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182 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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<title>Using Plans (FFTW 3.3.10)</title>
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<span id="Using-Plans"></span><div class="header">
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<p>
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Next: <a href="Basic-Interface.html" accesskey="n" rel="next">Basic Interface</a>, Previous: <a href="Data-Types-and-Files.html" accesskey="p" rel="prev">Data Types and Files</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="Using-Plans-1"></span><h3 class="section">4.2 Using Plans</h3>
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<p>Plans for all transform types in FFTW are stored as type
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<code>fftw_plan</code> (an opaque pointer type), and are created by one of the
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various planning routines described in the following sections.
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<span id="index-fftw_005fplan-1"></span>
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An <code>fftw_plan</code> contains all information necessary to compute the
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transform, including the pointers to the input and output arrays.
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</p>
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<div class="example">
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<pre class="example">void fftw_execute(const fftw_plan plan);
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</pre></div>
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<span id="index-fftw_005fexecute-1"></span>
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<p>This executes the <code>plan</code>, to compute the corresponding transform on
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the arrays for which it was planned (which must still exist). The plan
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is not modified, and <code>fftw_execute</code> can be called as many times as
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desired.
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</p>
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<p>To apply a given plan to a different array, you can use the new-array execute
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interface. See <a href="New_002darray-Execute-Functions.html">New-array Execute Functions</a>.
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</p>
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<p><code>fftw_execute</code> (and equivalents) is the only function in FFTW
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guaranteed to be thread-safe; see <a href="Thread-safety.html">Thread safety</a>.
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</p>
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<p>This function:
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</p><div class="example">
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<pre class="example">void fftw_destroy_plan(fftw_plan plan);
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</pre></div>
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<span id="index-fftw_005fdestroy_005fplan-1"></span>
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<p>deallocates the <code>plan</code> and all its associated data.
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</p>
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<p>FFTW’s planner saves some other persistent data, such as the
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accumulated wisdom and a list of algorithms available in the current
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configuration. If you want to deallocate all of that and reset FFTW
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to the pristine state it was in when you started your program, you can
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call:
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</p>
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<div class="example">
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<pre class="example">void fftw_cleanup(void);
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</pre></div>
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<span id="index-fftw_005fcleanup"></span>
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<p>After calling <code>fftw_cleanup</code>, all existing plans become undefined,
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and you should not attempt to execute them nor to destroy them. You can
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however create and execute/destroy new plans, in which case FFTW starts
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accumulating wisdom information again.
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</p>
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<p><code>fftw_cleanup</code> does not deallocate your plans, however. To prevent
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memory leaks, you must still call <code>fftw_destroy_plan</code> before
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executing <code>fftw_cleanup</code>.
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</p>
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<p>Occasionally, it may useful to know FFTW’s internal “cost” metric
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that it uses to compare plans to one another; this cost is
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proportional to an execution time of the plan, in undocumented units,
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if the plan was created with the <code>FFTW_MEASURE</code> or other
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timing-based options, or alternatively is a heuristic cost function
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for <code>FFTW_ESTIMATE</code> plans. (The cost values of measured and
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estimated plans are not comparable, being in different units. Also,
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costs from different FFTW versions or the same version compiled
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differently may not be in the same units. Plans created from wisdom
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have a cost of 0 since no timing measurement is performed for them.
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Finally, certain problems for which only one top-level algorithm was
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possible may have required no measurements of the cost of the whole
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plan, in which case <code>fftw_cost</code> will also return 0.) The cost
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metric for a given plan is returned by:
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</p>
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<div class="example">
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<pre class="example">double fftw_cost(const fftw_plan plan);
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</pre></div>
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<span id="index-fftw_005fcost"></span>
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<p>The following two routines are provided purely for academic purposes
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(that is, for entertainment).
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</p>
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<div class="example">
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<pre class="example">void fftw_flops(const fftw_plan plan,
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double *add, double *mul, double *fma);
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</pre></div>
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<span id="index-fftw_005fflops"></span>
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<p>Given a <code>plan</code>, set <code>add</code>, <code>mul</code>, and <code>fma</code> to an
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exact count of the number of floating-point additions, multiplications,
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and fused multiply-add operations involved in the plan’s execution. The
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total number of floating-point operations (flops) is <code>add + mul +
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2*fma</code>, or <code>add + mul + fma</code> if the hardware supports fused
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multiply-add instructions (although the number of FMA operations is only
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approximate because of compiler voodoo). (The number of operations
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should be an integer, but we use <code>double</code> to avoid overflowing
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<code>int</code> for large transforms; the arguments are of type <code>double</code>
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even for single and long-double precision versions of FFTW.)
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</p>
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<div class="example">
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<pre class="example">void fftw_fprint_plan(const fftw_plan plan, FILE *output_file);
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void fftw_print_plan(const fftw_plan plan);
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char *fftw_sprint_plan(const fftw_plan plan);
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</pre></div>
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<span id="index-fftw_005ffprint_005fplan"></span>
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<span id="index-fftw_005fprint_005fplan"></span>
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<p>This outputs a “nerd-readable” representation of the <code>plan</code> to
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the given file, to <code>stdout</code>, or two a newly allocated
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NUL-terminated string (which the caller is responsible for deallocating
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with <code>free</code>), respectively.
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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="Basic-Interface.html" accesskey="n" rel="next">Basic Interface</a>, Previous: <a href="Data-Types-and-Files.html" accesskey="p" rel="prev">Data Types and Files</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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