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Chris@19: FFTW's planner actually executes and times different possible FFT Chris@19: algorithms in order to pick the fastest plan for a given n. In Chris@19: order to do this in as short a time as possible, however, the timer must Chris@19: have a very high resolution, and to accomplish this we employ the Chris@19: hardware cycle counters that are available on most CPUs. Chris@19: Currently, FFTW supports the cycle counters on x86, PowerPC/POWER, Alpha, Chris@19: UltraSPARC (SPARC v9), IA64, PA-RISC, and MIPS processors. Chris@19: Chris@19:
Access to the cycle counters, unfortunately, is a compiler and/or
Chris@19: operating-system dependent task, often requiring inline assembly
Chris@19: language, and it may be that your compiler is not supported. If you are
Chris@19: not supported, FFTW will by default fall back on its estimator
Chris@19: (effectively using FFTW_ESTIMATE for all plans).
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Chris@19: You can add support by editing the file kernel/cycle.h; normally,
Chris@19: this will involve adapting one of the examples already present in order
Chris@19: to use the inline-assembler syntax for your C compiler, and will only
Chris@19: require a couple of lines of code. Anyone adding support for a new
Chris@19: system to cycle.h is encouraged to email us at fftw@fftw.org.
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If a cycle counter is not available on your system (e.g. some embedded
Chris@19: processor), and you don't want to use estimated plans, as a last resort
Chris@19: you can use the --with-slow-timer option to configure (on
Chris@19: Unix) or #define WITH_SLOW_TIMER in config.h (elsewhere).
Chris@19: This will use the much lower-resolution gettimeofday function, or even
Chris@19: clock if the former is unavailable, and planning will be
Chris@19: extremely slow.
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