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