It is really difficult to extract parallelism from sequentially written programs. It is even worse when it comes to restricted fine-grained parallelism, as on

It is really difficult to extract parallelism from sequentially written programs. It is even worse when it comes to restricted fine-grained parallelism, as on a GPU. I'm not aware of any really robust auto-parallelising compiler, and even compilers for specialised parallel languages struggle to reach performance close to hand-written code for more complex problems.