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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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Chris@42 3 <!-- This manual is for FFTW
Chris@42 4 (version 3.3.5, 30 July 2016).
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Chris@42 6 Copyright (C) 2003 Matteo Frigo.
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Chris@42 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: Planner Flags</title>
Chris@42 26
Chris@42 27 <meta name="description" content="FFTW 3.3.5: Planner Flags">
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Chris@42 35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
Chris@42 36 <link href="Basic-Interface.html#Basic-Interface" rel="up" title="Basic Interface">
Chris@42 37 <link href="Real_002ddata-DFTs.html#Real_002ddata-DFTs" rel="next" title="Real-data DFTs">
Chris@42 38 <link href="Complex-DFTs.html#Complex-DFTs" rel="prev" title="Complex DFTs">
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Chris@42 69 </head>
Chris@42 70
Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="Planner-Flags"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 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>
Chris@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="Planner-Flags-1"></a>
Chris@42 79 <h4 class="subsection">4.3.2 Planner Flags</h4>
Chris@42 80
Chris@42 81 <p>All of the planner routines in FFTW accept an integer <code>flags</code>
Chris@42 82 argument, which is a bitwise OR (&lsquo;<samp>|</samp>&rsquo;) of zero or more of the flag
Chris@42 83 constants defined below. These flags control the rigor (and time) of
Chris@42 84 the planning process, and can also impose (or lift) restrictions on the
Chris@42 85 type of transform algorithm that is employed.
Chris@42 86 </p>
Chris@42 87 <p><em>Important:</em> the planner overwrites the input array during
Chris@42 88 planning unless a saved plan (see <a href="Wisdom.html#Wisdom">Wisdom</a>) is available for that
Chris@42 89 problem, so you should initialize your input data after creating the
Chris@42 90 plan. The only exceptions to this are the <code>FFTW_ESTIMATE</code> and
Chris@42 91 <code>FFTW_WISDOM_ONLY</code> flags, as mentioned below.
Chris@42 92 </p>
Chris@42 93 <p>In all cases, if wisdom is available for the given problem that was
Chris@42 94 created with equal-or-greater planning rigor, then the more rigorous
Chris@42 95 wisdom is used. For example, in <code>FFTW_ESTIMATE</code> mode any available
Chris@42 96 wisdom is used, whereas in <code>FFTW_PATIENT</code> mode only wisdom created
Chris@42 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>.
Chris@42 98 </p>
Chris@42 99 <a name="Planning_002drigor-flags"></a>
Chris@42 100 <h4 class="subsubheading">Planning-rigor flags</h4>
Chris@42 101 <ul>
Chris@42 102 <li> <a name="index-FFTW_005fESTIMATE-2"></a>
Chris@42 103 <code>FFTW_ESTIMATE</code> specifies that, instead of actual measurements of
Chris@42 104 different algorithms, a simple heuristic is used to pick a (probably
Chris@42 105 sub-optimal) plan quickly. With this flag, the input/output arrays are
Chris@42 106 not overwritten during planning.
Chris@42 107
Chris@42 108 </li><li> <a name="index-FFTW_005fMEASURE-2"></a>
Chris@42 109 <code>FFTW_MEASURE</code> tells FFTW to find an optimized plan by actually
Chris@42 110 <em>computing</em> several FFTs and measuring their execution time.
Chris@42 111 Depending on your machine, this can take some time (often a few
Chris@42 112 seconds). <code>FFTW_MEASURE</code> is the default planning option.
Chris@42 113
Chris@42 114 </li><li> <a name="index-FFTW_005fPATIENT-2"></a>
Chris@42 115 <code>FFTW_PATIENT</code> is like <code>FFTW_MEASURE</code>, but considers a wider
Chris@42 116 range of algorithms and often produces a &ldquo;more optimal&rdquo; plan
Chris@42 117 (especially for large transforms), but at the expense of several times
Chris@42 118 longer planning time (especially for large transforms).
Chris@42 119
Chris@42 120 </li><li> <a name="index-FFTW_005fEXHAUSTIVE-1"></a>
Chris@42 121 <code>FFTW_EXHAUSTIVE</code> is like <code>FFTW_PATIENT</code>, but considers an
Chris@42 122 even wider range of algorithms, including many that we think are
Chris@42 123 unlikely to be fast, to produce the most optimal plan but with a
Chris@42 124 substantially increased planning time.
Chris@42 125
Chris@42 126 </li><li> <a name="index-FFTW_005fWISDOM_005fONLY"></a>
Chris@42 127 <code>FFTW_WISDOM_ONLY</code> is a special planning mode in which the plan
Chris@42 128 is only created if wisdom is available for the given problem, and
Chris@42 129 otherwise a <code>NULL</code> plan is returned. This can be combined with
Chris@42 130 other flags, e.g. &lsquo;<samp>FFTW_WISDOM_ONLY | FFTW_PATIENT</samp>&rsquo; creates a
Chris@42 131 plan only if wisdom is available that was created in
Chris@42 132 <code>FFTW_PATIENT</code> or <code>FFTW_EXHAUSTIVE</code> mode. The
Chris@42 133 <code>FFTW_WISDOM_ONLY</code> flag is intended for users who need to detect
Chris@42 134 whether wisdom is available; for example, if wisdom is not available
Chris@42 135 one may wish to allocate new arrays for planning so that user data is
Chris@42 136 not overwritten.
Chris@42 137
Chris@42 138 </li></ul>
Chris@42 139
Chris@42 140 <a name="Algorithm_002drestriction-flags"></a>
Chris@42 141 <h4 class="subsubheading">Algorithm-restriction flags</h4>
Chris@42 142 <ul>
Chris@42 143 <li> <a name="index-FFTW_005fDESTROY_005fINPUT"></a>
Chris@42 144 <code>FFTW_DESTROY_INPUT</code> specifies that an out-of-place transform is
Chris@42 145 allowed to <em>overwrite its input</em> array with arbitrary data; this
Chris@42 146 can sometimes allow more efficient algorithms to be employed.
Chris@42 147 <a name="index-out_002dof_002dplace"></a>
Chris@42 148
Chris@42 149 </li><li> <a name="index-FFTW_005fPRESERVE_005fINPUT-1"></a>
Chris@42 150 <code>FFTW_PRESERVE_INPUT</code> specifies that an out-of-place transform must
Chris@42 151 <em>not change its input</em> array. This is ordinarily the
Chris@42 152 <em>default</em>, except for c2r and hc2r (i.e. complex-to-real)
Chris@42 153 transforms for which <code>FFTW_DESTROY_INPUT</code> is the default. In the
Chris@42 154 latter cases, passing <code>FFTW_PRESERVE_INPUT</code> will attempt to use
Chris@42 155 algorithms that do not destroy the input, at the expense of worse
Chris@42 156 performance; for multi-dimensional c2r transforms, however, no
Chris@42 157 input-preserving algorithms are implemented and the planner will return
Chris@42 158 <code>NULL</code> if one is requested.
Chris@42 159 <a name="index-c2r-1"></a>
Chris@42 160 <a name="index-hc2r-1"></a>
Chris@42 161
Chris@42 162 </li><li> <a name="index-FFTW_005fUNALIGNED"></a>
Chris@42 163 <a name="index-alignment-1"></a>
Chris@42 164 <a name="index-fftw_005fmalloc-4"></a>
Chris@42 165 <a name="index-fftw_005falignment_005fof"></a>
Chris@42 166 <code>FFTW_UNALIGNED</code> specifies that the algorithm may not impose any
Chris@42 167 unusual alignment requirements on the input/output arrays (i.e. no
Chris@42 168 SIMD may be used). This flag is normally <em>not necessary</em>, since
Chris@42 169 the planner automatically detects misaligned arrays. The only use for
Chris@42 170 this flag is if you want to use the new-array execute interface to
Chris@42 171 execute a given plan on a different array that may not be aligned like
Chris@42 172 the original. (Using <code>fftw_malloc</code> makes this flag unnecessary
Chris@42 173 even then. You can also use <code>fftw_alignment_of</code> to detect
Chris@42 174 whether two arrays are equivalently aligned.)
Chris@42 175
Chris@42 176 </li></ul>
Chris@42 177
Chris@42 178 <a name="Limiting-planning-time"></a>
Chris@42 179 <h4 class="subsubheading">Limiting planning time</h4>
Chris@42 180
Chris@42 181 <div class="example">
Chris@42 182 <pre class="example">extern void fftw_set_timelimit(double seconds);
Chris@42 183 </pre></div>
Chris@42 184 <a name="index-fftw_005fset_005ftimelimit"></a>
Chris@42 185
Chris@42 186 <p>This function instructs FFTW to spend at most <code>seconds</code> seconds
Chris@42 187 (approximately) in the planner. If <code>seconds ==
Chris@42 188 FFTW_NO_TIMELIMIT</code> (the default value, which is negative), then
Chris@42 189 planning time is unbounded. Otherwise, FFTW plans with a
Chris@42 190 progressively wider range of algorithms until the the given time limit
Chris@42 191 is reached or the given range of algorithms is explored, returning the
Chris@42 192 best available plan.
Chris@42 193 <a name="index-FFTW_005fNO_005fTIMELIMIT"></a>
Chris@42 194 </p>
Chris@42 195
Chris@42 196 <p>For example, specifying <code>FFTW_PATIENT</code> first plans in
Chris@42 197 <code>FFTW_ESTIMATE</code> mode, then in <code>FFTW_MEASURE</code> mode, then
Chris@42 198 finally (time permitting) in <code>FFTW_PATIENT</code>. If
Chris@42 199 <code>FFTW_EXHAUSTIVE</code> is specified instead, the planner will further
Chris@42 200 progress to <code>FFTW_EXHAUSTIVE</code> mode.
Chris@42 201 </p>
Chris@42 202 <p>Note that the <code>seconds</code> argument specifies only a rough limit; in
Chris@42 203 practice, the planner may use somewhat more time if the time limit is
Chris@42 204 reached when the planner is in the middle of an operation that cannot
Chris@42 205 be interrupted. At the very least, the planner will complete planning
Chris@42 206 in <code>FFTW_ESTIMATE</code> mode (which is thus equivalent to a time limit
Chris@42 207 of 0).
Chris@42 208 </p>
Chris@42 209
Chris@42 210 <hr>
Chris@42 211 <div class="header">
Chris@42 212 <p>
Chris@42 213 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>
Chris@42 214 </div>
Chris@42 215
Chris@42 216
Chris@42 217
Chris@42 218 </body>
Chris@42 219 </html>