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2 <head>
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3 <title>Cycle Counters - FFTW 3.2.1</title>
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4 <meta http-equiv="Content-Type" content="text/html">
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5 <meta name="description" content="FFTW 3.2.1">
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6 <meta name="generator" content="makeinfo 4.8">
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7 <link title="Top" rel="start" href="index.html#Top">
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8 <link rel="up" href="Installation-and-Customization.html#Installation-and-Customization" title="Installation and Customization">
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9 <link rel="prev" href="Installation-on-non_002dUnix-systems.html#Installation-on-non_002dUnix-systems" title="Installation on non-Unix systems">
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10 <link rel="next" href="Generating-your-own-code.html#Generating-your-own-code" title="Generating your own code">
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11 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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12 <!--
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13 This manual is for FFTW
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14 (version 3.2.1, 5 February 2009).
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15
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16 Copyright (C) 2003 Matteo Frigo.
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17
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18 Copyright (C) 2003 Massachusetts Institute of Technology.
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19
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20 Permission is granted to make and distribute verbatim copies of
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21 this manual provided the copyright notice and this permission
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22 notice are preserved on all copies.
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23
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24 Permission is granted to copy and distribute modified versions of
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25 this manual under the conditions for verbatim copying, provided
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26 that the entire resulting derived work is distributed under the
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27 terms of a permission notice identical to this one.
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28
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29 Permission is granted to copy and distribute translations of this
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30 manual into another language, under the above conditions for
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31 modified versions, except that this permission notice may be
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32 stated in a translation approved by the Free Software Foundation.
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33 -->
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45 --></style>
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46 </head>
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47 <body>
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48 <div class="node">
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49 <p>
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50 <a name="Cycle-Counters"></a>
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51 Next: <a rel="next" accesskey="n" href="Generating-your-own-code.html#Generating-your-own-code">Generating your own code</a>,
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52 Previous: <a rel="previous" accesskey="p" href="Installation-on-non_002dUnix-systems.html#Installation-on-non_002dUnix-systems">Installation on non-Unix systems</a>,
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53 Up: <a rel="up" accesskey="u" href="Installation-and-Customization.html#Installation-and-Customization">Installation and Customization</a>
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54 <hr>
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55 </div>
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56
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57 <h3 class="section">9.3 Cycle Counters</h3>
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58
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59 <p><a name="index-cycle-counter-373"></a>
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60 FFTW's planner actually executes and times different possible FFT
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61 algorithms in order to pick the fastest plan for a given n. In
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62 order to do this in as short a time as possible, however, the timer must
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63 have a very high resolution, and to accomplish this we employ the
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64 hardware <dfn>cycle counters</dfn> that are available on most CPUs.
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65 Currently, FFTW supports the cycle counters on x86, PowerPC/POWER, Alpha,
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66 UltraSPARC (SPARC v9), IA64, PA-RISC, and MIPS processors.
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67
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68 <p><a name="index-compiler-374"></a>Access to the cycle counters, unfortunately, is a compiler and/or
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69 operating-system dependent task, often requiring inline assembly
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70 language, and it may be that your compiler is not supported. If you are
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71 <em>not</em> supported, FFTW will by default fall back on its estimator
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72 (effectively using <code>FFTW_ESTIMATE</code> for all plans).
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73 <a name="index-FFTW_005fESTIMATE-375"></a>
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74 You can add support by editing the file <code>kernel/cycle.h</code>; normally,
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75 this will involve adapting one of the examples already present in order
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76 to use the inline-assembler syntax for your C compiler, and will only
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77 require a couple of lines of code. Anyone adding support for a new
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78 system to <code>cycle.h</code> is encouraged to email us at <a href="mailto:fftw@fftw.org">fftw@fftw.org</a>.
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79
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80 <p>If a cycle counter is not available on your system (e.g. some embedded
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81 processor), and you don't want to use estimated plans, as a last resort
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82 you can use the <code>--with-slow-timer</code> option to <code>configure</code> (on
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83 Unix) or <code>#define WITH_SLOW_TIMER</code> in <code>config.h</code> (elsewhere).
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84 This will use the much lower-resolution <code>gettimeofday</code> function, or even
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85 <code>clock</code> if the former is unavailable, and planning will be
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86 extremely slow.
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87
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88 <!-- -->
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89 </body></html>
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90
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