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205 lines
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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<title>Planner Flags (FFTW 3.3.10)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Basic-Interface.html" rel="up" title="Basic Interface">
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<link href="Real_002ddata-DFTs.html" rel="next" title="Real-data DFTs">
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<span id="Planner-Flags"></span><div class="header">
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<p>
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Next: <a href="Real_002ddata-DFTs.html" accesskey="n" rel="next">Real-data DFTs</a>, Previous: <a href="Complex-DFTs.html" accesskey="p" rel="prev">Complex DFTs</a>, Up: <a href="Basic-Interface.html" accesskey="u" rel="up">Basic Interface</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="Planner-Flags-1"></span><h4 class="subsection">4.3.2 Planner Flags</h4>
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<p>All of the planner routines in FFTW accept an integer <code>flags</code>
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argument, which is a bitwise OR (‘<samp>|</samp>’) of zero or more of the flag
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constants defined below. These flags control the rigor (and time) of
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the planning process, and can also impose (or lift) restrictions on the
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type of transform algorithm that is employed.
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</p>
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<p><em>Important:</em> the planner overwrites the input array during
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planning unless a saved plan (see <a href="Wisdom.html">Wisdom</a>) is available for that
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problem, so you should initialize your input data after creating the
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plan. The only exceptions to this are the <code>FFTW_ESTIMATE</code> and
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<code>FFTW_WISDOM_ONLY</code> flags, as mentioned below.
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</p>
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<p>In all cases, if wisdom is available for the given problem that was
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created with equal-or-greater planning rigor, then the more rigorous
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wisdom is used. For example, in <code>FFTW_ESTIMATE</code> mode any available
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wisdom is used, whereas in <code>FFTW_PATIENT</code> mode only wisdom created
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in patient or exhaustive mode can be used. See <a href="Words-of-Wisdom_002dSaving-Plans.html">Words of Wisdom-Saving Plans</a>.
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</p>
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<span id="Planning_002drigor-flags"></span><h4 class="subsubheading">Planning-rigor flags</h4>
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<ul>
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<li> <span id="index-FFTW_005fESTIMATE-2"></span>
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<code>FFTW_ESTIMATE</code> specifies that, instead of actual measurements of
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different algorithms, a simple heuristic is used to pick a (probably
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sub-optimal) plan quickly. With this flag, the input/output arrays are
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not overwritten during planning.
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</li><li> <span id="index-FFTW_005fMEASURE-2"></span>
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<code>FFTW_MEASURE</code> tells FFTW to find an optimized plan by actually
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<em>computing</em> several FFTs and measuring their execution time.
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Depending on your machine, this can take some time (often a few
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seconds). <code>FFTW_MEASURE</code> is the default planning option.
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</li><li> <span id="index-FFTW_005fPATIENT-2"></span>
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<code>FFTW_PATIENT</code> is like <code>FFTW_MEASURE</code>, but considers a wider
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range of algorithms and often produces a “more optimal” plan
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(especially for large transforms), but at the expense of several times
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longer planning time (especially for large transforms).
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</li><li> <span id="index-FFTW_005fEXHAUSTIVE-1"></span>
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<code>FFTW_EXHAUSTIVE</code> is like <code>FFTW_PATIENT</code>, but considers an
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even wider range of algorithms, including many that we think are
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unlikely to be fast, to produce the most optimal plan but with a
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substantially increased planning time.
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</li><li> <span id="index-FFTW_005fWISDOM_005fONLY"></span>
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<code>FFTW_WISDOM_ONLY</code> is a special planning mode in which the plan
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is only created if wisdom is available for the given problem, and
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otherwise a <code>NULL</code> plan is returned. This can be combined with
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other flags, e.g. ‘<samp>FFTW_WISDOM_ONLY | FFTW_PATIENT</samp>’ creates a
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plan only if wisdom is available that was created in
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<code>FFTW_PATIENT</code> or <code>FFTW_EXHAUSTIVE</code> mode. The
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<code>FFTW_WISDOM_ONLY</code> flag is intended for users who need to detect
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whether wisdom is available; for example, if wisdom is not available
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one may wish to allocate new arrays for planning so that user data is
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not overwritten.
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</li></ul>
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<span id="Algorithm_002drestriction-flags"></span><h4 class="subsubheading">Algorithm-restriction flags</h4>
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<ul>
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<li> <span id="index-FFTW_005fDESTROY_005fINPUT"></span>
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<code>FFTW_DESTROY_INPUT</code> specifies that an out-of-place transform is
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allowed to <em>overwrite its input</em> array with arbitrary data; this
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can sometimes allow more efficient algorithms to be employed.
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<span id="index-out_002dof_002dplace"></span>
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</li><li> <span id="index-FFTW_005fPRESERVE_005fINPUT-1"></span>
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<code>FFTW_PRESERVE_INPUT</code> specifies that an out-of-place transform must
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<em>not change its input</em> array. This is ordinarily the
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<em>default</em>, except for c2r and hc2r (i.e. complex-to-real)
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transforms for which <code>FFTW_DESTROY_INPUT</code> is the default. In the
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latter cases, passing <code>FFTW_PRESERVE_INPUT</code> will attempt to use
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algorithms that do not destroy the input, at the expense of worse
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performance; for multi-dimensional c2r transforms, however, no
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input-preserving algorithms are implemented and the planner will return
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<code>NULL</code> if one is requested.
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<span id="index-c2r-1"></span>
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<span id="index-hc2r-1"></span>
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</li><li> <span id="index-FFTW_005fUNALIGNED"></span>
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<span id="index-alignment-1"></span>
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<span id="index-fftw_005fmalloc-4"></span>
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<span id="index-fftw_005falignment_005fof"></span>
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<code>FFTW_UNALIGNED</code> specifies that the algorithm may not impose any
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unusual alignment requirements on the input/output arrays (i.e. no
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SIMD may be used). This flag is normally <em>not necessary</em>, since
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the planner automatically detects misaligned arrays. The only use for
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this flag is if you want to use the new-array execute interface to
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execute a given plan on a different array that may not be aligned like
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the original. (Using <code>fftw_malloc</code> makes this flag unnecessary
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even then. You can also use <code>fftw_alignment_of</code> to detect
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whether two arrays are equivalently aligned.)
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</li></ul>
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<span id="Limiting-planning-time"></span><h4 class="subsubheading">Limiting planning time</h4>
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<div class="example">
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<pre class="example">extern void fftw_set_timelimit(double seconds);
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</pre></div>
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<span id="index-fftw_005fset_005ftimelimit"></span>
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<p>This function instructs FFTW to spend at most <code>seconds</code> seconds
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(approximately) in the planner. If <code>seconds ==
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FFTW_NO_TIMELIMIT</code> (the default value, which is negative), then
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planning time is unbounded. Otherwise, FFTW plans with a
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progressively wider range of algorithms until the given time limit
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is reached or the given range of algorithms is explored, returning the
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best available plan.
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<span id="index-FFTW_005fNO_005fTIMELIMIT"></span>
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</p>
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<p>For example, specifying <code>FFTW_PATIENT</code> first plans in
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<code>FFTW_ESTIMATE</code> mode, then in <code>FFTW_MEASURE</code> mode, then
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finally (time permitting) in <code>FFTW_PATIENT</code>. If
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<code>FFTW_EXHAUSTIVE</code> is specified instead, the planner will further
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progress to <code>FFTW_EXHAUSTIVE</code> mode.
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</p>
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<p>Note that the <code>seconds</code> argument specifies only a rough limit; in
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practice, the planner may use somewhat more time if the time limit is
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reached when the planner is in the middle of an operation that cannot
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be interrupted. At the very least, the planner will complete planning
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in <code>FFTW_ESTIMATE</code> mode (which is thus equivalent to a time limit
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of 0).
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Real_002ddata-DFTs.html" accesskey="n" rel="next">Real-data DFTs</a>, Previous: <a href="Complex-DFTs.html" accesskey="p" rel="prev">Complex DFTs</a>, Up: <a href="Basic-Interface.html" accesskey="u" rel="up">Basic Interface</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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