frameworkBundle = Bundle(for: SomeClassFromMyShaderFramework.self)
guard let defaultLibrary = try? device.makeDefaultLibrary(bundle: frameworkBundle) else {
    fatalError("Could not load default library from specified bundle")
}

This does require that you have at least one publicly-visible class in the framework containing your shaders, but that can be as simple as declaring an empty class strictly for the purpose of doing the bundle look-up:

public class SomeClassFromMyShaderFramework {}