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| author | Chris Cannam |
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| date | Wed, 20 Mar 2013 15:35:50 +0000 |
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| 1 <html lang="en"> | |
| 2 <head> | |
| 3 <title>Cycle Counters - FFTW 3.3.3</title> | |
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| 12 <!-- | |
| 13 This manual is for FFTW | |
| 14 (version 3.3.3, 25 November 2012). | |
| 15 | |
| 16 Copyright (C) 2003 Matteo Frigo. | |
| 17 | |
| 18 Copyright (C) 2003 Massachusetts Institute of Technology. | |
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| 20 Permission is granted to make and distribute verbatim copies of | |
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| 49 <a name="Cycle-Counters"></a> | |
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| 54 <hr> | |
| 55 </div> | |
| 56 | |
| 57 <h3 class="section">10.3 Cycle Counters</h3> | |
| 58 | |
| 59 <p><a name="index-cycle-counter-619"></a> | |
| 60 FFTW's planner actually executes and times different possible FFT | |
| 61 algorithms in order to pick the fastest plan for a given n. In | |
| 62 order to do this in as short a time as possible, however, the timer must | |
| 63 have a very high resolution, and to accomplish this we employ the | |
| 64 hardware <dfn>cycle counters</dfn> that are available on most CPUs. | |
| 65 Currently, FFTW supports the cycle counters on x86, PowerPC/POWER, Alpha, | |
| 66 UltraSPARC (SPARC v9), IA64, PA-RISC, and MIPS processors. | |
| 67 | |
| 68 <p><a name="index-compiler-620"></a>Access to the cycle counters, unfortunately, is a compiler and/or | |
| 69 operating-system dependent task, often requiring inline assembly | |
| 70 language, and it may be that your compiler is not supported. If you are | |
| 71 <em>not</em> supported, FFTW will by default fall back on its estimator | |
| 72 (effectively using <code>FFTW_ESTIMATE</code> for all plans). | |
| 73 <a name="index-FFTW_005fESTIMATE-621"></a> | |
| 74 You can add support by editing the file <code>kernel/cycle.h</code>; normally, | |
| 75 this will involve adapting one of the examples already present in order | |
| 76 to use the inline-assembler syntax for your C compiler, and will only | |
| 77 require a couple of lines of code. Anyone adding support for a new | |
| 78 system to <code>cycle.h</code> is encouraged to email us at <a href="mailto:fftw@fftw.org">fftw@fftw.org</a>. | |
| 79 | |
| 80 <p>If a cycle counter is not available on your system (e.g. some embedded | |
| 81 processor), and you don't want to use estimated plans, as a last resort | |
| 82 you can use the <code>--with-slow-timer</code> option to <code>configure</code> (on | |
| 83 Unix) or <code>#define WITH_SLOW_TIMER</code> in <code>config.h</code> (elsewhere). | |
| 84 This will use the much lower-resolution <code>gettimeofday</code> function, or even | |
| 85 <code>clock</code> if the former is unavailable, and planning will be | |
| 86 extremely slow. | |
| 87 | |
| 88 <!-- --> | |
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| 90 |