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<h3 class="section">10.3 Cycle Counters</h3>

<p><a name="index-cycle-counter-619"></a>
FFTW's planner actually executes and times different possible FFT
algorithms in order to pick the fastest plan for a given n.  In
order to do this in as short a time as possible, however, the timer must
have a very high resolution, and to accomplish this we employ the
hardware <dfn>cycle counters</dfn> that are available on most CPUs.
Currently, FFTW supports the cycle counters on x86, PowerPC/POWER, Alpha,
UltraSPARC (SPARC v9), IA64, PA-RISC, and MIPS processors.

   <p><a name="index-compiler-620"></a>Access to the cycle counters, unfortunately, is a compiler and/or
operating-system dependent task, often requiring inline assembly
language, and it may be that your compiler is not supported.  If you are
<em>not</em> supported, FFTW will by default fall back on its estimator
(effectively using <code>FFTW_ESTIMATE</code> for all plans).
<a name="index-FFTW_005fESTIMATE-621"></a>
You can add support by editing the file <code>kernel/cycle.h</code>; normally,
this will involve adapting one of the examples already present in order
to use the inline-assembler syntax for your C compiler, and will only
require a couple of lines of code.  Anyone adding support for a new
system to <code>cycle.h</code> is encouraged to email us at <a href="mailto:fftw@fftw.org">fftw@fftw.org</a>.

   <p>If a cycle counter is not available on your system (e.g. some embedded
processor), and you don't want to use estimated plans, as a last resort
you can use the <code>--with-slow-timer</code> option to <code>configure</code> (on
Unix) or <code>#define WITH_SLOW_TIMER</code> in <code>config.h</code> (elsewhere).
This will use the much lower-resolution <code>gettimeofday</code> function, or even
<code>clock</code> if the former is unavailable, and planning will be
extremely slow.

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author	Chris Cannam
date	Wed, 20 Mar 2013 15:35:50 +0000
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