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1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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2 <html>
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3 <!-- This manual is for FFTW
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4 (version 3.3.8, 24 May 2018).
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5
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6 Copyright (C) 2003 Matteo Frigo.
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7
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8 Copyright (C) 2003 Massachusetts Institute of Technology.
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9
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10 Permission is granted to make and distribute verbatim copies of this
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11 manual provided the copyright notice and this permission notice are
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12 preserved on all copies.
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13
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14 Permission is granted to copy and distribute modified versions of this
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15 manual under the conditions for verbatim copying, provided that the
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16 entire resulting derived work is distributed under the terms of a
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17 permission notice identical to this one.
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18
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19 Permission is granted to copy and distribute translations of this manual
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20 into another language, under the above conditions for modified versions,
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22 approved by the Free Software Foundation. -->
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23 <!-- Created by GNU Texinfo 6.3, http://www.gnu.org/software/texinfo/ -->
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24 <head>
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25 <title>FFTW 3.3.8: Planner Flags</title>
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26
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27 <meta name="description" content="FFTW 3.3.8: Planner Flags">
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28 <meta name="keywords" content="FFTW 3.3.8: Planner Flags">
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33 <link href="index.html#Top" rel="start" title="Top">
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34 <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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36 <link href="Basic-Interface.html#Basic-Interface" rel="up" title="Basic Interface">
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37 <link href="Real_002ddata-DFTs.html#Real_002ddata-DFTs" rel="next" title="Real-data DFTs">
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38 <link href="Complex-DFTs.html#Complex-DFTs" rel="prev" title="Complex DFTs">
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64 -->
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65 </style>
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66
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67
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68 </head>
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69
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70 <body lang="en">
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71 <a name="Planner-Flags"></a>
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72 <div class="header">
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73 <p>
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74 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> [<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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75 </div>
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76 <hr>
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77 <a name="Planner-Flags-1"></a>
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78 <h4 class="subsection">4.3.2 Planner Flags</h4>
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79
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80 <p>All of the planner routines in FFTW accept an integer <code>flags</code>
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81 argument, which is a bitwise OR (‘<samp>|</samp>’) of zero or more of the flag
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82 constants defined below. These flags control the rigor (and time) of
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83 the planning process, and can also impose (or lift) restrictions on the
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84 type of transform algorithm that is employed.
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85 </p>
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86 <p><em>Important:</em> the planner overwrites the input array during
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87 planning unless a saved plan (see <a href="Wisdom.html#Wisdom">Wisdom</a>) is available for that
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88 problem, so you should initialize your input data after creating the
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89 plan. The only exceptions to this are the <code>FFTW_ESTIMATE</code> and
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90 <code>FFTW_WISDOM_ONLY</code> flags, as mentioned below.
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91 </p>
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92 <p>In all cases, if wisdom is available for the given problem that was
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93 created with equal-or-greater planning rigor, then the more rigorous
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94 wisdom is used. For example, in <code>FFTW_ESTIMATE</code> mode any available
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95 wisdom is used, whereas in <code>FFTW_PATIENT</code> mode only wisdom created
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96 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>.
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97 </p>
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98 <a name="Planning_002drigor-flags"></a>
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99 <h4 class="subsubheading">Planning-rigor flags</h4>
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100 <ul>
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101 <li> <a name="index-FFTW_005fESTIMATE-2"></a>
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102 <code>FFTW_ESTIMATE</code> specifies that, instead of actual measurements of
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103 different algorithms, a simple heuristic is used to pick a (probably
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104 sub-optimal) plan quickly. With this flag, the input/output arrays are
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105 not overwritten during planning.
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106
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107 </li><li> <a name="index-FFTW_005fMEASURE-2"></a>
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108 <code>FFTW_MEASURE</code> tells FFTW to find an optimized plan by actually
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109 <em>computing</em> several FFTs and measuring their execution time.
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110 Depending on your machine, this can take some time (often a few
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111 seconds). <code>FFTW_MEASURE</code> is the default planning option.
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112
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113 </li><li> <a name="index-FFTW_005fPATIENT-2"></a>
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114 <code>FFTW_PATIENT</code> is like <code>FFTW_MEASURE</code>, but considers a wider
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115 range of algorithms and often produces a “more optimal” plan
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116 (especially for large transforms), but at the expense of several times
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117 longer planning time (especially for large transforms).
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118
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119 </li><li> <a name="index-FFTW_005fEXHAUSTIVE-1"></a>
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120 <code>FFTW_EXHAUSTIVE</code> is like <code>FFTW_PATIENT</code>, but considers an
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121 even wider range of algorithms, including many that we think are
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122 unlikely to be fast, to produce the most optimal plan but with a
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123 substantially increased planning time.
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124
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125 </li><li> <a name="index-FFTW_005fWISDOM_005fONLY"></a>
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126 <code>FFTW_WISDOM_ONLY</code> is a special planning mode in which the plan
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127 is only created if wisdom is available for the given problem, and
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128 otherwise a <code>NULL</code> plan is returned. This can be combined with
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129 other flags, e.g. ‘<samp>FFTW_WISDOM_ONLY | FFTW_PATIENT</samp>’ creates a
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130 plan only if wisdom is available that was created in
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131 <code>FFTW_PATIENT</code> or <code>FFTW_EXHAUSTIVE</code> mode. The
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132 <code>FFTW_WISDOM_ONLY</code> flag is intended for users who need to detect
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133 whether wisdom is available; for example, if wisdom is not available
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134 one may wish to allocate new arrays for planning so that user data is
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135 not overwritten.
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136
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137 </li></ul>
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138
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139 <a name="Algorithm_002drestriction-flags"></a>
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140 <h4 class="subsubheading">Algorithm-restriction flags</h4>
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141 <ul>
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142 <li> <a name="index-FFTW_005fDESTROY_005fINPUT"></a>
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143 <code>FFTW_DESTROY_INPUT</code> specifies that an out-of-place transform is
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144 allowed to <em>overwrite its input</em> array with arbitrary data; this
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145 can sometimes allow more efficient algorithms to be employed.
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146 <a name="index-out_002dof_002dplace"></a>
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147
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148 </li><li> <a name="index-FFTW_005fPRESERVE_005fINPUT-1"></a>
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149 <code>FFTW_PRESERVE_INPUT</code> specifies that an out-of-place transform must
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150 <em>not change its input</em> array. This is ordinarily the
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151 <em>default</em>, except for c2r and hc2r (i.e. complex-to-real)
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152 transforms for which <code>FFTW_DESTROY_INPUT</code> is the default. In the
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153 latter cases, passing <code>FFTW_PRESERVE_INPUT</code> will attempt to use
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154 algorithms that do not destroy the input, at the expense of worse
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155 performance; for multi-dimensional c2r transforms, however, no
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156 input-preserving algorithms are implemented and the planner will return
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157 <code>NULL</code> if one is requested.
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158 <a name="index-c2r-1"></a>
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159 <a name="index-hc2r-1"></a>
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160
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161 </li><li> <a name="index-FFTW_005fUNALIGNED"></a>
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162 <a name="index-alignment-1"></a>
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163 <a name="index-fftw_005fmalloc-4"></a>
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164 <a name="index-fftw_005falignment_005fof"></a>
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165 <code>FFTW_UNALIGNED</code> specifies that the algorithm may not impose any
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166 unusual alignment requirements on the input/output arrays (i.e. no
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167 SIMD may be used). This flag is normally <em>not necessary</em>, since
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168 the planner automatically detects misaligned arrays. The only use for
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169 this flag is if you want to use the new-array execute interface to
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170 execute a given plan on a different array that may not be aligned like
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171 the original. (Using <code>fftw_malloc</code> makes this flag unnecessary
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172 even then. You can also use <code>fftw_alignment_of</code> to detect
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173 whether two arrays are equivalently aligned.)
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174
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175 </li></ul>
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176
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177 <a name="Limiting-planning-time"></a>
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178 <h4 class="subsubheading">Limiting planning time</h4>
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179
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180 <div class="example">
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181 <pre class="example">extern void fftw_set_timelimit(double seconds);
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182 </pre></div>
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183 <a name="index-fftw_005fset_005ftimelimit"></a>
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184
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185 <p>This function instructs FFTW to spend at most <code>seconds</code> seconds
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186 (approximately) in the planner. If <code>seconds ==
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187 FFTW_NO_TIMELIMIT</code> (the default value, which is negative), then
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188 planning time is unbounded. Otherwise, FFTW plans with a
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189 progressively wider range of algorithms until the the given time limit
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190 is reached or the given range of algorithms is explored, returning the
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191 best available plan.
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192 <a name="index-FFTW_005fNO_005fTIMELIMIT"></a>
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193 </p>
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194
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195 <p>For example, specifying <code>FFTW_PATIENT</code> first plans in
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196 <code>FFTW_ESTIMATE</code> mode, then in <code>FFTW_MEASURE</code> mode, then
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197 finally (time permitting) in <code>FFTW_PATIENT</code>. If
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198 <code>FFTW_EXHAUSTIVE</code> is specified instead, the planner will further
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199 progress to <code>FFTW_EXHAUSTIVE</code> mode.
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200 </p>
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201 <p>Note that the <code>seconds</code> argument specifies only a rough limit; in
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202 practice, the planner may use somewhat more time if the time limit is
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203 reached when the planner is in the middle of an operation that cannot
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204 be interrupted. At the very least, the planner will complete planning
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205 in <code>FFTW_ESTIMATE</code> mode (which is thus equivalent to a time limit
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206 of 0).
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207 </p>
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208
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209 <hr>
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210 <div class="header">
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211 <p>
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212 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> [<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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213 </div>
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214
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215
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216
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217 </body>
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218 </html>
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