A C++ library that takes GLSL shaders, does some GPU-independent optimizations on them and outputs GLSL or Metal source back. Optimizations are function inlining, dead code removal, copy propagation, constant folding, constant propagation, arithmetic optimizations and so on. Apparently quite a few mobile platforms are pretty bad at optimizing shaders; and unfortunately they also lack offline shader compilers. So using a GLSL optimizer offline before can make the shader run much faster on a platform like that. See performance numbers in this blog post.