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Add FFTW3
author Chris Cannam
date Wed, 20 Mar 2013 15:35:50 +0000
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Chris@10 3 <title>Planner Flags - FFTW 3.3.3</title>
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Chris@10 13 This manual is for FFTW
Chris@10 14 (version 3.3.3, 25 November 2012).
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Chris@10 16 Copyright (C) 2003 Matteo Frigo.
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Chris@10 47 <body>
Chris@10 48 <div class="node">
Chris@10 49 <a name="Planner-Flags"></a>
Chris@10 50 <p>
Chris@10 51 Next:&nbsp;<a rel="next" accesskey="n" href="Real_002ddata-DFTs.html#Real_002ddata-DFTs">Real-data DFTs</a>,
Chris@10 52 Previous:&nbsp;<a rel="previous" accesskey="p" href="Complex-DFTs.html#Complex-DFTs">Complex DFTs</a>,
Chris@10 53 Up:&nbsp;<a rel="up" accesskey="u" href="Basic-Interface.html#Basic-Interface">Basic Interface</a>
Chris@10 54 <hr>
Chris@10 55 </div>
Chris@10 56
Chris@10 57 <h4 class="subsection">4.3.2 Planner Flags</h4>
Chris@10 58
Chris@10 59 <p>All of the planner routines in FFTW accept an integer <code>flags</code>
Chris@10 60 argument, which is a bitwise OR (&lsquo;<samp><span class="samp">|</span></samp>&rsquo;) of zero or more of the flag
Chris@10 61 constants defined below. These flags control the rigor (and time) of
Chris@10 62 the planning process, and can also impose (or lift) restrictions on the
Chris@10 63 type of transform algorithm that is employed.
Chris@10 64
Chris@10 65 <p><em>Important:</em> the planner overwrites the input array during
Chris@10 66 planning unless a saved plan (see <a href="Wisdom.html#Wisdom">Wisdom</a>) is available for that
Chris@10 67 problem, so you should initialize your input data after creating the
Chris@10 68 plan. The only exceptions to this are the <code>FFTW_ESTIMATE</code> and
Chris@10 69 <code>FFTW_WISDOM_ONLY</code> flags, as mentioned below.
Chris@10 70
Chris@10 71 <p>In all cases, if wisdom is available for the given problem that was
Chris@10 72 created with equal-or-greater planning rigor, then the more rigorous
Chris@10 73 wisdom is used. For example, in <code>FFTW_ESTIMATE</code> mode any available
Chris@10 74 wisdom is used, whereas in <code>FFTW_PATIENT</code> mode only wisdom created
Chris@10 75 in patient or exhaustive mode can be used. See <a href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans">Words of Wisdom-Saving Plans</a>.
Chris@10 76
Chris@10 77 <h5 class="subsubheading">Planning-rigor flags</h5>
Chris@10 78
Chris@10 79 <ul>
Chris@10 80 <li><a name="index-FFTW_005fESTIMATE-171"></a><code>FFTW_ESTIMATE</code> specifies that, instead of actual measurements of
Chris@10 81 different algorithms, a simple heuristic is used to pick a (probably
Chris@10 82 sub-optimal) plan quickly. With this flag, the input/output arrays are
Chris@10 83 not overwritten during planning.
Chris@10 84
Chris@10 85 <li><a name="index-FFTW_005fMEASURE-172"></a><code>FFTW_MEASURE</code> tells FFTW to find an optimized plan by actually
Chris@10 86 <em>computing</em> several FFTs and measuring their execution time.
Chris@10 87 Depending on your machine, this can take some time (often a few
Chris@10 88 seconds). <code>FFTW_MEASURE</code> is the default planning option.
Chris@10 89
Chris@10 90 <li><a name="index-FFTW_005fPATIENT-173"></a><code>FFTW_PATIENT</code> is like <code>FFTW_MEASURE</code>, but considers a wider
Chris@10 91 range of algorithms and often produces a &ldquo;more optimal&rdquo; plan
Chris@10 92 (especially for large transforms), but at the expense of several times
Chris@10 93 longer planning time (especially for large transforms).
Chris@10 94
Chris@10 95 <li><a name="index-FFTW_005fEXHAUSTIVE-174"></a><code>FFTW_EXHAUSTIVE</code> is like <code>FFTW_PATIENT</code>, but considers an
Chris@10 96 even wider range of algorithms, including many that we think are
Chris@10 97 unlikely to be fast, to produce the most optimal plan but with a
Chris@10 98 substantially increased planning time.
Chris@10 99
Chris@10 100 <li><a name="index-FFTW_005fWISDOM_005fONLY-175"></a><code>FFTW_WISDOM_ONLY</code> is a special planning mode in which the plan
Chris@10 101 is only created if wisdom is available for the given problem, and
Chris@10 102 otherwise a <code>NULL</code> plan is returned. This can be combined with
Chris@10 103 other flags, e.g. &lsquo;<samp><span class="samp">FFTW_WISDOM_ONLY | FFTW_PATIENT</span></samp>&rsquo; creates a
Chris@10 104 plan only if wisdom is available that was created in
Chris@10 105 <code>FFTW_PATIENT</code> or <code>FFTW_EXHAUSTIVE</code> mode. The
Chris@10 106 <code>FFTW_WISDOM_ONLY</code> flag is intended for users who need to detect
Chris@10 107 whether wisdom is available; for example, if wisdom is not available
Chris@10 108 one may wish to allocate new arrays for planning so that user data is
Chris@10 109 not overwritten.
Chris@10 110
Chris@10 111 </ul>
Chris@10 112
Chris@10 113 <h5 class="subsubheading">Algorithm-restriction flags</h5>
Chris@10 114
Chris@10 115 <ul>
Chris@10 116 <li><a name="index-FFTW_005fDESTROY_005fINPUT-176"></a><code>FFTW_DESTROY_INPUT</code> specifies that an out-of-place transform is
Chris@10 117 allowed to <em>overwrite its input</em> array with arbitrary data; this
Chris@10 118 can sometimes allow more efficient algorithms to be employed.
Chris@10 119 <a name="index-out_002dof_002dplace-177"></a>
Chris@10 120 <li><a name="index-FFTW_005fPRESERVE_005fINPUT-178"></a><code>FFTW_PRESERVE_INPUT</code> specifies that an out-of-place transform must
Chris@10 121 <em>not change its input</em> array. This is ordinarily the
Chris@10 122 <em>default</em>, except for c2r and hc2r (i.e. complex-to-real)
Chris@10 123 transforms for which <code>FFTW_DESTROY_INPUT</code> is the default. In the
Chris@10 124 latter cases, passing <code>FFTW_PRESERVE_INPUT</code> will attempt to use
Chris@10 125 algorithms that do not destroy the input, at the expense of worse
Chris@10 126 performance; for multi-dimensional c2r transforms, however, no
Chris@10 127 input-preserving algorithms are implemented and the planner will return
Chris@10 128 <code>NULL</code> if one is requested.
Chris@10 129 <a name="index-c2r-179"></a><a name="index-hc2r-180"></a>
Chris@10 130 <li><a name="index-FFTW_005fUNALIGNED-181"></a><a name="index-alignment-182"></a><code>FFTW_UNALIGNED</code> specifies that the algorithm may not impose any
Chris@10 131 unusual alignment requirements on the input/output arrays (i.e. no
Chris@10 132 SIMD may be used). This flag is normally <em>not necessary</em>, since
Chris@10 133 the planner automatically detects misaligned arrays. The only use for
Chris@10 134 this flag is if you want to use the new-array execute interface to
Chris@10 135 execute a given plan on a different array that may not be aligned like
Chris@10 136 the original. (Using <code>fftw_malloc</code> makes this flag unnecessary
Chris@10 137 even then.)
Chris@10 138
Chris@10 139 </ul>
Chris@10 140
Chris@10 141 <h5 class="subsubheading">Limiting planning time</h5>
Chris@10 142
Chris@10 143 <pre class="example"> extern void fftw_set_timelimit(double seconds);
Chris@10 144 </pre>
Chris@10 145 <p><a name="index-fftw_005fset_005ftimelimit-183"></a>
Chris@10 146 This function instructs FFTW to spend at most <code>seconds</code> seconds
Chris@10 147 (approximately) in the planner. If <code>seconds ==
Chris@10 148 FFTW_NO_TIMELIMIT</code> (the default value, which is negative), then
Chris@10 149 planning time is unbounded. Otherwise, FFTW plans with a
Chris@10 150 progressively wider range of algorithms until the the given time limit
Chris@10 151 is reached or the given range of algorithms is explored, returning the
Chris@10 152 best available plan.
Chris@10 153 <a name="index-FFTW_005fNO_005fTIMELIMIT-184"></a>
Chris@10 154
Chris@10 155 <p>For example, specifying <code>FFTW_PATIENT</code> first plans in
Chris@10 156 <code>FFTW_ESTIMATE</code> mode, then in <code>FFTW_MEASURE</code> mode, then
Chris@10 157 finally (time permitting) in <code>FFTW_PATIENT</code>. If
Chris@10 158 <code>FFTW_EXHAUSTIVE</code> is specified instead, the planner will further
Chris@10 159 progress to <code>FFTW_EXHAUSTIVE</code> mode.
Chris@10 160
Chris@10 161 <p>Note that the <code>seconds</code> argument specifies only a rough limit; in
Chris@10 162 practice, the planner may use somewhat more time if the time limit is
Chris@10 163 reached when the planner is in the middle of an operation that cannot
Chris@10 164 be interrupted. At the very least, the planner will complete planning
Chris@10 165 in <code>FFTW_ESTIMATE</code> mode (which is thus equivalent to a time limit
Chris@10 166 of 0).
Chris@10 167
Chris@10 168 <!-- =========> -->
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Chris@10 170