Re: fftw: Usage of SSE in 64bit?

["Steven G. Johnson" <stevenj@alum.mit.edu>]

(Reposting, as this message does not seem to have gone through.)

I am one of the FFTW developers, and wanted to comment on this.

Yes, you should definitely use --enable-sse/--enable-sse2 flags in when 
compiling single/double precision versions of FFTW on all x86 and x86-64 
platforms.  This is *not* just a matter of compiler flags -- it enables 
the compilation of special computational kernels in FFTW that explicitly 
use SSE/SSE2 intrinsics.

In addition to x86-64, note that this is SAFE to enable in general for 
all 32-bit x86 platforms.  FFTW checks at runtime to see whether the 
processor supports SSE/SSE2 and disables its SSE/SSE2 code if not. 
(Similarly for Altivec on PowerPC, and similarly in the next release for 
AVX instructions.)

In general, I would recommend that the packager read the FFTW 
installation manual closely, since it documents these options.
    http://fftw.org/doc/Installation-on-Unix.html

For benchmarking, I would recommend using the "bench" program that comes 
with FFTW. e.g. you can compare for a size-1024 FFT with and without the 
SSE/SSE2 kernels just by doing:
    ./bench -opatient 1024
    ./bench -opatient -onosimd 1024
On my 64-bit Intel Xeon E5440 running FFTW 3.2.2 and Debian GNU/Linux, 
the SSE/SSE2 version is faster for size 1024 by a factor of 1.7 in 
double precision and by a factor of 3.4 in single precision.

Regards,
Steven G. Johnson