194 lines
8.6 KiB
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194 lines
8.6 KiB
HTML
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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>New-array Execute Functions (FFTW 3.3.10)</title>
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<link href="index.html" rel="start" title="Top">
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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="FFTW-Reference.html" rel="up" title="FFTW Reference">
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<link href="Wisdom.html" rel="next" title="Wisdom">
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</head>
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<span id="New_002darray-Execute-Functions"></span><div class="header">
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<p>
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Next: <a href="Wisdom.html" accesskey="n" rel="next">Wisdom</a>, Previous: <a href="Guru-Interface.html" accesskey="p" rel="prev">Guru Interface</a>, Up: <a href="FFTW-Reference.html" accesskey="u" rel="up">FFTW Reference</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="New_002darray-Execute-Functions-1"></span><h3 class="section">4.6 New-array Execute Functions</h3>
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<span id="index-execute-2"></span>
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<span id="index-new_002darray-execution"></span>
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<p>Normally, one executes a plan for the arrays with which the plan was
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created, by calling <code>fftw_execute(plan)</code> as described in <a href="Using-Plans.html">Using Plans</a>.
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<span id="index-fftw_005fexecute-2"></span>
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However, it is possible for sophisticated users to apply a given plan
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to a <em>different</em> array using the “new-array execute” functions
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detailed below, provided that the following conditions are met:
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</p>
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<ul>
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<li> The array size, strides, etcetera are the same (since those are set by
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the plan).
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</li><li> The input and output arrays are the same (in-place) or different
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(out-of-place) if the plan was originally created to be in-place or
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out-of-place, respectively.
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</li><li> For split arrays, the separations between the real and imaginary
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parts, <code>ii-ri</code> and <code>io-ro</code>, are the same as they were for
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the input and output arrays when the plan was created. (This
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condition is automatically satisfied for interleaved arrays.)
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</li><li> The <em>alignment</em> of the new input/output arrays is the same as that
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of the input/output arrays when the plan was created, unless the plan
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was created with the <code>FFTW_UNALIGNED</code> flag.
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<span id="index-FFTW_005fUNALIGNED-1"></span>
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Here, the alignment is a platform-dependent quantity (for example, it is
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the address modulo 16 if SSE SIMD instructions are used, but the address
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modulo 4 for non-SIMD single-precision FFTW on the same machine). In
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general, only arrays allocated with <code>fftw_malloc</code> are guaranteed to
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be equally aligned (see <a href="SIMD-alignment-and-fftw_005fmalloc.html">SIMD alignment and fftw_malloc</a>).
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</li></ul>
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<span id="index-alignment-2"></span>
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<p>The alignment issue is especially critical, because if you don’t use
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<code>fftw_malloc</code> then you may have little control over the alignment
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of arrays in memory. For example, neither the C++ <code>new</code> function
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nor the Fortran <code>allocate</code> statement provide strong enough
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guarantees about data alignment. If you don’t use <code>fftw_malloc</code>,
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therefore, you probably have to use <code>FFTW_UNALIGNED</code> (which
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disables most SIMD support). If possible, it is probably better for
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you to simply create multiple plans (creating a new plan is quick once
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one exists for a given size), or better yet re-use the same array for
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your transforms.
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</p>
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<span id="index-fftw_005falignment_005fof-1"></span>
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<p>For rare circumstances in which you cannot control the alignment of
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allocated memory, but wish to determine where a given array is
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aligned like the original array for which a plan was created, you can
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use the <code>fftw_alignment_of</code> function:
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</p><div class="example">
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<pre class="example">int fftw_alignment_of(double *p);
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</pre></div>
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<p>Two arrays have equivalent alignment (for the purposes of applying a
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plan) if and only if <code>fftw_alignment_of</code> returns the same value
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for the corresponding pointers to their data (typecast to <code>double*</code>
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if necessary).
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</p>
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<p>If you are tempted to use the new-array execute interface because you
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want to transform a known bunch of arrays of the same size, you should
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probably go use the advanced interface instead (see <a href="Advanced-Interface.html">Advanced Interface</a>)).
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</p>
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<p>The new-array execute functions are:
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</p>
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<div class="example">
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<pre class="example">void fftw_execute_dft(
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const fftw_plan p,
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fftw_complex *in, fftw_complex *out);
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void fftw_execute_split_dft(
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const fftw_plan p,
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double *ri, double *ii, double *ro, double *io);
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void fftw_execute_dft_r2c(
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const fftw_plan p,
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double *in, fftw_complex *out);
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void fftw_execute_split_dft_r2c(
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const fftw_plan p,
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double *in, double *ro, double *io);
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void fftw_execute_dft_c2r(
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const fftw_plan p,
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fftw_complex *in, double *out);
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void fftw_execute_split_dft_c2r(
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const fftw_plan p,
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double *ri, double *ii, double *out);
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void fftw_execute_r2r(
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const fftw_plan p,
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double *in, double *out);
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</pre></div>
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<span id="index-fftw_005fexecute_005fdft"></span>
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<span id="index-fftw_005fexecute_005fsplit_005fdft"></span>
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<span id="index-fftw_005fexecute_005fdft_005fr2c"></span>
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<span id="index-fftw_005fexecute_005fsplit_005fdft_005fr2c"></span>
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<span id="index-fftw_005fexecute_005fdft_005fc2r"></span>
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<span id="index-fftw_005fexecute_005fsplit_005fdft_005fc2r"></span>
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<span id="index-fftw_005fexecute_005fr2r"></span>
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<p>These execute the <code>plan</code> to compute the corresponding transform on
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the input/output arrays specified by the subsequent arguments. The
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input/output array arguments have the same meanings as the ones passed
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to the guru planner routines in the preceding sections. The <code>plan</code>
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is not modified, and these routines can be called as many times as
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desired, or intermixed with calls to the ordinary <code>fftw_execute</code>.
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</p>
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<p>The <code>plan</code> <em>must</em> have been created for the transform type
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corresponding to the execute function, e.g. it must be a complex-DFT
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plan for <code>fftw_execute_dft</code>. Any of the planner routines for that
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transform type, from the basic to the guru interface, could have been
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used to create the plan, however.
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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="Wisdom.html" accesskey="n" rel="next">Wisdom</a>, Previous: <a href="Guru-Interface.html" accesskey="p" rel="prev">Guru Interface</a>, Up: <a href="FFTW-Reference.html" accesskey="u" rel="up">FFTW Reference</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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