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1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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2 <html>
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3 <!-- This manual is for FFTW
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4 (version 3.3.5, 30 July 2016).
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5
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6 Copyright (C) 2003 Matteo Frigo.
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7
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8 Copyright (C) 2003 Massachusetts Institute of Technology.
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9
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10 Permission is granted to make and distribute verbatim copies of this
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11 manual provided the copyright notice and this permission notice are
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12 preserved on all copies.
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13
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14 Permission is granted to copy and distribute modified versions of this
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15 manual under the conditions for verbatim copying, provided that the
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16 entire resulting derived work is distributed under the terms of a
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17 permission notice identical to this one.
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18
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19 Permission is granted to copy and distribute translations of this manual
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20 into another language, under the above conditions for modified versions,
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21 except that this permission notice may be stated in a translation
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22 approved by the Free Software Foundation. -->
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23 <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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24 <head>
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25 <title>FFTW 3.3.5: Cycle Counters</title>
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26
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27 <meta name="description" content="FFTW 3.3.5: Cycle Counters">
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28 <meta name="keywords" content="FFTW 3.3.5: Cycle Counters">
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33 <link href="index.html#Top" rel="start" title="Top">
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34 <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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36 <link href="Installation-and-Customization.html#Installation-and-Customization" rel="up" title="Installation and Customization">
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37 <link href="Generating-your-own-code.html#Generating-your-own-code" rel="next" title="Generating your own code">
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38 <link href="Installation-on-non_002dUnix-systems.html#Installation-on-non_002dUnix-systems" rel="prev" title="Installation on non-Unix systems">
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39 <style type="text/css">
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64 ul.no-bullet {list-style: none}
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65 -->
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66 </style>
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67
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68
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69 </head>
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70
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71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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72 <a name="Cycle-Counters"></a>
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73 <div class="header">
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74 <p>
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75 Next: <a href="Generating-your-own-code.html#Generating-your-own-code" accesskey="n" rel="next">Generating your own code</a>, Previous: <a href="Installation-on-non_002dUnix-systems.html#Installation-on-non_002dUnix-systems" accesskey="p" rel="prev">Installation on non-Unix systems</a>, Up: <a href="Installation-and-Customization.html#Installation-and-Customization" accesskey="u" rel="up">Installation and Customization</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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76 </div>
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77 <hr>
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78 <a name="Cycle-Counters-1"></a>
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79 <h3 class="section">10.3 Cycle Counters</h3>
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80 <a name="index-cycle-counter-1"></a>
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81
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82 <p>FFTW’s planner actually executes and times different possible FFT
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83 algorithms in order to pick the fastest plan for a given <em>n</em>. In
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84 order to do this in as short a time as possible, however, the timer must
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85 have a very high resolution, and to accomplish this we employ the
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86 hardware <em>cycle counters</em> that are available on most CPUs.
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87 Currently, FFTW supports the cycle counters on x86, PowerPC/POWER, Alpha,
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88 UltraSPARC (SPARC v9), IA64, PA-RISC, and MIPS processors.
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89 </p>
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90 <a name="index-compiler-3"></a>
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91 <p>Access to the cycle counters, unfortunately, is a compiler and/or
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92 operating-system dependent task, often requiring inline assembly
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93 language, and it may be that your compiler is not supported. If you are
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94 <em>not</em> supported, FFTW will by default fall back on its estimator
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95 (effectively using <code>FFTW_ESTIMATE</code> for all plans).
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96 <a name="index-FFTW_005fESTIMATE-3"></a>
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97 </p>
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98 <p>You can add support by editing the file <code>kernel/cycle.h</code>; normally,
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99 this will involve adapting one of the examples already present in order
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100 to use the inline-assembler syntax for your C compiler, and will only
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101 require a couple of lines of code. Anyone adding support for a new
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102 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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103 </p>
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104 <p>If a cycle counter is not available on your system (e.g. some embedded
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105 processor), and you don’t want to use estimated plans, as a last resort
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106 you can use the <code>--with-slow-timer</code> option to <code>configure</code> (on
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107 Unix) or <code>#define WITH_SLOW_TIMER</code> in <code>config.h</code> (elsewhere).
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108 This will use the much lower-resolution <code>gettimeofday</code> function, or even
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109 <code>clock</code> if the former is unavailable, and planning will be
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110 extremely slow.
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111 </p>
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112 <hr>
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113 <div class="header">
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114 <p>
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115 Next: <a href="Generating-your-own-code.html#Generating-your-own-code" accesskey="n" rel="next">Generating your own code</a>, Previous: <a href="Installation-on-non_002dUnix-systems.html#Installation-on-non_002dUnix-systems" accesskey="p" rel="prev">Installation on non-Unix systems</a>, Up: <a href="Installation-and-Customization.html#Installation-and-Customization" accesskey="u" rel="up">Installation and Customization</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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116 </div>
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117
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118
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119
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120 </body>
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121 </html>
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