fftw3: Cycle Counters
10.3 Cycle Counters
===================
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
(effectively using 'FFTW_ESTIMATE' for all plans).
You can add support by editing the file 'kernel/cycle.h'; normally,
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
system to 'cycle.h' is encouraged to email us at <fftw@fftw.org>.
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 'configure'
(on Unix) or '#define WITH_SLOW_TIMER' in 'config.h' (elsewhere). This
will use the much lower-resolution 'gettimeofday' function, or even
'clock' if the former is unavailable, and planning will be extremely
slow.
Info Catalog
fftw3: Installation on non-Unix systems
fftw3: Installation and Customization
fftw3: Generating your own code