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<h3 class="section">4.6 New-array Execute Functions</h3>

<p><a name="index-execute-264"></a><a name="index-new_002darray-execution-265"></a>
Normally, one executes a plan for the arrays with which the plan was
created, by calling <code>fftw_execute(plan)</code> as described in <a href="Using-Plans.html#Using-Plans">Using Plans</a>.
<a name="index-fftw_005fexecute-266"></a>However, it is possible for sophisticated users to apply a given plan
to a <em>different</em> array using the &ldquo;new-array execute&rdquo; functions
detailed below, provided that the following conditions are met:

     <ul>
<li>The array size, strides, etcetera are the same (since those are set by
the plan).

     <li>The input and output arrays are the same (in-place) or different
(out-of-place) if the plan was originally created to be in-place or
out-of-place, respectively.

     <li>For split arrays, the separations between the real and imaginary
parts, <code>ii-ri</code> and <code>io-ro</code>, are the same as they were for
the input and output arrays when the plan was created.  (This
condition is automatically satisfied for interleaved arrays.)

     <li>The <dfn>alignment</dfn> of the new input/output arrays is the same as that
of the input/output arrays when the plan was created, unless the plan
was created with the <code>FFTW_UNALIGNED</code> flag.
<a name="index-FFTW_005fUNALIGNED-267"></a>Here, the alignment is a platform-dependent quantity (for example, it is
the address modulo 16 if SSE SIMD instructions are used, but the address
modulo 4 for non-SIMD single-precision FFTW on the same machine).  In
general, only arrays allocated with <code>fftw_malloc</code> are guaranteed to
be equally aligned (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).

   </ul>

   <p><a name="index-alignment-268"></a>The alignment issue is especially critical, because if you don't use
<code>fftw_malloc</code> then you may have little control over the alignment
of arrays in memory.  For example, neither the C++ <code>new</code> function
nor the Fortran <code>allocate</code> statement provide strong enough
guarantees about data alignment.  If you don't use <code>fftw_malloc</code>,
therefore, you probably have to use <code>FFTW_UNALIGNED</code> (which
disables most SIMD support).  If possible, it is probably better for
you to simply create multiple plans (creating a new plan is quick once
one exists for a given size), or better yet re-use the same array for
your transforms.

   <p>If you are tempted to use the new-array execute interface because you
want to transform a known bunch of arrays of the same size, you should
probably go use the advanced interface instead (see <a href="Advanced-Interface.html#Advanced-Interface">Advanced Interface</a>)).

   <p>The new-array execute functions are:

<pre class="example">     void fftw_execute_dft(
          const fftw_plan p,
          fftw_complex *in, fftw_complex *out);

     void fftw_execute_split_dft(
          const fftw_plan p,
          double *ri, double *ii, double *ro, double *io);

     void fftw_execute_dft_r2c(
          const fftw_plan p,
          double *in, fftw_complex *out);

     void fftw_execute_split_dft_r2c(
          const fftw_plan p,
          double *in, double *ro, double *io);

     void fftw_execute_dft_c2r(
          const fftw_plan p,
          fftw_complex *in, double *out);

     void fftw_execute_split_dft_c2r(
          const fftw_plan p,
          double *ri, double *ii, double *out);

     void fftw_execute_r2r(
          const fftw_plan p,
          double *in, double *out);
</pre>
   <p><a name="index-fftw_005fexecute_005fdft-269"></a><a name="index-fftw_005fexecute_005fsplit_005fdft-270"></a><a name="index-fftw_005fexecute_005fdft_005fr2c-271"></a><a name="index-fftw_005fexecute_005fsplit_005fdft_005fr2c-272"></a><a name="index-fftw_005fexecute_005fdft_005fc2r-273"></a><a name="index-fftw_005fexecute_005fsplit_005fdft_005fc2r-274"></a><a name="index-fftw_005fexecute_005fr2r-275"></a>
These execute the <code>plan</code> to compute the corresponding transform on
the input/output arrays specified by the subsequent arguments.  The
input/output array arguments have the same meanings as the ones passed
to the guru planner routines in the preceding sections.  The <code>plan</code>
is not modified, and these routines can be called as many times as
desired, or intermixed with calls to the ordinary <code>fftw_execute</code>.

   <p>The <code>plan</code> <em>must</em> have been created for the transform type
corresponding to the execute function, e.g. it must be a complex-DFT
plan for <code>fftw_execute_dft</code>.  Any of the planner routines for that
transform type, from the basic to the guru interface, could have been
used to create the plan, however.

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author	Chris Cannam
date	Thu, 09 May 2013 10:52:46 +0100
parents	37bf6b4a2645
children