dfftopt(3P) Sun Performance Library dfftopt(3P)NAMEdfftopt - compute the length of the closest fast FFT
SYNOPSIS
INTEGER FUNCTION DFFTOPT(LEN)
INTEGER LEN
INTEGER*8 FUNCTION DFFTOPT_64(LEN)
INTEGER*8 LEN
F95 INTERFACE
INTEGER FUNCTION DFFTOPT(LEN)
INTEGER :: LEN
INTEGER(8) FUNCTION DFFTOPT_64(LEN)INTEGER(8) :: LEN
C INTERFACE
#include <sunperf.h>
int dfftopt(int len);
long dfftopt_64(long len);
PURPOSEdfftopt computes the length of the closest fast FFT. Fast Fourier
transform algorithms, including those used in Performance Library, work
best with vector lengths that are products of small primes. For exam‐
ple, an FFT of length 32=2**5 will run faster than an FFT of prime
length 31 because 32 is a product of small primes and 31 is not. If
your application is such that you can taper or zero pad your vector to
a larger length then this function may help you select a better length
and run your FFT faster.
DFFTOPT will return an integer no smaller than the input argument N
that is the closest number that is the product of small primes.
DFFTOPT will return 16 for an input of N=16 and return 18=2*3*3 for an
input of N=17.
Note that the length computed here is not guaranteed to be optimal,
only to be a
product of small primes. Also, the value returned may change as the
underlying
FFTs become capable of handling larger primes. For example, passing
in N=51 to day will return 52=2*2*13 rather than 51=3*17 because the
FFTs in Performance Li brary do not have fast radix 17 code. In the
future, radix 17 code may be added
and then N=51 will return 51.
6 Mar 2009 dfftopt(3P)
