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<title>FFTW 3.3.5: Plan execution in Fortran</title>

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<a name="Plan-execution-in-Fortran"></a>
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Next: <a href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran" accesskey="n" rel="next">Allocating aligned memory in Fortran</a>, Previous: <a href="FFTW-Fortran-type-reference.html#FFTW-Fortran-type-reference" accesskey="p" rel="prev">FFTW Fortran type reference</a>, Up: <a href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" accesskey="u" rel="up">Calling FFTW from Modern Fortran</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>
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<hr>
<a name="Plan-execution-in-Fortran-1"></a>
<h3 class="section">7.4 Plan execution in Fortran</h3>

<p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
which executes the plan to perform the transform on the input/output
arrays passed when the plan was created (see <a href="Using-Plans.html#Using-Plans">Using Plans</a>).  The
corresponding subroutine call in modern Fortran is:
</p><div class="example">
<pre class="example"> call fftw_execute(plan)
</pre></div>
<a name="index-fftw_005fexecute-6"></a>

<p>However, we have had reports that this causes problems with some
recent optimizing Fortran compilers.  The problem is, because the
input/output arrays are not passed as explicit arguments to
<code>fftw_execute</code>, the semantics of Fortran (unlike C) allow the
compiler to assume that the input/output arrays are not changed by
<code>fftw_execute</code>.  As a consequence, certain compilers end up
repositioning the call to <code>fftw_execute</code>, assuming incorrectly
that it does nothing to the arrays.
</p>
<p>There are various workarounds to this, but the safest and simplest
thing is to not use <code>fftw_execute</code> in Fortran.  Instead, use the
functions described in <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>, which take
the input/output arrays as explicit arguments.  For example, if the
plan is for a complex-data DFT and was created for the arrays
<code>in</code> and <code>out</code>, you would do:
</p><div class="example">
<pre class="example"> call fftw_execute_dft(plan, in, out)
</pre></div>
<a name="index-fftw_005fexecute_005fdft-3"></a>

<p>There are a few things to be careful of, however:
</p>
<ul>
<li> <a name="index-fftw_005fexecute_005fdft_005fr2c-2"></a>
<a name="index-fftw_005fexecute_005fdft_005fc2r-1"></a>
<a name="index-fftw_005fexecute_005fr2r-1"></a>
You must use the correct type of execute function, matching the way
the plan was created.  Complex DFT plans should use
<code>fftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
<code>fftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
use <code>fftw_execute_dft_c2r</code>.  The various r2r plans should use
<code>fftw_execute_r2r</code>.  Fortunately, if you use the wrong one you
will get a compile-time type-mismatch error (unlike legacy Fortran).

</li><li> You should normally pass the same input/output arrays that were used when
creating the plan.  This is always safe.

</li><li> <em>If</em> you pass <em>different</em> input/output arrays compared to
those used when creating the plan, you must abide by all the
restrictions of the new-array execute functions (see <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>).  The most tricky of these is the
requirement that the new arrays have the same alignment as the
original arrays; the best (and possibly only) way to guarantee this
is to use the &lsquo;<samp>fftw_alloc</samp>&rsquo; functions to allocate your arrays (see <a href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran">Allocating aligned memory in Fortran</a>). Alternatively, you can
use the <code>FFTW_UNALIGNED</code> flag when creating the
plan, in which case the plan does not depend on the alignment, but
this may sacrifice substantial performance on architectures (like x86)
with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
<a name="index-FFTW_005fUNALIGNED-2"></a>

</li></ul>

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Next: <a href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran" accesskey="n" rel="next">Allocating aligned memory in Fortran</a>, Previous: <a href="FFTW-Fortran-type-reference.html#FFTW-Fortran-type-reference" accesskey="p" rel="prev">FFTW Fortran type reference</a>, Up: <a href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" accesskey="u" rel="up">Calling FFTW from Modern Fortran</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>
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author	Chris Cannam <cannam@all-day-breakfast.com>
date	Mon, 02 Mar 2020 14:03:47 +0000
parents	7867fa7e1b6b
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