GPU code

[worktycho]

Again GPU side is easy. Just use a cache object and callback and keeping memory on GPU is also not difficult. GPU wise I could write an api with minimal additional cost. Because you build up an AST for the codegen you can optimise the memory access pattern. If the operations are pure then the optimisation is easy. The problem is when executing on platforms without GPUs like cheap servers. I experimented with this sort of interface a while back and it costs up to 20-30x performance loss on the CPU.