DCompute Metal Backend

[Asadbek]

Hello everyone,

I am creating this thread to share my progress on building a 
Metal GPU target for DCompute. Apologies for the delay in sharing 
this as promised earlier!

I started working on this back in late April. I have set up basic 
compiler support to lower to correct, Apple-compliant LLVM IR 
(AIR). Since Apple uses a custom fork of LLVM, getting this to 
work involved a fair amount of reverse engineering, but the 
baseline is now functional.

Here is the draft PR on LDC: 
https://github.com/ldc-developers/ldc/pull/5118

For context, GPU programming splits code into two categories: 
device code and host code. Device code is sent to the GPU runtime 
and mostly contains shaders. Because DCompute is a compute 
library (where compute shaders are called kernels), the device 
code that DCompute sends to the GPU contains only kernels.

Host code facilitates execution between the GPU and the CPU. It 
handles copying memory buffers to the GPU address space, sending 
the device code binary to the GPU, and waiting for the results to 
process them (though it does not necessarily need to wait and 
block the running thread while waiting). I have also set up a 
basic host code implementation.

Here is the PR on DCompute for the host code: 
https://github.com/libmir/dcompute/pull/99

Currently, basic kernels can successfully execute on Apple Metal 
GPUs. I will continue development and share future updates in 
this thread.

Special thanks to Nicholas and Gaofei for their help and 
suggestions throughout the development process!