The directory genfft contains the programs that were used to
generate FFTW's “codelets,” which are hard-coded transforms of small
sizes.
We do not expect casual users to employ the generator, which is a rather
sophisticated program that generates directed acyclic graphs of FFT
algorithms and performs algebraic simplifications on them. It was
written in Objective Caml, a dialect of ML, which is available at
http://caml.inria.fr/ocaml/index.en.html.
If you have Objective Caml installed (along with recent versions of
GNU autoconf, automake, and libtool), then you
can change the set of codelets that are generated or play with the
generation options. The set of generated codelets is specified by the
{dft,rdft}/{codelets,simd}/*/Makefile.am files. For example, you can add
efficient REDFT codelets of small sizes by modifying
rdft/codelets/r2r/Makefile.am.
After you modify any Makefile.am files, you can type sh
bootstrap.sh in the top-level directory followed by make to
re-generate the files.
We do not provide more details about the code-generation process, since we do not expect that most users will need to generate their own code. However, feel free to contact us at fftw@fftw.org if you are interested in the subject.
You might find it interesting to learn Caml and/or some modern programming techniques that we used in the generator (including monadic programming), especially if you heard the rumor that Java and object-oriented programming are the latest advancement in the field. The internal operation of the codelet generator is described in the paper, “A Fast Fourier Transform Compiler,” by M. Frigo, which is available from the FFTW home page and also appeared in the Proceedings of the 1999 ACM SIGPLAN Conference on Programming Language Design and Implementation (PLDI).