Enqueuing kernels from kernels on GPU/CPU

Graphics processing units (GPUs) and other compute units are allowed to enqueue tasks for themselves by themselves, without needing a host processor to queue the work for the GPU. Built-in functions enable kernels to enqueue kernels for execution on a device. In some embodiments, ndrange kernels execute over an N-dimensional range to provide data-parallel operations. Task kernels provide task-parallel operations. In some embodiments, kernels may be defined using clang block syntax. The order of execution of commands on a compute unit may be constrained or allow execution of commands out-of-order. Compute units may control when kernels enqueued by the compute unit begins execution.