Avoid inconsistencies in kernels that contain code written by scripts about kernels

[jeffhammond]

You might be interested in reusing my stencil code generator.

The OpenCL one - https://github.com/ParRes/Kernels/blob/master/Cxx11/generate-opencl-stencil.py - is straightforward and easy to adapt to C89.

The C++ one is more complicated because it generates seven different types of loop syntax (one for each programming model it supports): https://github.com/ParRes/Kernels/blob/master/Cxx11/generate-cxx-stencil.py

Note that the code is explicitly instantiated for every case (e.g. https://github.com/ParRes/Kernels/blob/master/Cxx11/stencil_seq.hpp) and the driver needs to branch into them (e.g. https://github.com/ParRes/Kernels/blob/master/Cxx11/stencil-vector.cc#L177). One could of course replace the default case with a generic implementation, but I figure that limiting the user to radius=9 by default isn't a bad thing.

For OpenCL, I generated the code on the fly because the OpenCL programming model is happy with that, whereas C++ codes have to include the header. Including the header with 2x9 stencil kernels (~90KB of code) increases compilation time significantly (a few seconds with an optimizing compiler like ICC). One could easily reduce compile-time by reducing the default maximum radius to something like 5.

I have thought about using JiT for C++, but that entails invoking the compiler, generating a shared library, then dlopen-ing the shared library, which seems like a pain. It's also not viable in a variety of scenarios, e.g. Cray machines.