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FFTW's planner actually executes and times different possible FFT algorithms in order to pick the fastest plan for a given n. In order to do this in as short a time as possible, however, the timer must have a very high resolution, and to accomplish this we employ the hardware cycle counters that are available on most CPUs. Currently, FFTW supports the cycle counters on x86, PowerPC/POWER, Alpha, UltraSPARC (SPARC v9), IA64, PA-RISC, and MIPS processors.
Access to the cycle counters, unfortunately, is a compiler and/or
operating-system dependent task, often requiring inline assembly
language, and it may be that your compiler is not supported. If you are
not supported, FFTW will by default fall back on its estimator
FFTW_ESTIMATE for all plans).
You can add support by editing the file
this will involve adapting one of the examples already present in order
to use the inline-assembler syntax for your C compiler, and will only
require a couple of lines of code. Anyone adding support for a new
cycle.h is encouraged to email us at email@example.com.
If a cycle counter is not available on your system (e.g. some embedded
processor), and you don't want to use estimated plans, as a last resort
you can use the
--with-slow-timer option to
#define WITH_SLOW_TIMER in
This will use the much slower
gettimeofday function, or even
clock if the former is unavailable, and planning will be