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Current fftw source
author Chris Cannam <cannam@all-day-breakfast.com>
date Tue, 18 Oct 2016 13:40:26 +0100
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72 <a name="Cycle-Counters"></a>
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74 <p>
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> &nbsp; [<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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78 <a name="Cycle-Counters-1"></a>
79 <h3 class="section">10.3 Cycle Counters</h3>
80 <a name="index-cycle-counter-1"></a>
81
82 <p>FFTW&rsquo;s planner actually executes and times different possible FFT
83 algorithms in order to pick the fastest plan for a given <em>n</em>. In
84 order to do this in as short a time as possible, however, the timer must
85 have a very high resolution, and to accomplish this we employ the
86 hardware <em>cycle counters</em> that are available on most CPUs.
87 Currently, FFTW supports the cycle counters on x86, PowerPC/POWER, Alpha,
88 UltraSPARC (SPARC v9), IA64, PA-RISC, and MIPS processors.
89 </p>
90 <a name="index-compiler-3"></a>
91 <p>Access to the cycle counters, unfortunately, is a compiler and/or
92 operating-system dependent task, often requiring inline assembly
93 language, and it may be that your compiler is not supported. If you are
94 <em>not</em> supported, FFTW will by default fall back on its estimator
95 (effectively using <code>FFTW_ESTIMATE</code> for all plans).
96 <a name="index-FFTW_005fESTIMATE-3"></a>
97 </p>
98 <p>You can add support by editing the file <code>kernel/cycle.h</code>; normally,
99 this will involve adapting one of the examples already present in order
100 to use the inline-assembler syntax for your C compiler, and will only
101 require a couple of lines of code. Anyone adding support for a new
102 system to <code>cycle.h</code> is encouraged to email us at <a href="mailto:fftw@fftw.org">fftw@fftw.org</a>.
103 </p>
104 <p>If a cycle counter is not available on your system (e.g. some embedded
105 processor), and you don&rsquo;t want to use estimated plans, as a last resort
106 you can use the <code>--with-slow-timer</code> option to <code>configure</code> (on
107 Unix) or <code>#define WITH_SLOW_TIMER</code> in <code>config.h</code> (elsewhere).
108 This will use the much lower-resolution <code>gettimeofday</code> function, or even
109 <code>clock</code> if the former is unavailable, and planning will be
110 extremely slow.
111 </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> &nbsp; [<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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