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10.3 Cycle Counters

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FFTW’s planner actually executes and times different possible FFT Chris@82: algorithms in order to pick the fastest plan for a given n. In Chris@82: order to do this in as short a time as possible, however, the timer must Chris@82: have a very high resolution, and to accomplish this we employ the Chris@82: hardware cycle counters that are available on most CPUs. Chris@82: Currently, FFTW supports the cycle counters on x86, PowerPC/POWER, Alpha, Chris@82: UltraSPARC (SPARC v9), IA64, PA-RISC, and MIPS processors. Chris@82:

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Access to the cycle counters, unfortunately, is a compiler and/or Chris@82: operating-system dependent task, often requiring inline assembly Chris@82: language, and it may be that your compiler is not supported. If you are Chris@82: not supported, FFTW will by default fall back on its estimator Chris@82: (effectively using FFTW_ESTIMATE for all plans). Chris@82: Chris@82:

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You can add support by editing the file kernel/cycle.h; normally, Chris@82: this will involve adapting one of the examples already present in order Chris@82: to use the inline-assembler syntax for your C compiler, and will only Chris@82: require a couple of lines of code. Anyone adding support for a new Chris@82: system to cycle.h is encouraged to email us at fftw@fftw.org. Chris@82:

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If a cycle counter is not available on your system (e.g. some embedded Chris@82: processor), and you don’t want to use estimated plans, as a last resort Chris@82: you can use the --with-slow-timer option to configure (on Chris@82: Unix) or #define WITH_SLOW_TIMER in config.h (elsewhere). Chris@82: This will use the much lower-resolution gettimeofday function, or even Chris@82: clock if the former is unavailable, and planning will be Chris@82: extremely slow. Chris@82:

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