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Replace these with versions built using an older toolset (so as to avoid ABI compatibilities when linking on Ubuntu 14.04 for packaging purposes)
author Chris Cannam
date Fri, 07 Feb 2020 11:51:13 +0000
parents 37bf6b4a2645
children
rev   line source
Chris@10 1 <html lang="en">
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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 46 </head>
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