Based another comment by someone who studied the source, it sounds like the ocaml code generates C code for the various optimized kernels. Wonder how the C kernel chooser code looks like.

FWIW, proper hygienic macros and scientific focused array types makes this sort of meta-programming optimization in Julia kinda fun.

Though just using type dispatching / multi-methods might be all that’s needed. It all makes me want to have a reason to write that kind of optimized code... For example if you can lift the size, variable, and variability into types you could match on your config roughly like: “fft(X :: Pwr2_64, Y :: FWD, Dir :: REV, Var :: BIDIR, Z) = 2^4-2^(log2(X))”.