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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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cannam@127 3 <!-- This manual is for FFTW
cannam@127 4 (version 3.3.5, 30 July 2016).
cannam@127 5
cannam@127 6 Copyright (C) 2003 Matteo Frigo.
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cannam@127 24 <head>
cannam@127 25 <title>FFTW 3.3.5: Planner Flags</title>
cannam@127 26
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cannam@127 70
cannam@127 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
cannam@127 72 <a name="Planner-Flags"></a>
cannam@127 73 <div class="header">
cannam@127 74 <p>
cannam@127 75 Next: <a href="Real_002ddata-DFTs.html#Real_002ddata-DFTs" accesskey="n" rel="next">Real-data DFTs</a>, Previous: <a href="Complex-DFTs.html#Complex-DFTs" accesskey="p" rel="prev">Complex DFTs</a>, Up: <a href="Basic-Interface.html#Basic-Interface" accesskey="u" rel="up">Basic Interface</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>
cannam@127 76 </div>
cannam@127 77 <hr>
cannam@127 78 <a name="Planner-Flags-1"></a>
cannam@127 79 <h4 class="subsection">4.3.2 Planner Flags</h4>
cannam@127 80
cannam@127 81 <p>All of the planner routines in FFTW accept an integer <code>flags</code>
cannam@127 82 argument, which is a bitwise OR (&lsquo;<samp>|</samp>&rsquo;) of zero or more of the flag
cannam@127 83 constants defined below. These flags control the rigor (and time) of
cannam@127 84 the planning process, and can also impose (or lift) restrictions on the
cannam@127 85 type of transform algorithm that is employed.
cannam@127 86 </p>
cannam@127 87 <p><em>Important:</em> the planner overwrites the input array during
cannam@127 88 planning unless a saved plan (see <a href="Wisdom.html#Wisdom">Wisdom</a>) is available for that
cannam@127 89 problem, so you should initialize your input data after creating the
cannam@127 90 plan. The only exceptions to this are the <code>FFTW_ESTIMATE</code> and
cannam@127 91 <code>FFTW_WISDOM_ONLY</code> flags, as mentioned below.
cannam@127 92 </p>
cannam@127 93 <p>In all cases, if wisdom is available for the given problem that was
cannam@127 94 created with equal-or-greater planning rigor, then the more rigorous
cannam@127 95 wisdom is used. For example, in <code>FFTW_ESTIMATE</code> mode any available
cannam@127 96 wisdom is used, whereas in <code>FFTW_PATIENT</code> mode only wisdom created
cannam@127 97 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>.
cannam@127 98 </p>
cannam@127 99 <a name="Planning_002drigor-flags"></a>
cannam@127 100 <h4 class="subsubheading">Planning-rigor flags</h4>
cannam@127 101 <ul>
cannam@127 102 <li> <a name="index-FFTW_005fESTIMATE-2"></a>
cannam@127 103 <code>FFTW_ESTIMATE</code> specifies that, instead of actual measurements of
cannam@127 104 different algorithms, a simple heuristic is used to pick a (probably
cannam@127 105 sub-optimal) plan quickly. With this flag, the input/output arrays are
cannam@127 106 not overwritten during planning.
cannam@127 107
cannam@127 108 </li><li> <a name="index-FFTW_005fMEASURE-2"></a>
cannam@127 109 <code>FFTW_MEASURE</code> tells FFTW to find an optimized plan by actually
cannam@127 110 <em>computing</em> several FFTs and measuring their execution time.
cannam@127 111 Depending on your machine, this can take some time (often a few
cannam@127 112 seconds). <code>FFTW_MEASURE</code> is the default planning option.
cannam@127 113
cannam@127 114 </li><li> <a name="index-FFTW_005fPATIENT-2"></a>
cannam@127 115 <code>FFTW_PATIENT</code> is like <code>FFTW_MEASURE</code>, but considers a wider
cannam@127 116 range of algorithms and often produces a &ldquo;more optimal&rdquo; plan
cannam@127 117 (especially for large transforms), but at the expense of several times
cannam@127 118 longer planning time (especially for large transforms).
cannam@127 119
cannam@127 120 </li><li> <a name="index-FFTW_005fEXHAUSTIVE-1"></a>
cannam@127 121 <code>FFTW_EXHAUSTIVE</code> is like <code>FFTW_PATIENT</code>, but considers an
cannam@127 122 even wider range of algorithms, including many that we think are
cannam@127 123 unlikely to be fast, to produce the most optimal plan but with a
cannam@127 124 substantially increased planning time.
cannam@127 125
cannam@127 126 </li><li> <a name="index-FFTW_005fWISDOM_005fONLY"></a>
cannam@127 127 <code>FFTW_WISDOM_ONLY</code> is a special planning mode in which the plan
cannam@127 128 is only created if wisdom is available for the given problem, and
cannam@127 129 otherwise a <code>NULL</code> plan is returned. This can be combined with
cannam@127 130 other flags, e.g. &lsquo;<samp>FFTW_WISDOM_ONLY | FFTW_PATIENT</samp>&rsquo; creates a
cannam@127 131 plan only if wisdom is available that was created in
cannam@127 132 <code>FFTW_PATIENT</code> or <code>FFTW_EXHAUSTIVE</code> mode. The
cannam@127 133 <code>FFTW_WISDOM_ONLY</code> flag is intended for users who need to detect
cannam@127 134 whether wisdom is available; for example, if wisdom is not available
cannam@127 135 one may wish to allocate new arrays for planning so that user data is
cannam@127 136 not overwritten.
cannam@127 137
cannam@127 138 </li></ul>
cannam@127 139
cannam@127 140 <a name="Algorithm_002drestriction-flags"></a>
cannam@127 141 <h4 class="subsubheading">Algorithm-restriction flags</h4>
cannam@127 142 <ul>
cannam@127 143 <li> <a name="index-FFTW_005fDESTROY_005fINPUT"></a>
cannam@127 144 <code>FFTW_DESTROY_INPUT</code> specifies that an out-of-place transform is
cannam@127 145 allowed to <em>overwrite its input</em> array with arbitrary data; this
cannam@127 146 can sometimes allow more efficient algorithms to be employed.
cannam@127 147 <a name="index-out_002dof_002dplace"></a>
cannam@127 148
cannam@127 149 </li><li> <a name="index-FFTW_005fPRESERVE_005fINPUT-1"></a>
cannam@127 150 <code>FFTW_PRESERVE_INPUT</code> specifies that an out-of-place transform must
cannam@127 151 <em>not change its input</em> array. This is ordinarily the
cannam@127 152 <em>default</em>, except for c2r and hc2r (i.e. complex-to-real)
cannam@127 153 transforms for which <code>FFTW_DESTROY_INPUT</code> is the default. In the
cannam@127 154 latter cases, passing <code>FFTW_PRESERVE_INPUT</code> will attempt to use
cannam@127 155 algorithms that do not destroy the input, at the expense of worse
cannam@127 156 performance; for multi-dimensional c2r transforms, however, no
cannam@127 157 input-preserving algorithms are implemented and the planner will return
cannam@127 158 <code>NULL</code> if one is requested.
cannam@127 159 <a name="index-c2r-1"></a>
cannam@127 160 <a name="index-hc2r-1"></a>
cannam@127 161
cannam@127 162 </li><li> <a name="index-FFTW_005fUNALIGNED"></a>
cannam@127 163 <a name="index-alignment-1"></a>
cannam@127 164 <a name="index-fftw_005fmalloc-4"></a>
cannam@127 165 <a name="index-fftw_005falignment_005fof"></a>
cannam@127 166 <code>FFTW_UNALIGNED</code> specifies that the algorithm may not impose any
cannam@127 167 unusual alignment requirements on the input/output arrays (i.e. no
cannam@127 168 SIMD may be used). This flag is normally <em>not necessary</em>, since
cannam@127 169 the planner automatically detects misaligned arrays. The only use for
cannam@127 170 this flag is if you want to use the new-array execute interface to
cannam@127 171 execute a given plan on a different array that may not be aligned like
cannam@127 172 the original. (Using <code>fftw_malloc</code> makes this flag unnecessary
cannam@127 173 even then. You can also use <code>fftw_alignment_of</code> to detect
cannam@127 174 whether two arrays are equivalently aligned.)
cannam@127 175
cannam@127 176 </li></ul>
cannam@127 177
cannam@127 178 <a name="Limiting-planning-time"></a>
cannam@127 179 <h4 class="subsubheading">Limiting planning time</h4>
cannam@127 180
cannam@127 181 <div class="example">
cannam@127 182 <pre class="example">extern void fftw_set_timelimit(double seconds);
cannam@127 183 </pre></div>
cannam@127 184 <a name="index-fftw_005fset_005ftimelimit"></a>
cannam@127 185
cannam@127 186 <p>This function instructs FFTW to spend at most <code>seconds</code> seconds
cannam@127 187 (approximately) in the planner. If <code>seconds ==
cannam@127 188 FFTW_NO_TIMELIMIT</code> (the default value, which is negative), then
cannam@127 189 planning time is unbounded. Otherwise, FFTW plans with a
cannam@127 190 progressively wider range of algorithms until the the given time limit
cannam@127 191 is reached or the given range of algorithms is explored, returning the
cannam@127 192 best available plan.
cannam@127 193 <a name="index-FFTW_005fNO_005fTIMELIMIT"></a>
cannam@127 194 </p>
cannam@127 195
cannam@127 196 <p>For example, specifying <code>FFTW_PATIENT</code> first plans in
cannam@127 197 <code>FFTW_ESTIMATE</code> mode, then in <code>FFTW_MEASURE</code> mode, then
cannam@127 198 finally (time permitting) in <code>FFTW_PATIENT</code>. If
cannam@127 199 <code>FFTW_EXHAUSTIVE</code> is specified instead, the planner will further
cannam@127 200 progress to <code>FFTW_EXHAUSTIVE</code> mode.
cannam@127 201 </p>
cannam@127 202 <p>Note that the <code>seconds</code> argument specifies only a rough limit; in
cannam@127 203 practice, the planner may use somewhat more time if the time limit is
cannam@127 204 reached when the planner is in the middle of an operation that cannot
cannam@127 205 be interrupted. At the very least, the planner will complete planning
cannam@127 206 in <code>FFTW_ESTIMATE</code> mode (which is thus equivalent to a time limit
cannam@127 207 of 0).
cannam@127 208 </p>
cannam@127 209
cannam@127 210 <hr>
cannam@127 211 <div class="header">
cannam@127 212 <p>
cannam@127 213 Next: <a href="Real_002ddata-DFTs.html#Real_002ddata-DFTs" accesskey="n" rel="next">Real-data DFTs</a>, Previous: <a href="Complex-DFTs.html#Complex-DFTs" accesskey="p" rel="prev">Complex DFTs</a>, Up: <a href="Basic-Interface.html#Basic-Interface" accesskey="u" rel="up">Basic Interface</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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