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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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72 <a name="New_002darray-Execute-Functions"></a>
73 <div class="header">
74 <p>
75 Next: <a href="Wisdom.html#Wisdom" accesskey="n" rel="next">Wisdom</a>, Previous: <a href="Guru-Interface.html#Guru-Interface" accesskey="p" rel="prev">Guru Interface</a>, Up: <a href="FFTW-Reference.html#FFTW-Reference" accesskey="u" rel="up">FFTW Reference</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="New_002darray-Execute-Functions-1"></a>
79 <h3 class="section">4.6 New-array Execute Functions</h3>
80 <a name="index-execute-2"></a>
81 <a name="index-new_002darray-execution"></a>
82
83 <p>Normally, one executes a plan for the arrays with which the plan was
84 created, by calling <code>fftw_execute(plan)</code> as described in <a href="Using-Plans.html#Using-Plans">Using Plans</a>.
85 <a name="index-fftw_005fexecute-2"></a>
86 However, it is possible for sophisticated users to apply a given plan
87 to a <em>different</em> array using the &ldquo;new-array execute&rdquo; functions
88 detailed below, provided that the following conditions are met:
89 </p>
90 <ul>
91 <li> The array size, strides, etcetera are the same (since those are set by
92 the plan).
93
94 </li><li> The input and output arrays are the same (in-place) or different
95 (out-of-place) if the plan was originally created to be in-place or
96 out-of-place, respectively.
97
98 </li><li> For split arrays, the separations between the real and imaginary
99 parts, <code>ii-ri</code> and <code>io-ro</code>, are the same as they were for
100 the input and output arrays when the plan was created. (This
101 condition is automatically satisfied for interleaved arrays.)
102
103 </li><li> The <em>alignment</em> of the new input/output arrays is the same as that
104 of the input/output arrays when the plan was created, unless the plan
105 was created with the <code>FFTW_UNALIGNED</code> flag.
106 <a name="index-FFTW_005fUNALIGNED-1"></a>
107 Here, the alignment is a platform-dependent quantity (for example, it is
108 the address modulo 16 if SSE SIMD instructions are used, but the address
109 modulo 4 for non-SIMD single-precision FFTW on the same machine). In
110 general, only arrays allocated with <code>fftw_malloc</code> are guaranteed to
111 be equally aligned (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
112
113 </li></ul>
114
115 <a name="index-alignment-2"></a>
116 <p>The alignment issue is especially critical, because if you don&rsquo;t use
117 <code>fftw_malloc</code> then you may have little control over the alignment
118 of arrays in memory. For example, neither the C++ <code>new</code> function
119 nor the Fortran <code>allocate</code> statement provide strong enough
120 guarantees about data alignment. If you don&rsquo;t use <code>fftw_malloc</code>,
121 therefore, you probably have to use <code>FFTW_UNALIGNED</code> (which
122 disables most SIMD support). If possible, it is probably better for
123 you to simply create multiple plans (creating a new plan is quick once
124 one exists for a given size), or better yet re-use the same array for
125 your transforms.
126 </p>
127 <a name="index-fftw_005falignment_005fof-1"></a>
128 <p>For rare circumstances in which you cannot control the alignment of
129 allocated memory, but wish to determine where a given array is
130 aligned like the original array for which a plan was created, you can
131 use the <code>fftw_alignment_of</code> function:
132 </p><div class="example">
133 <pre class="example">int fftw_alignment_of(double *p);
134 </pre></div>
135 <p>Two arrays have equivalent alignment (for the purposes of applying a
136 plan) if and only if <code>fftw_alignment_of</code> returns the same value
137 for the corresponding pointers to their data (typecast to <code>double*</code>
138 if necessary).
139 </p>
140 <p>If you are tempted to use the new-array execute interface because you
141 want to transform a known bunch of arrays of the same size, you should
142 probably go use the advanced interface instead (see <a href="Advanced-Interface.html#Advanced-Interface">Advanced Interface</a>)).
143 </p>
144 <p>The new-array execute functions are:
145 </p>
146 <div class="example">
147 <pre class="example">void fftw_execute_dft(
148 const fftw_plan p,
149 fftw_complex *in, fftw_complex *out);
150
151 void fftw_execute_split_dft(
152 const fftw_plan p,
153 double *ri, double *ii, double *ro, double *io);
154
155 void fftw_execute_dft_r2c(
156 const fftw_plan p,
157 double *in, fftw_complex *out);
158
159 void fftw_execute_split_dft_r2c(
160 const fftw_plan p,
161 double *in, double *ro, double *io);
162
163 void fftw_execute_dft_c2r(
164 const fftw_plan p,
165 fftw_complex *in, double *out);
166
167 void fftw_execute_split_dft_c2r(
168 const fftw_plan p,
169 double *ri, double *ii, double *out);
170
171 void fftw_execute_r2r(
172 const fftw_plan p,
173 double *in, double *out);
174 </pre></div>
175 <a name="index-fftw_005fexecute_005fdft"></a>
176 <a name="index-fftw_005fexecute_005fsplit_005fdft"></a>
177 <a name="index-fftw_005fexecute_005fdft_005fr2c"></a>
178 <a name="index-fftw_005fexecute_005fsplit_005fdft_005fr2c"></a>
179 <a name="index-fftw_005fexecute_005fdft_005fc2r"></a>
180 <a name="index-fftw_005fexecute_005fsplit_005fdft_005fc2r"></a>
181 <a name="index-fftw_005fexecute_005fr2r"></a>
182
183 <p>These execute the <code>plan</code> to compute the corresponding transform on
184 the input/output arrays specified by the subsequent arguments. The
185 input/output array arguments have the same meanings as the ones passed
186 to the guru planner routines in the preceding sections. The <code>plan</code>
187 is not modified, and these routines can be called as many times as
188 desired, or intermixed with calls to the ordinary <code>fftw_execute</code>.
189 </p>
190 <p>The <code>plan</code> <em>must</em> have been created for the transform type
191 corresponding to the execute function, e.g. it must be a complex-DFT
192 plan for <code>fftw_execute_dft</code>. Any of the planner routines for that
193 transform type, from the basic to the guru interface, could have been
194 used to create the plan, however.
195 </p>
196 <hr>
197 <div class="header">
198 <p>
199 Next: <a href="Wisdom.html#Wisdom" accesskey="n" rel="next">Wisdom</a>, Previous: <a href="Guru-Interface.html#Guru-Interface" accesskey="p" rel="prev">Guru Interface</a>, Up: <a href="FFTW-Reference.html#FFTW-Reference" accesskey="u" rel="up">FFTW Reference</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>
200 </div>
201
202
203
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