FFTW certainly isn't simply written on OCaml. Apart from higher-level C, at base it has the sort of low-level micro-architecture-specific kernels you'd expect (

FFTW certainly isn't simply written on OCaml. Apart from higher-level C, at base it has the sort of low-level micro-architecture-specific kernels you'd expect (using C intrinsics rather than assembler in this case). [There was useful advice in https://stackoverflow.com/a/3058546 https://stackoverflow.com/a/3058546 for example, and doubtless more recently, concerning trolling.]