Common Lisp vs. C/ASM: Battle FFT October 16th, 2009
Patrick Stein

Dr. David McClain of Refined Audiometrics sent me his code for interfacing with the Apple vDSP library. Here it is along with a Makefile that I put together for it: vdsp-wrapper.tar.gz. I have not actually run the code myself. I started to convert his Lispworks FLI code to CFFI code, but bailed on it in favor of making a minimal CFFI interface to FFTW.

Dr. McClain says that Apple’s vDSP library usually gains him 20% or so versus FFTW. Further, there may well be plenty of alignment issues that should be taken into account in allocating the foreign buffers. Regardless, here is my very basic wrapper around FFTW’s fftw_plan_dft_1d() function: fftw.lisp.

To get this working with SBCL on my Mac, I needed to get FFTW compiling 32-bit shared libraries. By default, it compiles 64-bit static libraries on my system.

####### for 32-bit
% CC="gcc -arch i386" ./configure --prefix=/usr/local --enable-shared

Before I recompiled for 32-bit though, I manually moved the libfftw3.* files in /usr/local/lib into libfftw3_64.* files (of course, moving both the symlink libfftw3.dylib and its target).

Doing the FFI to FFTW does a 1,048,576-sample buffer in 0.202 seconds under SBCL 1.0.30 versus 0.72 seconds for my all-Lisp version. Under Clozure-64, it takes 0.91 seconds versus nearly 30 seconds for the all-Lisp version. I should check to make sure FFTW performs as well when my buffer is randomly filled instead of filled with a constant value. But, yep. There you have it.

So, certainly, if you need high-performance FFTs, you’ll want to FFI wrap something rather than use my all-Lisp version. If you don’t need such high-performance, I give you no-hassle portability.

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