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109 lines
5.4 KiB
HTML
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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preserved on all copies.
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Permission is granted to copy and distribute modified versions of this
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manual under the conditions for verbatim copying, provided that the
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entire resulting derived work is distributed under the terms of a
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permission notice identical to this one.
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Permission is granted to copy and distribute translations of this manual
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approved by the Free Software Foundation. -->
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<head>
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<title>Cycle Counters (FFTW 3.3.10)</title>
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<meta name="description" content="Cycle Counters (FFTW 3.3.10)">
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Installation-and-Customization.html" rel="up" title="Installation and Customization">
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<link href="Generating-your-own-code.html" rel="next" title="Generating your own code">
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</head>
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<body lang="en">
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<span id="Cycle-Counters"></span><div class="header">
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<p>
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Next: <a href="Generating-your-own-code.html" accesskey="n" rel="next">Generating your own code</a>, Previous: <a href="Installation-on-non_002dUnix-systems.html" accesskey="p" rel="prev">Installation on non-Unix systems</a>, Up: <a href="Installation-and-Customization.html" 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" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="Cycle-Counters-1"></span><h3 class="section">10.3 Cycle Counters</h3>
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<span id="index-cycle-counter-1"></span>
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<p>FFTW’s planner actually executes and times different possible FFT
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algorithms in order to pick the fastest plan for a given <em>n</em>. In
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order to do this in as short a time as possible, however, the timer must
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have a very high resolution, and to accomplish this we employ the
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hardware <em>cycle counters</em> that are available on most CPUs.
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Currently, FFTW supports the cycle counters on x86, PowerPC/POWER, Alpha,
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UltraSPARC (SPARC v9), IA64, PA-RISC, and MIPS processors.
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</p>
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<span id="index-compiler-3"></span>
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<p>Access to the cycle counters, unfortunately, is a compiler and/or
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operating-system dependent task, often requiring inline assembly
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language, and it may be that your compiler is not supported. If you are
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<em>not</em> supported, FFTW will by default fall back on its estimator
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(effectively using <code>FFTW_ESTIMATE</code> for all plans).
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<span id="index-FFTW_005fESTIMATE-3"></span>
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</p>
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<p>You can add support by editing the file <code>kernel/cycle.h</code>; normally,
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this will involve adapting one of the examples already present in order
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to use the inline-assembler syntax for your C compiler, and will only
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require a couple of lines of code. Anyone adding support for a new
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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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</p>
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<p>If a cycle counter is not available on your system (e.g. some embedded
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processor), and you don’t want to use estimated plans, as a last resort
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you can use the <code>--with-slow-timer</code> option to <code>configure</code> (on
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Unix) or <code>#define WITH_SLOW_TIMER</code> in <code>config.h</code> (elsewhere).
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This will use the much lower-resolution <code>gettimeofday</code> function, or even
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<code>clock</code> if the former is unavailable, and planning will be
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extremely slow.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Generating-your-own-code.html" accesskey="n" rel="next">Generating your own code</a>, Previous: <a href="Installation-on-non_002dUnix-systems.html" accesskey="p" rel="prev">Installation on non-Unix systems</a>, Up: <a href="Installation-and-Customization.html" 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" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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